| 1. |
- Afram, Gabriel, et al.
(författare)
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Reduced intensity conditioning increases risk of severe cGVHD : identification of risk factors for cGVHD in a multicenter setting
- 2018
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Ingår i: Medical Oncology. - : Springer Science and Business Media LLC. - 1357-0560 .- 1559-131X. ; 35:6
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Tidskriftsartikel (refereegranskat)abstract
- Chronic graft-versus-host disease (cGVHD) remains a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HSCT). Aim is to identify risk factors for the development of cGVHD in a multicenter setting. Patients transplanted between 2000 and 2006 were analyzed (n = 820). Donors were HLA-identical siblings (57%), matched unrelated donors (30%), and HLA-A, B or DR antigen mismatched (13%). Reduced intensity conditioning (RIC) was given to 65% of patients. Overall incidence of cGVHD was 46% for patients surviving more than 100 days after HSCT (n = 747). Older patient age [HR 1.15, p < 0.001], prior acute GVHD [1.30, p = 0.024], and RIC [1.36, p = 0.028] increased overall cGVHD. In addition, RIC [4.85, p < 0.001], prior aGVHD [2.14, p = 0.001] and female donor to male recipient [1.80, p = 0.008] increased the risk of severe cGVHD. ATG had a protective effect for both overall [0.41, p < 0.001] and severe cGVHD [0.20, p < 0.001]. Relapse-free survival (RFS) was impaired in patients with severe cGVHD. RIC, prior aGVHD, and female-to-male donation increase the risk of severe cGVHD. ATG reduces the risk of all grades of cGVHD without hampering RFS. GVHD prophylaxis may be tailored according to the risk profile of patients.
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| 2. |
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| 3. |
- Pérez-Simón, Jose A, et al.
(författare)
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Evaluation of prognostic factors among patients with chronic graft-versus-host disease
- 2012
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Ingår i: Haematologica. - : Ferrata Storti Foundation (Haematologica). - 0390-6078 .- 1592-8721. ; 97:8, s. 1187-1195
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND:Chronic graft-versus-host disease (cGVHD) is a major complication after allogeneic stem cell transplantation with an adverse effect on both mortality and morbidity. In 2005, the National Institute of Health proposed new criteria for diagnosis and classification of chronic graft-versus-host disease for clinical trials. New sub-categories were recognized such as late onset acute graft-versus-host disease and overlap syndrome.DESIGN AND METHODS:We evaluated the prognostic impact of the new sub-categories as well as the clinical scoring system proposed by the National Institute of Health in a retrospective, multicenter study of 820 patients undergoing allogeneic stem cell transplantation between 2000 and 2006 at 3 different institutions. Patients were retrospectively categorized according to the National Institute of Health criteria from patients' medical histories.RESULTS:As far as the new sub-categories are concerned, in univariate analysis diagnosis of overlap syndrome adversely affected the outcome. Also, the number of organs involved for a cut-off value of 4 significantly influenced both cGVHD related mortality and survival. In multivariate analysis, in addition to NIH score, platelet count and performance score at the time of cGVHD diagnosis, plus gut involvement, significantly influenced outcome. These 3 variables allowed us to develop a simple score system which identifies 4 subgroups of patients with 84%, 64%, 43% and 0% overall survival at five years after cGVHD diagnosis (score 0: HR=15.96 (95% CI: 6.85-37.17), P<0.001; score 1: HR=5.47 (95% CI: 2.6-11.5), P<0.001; score 2: HR=2.8 (95% CI: 1.32-5.93), P=0.007).CONCLUSIONS:In summary, we have identified a powerful and simple tool to discriminate different subgroups of patients in terms of chronic graft-versus-host disease related mortality and survival.
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| 4. |
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| 5. |
- Anderson, Bruce T., et al.
(författare)
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Persistent anomalies of the extratropical Northern Hemisphere wintertime circulation as an initiator of El Nino/Southern Oscillation events
- 2017
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Climates across both hemispheres are strongly influenced by tropical Pacific variability associated with the El Nino/Southern Oscillation (ENSO). Conversely, extratropical variability also can affect the tropics. In particular, seasonal-mean alterations of near-surface winds associated with the North Pacific Oscillation (NPO) serve as a significant extratropical forcing agent of ENSO. However, it is still unclear what dynamical processes give rise to year-to-year shifts in these long-lived NPO anomalies. Here we show that intraseasonal variability in boreal winter pressure patterns over the Central North Pacific (CNP) imparts a significant signature upon the seasonal-mean circulations characteristic of the NPO. Further we show that the seasonal-mean signature results in part from year-to-year variations in persistent, quasi-stationary low-pressure intrusions into the subtropics of the CNP, accompanied by the establishment of persistent, quasi-stationary high-pressure anomalies over high latitudes of the CNP. Overall, we find that the frequency of these persistent extratropical anomalies (PEAs) during a given winter serves as a key modulator of intraseasonal variability in extratropical North Pacific circulations and, through their influence on the seasonal-mean circulations in and around the southern lobe of the NPO, the state of the equatorial Pacific 9-12 months later.
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| 6. |
- Beal, Jacob, et al.
(författare)
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Robust estimation of bacterial cell count from optical density
- 2020
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Ingår i: Communications Biology. - : Springer Science and Business Media LLC. - 2399-3642. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data.
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| 7. |
- Berger, Marit, 1986-
(författare)
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Modelling the early to mid-Holocene Arctic climate
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the recent past it has become evident that the Earth's climate is changing, and that human activity play a significant role in these changes. One of the regions where the ongoing climate change has been most evident is in the Arctic: the surface temperature has increased twice as much in this region as compared to the global average, in addition, a significant decline in the Arctic sea-ice extent has been observed in the past decades. Climate model studies of past climates are important tools to understand the ongoing climate change and how the Earth's climate may respond to changes in the forcing.This thesis includes studies of the Arctic climate in simulations of the early and mid-Holocene, 9 000 and 6 000 years before present. Changes in the Earth's orbital parameters resulted in increased summer insolation as compared to present day, especially at high northern latitudes. Geological data imply that the surface temperatures in the early to mid Holocene were similar to those projected for the near future. In addition, the geological data implies that the Arctic sea ice cover was significantly reduced in this period. This makes the early to mid-Holocene an interesting period to study with respect to the changes observed in the region at present.Several model studies of the mid-Holocene have been performed through the Paleoclimate Modeling Intercomparison Project (PMIP1 to PMIP3). The simulations have been performed with climate models of varying complexity, from atmosphere-only models in the first phase to fully coupled models with the same resolution as used for future climate simulations in the third phase. The first part of this thesis investigates the simulated sea ice in the pre-industrial and mid-Holocene simulations included in the PMIP2 and PMIP3 ensemble. As the complexity of the models increases, the models simulate smaller extents and thinner sea ice in the Arctic; the sea-ice extent suggested by the proxy data for the mid-Holocene is however not reproduced by the majority of the models.One possible explanation for the discrepancy between the simulated and reconstructed Arctic sea ice extent is missing or inadequate representations of important processes. The representation of atmospheric aerosol direct and indirect effects in past climates is a candidate process. Previous studies of deeper time periods have concluded that the representation of the direct and indirect effects of the atmospheric aerosols can influence the simulated climates, and reduce the equator to pole temperature gradient in past warm climates, in better agreement with reconstructions. The second part of the thesis investigates the influence of aerosol on the early Holocene climate. The indirect effect of reduced aerosol concentrations as compared to the present day is found to cause an amplification of the warming, especially in the Arctic region. A better agreement with reconstructed Arctic sea ice extent is thus achieved.
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| 8. |
- Blanco, Joaquin, 1985-, et al.
(författare)
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A Cloud-Controlling Factor Perspective on the Hemispheric Asymmetry of Extratropical Cloud Albedo
- 2023
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 36:6, s. 1793-1804
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Tidskriftsartikel (refereegranskat)abstract
- The Northern and Southern Hemispheres reflect on average almost equal amounts of sunlight due to compensating hemispheric asymmetries in clear-sky and cloud albedo. Recent work indicates that the cloud albedo asymmetry is largely due to clouds in extratropical oceanic regions. Here, we investigate the proximate causes of this extratropical cloud albedo asymmetry using a cloud-controlling factor (CCF) approach. We develop a simple index that measures the skill of CCFs, either individually or in combination, in predicting the asymmetry. The index captures the contribution to the asymmetry due to interhemispheric differences in the probability distribution function of daily CCF values. Cloud albedo is quantified using daily MODIS satellite retrievals, and is related to range of CCFs derived from the ERA5 product. We find that sea surface temperature is the CCF that individually explains the largest fraction of the asymmetry, followed by surface wind. The asymmetry is predominantly due to low clouds, and our results are consistent with prior local-scale modeling work showing that marine boundary layer clouds become thicker and more extensive as surface wind increases and surface temperature cools. The asymmetry is consistent with large-scale control of storm-track intensity and surface winds by meridional temperature gradients: persistently cold and windy conditions in the Southern Hemisphere keep cloud albedo high year-round. Our results have important implications for global-scale cloud feedbacks and contribute to efforts to develop a theory for planetary albedo and its symmetry.
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| 9. |
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| 10. |
- Caballero, Rodrigo, et al.
(författare)
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Midlatitude Eddies, Storm-Track Diffusivity, and Poleward Moisture Transport in Warm Climates
- 2012
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 69:11, s. 3237-3250
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Tidskriftsartikel (refereegranskat)abstract
- Recent work using both simplified and comprehensive GCMs has shown that poleward moisture transport across midlatitudes follows Clausius-Clapeyron scaling at temperatures close to modern, but that it reaches a maximum at sufficiently elevated temperatures and then decreases with further warming. This study explores the reasons for this nonmonotonic behavior using a sequence of NCAR Community Atmosphere Model, version 3 (CAM3) simulations in an aquaplanet configuration spanning a broad range of climates. No significant change is found in the scale, structure, or organization of midlatitude eddies across these simulations. Instead, the high-temperature decrease in poleward moisture transport is attributed to the combined effect of decreasing eddy velocities and contracting mixing lengths. The contraction in mixing length is, in turn, a consequence of the decreasing eddy velocities in combination with constant eddy decorrelation time scales.
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| 11. |
- Caballero, Rodrigo, et al.
(författare)
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State-dependent climate sensitivity in past warm climates and its implications for future climate projections
- 2013
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 110:35, s. 14162-14167
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Tidskriftsartikel (refereegranskat)abstract
- Projections of future climate depend critically on refined estimates of climate sensitivity. Recent progress in temperature proxies dramatically increases the magnitude of warming reconstructed from early Paleogene greenhouse climates and demands a close examination of the forcing and feedback mechanisms that maintained this warmth and the broad dynamic range that these paleoclimate records attest to. Here, we show that several complementary resolutions to these questions are possible in the context of model simulations using modern and early Paleogene configurations. We find that (i) changes in boundary conditions representative of slow Earth system feedbacks play an important role in maintaining elevated early Paleogene temperatures, (ii) radiative forcing by carbon dioxide deviates significantly from pure logarithmic behavior at concentrations relevant for simulation of the early Paleogene, and (iii) fast or Charney climate sensitivity in this model increases sharply as the climate warms. Thus, increased forcing and increased slow and fast sensitivity can all play a substantial role in maintaining early Paleogene warmth. This poses an equifinality problem: The same climate can be maintained by a different mix of these ingredients; however, at present, the mix cannot be constrained directly from climate proxy data. The implications of strongly state-dependent fast sensitivity reach far beyond the early Paleogene. The study of past warm climates may not narrow uncertainty in future climate projections in coming centuries because fast climate sensitivity may itself be state-dependent, but proxies and models are both consistent with significant increases in fast sensitivity with increasing temperature.
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| 12. |
- Caballero, Rodrigo, et al.
(författare)
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Surface superrotation
- 2018
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 75:10, s. 3671-3689
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Tidskriftsartikel (refereegranskat)abstract
- Equatorial superrotation is commonly observed in simulations of Earth and planetary climates, but is almost without exception found to occur only at upper levels, with zero or easterly winds at the surface. Surface superrotation—a state with climatological zonal-mean westerlies at the equatorial surface—would lead to a major reorganization of the tropical ocean circulation with important consequences for global climate. Here, we examine the mechanisms that give rise to surface superrotation. We identify four theoretical scenarios under which surface superrotation may be achieved. Using an axisymmetric model forced by prescribed zonal-mean torques, we provide concrete examples of surface superrotation under all four scenarios. We also find that we can induce surface superrotation in a full-complexity atmospheric general circulation model, albeit in an extreme parameter range (in particular, convective momentum transport is artificially increased by almost an order of magnitude). We conclude that a transition to surface superrotation is unlikely in Earth-like climates, including ancient or future warm climates, though this conclusion is subject to the currently large uncertainties in the parameterization of convective momentum transport.
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| 13. |
- Carlson, Henrik, et al.
(författare)
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Atmospheric circulation and hydroclimate impacts of alternative warming scenarios for the Eocene
- 2017
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 13:8, s. 1037-1048
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Tidskriftsartikel (refereegranskat)abstract
- Recent work in modelling the warm climates of the early Eocene shows that it is possible to obtain a reasonable global match between model surface temperature and proxy reconstructions, but only by using extremely high atmospheric CO2 concentrations or more modest CO2 levels complemented by a reduction in global cloud albedo. Understanding the mix of radiative forcing that gave rise to Eocene warmth has important implications for constraining Earth's climate sensitivity, but progress in this direction is hampered by the lack of direct proxy constraints on cloud properties. Here, we explore the potential for distinguishing among different radiative forcing scenarios via their impact on regional climate changes. We do this by comparing climate model simulations of two end-member scenarios: one in which the climate is warmed entirely by CO2 (which we refer to as the greenhouse gas (GHG) scenario) and another in which it is warmed entirely by reduced cloud albedo (which we refer to as the low CO2-thin clouds or LCTC scenario). The two simulations have an almost identical global-mean surface temperature and equator-to-pole temperature difference, but the LCTC scenario has similar to 11% greater global-mean precipitation than the GHG scenario. The LCTC scenario also has cooler midlatitude continents and warmer oceans than the GHG scenario and a tropical climate which is significantly more El Nino-like. Extremely high warm-season temperatures in the subtropics are mitigated in the LCTC scenario, while cool-season temperatures are lower at all latitudes. These changes appear large enough to motivate further, more detailed study using other climate models and a more realistic set of modelling assumptions.
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| 14. |
- Carlson, Henrik, 1986-
(författare)
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Atmospheric dynamics and the hydrologic cycle in warm climates
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Past warm climates represent one extreme of Earth's known climate states. Here, we study warm climates in both idealized simulations and full-complexity general circulation model (GCM) simulations of the early Eocene epoch, approximately 50 million years ago.In increasingly warmer idealized aquaplanet simulations, the amplitude of intra-seasonal tropical variability is enhanced. The anomalies propagate eastward in the tropics and resemble the observed Madden-Julian Oscillation (MJO). The strong MJO anomalies drive a momentum convergence on the equator that causes westerly winds in the troposphere, a state known as superrotation. The results in this thesis show that superrotation further enhances the MJO by affecting the penetration of midlatitude eddies into the deep tropics. An additional question is how a super-rotating atmosphere, a dramatically different general circulation regime compared to today, will affect the climate, potentially via changes in cloud distributions and ocean circulation. If the superrotation extends down to the surface near the equator, surface westerly winds will drive equatorial downwelling in the eastern equatorial Pacific Ocean, rather than upwelling as in the present climate. Here, we show that surface superrotation is unlikely in past warm climates, although this in part depends on the intensity of the vertical momentum transfer associated with cumulus convection and how this process is represented in a specific GCM. There is, currently, no consensus on what the specific mix of forcings was that caused the warm climates of the early Eocene. High greenhouse gases likely played a significant role, but simulations with reasonable greenhouse gas concentrations cannot reproduce the high temperatures estimated by proxy data. Here, we investigate both an early Eocene climate forced by high greenhouse gas concentrations and one forced by optically thinner clouds, with artificially increased cloud droplet radius that causes increased solar radiation at the surface. Both alternative warming scenarios produce nearly identical zonal mean temperatures, but the hydrologic cycle differs; the thinner clouds scenario has 11% larger global mean precipitation. Moreover, the results in this thesis indicate that a reasonable estimate of vegetation, based on the model simulation, is likely necessary to evaluate alternative warming scenarios with proxy data.
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| 15. |
- Carlson, Henrik, et al.
(författare)
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Enhanced MJO and transition to superrotation in warm climates
- 2016
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Ingår i: Journal of Advances in Modeling Earth Systems. - 1942-2466. ; 8:1, s. 304-318
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Tidskriftsartikel (refereegranskat)abstract
- Using the NCAR CAM3 model in aquaplanet configuration, we perform a suite of simulations spanning a broad range of warm climates. The simulations show a spontaneous transition to superrotation, i.e., westerly winds at upper levels above the equator. The momentum convergence leading to superrotation is driven by eastward-propagating equatorial waves with structure similar to the modern Madden-Julian Oscillation (MJO), whose amplitude increases strongly with temperature. We analyze the moist static energy (MSE) budget of the model's MJO to identify mechanisms leading to its enhanced amplitude. Two such mechanisms are identified: a rapid increase of mean low-level MSE with rising temperature, as found in previous work, and reduced damping of the MJO by synoptic-scale eddies. Both effects imply a reduced gross moist stability and enhanced MJO amplitude. The reduced eddy damping is caused by the transition to superrotation, which allows a greater penetration of extratropical eddies into the equatorial zone; the dominant effect of this greater penetration is to flatten the meridional gradient zonal-mean MSE, which effectively impedes the generation of anomalous MSE divergence by MJO-modulated eddies. This mechanism implies a positive feedback between superrotation and the MJO which may hasten the transition into a strongly superrotating state.
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| 16. |
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| 17. |
- Chabangborn, Akkaneewut, 1977-
(författare)
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The Asian monsoon - 50-7 ka BP
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Asian monsoon is one of the largest climatic systems on Earth. It covers an area from the Arabian Sea to the South China Sea and from northern Australia to northern China with the world’s highest population density. Moreover, the Asian monsoon transports heat energy and humidity to higher latitudes. In order to better understand the behaviour of the Asian monsoon and its environmental impact, its variability between 50 and 7 ka BP is analysed using paleo-data compilation, data-model comparisons, and lake sediment analysis. The main results presented here are from the compilation of the Asian monsoon variability during the last glacial maximum (LGM) (23 - 19 ka BP) which is presumed to be under persistence cool and dry climatic conditions. The pattern of reconstructed and simulated precipitation agrees well in most of the region. However, the data-model discrepancies show in some areas, which may come from low resolution of the model or the local topographic effect. The reconstructed SSTs are well correlation with simulated SSTs, except in the Arabian Sea. The LGM Asian monsoon changes around 20 – 19 ka BP. The simulated ITCZ varies between 5°N and 15°N in the west and the east of the Asian monsoon region. However, the reconstructed ITCZ is ~5°N in the Arabian Sea, shifts northward in the Bay of Bengal, reaches ~30°N over central of China and migrates southward in the South China Sea. The ITCZ is likely shift northward after 20 ka BP. The climatic change might have been triggered by several factors, e.g., an increased land-sea thermal contrast and a variation of Pacific water inflow.
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| 18. |
- Datseris, George, et al.
(författare)
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Minimal Recipes for Global Cloudiness
- 2022
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 49:20
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Tidskriftsartikel (refereegranskat)abstract
- Clouds are primary modulators of Earth's energy balance. It is thus important to understand the links connecting variabilities in cloudiness to variabilities in other state variables of the climate system, and also describe how these links would change in a changing climate. A conceptual model of global cloudiness can help elucidate these points. In this work we derive simple representations of cloudiness, that can be useful in creating a theory of global cloudiness. These representations illustrate how both spatial and temporal variability of cloudiness can be expressed in terms of basic state variables. Specifically, cloud albedo is captured by a nonlinear combination of pressure velocity and a measure of the low-level stability, and cloud longwave effect is captured by surface temperature, pressure velocity, and standard deviation of pressure velocity. We conclude with a short discussion on the usefulness of this work in the context of global warming response studies.
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| 19. |
- de Boer, Agatha M., et al.
(författare)
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Processes driving thunderstorms over the Agulhas Current
- 2013
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. - : American Geophysical Union (AGU). - 2169-897X. ; 118:5, s. 2220-2228
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Tidskriftsartikel (refereegranskat)abstract
- Lightning occurs predominantly over land and is not common over the open ocean. We study here one oceanic region in which thunderstorms are frequently found, namely the warm Agulhas Current off the southeast coast of South Africa. The seasonal and interannual lightning variability is derived from satellite and terrestrial data sets. Favorable climatic conditions for lightning are investigated using both ERA-Interim and NCEP/NCAR reanalysis data. We find peak lightning in austral autumn over the Agulhas Current but with low seasonality (i.e., there is considerable lightning throughout the year). While the climatological wind direction varies strongly with latitude and season, the wind direction is predominantly northerly throughout the region during thunderstorms. A composite of sea level pressure during thunderstorm days indicates that thunderstorms are related to eastward-propagating synoptic-scale wave trains passing through the Agulhas Current region. The strong convective activity during thunderstorms occur in the warm sector of a cyclone and is associated with horizontal convergence and lifting of warm, moist surface air originating over the warm Agulhas Current.
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| 20. |
- de Erausquin, Gabriel A, et al.
(författare)
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Chronic neuropsychiatric sequelae of SARS-CoV-2: Protocol and methods from the Alzheimer's Association Global Consortium.
- 2022
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Ingår i: Alzheimer's & dementia (New York, N. Y.). - : Wiley. - 2352-8737. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- Coronavirus disease 2019 (COVID-19) has caused >3.5 million deaths worldwide and affected >160 million people. At least twice as many have been infected but remained asymptomatic or minimally symptomatic. COVID-19 includes central nervous system manifestations mediated by inflammation and cerebrovascular, anoxic, and/or viral neurotoxicity mechanisms. More than one third of patients with COVID-19 develop neurologic problems during the acute phase of the illness, including loss of sense of smell or taste, seizures, and stroke. Damage or functional changes to the brain may result in chronic sequelae. The risk of incident cognitive and neuropsychiatric complications appears independent from the severity of the original pulmonary illness. It behooves the scientific and medical community to attempt to understand the molecular and/or systemic factors linking COVID-19 to neurologic illness, both short and long term.This article describes what is known so far in terms of links among COVID-19, the brain, neurological symptoms, and Alzheimer's disease (AD) and related dementias. We focus on risk factors and possible molecular, inflammatory, and viral mechanisms underlying neurological injury. We also provide a comprehensive description of the Alzheimer's Association Consortium on Chronic Neuropsychiatric Sequelae of SARS-CoV-2 infection (CNS SC2) harmonized methodology to address these questions using a worldwide network of researchers and institutions.Successful harmonization of designs and methods was achieved through a consensus process initially fragmented by specific interest groups (epidemiology, clinical assessments, cognitive evaluation, biomarkers, and neuroimaging). Conclusions from subcommittees were presented to the whole group and discussed extensively. Presently data collection is ongoing at 19 sites in 12 countries representing Asia, Africa, the Americas, and Europe.The Alzheimer's Association Global Consortium harmonized methodology is proposed as a model to study long-term neurocognitive sequelae of SARS-CoV-2 infection.The following review describes what is known so far in terms of molecular and epidemiological links among COVID-19, the brain, neurological symptoms, and AD and related dementias (ADRD)The primary objective of this large-scale collaboration is to clarify the pathogenesis of ADRD and to advance our understanding of the impact of a neurotropic virus on the long-term risk of cognitive decline and other CNS sequelae. No available evidence supports the notion that cognitive impairment after SARS-CoV-2 infection is a form of dementia (ADRD or otherwise). The longitudinal methodologies espoused by the consortium are intended to provide data to answer this question as clearly as possible controlling for possible confounders. Our specific hypothesis is that SARS-CoV-2 triggers ADRD-like pathology following the extended olfactory cortical network (EOCN) in older individuals with specific genetic susceptibility.The proposed harmonization strategies and flexible study designs offer the possibility to include large samples of under-represented racial and ethnic groups, creating a rich set of harmonized cohorts for future studies of the pathophysiology, determinants, long-term consequences, and trends in cognitive aging, ADRD, and vascular disease.We provide a framework for current and future studies to be carried out within the Consortium. and offers a "green paper" to the research community with a very broad, global base of support, on tools suitable for low- and middle-income countries aimed to compare and combine future longitudinal data on the topic.The Consortium proposes a combination of design and statistical methods as a means of approaching causal inference of the COVID-19 neuropsychiatric sequelae. We expect that deep phenotyping of neuropsychiatric sequelae may provide a series of candidate syndromes with phenomenological and biological characterization that can be further explored. By generating high-quality harmonized data across sites we aim to capture both descriptive and, where possible, causal associations.
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| 21. |
- Dimitrelos, Antonios, 1986-
(författare)
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A large-eddy simulation perspective on Arctic airmass transformation and low-level cloud evolution
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Arctic is currently warming faster than other regions of the Earth. Many processes and feedbacks contribute to the enhanced warming. Among these are the radiative effects of clouds. Arctic mixed-phase clouds, which contain both liquid and ice condensate, have high longevity and can exert significant surface warming since the amount of solar radiation in the region is relatively low and the surface reflectivity often is high. In this thesis, we study these clouds utilizing a large-eddy model coupled with one-dimensional thermodynamic sea ice model. The main aim is to understand the interactions between cloud dynamics, microphysics, radiation, and turbulent processes and how these together govern the life cycle and surface warming of the clouds. By comparing a group of models with observations from the summertime high Arctic, we confirm the hypothesis that when aerosol concentrations are low, a small increase in their number concentration can increase the liquid water content of the cloud and in turn, the surface warming. Idealized simulations of moist intrusions into the Arctic show that the surface temperature may increase by more than 15o C if we allow clouds to form during a moist intrusion compared to if the atmosphere is cloud free. The simulations also show that the large-scale divergence rate strongly impacts the maintenance of the liquid layer at the top of these clouds. A main finding of the thesis is that the temperature of the cloud that forms during a moist intrusion is close to the initial dew point temperature. Thus, the surface warming induced by the clouds depends mostly on the initial humidity of the air mass rather than the initial temperature. In addition, the stability of the initial dew point temperature profile largely controls the turbulent state of the cloud. If the profile is unstable, then the cloud can transform from a thin, stable stratus to a deeper stratocumulus cloud, which also enhances the surface warming. Consequently, both the initial amount and the vertical structure of the initial moisture of the intrusion are important for the warming of the sea ice. A change in the number of cloud condensation nuclei does not affect the cloud evolution considerably provided that there is a continuous supply of these nuclei. However, if cloud condensation nuclei sources are absent then the cloud may remain in its stable state. Furthermore, a decrease in the cloud ice condensate, which may be caused by a lack of ice nucleation particles, may delay the transformation of the cloud into a stratocumulus. These results suggest that any future change in aerosol loading and atmospheric moisture transport into the Arctic may alter the surface longwave cloud radiative effect and cause changes in the sea ice evolution.
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| 22. |
- Dimitrelos, Antonios, et al.
(författare)
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A Sensitivity Study of Arctic Air-Mass Transformation Using Large Eddy Simulation
- 2020
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Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 125:6
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Tidskriftsartikel (refereegranskat)abstract
- Arctic air mass transformation is linked to the evolution of low-level mixed-phase clouds. These clouds can alter the structure of the boundary layer and modify the surface energy budget. In this study, we use three-dimensional large eddy simulation and a bulk sea ice model to examine the lifecycle of clouds formed during wintertime advection of moist and warm air over sea ice, following a Lagrangian perspective. We investigate the stages of cloud formation, evolution, and decay. The results show that radiative cooling at the surface gives rise to fog formation which subsequently rises and transforms into a mixed-phase cloud. In our baseline simulation, the cloud persists for about 5 days and increases the surface temperature by on average 17 degrees C. Sensitivity tests show that the lifetime of the cloud is sensitive to changes in the vapor supply at cloud top. This flux is mainly impacted by changes in the divergence rate; an imposed convergence decreases the lifetime to 2 days while an imposed large-scale divergence increases the lifetime to more than 6 days. The largest difference in cloud radiative properties is found in the experiment with increased ice crystal number concentrations. In this case, the lifetime of the cloud is similar compared to baseline but the amount of liquid water is clearly depleted throughout the whole cloud sequence and the surface temperature is on average 6 degrees C cooler. The cloud condensation nuclei concentration has a weaker effect on the radiative properties and lifetime of the cloud. Plain Language Summary Arctic air mass transformation is a process in which an air mass originating over the open ocean enters the high Arctic and cools. Low-altitude clouds form and are often very persistent. They can exist for several days and warm the surface by emitting infrared radiation towards the surface. In this study, we have investigated the effect of the cloud on the surface energy budget by conducting large eddy simulations. In the model code we have incorporated a module that considers the thermodynamics of the sea ice surface. Knowing the sensitivity of these clouds to different parameters and physical processes will make us capable of predicting the cloud lifetime and radiative properties, and thus the induced warming effect on the sea ice surface. We have found that an increased ice crystal number concentration leads to a tenuous form of the cloud that only weakly warms the surface. An imposed large-scale ascent or descent affects the cloud lifetime by more than a day. Increasing the number of cloud condensation nuclei enhances the warming effect of the cloud.
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| 23. |
- Dimitrelos, Antonios, 1986-, et al.
(författare)
-
Controls on surface warming by winter Arctic moist intrusions in idealized large-eddy simulations
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The main energy input to the polar regions in winter is the advection of warm, moist air from lower latitudes. This makes the polar climate sensitive to the temperature and moisture of extra-polar air. Here, we study this sensitivity from an air-mass transformation perspective. We perform simulations of an idealized maritime air mass brought into contact with sea ice employing a three-dimensional large-eddy simulation model coupled to a one-dimensional multilayer sea ice model. We study the response of cloud dynamics and surface warming during the air-mass transformation process to varying initial temperature and humidity conditions of the air mass. We find in all cases that a mixed-phase cloud is formed, initially near the surface but rising continuously with time. Surface warming of the sea ice is driven by downward longwave surface fluxes, which are largerly controlled by the temperature and optical depth of the cloud. Cloud temperature, in turn, is robustly constarined by the initial dewpoint temperature of the air mass. Since dewpoint only depends on moisture, the overall result is that surface warming depends almost exclusively on initial humidity and is largerly independent of initial temperature. We discuss possible climate implications of this result, in particular for polar amplification of surface warming and the role played by atmospheric energy transport.
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| 24. |
- Dimitrelos, Antonios, et al.
(författare)
-
Controls on Surface Warming by Winter Arctic Moist Intrusions in Idealized Large-Eddy Simulations
- 2023
-
Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 36:5, s. 1287-1300
-
Tidskriftsartikel (refereegranskat)abstract
- The main energy input to the polar regions in winter is the advection of warm, moist air from lower lati-tudes. This makes the polar climate sensitive to the temperature and moisture of extrapolar air. Here, we study this sensi-tivity from an air-mass transformation perspective. We perform simulations of an idealized maritime air mass brought into contact with sea ice employing a three-dimensional large-eddy simulation model coupled to a one-dimensional multilayer sea ice model. We study the response of cloud dynamics and surface warming during the air-mass transformation process to varying initial temperature and humidity conditions of the air mass. We find in all cases that a mixed-phase cloud is formed, initially near the surface but rising continuously with time. Surface warming of the sea ice is driven by downward longwave surface fluxes, which are largely controlled by the temperature and optical depth of the cloud. Cloud tempera-ture, in turn, is robustly constrained by the initial dewpoint temperature of the air mass. Since dewpoint only depends on moisture, the overall result is that surface warming depends almost exclusively on initial humidity and is largely indepen-dent of initial temperature. We discuss possible climate implications of this result-in particular, for polar amplification of surface warming and the role played by atmospheric energy transports.
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| 25. |
- Dimitrelos, Antonios, 1986-, et al.
(författare)
-
The turbulent state of Arctic mixed-phase clouds under different conditions of moisture, aerosols, and ice water content
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Previous studies have shown that low-level mixed-phase clouds (MPCs) that form during moist intrusions into the Arctic can exist in either a stable (stratus) or a convective (stratocumulus) state. Here, we examine the process and conditions that promote a transition between the two states through idealized simulations using a three-dimensional large-eddy simulation model coupled with a one-dimensional multilayer sea ice model. We find that the vertical distribution of the initial dew point temperature (Td) profile has a fundamental influence on whether a transition from stable to convective conditions occurs or not. If the initial moisture content of the advected airmass decreased rapidly with height, i.e. if the Td profile is unstable, then a turbulent transition is likely to occur and a stratocumulus cloud can form. However, the availability and properties of aerosols and the cloud ice content can delay or even prevent stratocumulus formation, regardless if the conditions in terms of the initial Td profile are favorable. If no suitable cloud condensation nuclei are available at the base of the cloud at the time of the transition, then no new droplets form, the buoyancy remains low and the cloud remains in its stable state. Furthermore, a decrease in cloud ice water content results in a more stably stratified cloud layer and a delay in the transition. The transition from the stable to the convective state has a substantial effect on the surface warming induced by the cloud in the model; simulations with a transition generally show larger surface warming than simulations without a transition. Our results suggest that the low-level mixed-phase cloud evolution and the thermodynamic transition of an airmass during a moist intrusion are closely linked to the aerosol processing by the cloud, i.e. a chemical transformation, and that the two processes should be considered simultaneously.
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| 26. |
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| 27. |
- Embert, Dirk, et al.
(författare)
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Priority areas for amphibian conservation in a neotropical megadiverse country : the need for alternative, non place based, conservation
- 2011
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Ingår i: Biodiversity and Conservation. - : Springer Science and Business Media LLC. - 0960-3115 .- 1572-9710. ; 20:7, s. 1557-1570
-
Tidskriftsartikel (refereegranskat)abstract
- Amphibians constitute one of the most threatened species group worldwide. Because they are affected by various threats such as habitat destruction, chytrid fungus, climate change, invasive species and human use, different approaches are needed for their conservation. In this paper we examine patterns of alpha diversity, endemism and threatened species distributions of amphibians in Bolivia. We modeled distribution data using ecological niche modeling implemented with MaxEnt (Maximum Entropy Modeling). In contrast with previous studies, we validated each individual map with specialists. Important areas for amphibian conservation were defined by overlapping species richness, endemism richness and concentration of threatened species, and then excluding all unsuitable habitats. Our goal was to identify high-priority sites for the conservation of amphibians in Bolivia. We discuss a place-based strategy (identification of priority areas) and the need for further non place-based strategies. We highlight the importance of strengthening two existing protected areas that together host up to 40% of Bolivia's threatened amphibian species, and we propose alternative strategies such as ex situ conservation for those species not covered by the priority areas. Bolivia's General Directorate for Biodiversity and Protected Areas (in Spanish, Direccin General de Biodiversidad y Areas Protegidas (DGBAP)) and Bolivian experts are jointly using the here presented results to define a national strategy of amphibian conservation for safeguarding this important element of Bolivia's biodiversity.
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| 28. |
- Engelke, Wito, 1983-, et al.
(författare)
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Topology-Based Feature Design and Tracking for Multi-center Cyclones
- 2021
-
Ingår i: Topological Methods in Data Analysis and Visualization VI. - Cham : Springer. - 9783030834999 - 9783030835002 ; , s. 71-85
-
Bokkapitel (refereegranskat)abstract
- In this paper, we propose a concept to design, track, and compare application-specific feature definitions expressed as sets of critical points. Our work has been inspired by the observation that in many applications a large variety of different feature definitions for the same concept are used. Often, these definitions compete with each other and it is unclear which definition should be used in which context. A prominent example is the definition of cyclones in climate research. Despite the differences, frequently these feature definitions can be related to topological concepts.
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| 29. |
- Goldner, A., et al.
(författare)
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Does Antarctic glaciation cool the world?
- 2013
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 9:1, s. 173-189
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, we compare the simulated climatic impact of adding an Antarctic ice sheet (AIS) to the greenhouse world of the Eocene and removing the AIS from the modern world. The modern global mean surface temperature anomaly (Delta T) induced by Antarctic Glaciation depends on the background CO2 levels and ranges from -1.22 to -0.18 K. The Eocene Delta T is nearly constant at similar to-0.25 K. We calculate an climate sensitivity parameter S[Antarctica] which we define as Delta T divided by the change in effective radiative forcing (Delta Q(Antarctica)) which includes some fast feedbacks imposed by prescribing the glacial properties of Antarctica. The main difference between the modern and Eocene responses is that a negative cloud feedback warms much of the Earth's surface as a large AIS is introduced in the Eocene, whereas this cloud feedback is weakly positive and acts in combination with positive sea-ice feedbacks to enhance cooling introduced by adding an ice sheet in the modern. Because of the importance of cloud feedbacks in determining the final temperature sensitivity of the AIS, our results are likely to be model dependent. Nevertheless, these model results suggest that the effective radiative forcing and feedbacks induced by the AIS did not significantly decrease global mean surface temperature across the Eocene-Oligocene transition (EOT -34.1 to 33.6 Ma) and that other factors like declining atmospheric CO2 are more important for cooling across the EOT. The results illustrate that the efficacy of AIS forcing in the Eocene is not necessarily close to one and is likely to be model and state dependent. This implies that using EOT paleoclimate proxy data by itself to estimate climate sensitivity for future climate prediction requires climate models and consequently these estimates will have large uncertainty, largely due to uncertainties in modelling low clouds.
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| 30. |
- Goldner, Aaron, et al.
(författare)
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Implications of the permanent El Niño teleconnection "blueprint" for past global and North American hydroclimatology
- 2011
-
Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 7, s. 723-743
-
Tidskriftsartikel (refereegranskat)abstract
- Substantial evidence exists for wetter-than-modern continental conditions in North America during the pre-Quaternary warm climate intervals. This is in apparent conflict with the robust global prediction for future climate change of a northward expansion of the subtropical dry zones that should drive aridification of many semiarid regions. Indeed, areas of expected future aridification include much of western North America, where extensive paleoenvironmental records are documented to have been much wetter before the onset of Quaternary ice ages. It has also been proposed that climates previous to the Quaternary may have been characterized as being in a state with warmer-than-modern eastern equatorial sea surface temperatures (SSTs). Because equatorial Pacific SSTs exert strong controls on midlatitude atmospheric circulation and the global hydrologic cycle, the teleconnected response from this permanent El Niño-like mean state has been proposed as a useful analogue model, or "blueprint", for understanding global climatological anomalies in the past. The present study quantitatively explores the implications of this blueprint for past climates with a specific focus on the Miocene and Pliocene, using a global climate model (CAM3.0) and a nested high-resolution climate model (RegCM3) to study the hydrologic impacts on global and North American climate of a change in mean SSTs resembling that which occurs during modern El Niño events. We find that the global circulation response to a permanent El Niño resembles a large, long El Niño event. This state also exhibits equatorial super-rotation, which would represent a fundamental change to the tropical circulations. We also find a southward shift in winter storm tracks in the Pacific and Atlantic, which affects precipitation and temperature over the mid-latitudes. In addition, summertime precipitation increases over the majority of the continental United States. These increases in precipitation are controlled by shifts in the subtropical jet and secondary atmospheric feedbacks. Based on these results and the data proxy comparison, we conclude that a permanent El Niño like state is one potential explanation of wetter-than-modern conditions observed in paleoclimate-proxy records, particularly over the western United States.
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| 31. |
- Hadas, Or, et al.
(författare)
-
The role of baroclinic activity in controlling Earth's albedo in the present and future climates
- 2023
-
Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 120:5
-
Tidskriftsartikel (refereegranskat)abstract
- Clouds are one of the most influential components of Earth's climate system. Specifically, the midlatitude clouds play a vital role in shaping Earth's albedo. This study investigates the connection between baroclinic activity, which dominates the midlatitude climate, and cloud-albedo and how it relates to Earth's existing hemispheric albedo symmetry. We show that baroclinic activity and cloud-albedo are highly correlated. By using Lagrangian tracking of cyclones and anticyclones and analyzing their individual cloud properties at different vertical levels, we explain why their cloud-albedo increases monotonically with intensity. We find that while for anticyclones, the relation between strength and cloudiness is mostly linear, for cyclones, in which clouds are more prevalent, the relation saturates with strength. Using the cloud-albedo strength relationships and the climatology of baroclinic activity, we demonstrate that the observed hemispheric difference in cloud-albedo is well explained by the difference in the population of cyclones and anticyclones, which counter-balances the difference in clear-sky albedo. Finally, we discuss the robustness of the hemispheric albedo symmetry in the future climate. Seemingly, the symmetry should break, as the northern hemisphere's storm track response differs from that of the southern hemisphere due to Arctic amplification. However, we show that the saturation of the cloud response to storm intensity implies that the increase in the skewness of the southern hemisphere storm distribution toward strong storms will decrease future cloud-albedo in the southern hemisphere. This complex response explains how albedo symmetry might persist even with the predicted asymmetric hemispheric change in baroclinicity under climate change.
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| 32. |
- Hanley, John, 1982-
(författare)
-
Extreme Storms in the North Atlantic and Europe
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A study of the most extreme cyclones affecting the North Atlantic and Europe is presented with particular focus on extreme windstorms over the densely populated area of Western Europe, whose associated high surface wind speeds are capable of causing extensive structural damage and occasionally a loss of life.A novel cyclone identification and tracking algorithm is presented which explicitly recognises ‘multi-centre cyclones’ (MCCs), defined as cyclonic systems which contain two or three sea-level pressure minima. The method also recognises cyclone merging and splitting events and reduces the number of tracks which would have been spuriously split at some point in their life-cycle. MCC frequency is shown to increase with storm intensity, with approximately 60% of the top 30% of cyclones constituting MCCs at some point in their life-cycle.The first findings of the IMILAST (Inter-comparison of MId-LAtitudeSTorm diagnostics) project, an intercomparison study of 15 cyclone identification and tracking algorithms, are presented. Each method was applied to a 20 year period of the ERA-Interim dataset and results for cyclone frequency, intensity, life-cycle and track location were compared across the methods.The relationship between the evolution of the most intense wind storms affecting Western Europe (Britain and Ireland, Scandinavia, and Western Continental Europe) and the large-scale atmospheric flow is investigated using an automated cyclone tracking algorithm and an objective measure of cyclone destructiveness applied to ERA40 and ERA-Interim reanalysis data as well as EC-Earth model output data at two different spatial resolutions. Composite analyses reveal a clear connection between the precise location of upper-level anti-cyclonic wave breaking and cold air intrusion from the north and the position and orientation of an intense jet; this, in turn, plays a crucial role in determining into which region a developing extreme storm will be steered.
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| 33. |
- Hanley, John, 1982-, et al.
(författare)
-
Objective identification and tracking of multi-centre cyclones in the ERA-Interim reanalysis dataset
- 2012
-
Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 138:664, s. 612-625
-
Tidskriftsartikel (refereegranskat)abstract
- We present a novel cyclone identification and tracking method that explicitly recognizes multicentre cyclones (MCCs), defined as a cyclonic system with two or three sea-level pressure minima within its outermost contour. The method allows for the recognition of cyclone merger and splitting events in a natural way, and provides a consistent measure of the cyclone extent. Using the ERA-Interim reanalysis dataset, we compute a climatology using this method and show that MCCs occur in about 32 of all cyclone tracks and are much more prevalent in more intense storms. We also show that the method permits reconnection of tracks that would have been spuriously split using a conventional method. We present spatial maps of cyclone mergers, splitting, genesis and lysis using the method and also compute statistics of precipitation falling within cyclones, showing that it is strongly concentrated in the most intense cyclones.
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| 34. |
- Hanley, John, 1982-, et al.
(författare)
-
Rossby wave breaking and extreme windstorms over Western Europe
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the role of upper-level Rossby wave breaking in the evolution ofthemost extreme windstorms affecting three regions inWestern Europe: Britainand Ireland, Scandinavia and Western Continental Europe. Using ERA40and ERA-Interim reanalysis data along with EC-Earth model output at twodifferent spatial resolutions, we first construct an extreme wind climatologyover the selected regions and inter-compare the model climatology with thatcomputed from the reanalysis data. Using a storm destructiveness measure, wethen select the top 25 most destructive storms in each region from a multidecadalclimatology in each of our four datasets; track-by-track analysis revealsa good agreement in the trajectories and evolution of these storms in bothmodel resolutions compared to the reanalysis data. Temporal MSLP reanalysiscomposites demonstrate that in each region, there exists a set of large-scaleconditions conducive to the development of these storms; similar composites ofmodel output data show that these surface conditions are broadly well capturedby both model resolutions. Temporal composites of potential temperature onthe 2-PVU surface using reanalysis data reveal that these regional large-scalesurface patterns can be associated with exceptional cyclonic and anti-cyclonicwave breaking occurring contemporaneously in the North Atlantic; the preciselocation of these wave breaking events controls the position and orientation ofan intense upper-level jet which in turn determines into which region the stormsare steered. Similar composites using model output data show qualitativelythe same picture, but with an overall positive bias most likely due to a lowertropopause height in the model.
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| 35. |
- Hanley, John, 1982-, et al.
(författare)
-
The role of large-scale atmospheric flow and Rossby wave breaking in the evolution of extreme windstorms over Europe
- 2012
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 39, s. L21708-
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the relationship between large-scale atmospheric flow and the evolution of the most extreme windstorms affecting Western Continental Europe. The 25 most destructive Western Continental European wind storms are selected from a 43-year climatology. 22 of these storms are grouped as having a similar trajectory and evolution. We show that these storms typically occur during particularly strong and persistent positive NAO anomalies which peak approximately 2 days before the storms' peak intensity; the NAO pattern then shifts eastward to a position over the European continent when the storms strike Europe. A temporal composite of potential temperature on the 2-PVU surface suggests that this NAO shift is the result of simultaneous cyclonic and anticyclonic wave breaking penetrating further to the east than during a typical high-NAO event. This creates an extremely intense, zonally-orientated jet over the North Atlantic whose baroclinicity favours explosive intensification of storms while steering them into Western Continental Europe.
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| 36. |
- Harnik, Nili, et al.
(författare)
-
The Circumglobal North American wave pattern and its relation to cold events in eastern North America
- 2016
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 43:20, s. 11015-11023
-
Tidskriftsartikel (refereegranskat)abstract
- Extreme large-scale North American cold events are associated with strong undulations in the tropospheric jet stream which bring cold polar air southward over the continent. Here we propose that these jet undulations are associated with the North American part of the Circumglobal Teleconnection Patterna pair of zonally oriented waves of zonal wave number 5 which are in zonal quadrature with each other. While the Pacific/North American pattern is associated with the first circumglobal wave pattern, North American extreme cold events are associated with the second pattern. The 300hPa meridional wind and surface temperature anomalies associated with the Circumglobal North American wave packet are similar to those associated with the strongest eastern U.S. cold events. Both types of events are associated with a wave packet propagating all the way from Asia across the Pacific and across North America, with cold temperature anomalies spreading southeastward from Canada over the continent.
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| 37. |
- Hartung, Kerstin, 1989-
(författare)
-
Paths to improving atmospheric models across scales : The importance of the unresolved scales
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Correct representation of physical processes, the parametrizations, and their interaction with the resolved circulation is crucial for the performance of numerical models. Here, focus is put on understanding model biases and developing tools to alleviate existing biases. Atmospheric blocking can divert the typical atmospheric flow for several days up to weeks and thereby impacts the mean climate of the region experiencing blocking. Models typically underestimate the frequency of atmospheric blocking. Based on results from the global climate model EC-Earth, it is found that the atmospheric model resolution is not strongly influencing the representation of atmospheric blocking once the grid reaches about 80 km grid length in the horizontal. Updating several physical parametrizations, and thereby the model version, is the largest contributor to advancements in simulating atmospheric blocking. The importance of the topography for the large-scale atmospheric flow is further investigated with the reanalysis ERA-Interim by applying a simplified theoretical analysis. It is found that the idealized topographic forcing theory can explain some part of the observed large-scale properties of the flow, though the method does mainly produce relative results. The explained part of the large-scale structure is increased during periods of northwesterly flow and when the flow impinges the mountain ridge almost orthogonally.Small-scale processes acting in air masses transported from midlatitudes to the Arctic are also discussed. Numerical models often struggle with representing the stable conditions in the Arctic and tend to underestimate the downward longwave impact during cloudy conditions. A comparison of single-column models (SCMs) indicates that most models can capture the bimodal longwave distribution which develops from alternating cloudy and clear-sky conditions. SCMs are often used for model development as they allow to decouple the parametrized physical processes from the large-scale environment and enable many parameter sensitivity tests. A new tool is presented which can be used for the development of physical parametrizations in marine and polar conditions. It combines one-dimensional models of the atmosphere and ocean, including sea-ice, into a coupled atmosphere-ocean SCM (AOSCM). The presented setup constitutes an advantage compared to SCMs of one component because the coupling is directly modelled and the interaction between the respective boundary layers does not dependent on prescribed boundary conditions.
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| 38. |
- Heifetz, Eyal, et al.
(författare)
-
An alternative view on the role of the beta-effect in the Rossby wave propagation mechanism
- 2014
-
Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 66, s. 22672-
-
Tidskriftsartikel (refereegranskat)abstract
- The role of the beta-effect in the Rossby wave propagation mechanism is examined in the linearised shallow water equations directly in momentum-height variables, without recourse to potential vorticity (PV). Rigorous asymptotic expansion of the equations, with respect to the small non-dimensionalised beta parameter, reveals in detail how the Coriolis force acting on the small ageostrophic terms translates the geostrophic leading-order solution to propagate westward in concert. This information cannot be obtained directly from the conventional PV perspective on the propagation mechanism. Furthermore, a comparison between the beta-effect in planetary Rossby waves and the sloping-bottom effect in promoting topographic Rossby waves shows that the ageostrophic terms play different roles in the two cases. This is despite the fact that from the PV viewpoint whether the advection of mean PV gradient is set up by changes in planetary vorticity or by mean depth is inconsequential.
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| 39. |
- Herbert, Corentin, et al.
(författare)
-
Atmospheric Bistability and Abrupt Transitions to Superrotation : Wave-Jet Resonance and Hadley Cell Feedbacks
- 2020
-
Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 77:1, s. 31-49
-
Tidskriftsartikel (refereegranskat)abstract
- Strong eastward jets at the equator have been observed in many planetary atmospheres and simulated in numerical models of varying complexity. However, the nature of the transition from a conventional state of the general circulation, with easterlies or weak westerlies in the tropics, to such a superrotating state remains unclear. Is it abrupt or continuous? This question may have far-reaching consequences, as it may provide a mechanism for abrupt climate change in a planetary atmosphere, both through the loss of stability of the conventional circulation and through potential noise-induced transitions in the bistability range. We study two previously suggested feedbacks that may lead to bistability between a conventional and a superrotating state: the Hadley cell feedback and a wave-jet resonance feedback. We delineate the regime of applicability of these two mechanisms in a simple model of zonal acceleration budget at the equator. Then we show using numerical simulations of the axisymmetric primitive equations that the wave-jet resonance feedback indeed leads to robust bistability, while the bistability governed by the Hadley cell feedback, although observed in our numerical simulations, is much more fragile in a multilevel model.
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| 40. |
- Hess, Georg, et al.
(författare)
-
Health-related quality of life data from a phase 3, international, randomized, open-label, multicenter study in patients with previously treated mantle cell lymphoma treated with ibrutinib versus temsirolimus
- 2017
-
Ingår i: Leukemia and Lymphoma. - : Informa UK Limited. - 1042-8194 .- 1029-2403. ; 58:12, s. 2824-2832
-
Tidskriftsartikel (refereegranskat)abstract
- Mantle cell lymphoma (MCL) is a rare, aggressive, incurable B-cell malignancy. Ibrutinib has been shown to be highly active for patients with relapsed/refractory (R/R) MCL. The RAY trial (MCL3001) was a phase 3, randomized, open-label, multicenter study that compared ibrutinib with temsirolimus in patients with R/R MCL. Active disease is frequently associated with impaired functional status and reduced well-being. Therefore, the current study employed two patient-reported outcome instruments, the Functional Assessment of Cancer Therapy-Lymphoma (FACT-Lym) and the EQ-5D-5L, to assess symptoms, well-being, health status, and health-related quality of life of patients on treatment within the RAY trial. We found that patients on ibrutinib had substantial improvement in FACT-Lym subscale and total scores, and had improvement in EQ-5D-5L utility and VAS scores compared with temsirolimus patients, indicating a superior well-being. These improvements in well-being correlated with clinical response, indicating that better health-related quality of life was associated with decreased disease burden.
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| 41. |
- Holmberg, Emma, et al.
(författare)
-
The link between European warm-temperature extremes and atmospheric persistence
- 2023
-
Ingår i: Earth System Dynamics. - : Copernicus Publications. - 2190-4979 .- 2190-4987. ; 14:4, s. 737-765
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the link between warm-temperature extremes in Europe and the persistence of large-scale atmospheric-circulation patterns for both winter and summer, along with some possible physical mechanisms connecting the two. We assess atmospheric persistence, leveraging concepts from dynamical systems theory, and reconcile this approach with the more conventional meteorological views of persistence. We find that wintertime warm spells are partly associated with persistent zonal advection at the surface level but display no statistically significant persistence anomaly in the mid-troposphere. For summertime heatwaves, we find a weak yet significant link to anomalously persistent circulation patterns in the mid-troposphere, while there are few significant persistence anomalies of the surface circulation pattern. We further find no evidence of a strong warm-temperature advection signal. This suggests that other radiative and dynamical processes, for example sensible heating and adiabatic warming, as well as local effects, could play a more important role than large-scale warm-temperature advection for these events. We thus argue that persistent atmospheric configurations are not a necessary requirement for warm-temperature extremes and that the results depend to a considerable extent on region and tropospheric level.
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| 42. |
- Huber, M., et al.
(författare)
-
The early Eocene equable climate problem revisited
- 2011
-
Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 7:2, s. 603-633
-
Tidskriftsartikel (refereegranskat)abstract
- The early Eocene quable climate problem, i.e. warm extratropical annual mean and above-freezing winter temperatures evidenced by proxy records, has remained as one of the great unsolved problems in paleoclimate. Recent progress in modeling and in paleoclimate proxy development provides an opportunity to revisit this problem to ascertain if the current generation of models can reproduce the past climate features without extensive modification. Here we have compiled early Eocene terrestrial temperature data and compared with climate model results using a consistent and rigorous methodology. We test the hypothesis that equable climates can be explained simply as a response to increased greenhouse gas forcing within the framework of the atmospheric component of the Community Climate System Model (version 3), a climate model in common use for predicting future climate change. We find that, with suitably large radiative forcing, the model and data are in general agreement for annual mean and cold month mean temperatures, and that the pattern of high latitude amplification recorded by proxies can be largely, but not perfectly, reproduced.
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| 43. |
- Hutchinson, David K., et al.
(författare)
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Arctic closure as a trigger for Atlantic overturning at the Eocene-Oligocene Transition
- 2019
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- The Eocene-Oligocene Transition (EOT), approximately 34 Ma ago, marks a period of major global cooling and inception of the Antarctic ice sheet. Proxies of deep circulation suggest a contemporaneous onset or strengthening of the Atlantic meridional overturning circulation (AMOC). Proxy evidence of gradual salinification of the North Atlantic and tectonically driven isolation of the Arctic suggest that closing the Arctic-Atlantic gateway could have triggered the AMOC at the EOT. We demonstrate this trigger of the AMOC using a new paleoclimate model with late Eocene boundary conditions. The control simulation reproduces Eocene observations of low Arctic salinities. Subsequent closure of the Arctic-Atlantic gateway triggers the AMOC by blocking freshwater inflow from the Arctic. Salt advection feedbacks then lead to cessation of overturning in the North Pacific. These circulation changes imply major warming of the North Atlantic Ocean, and simultaneous cooling of the North Pacific, but no interhemispheric change in temperatures.
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| 44. |
- Hutchinson, David K., et al.
(författare)
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Climate sensitivity and meridional overturning circulation in the late Eocene using GFDL CM2.1
- 2018
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 14:6, s. 789-810
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Tidskriftsartikel (refereegranskat)abstract
- The Eocene-Oligocene transition (EOT), which took place approximately 34 Ma ago, is an interval of great interest in Earth's climate history, due to the inception of the Antarctic ice sheet and major global cooling. Climate simulations of the transition are needed to help interpret proxy data, test mechanistic hypotheses for the transition and determine the climate sensitivity at the time. However, model studies of the EOT thus far typically employ control states designed for a different time period, or ocean resolution on the order of 3 degrees. Here we developed a new higher resolution palaeoclimate model configuration based on the GFDL CM2.1 climate model adapted to a late Eocene (38 Ma) palaeogeography reconstruction. The ocean and atmosphere horizontal resolutions are 1 degrees similar to 1.5 degrees and 3 degrees 3.75 ffi respectively. This represents a significant step forward in resolving the ocean geography, gateways and circulation in a coupled climate model of this period. We run the model under three different levels of atmospheric CO2: 400, 800 and 1600 ppm. The model exhibits relatively high sensitivity to CO2 compared with other recent model studies, and thus can capture the expected Eocene high latitude warmth within observed estimates of atmospheric CO2. However, the model does not capture the low meridional temperature gradient seen in proxies. Equatorial sea surface temperatures are too high in the model (3037 degrees C) compared with observations (max 32 degrees C), although observations are lacking in the warmest regions of the western Pacific. The model exhibits bipolar sinking in the North Pacific and Southern Ocean, which persists under all levels of CO2. North Atlantic surface salinities are too fresh to permit sinking (25-30 psu), due to surface transport from the very fresh Arctic (similar to 20 psu), where surface salinities approximately agree with Eocene proxy estimates. North Atlantic salinity increases by 1-2 psu when CO2 is halved, and similarly freshens when CO2 is doubled, due to changes in the hydrological cycle.
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| 45. |
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| 46. |
- Jönsson, Aiden, 1991-, et al.
(författare)
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Net northward ocean heat transport modulates mean hemispheric cloud asymmetries
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Clouds in the Southern Hemisphere (SH) extratropics make up for the Northern Hemisphere (NH)’s greater tropical cloud cover and clear-sky albedo, making Earth’s planetary albedo hemispherically symmetric over the satellite record. Knowledge of a mechanism for maintaining hemispheric albedo symmetry would prove valuable for understanding cloud responses to external forcings. Using simulations of an Earth-like aquaplanet, we investigate the role of ocean heat transport (OHT) in determining hemispheric differences in cloud cover. With increasing northward cross-equatorial OHT, the SH becomes dominant in low cloud cover at all latitudes, while NH increases in high clouds are negated by reductions in low clouds. We describe a dynamical link between the increasing SH extratropical cloud cover and increasing NH tropical cloud cover with more northward cross-equatorial OHT. We investigate the effects of clouds and condensation on AHT responses, which increase southward AHT through latent heating in the extratropics and radiative effects in lower latitudes, aiding in reducing the hemispheric energy contrast. Because SH cloud increases are greater than NH cloud reductions, increasing cloud asymmetry with more northward cross-equatorial OHT leads to net increases in global cloud cover and cooling.
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| 47. |
- Kleman, Johan, et al.
(författare)
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Pre-LGM Northern Hemisphere ice sheet topography
- 2013
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 9, s. 2365-2378
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Tidskriftsartikel (refereegranskat)abstract
- We here reconstruct the paleotopography of Northern Hemisphere ice sheets during the glacial maxima of marine isotope stages (MIS) 5b and 4.We employ a combined approach, blending geologically based reconstruction and numerical modeling, to arrive at probable ice sheet extents and topographies for each of these two time slices. For a physically based 3-D calculation based on geologically derived 2-D constraints, we use the University of Maine Ice Sheet Model (UMISM) to calculate ice sheet thickness and topography. The approach and ice sheet modeling strategy is designed to provide robust data sets of sufficient resolution for atmospheric circulation experiments for these previously elusive time periods. Two tunable parameters, a temperature scaling function applied to a spliced Vostok–GRIP record, and spatial adjustment of the climatic pole position, were employed iteratively to achieve a good fit to geological constraints where such were available. The model credibly reproduces the first-order pattern of size and location of geologically indicated ice sheets during marine isotope stages (MIS) 5b (86.2 kyr model age) and 4 (64 kyr model age). From the interglacial state of two north–south obstacles to atmospheric circulation (Rocky Mountains and Greenland), by MIS 5b the emergence of combined Quebec–central Arctic and Scandinavian–Barents-Kara ice sheets had increased the number of such highland obstacles to four. The number of major ice sheets remained constant through MIS 4, but the merging of the Cordilleran and the proto-Laurentide Ice Sheet produced a single continent-wide North American ice sheet at the LGM.
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| 48. |
- Kuang Soh, Wuu, et al.
(författare)
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Rising CO2 drives divergence in water use efficiency of evergreen and deciduous plants
- 2019
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 5:12
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Tidskriftsartikel (refereegranskat)abstract
- Intrinsic water use efficiency (iWUE), defined as the ratio of photosynthesis to stomatal conductance, is a key variable in plant physiology and ecology. Yet, how rising atmospheric CO2 concentration affects iWUE at broad species and ecosystem scales is poorly understood. In a field-based study of 244 woody angiosperm species across eight biomes over the past 25 years of increasing atmospheric CO2 (similar to 45 ppm), we show that iWUE in evergreen species has increased more rapidly than in deciduous species. Specifically, the difference in iWUE gain between evergreen and deciduous taxa diverges along a mean annual temperature gradient from tropical to boreal forests and follows similar observed trends in leaf functional traits such as leaf mass per area. Synthesis of multiple lines of evidence supports our findings. This study provides timely insights into the impact of Anthropocene climate change on forest ecosystems and will aid the development of next-generation trait-based vegetation models.
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| 49. |
- Lee, Sukyoung, et al.
(författare)
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Relation Between Arctic Moisture Flux and Tropical Temperature Biases in CMIP5 Simulations and Its Fingerprint in RCP8.5 Projections
- 2019
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:2, s. 1088-1096
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Tidskriftsartikel (refereegranskat)abstract
- Arctic moisture intrusions have played an important role in warming the Arctic over the past few decades. A prior study found that Coupled Model Intercomparison Project Phase 5 (CMIP5) models exhibit large regional biases in the moisture flux across 70 degrees N. It is shown here that the systematic misrepresentation of the moisture flux is related to the models' overprediction of zonal wavenumber k = 2 contribution and underprediction of k = 1 contribution to the flux. Models with a warmer tropical upper troposphere and El-Nino-like tropical surface temperature tend to simulate stronger k = 2 flux, while k = 1 flux is uncorrelated with tropical upper tropospheric temperature and is associated with La-Nina-like surface temperature. The models also overpredict the transient eddy moisture flux while underpredicting the stationary eddy flux. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show trends in moisture flux that is consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. Plain Language Summary The Arctic is the region where climate change has been most rapid. A growing body of work indicates that moisture intrusions into Arctic have played an important role in warming the Arctic over the past decades. Coupled Model Intercomparison Project Phase 5 (CMIP5) models have served as a critical tool for projecting future climate changes. Therefore, it is imperative to evaluate whether the physical processes governing moisture intrusions are accurately represented by the models. It is shown here that there is a systematic misrepresentation of the moisture flux into the Arctic related to the models' biased representation of tropical temperatures. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show moisture flux trends that are consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. It is common practice to regard averages across climate model as being the true response to climate forcing. The findings here question this widespread assumption and underscore the need to pay close attention to model biases and their causes.
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| 50. |
- Lindvall, Jenny, et al.
(författare)
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The impact of changes in parameterizations of surface drag and vertical diffusion on the large-scale circulation in the Community Atmosphere Model (CAM5)
- 2017
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Ingår i: Climate Dynamics. - : Springer. - 0930-7575 .- 1432-0894. ; 48:11, s. 3741-3758
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Tidskriftsartikel (refereegranskat)abstract
- Simulations with the Community Atmosphere Model version 5 (CAM5) are used to analyze the sensitivity of the large-scale circulation to changes in parameterizations of orographic surface drag and vertical diffusion. Many GCMs and NWP models use enhanced turbulent mixing in stable conditions to improve simulations, while CAM5 cuts off all turbulence at high stabilities and instead employs a strong orographic surface stress parameterization, known as turbulent mountain stress (TMS). TMS completely dominates the surface stress over land and reduces the near-surface wind speeds compared to simulations without TMS. It is found that TMS is generally beneficial for the large-scale circulation as it improves zonal wind speeds, Arctic sea level pressure and zonal anomalies of the 500-hPa stream function, compared to ERA-Interim. It also alleviates atmospheric blocking frequency biases in the Northern Hemisphere. Using a scheme that instead allows for a modest increase of turbulent diffusion at higher stabilities only in the planetary boundary layer (PBL) appears to in some aspects have a similar, although much smaller, beneficial effect as TMS. Enhanced mixing throughout the atmospheric column, however, degrades the CAM5 simulation. Evaluating the simulations in comparison with detailed measurements at two locations reveals that TMS is detrimental for the PBL at the flat grassland ARM Southern Great Plains site, giving too strong wind turning and too deep PBLs. At the Sodankyla forest site, the effect of TMS is smaller due to the larger local vegetation roughness. At both sites, all simulations substantially overestimate the boundary layer ageostrophic flow.
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