| 1. |
- Chiacchio, Marc, et al.
(författare)
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On the links between meteorological variables, aerosols, and tropical cyclone frequency in individual ocean basins
- 2017
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 122:2, s. 802-822
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Tidskriftsartikel (refereegranskat)abstract
- A generalized linear model based on Poisson regression has been used to assess the impact of environmental variables modulating tropical cyclone frequency in six main cyclone development areas: the East Pacific, West Pacific, North Atlantic, North Indian, South Indian, and South Pacific. The analysis covers the period 1980-2009 and focuses on widely used meteorological parameters including wind shear, sea surface temperature, and relative humidity from different reanalyses as well as aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model. Circulation indices are also included. Cyclone frequency is obtained from the International Best Track Archive for Climate Stewardship. A strong link is found between cyclone frequency and the relative sea surface temperature, Atlantic Meridional Mode, and wind shear with significant explained log likelihoods in the North Atlantic of 37%, 27%, and 28%, respectively. A significant impact of black carbon and organic aerosols on cyclone frequency is found over the North Indian Ocean, with explained log likelihoods of 27%. A weaker but still significant impact is found for observed dust aerosols in the North Atlantic with an explained log likelihood of 11%. Changes in lower stratospheric temperatures explain 28% of the log likelihood in the North Atlantic. Lower stratospheric temperatures from a subset of Coupled Model Intercomparison Project Phase 5 models properly simulate the warming and subsequent cooling of the lower stratosphere that follows a volcanic eruption but underestimates the cooling by about 0.5 degrees C.
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| 2. |
- Finke, Kathrin, et al.
(författare)
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Exceptionally persistent Eurasian cold events and their stratospheric link
- 2023
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Ingår i: Asia-Pacific Journal of Atmospheric Sciences. - : Springer Science and Business Media LLC. - 1976-7633 .- 1976-7951. ; 59:1, s. 95-111
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Tidskriftsartikel (refereegranskat)abstract
- Persistent boreal winter cold spells (PCEs) can heavily strain the economy and significantly impact everyday life. While sudden stratospheric warmings are considered a precursor for Eurasian (EUR) cold events, these temperature extremes may occur during the full range of stratospheric variability. We investigate PCEs relative to the prevailing stratospheric polar vortex regime before their onset, with a particular focus on extremely weak (SSW) and strong (SPV) stratospheric winds by performing (lagged) composite analysis based on ERA5 reanalysis. On average, SPV PCEs that are concentrated over central EUR, are colder, shorter and set in more abruptly compared to SSW PCEs. A quasi-stationary, mid-tropospheric anticyclone over the Arctic Ocean that blocks warm air advection toward EUR is connected to the canonical downward progression of the negative North Atlantic Oscillation for SSW PCEs. In contrast, during SPV PCEs, the anticyclone is part of a Rossby wave having an origin co-located with negative wave activity flux anomalies over and being influenced by stratospheric wave reflection toward the North Atlantic. Its slow east-ward propagation is likely related to Arctic surface warming and unusually weak zonal winds over EUR.
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| 3. |
- Finke, Kathrin, 1991-
(författare)
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Northern Hemispheric Cold Spells and their Tropospheric-Stratospheric Link
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cold spells have severe consequences for society. They require early warnings for elaborate mitigation strategies on sub-seasonal to seasonal time-scales. Intense stratospheric westerlies and a polar vortex breakdown (SSW) may enhance extended-range forecast skill for Eurasian and North American cold extremes through a dynamic coupling to the troposphere. Understanding the complex interplay remains a challenging task that requires further investigation.Since fine-grained observational stratospheric data is limited to the satellite era, climate model simulations, such as atmosphere-only simulations (AMIP) from the Coupled Model Intercomparison Project Phase 6, can be considered. Application of the common empirical orthogonal function method in Paper II, a tool for multimodel comparison and evaluation, unveiled differences in daily winter 2m temperatures (T2m) across four reanalyses while stratospheric geopotential height varies across AMIP models. Results show a link between a weak polar vortex and cold T2m anomalies over Eurasia in reanalysis data.In addition, quantile regression is a simple but proficient statistical method that neatly enables modeling the response variable’s complete conditional distribution. Thereby, information about extremes, which hide in the distribution’s tails, is extracted. Application to boreal winter ERA5 reanalysis data and teleconnection indices in Paper I reveals significant asymmetries in duration, strength, and direction of the stratosphere-troposphere connection across quantiles. Regionally specific, lagged composite analysis of ERA5 data in Paper III verifies the canonical warm stratosphere - cold Eurasia relation. However, persistent Eurasian cold spells may also coincide with a strong polar vortex. We find stratospheric reflection of upward propagating planetary waves toward the North Atlantic to potentially influence mid-tropospheric circulation anomalies that travel towards Eurasia. By interacting with a quasi-stationary anticyclone over the Barents Sea, which promotes a cold Eurasia, these circulation anomalies likely influence the persistence and strength of the cold spell.Paper IV discusses the relationship between the 2018/2019 winter SSW and the subsequent North American cold spell using the JRA-55 reanalysis. An unusual wave number 3 planetary wave pulse in the stratosphere led to a polar vortex split. Further, wave reflection at the stratospheric Aleutian high likely fostered the circulation configuration, i.e., positive North Pacific and negative North American geopotential height anomalies that facilitated the cold temperatures.
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| 4. |
- Finke, Kathrin, et al.
(författare)
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Tropospheric Response to Stratospheric Variability via Lagged Quantile Regression
- 2022
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 49:15
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Tidskriftsartikel (refereegranskat)abstract
- Stratospheric variability may affect tropospheric weather on time scales between a few weeks and a few months, thus providing great potential for sub-seasonal to seasonal weather forecasts of extremes. Unlike standard linear regression, which is based on the conditional mean of the response variable, lagged quantile regression is used here, which enables modeling the response variable's complete conditional distribution. Using the ERA5 reanalysis, we explore the relation of prominent teleconnection indices and gridded tropospheric fields to the stratosphere during boreal winter for various quantiles and lags. Significant differences in duration, strength, and direction of the stratosphere-troposphere connection in outer quantiles compared to the median are identified. In particular, tropospheric extremes and their relation to the polar vortex strength are discussed, and an outlook is given.
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| 5. |
- Ge, ShuCan, et al.
(författare)
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Characteristic analysis of layered PMSEs measured with different elevation angles at VHF based on an experimental case
- 2021
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Ingår i: Earth and Planetary Physics. - : Earth and Planetary Physics. - 2096-3955. ; 5:1, s. 42-51
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Tidskriftsartikel (refereegranskat)abstract
- Polar Mesosphere Summer Echoes (PMSEs) are very strong radar echoes observed at altitudes near the polar summer mesopause. One of the essential properties of these radar echoes is that they can give useful diagnostic information about the physics of the scattering process. In this paper, the related characteristics of PMSEs measured with the European Incoherent SCATter Very High Frequency (EISCAT VHF) 224 MHz radar on 13-15 July 2010 are studied at different elevation angles from 78 degrees to 90 degrees. It is found that the PMSEs peak power and strongest PMSEs average power occur at the same elevation angles. Also interesting is that the strongest PMSEs occur at off-vertical angles when a PMSEs has a layered (multilayer) structure. And reflection may have more significant effects on PMSEs when there are double or multilayer PMSEs. Possible explanations regarding these observations are discussed.
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| 6. |
- Hannachi, Abdel, 1961-, et al.
(författare)
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Common EOFs : a tool for multi-model comparison and evaluation
- 2023
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 60:5-6, s. 1689-1703
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Tidskriftsartikel (refereegranskat)abstract
- With the increase in the volume of climate model simulations for past, present and future climate, from various institutions across the globe, there is a need for efficient and robust methods for model comparison and/or evaluation. This manuscript discusses common empirical orthogonal function analysis with a step-wise algorithm, which can be used for the above objective. The method looks for simultaneous diagonalisation of several covariance matrices in a step-wise fashion ensuring thus simultaneous monotonic decrease of the eigenvalues in all groups, and allowing therefore for dimension reduction. The method is applied to a number of tropospheric and stratospheric fields from the main four reanalysis products, and also to several historical climate model simulations from CMIP6, the Coupled Model Intercomparison Project (Phase 6). Monthly means as well as winter daily gridded data are considered over the Northern Hemisphere. The method shows consistency between mass fields as well as mid-tropospheric and stratospheric fields of the reanalyses, but also reveals significant differences in the 2 m surface-air temperature in terms of explained variance. CMIP6 models, on the other hand, show differences reflected in the percentage of explained variance of the leading common EOFs with inter-group variation ranging from 5–10% in the troposphere to about 25% in the stratosphere. Higher order statistics within the leading common modes of variability, in addition to further merits of the method are also discussed.
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| 7. |
- Hannachi, Abdel, et al.
(författare)
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On the Nonlinearity of Winter Northern Hemisphere Atmospheric Variability
- 2019
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 76:1, s. 333-356
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Tidskriftsartikel (refereegranskat)abstract
- Nonlinearity in the Northern Hemisphere’s winter time atmospheric flow is investigated from both an intermediate complexity model of the extratropics and reanalyses. A long simulation is obtained using a three-level quasi-geostrophic model on the sphere. Kernel empirical orthogonal functions (EOFs), which help delineate complex structures, are used along with the local flow tendencies. Two fixed points are obtained, which are associated with strong bimodality in two-dimensional kernel PC space in consistency with conceptual low-order dynamics. The regimes reflect zonal and blocked flows. The analysis is then extended to ERA-40 and JRA-55 reanalyses using daily sea level pressure (SLP) and geopotential heights in the stratosphere (20-hPa) and troposphere (500-hPa). In the stratosphere, trimodality is obtained, representing disturbed, displaced and undisturbed states of the winter polar vortex. In the troposphere the probability density functions (PDFs), for both fields, within the two-dimensional (2D) kernel EOF space are strongly bimodal. The modes correspond broadly to opposite phases of the Arctic Oscillation with signature of negative North Atlantic Oscillation (NAO). Over the North Atlantic/European sector a trimodal PDF is also obtained with two strong and one weak modes. The strong modes are associated, respectively, with the north (or +NAO) and south (or –NAO) positions of the eddy-driven jet strteam. The third weak mode is interpreted as a transition path between the two positions. A climate change signal is also observed in the troposphere of the winter hemisphere, resulting in an increase (decrease) in the frequency of the polar high (low) consistent with an increase of zonal flow frequency.
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| 8. |
- Hannachi, Abdel
(författare)
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Regularised empirical orthogonal functions
- 2016
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 68
-
Tidskriftsartikel (refereegranskat)abstract
- Empirical orthogonal functions, extensively used in weather/climate research, suffer serious geometric drawbacks such as orthogonality in space and time and mixing. The present paper presents a different version, the regularised (or smooth) empirical orthogonal function (EOF) method, by including a regularisation constraint, which originates from the field of regression/correlation of continuous variables. The method includes an extra unknown, the smoothing parameter, and solves a generalised eigenvalue problem and can overcome, therefore, some shortcomings of EOFs. For example, the geometrical constraints satisfied by conventional EOFs are relaxed. In addition, the method can help alleviate the mixing drawback. It can also be used in combination with other methods, which are based on downscaling or dimensionality reduction. The method has been applied to sea level pressure and sea surface temperature and yields an optimal value of the smoothing parameter. The method shows, in particular, that the leading sea level pressure pattern, with substantially larger explained variance compared to its EOF counterpart, has a pronounced Arctic Oscillation compared to the mixed North Atlantic Oscillation/Arctic Oscillation pattern of the leading EOF. The analysis of the remaining leading patterns and the application to sea surface temperature field and trend EOFs are also discussed.
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| 9. |
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| 10. |
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| 11. |
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| 12. |
- Iqbal, Waheed, et al.
(författare)
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Analysis of the variability of the North Atlantic eddy-driven jet stream in CMIP5
- 2018
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 51:1-2, s. 235-247
-
Tidskriftsartikel (refereegranskat)abstract
- The North Atlantic eddy-driven jet is a dominant feature of extratropical climate and its variability is associated with the large-scale changes in the surface climate of midlatitudes. Variability of this jet is analysed in a set of General Circulation Models (GCMs) from the Coupled Model Inter-comparison Project phase-5 (CMIP5) over the North Atlantic region. The CMIP5 simulations for the 20th century climate (Historical) are compared with the ERA40 reanalysis data. The jet latitude index, wind speed and jet persistence are analysed in order to evaluate 11 CMIP5 GCMs and to compare them with those from CMIP3 integrations. The phase of mean seasonal cycle of jet latitude and wind speed from historical runs of CMIP5 GCMs are comparable to ERA40. The wind speed mean seasonal cycle by CMIP5 GCMs is overestimated in winter months. A positive (negative) jet latitude anomaly in historical simulations relative to ERA40 is observed in summer (winter). The ensemble mean of jet latitude biases in historical simulations of CMIP3 and CMIP5 with respect to ERA40 are and respectively. Thus indicating improvements in CMIP5 in comparison to the CMIP3 GCMs. The comparison of historical and future simulations of CMIP5 under RCP4.5 and RCP8.5 for the period 2076-2099, shows positive anomalies in the jet latitude implying a poleward shifted jet. The results from the analysed models offer no specific improvements in simulating the trimodality of the eddy-driven jet.
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| 13. |
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| 14. |
- Iqbal, Waheed, et al.
(författare)
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Mean climate and representation of jet streams in the CORDEX South Asia simulations by the regional climate model RCA4
- 2017
-
Ingår i: Journal of Theoretical and Applied Climatology. - : Springer Science and Business Media LLC. - 0177-798X .- 1434-4483. ; 129:1-2, s. 1-19
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Tidskriftsartikel (refereegranskat)abstract
- A number of simulations with the fourth release of the Rossby Center Regional Climate Model (RCA4) conducted within the COordinated Regional climate Downscaling EXperiment (CORDEX) framework for South Asia at 50 km horizontal resolution are evaluated for mean winter (December-March) and summer (June-September) climate during 1980-2005. The two driving data sets ERA-Interim reanalysis and the general circulation model EC-Earth have been analyzed besides the RCA4 simulations to address the added value. RCA4 successfully captures the mean climate in both the seasons. The biases in RCA4 appear to come from the driving data sets which are amplified after downscaling. The jet streams influencing the seasonal precipitation variability in both seasons are also analyzed. The spatial and quantitative analysis over CORDEX South Asia generally revealed the ability of RCA4 to capture the mean seasonal climate as well as the position and strength of the jet streams despite weak/strong jet representation in the driving data. The EC-Earth downscaled with RCA4 exhibited cold biases over the domain and a weak Somali jet over the Arabian Sea. Moreover, the moisture transport from the Arabian Sea during summer is pronounced in RCA4 simulations resulting in enhanced monsoon rainfall over northwestern parts of India. Both the Somali jet and the tropical easterly jet become stronger during strong summer monsoon years. However, there is robust impact of wet years in summer over the Somali jet. Wet-minus-dry composites in winter indicate strengthening (weakening) of the subtropical jet in RCA4 run by ERA-Interim (EC-Earth). The driving data have clear reflections on the RCA4 simulations.
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| 15. |
- Iqbal, Waheed, 1985-, et al.
(författare)
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Troposphere-Stratosphere Dynamical Coupling in Regard to the North Atlantic Eddy-Driven Jet Variability
- 2019
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Ingår i: Journal of the Meteorological Society of Japan. - : Meteorological Society of Japan. - 0026-1165 .- 2186-9057. ; 97:3, s. 657-671
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Tidskriftsartikel (refereegranskat)abstract
- For several decades, the interaction between the troposphere and the stratosphere has attracted the attention of climate scientists, not least for the benefit it has on understanding dynamical processes and predictability. This interaction has been revived recently in regard to downward disturbance propagation effects on tropospheric circulations. In the current study, we investigate such interactions over the North Atlantic region in relation to the eddy-driven jet stream. The atmospheric low-frequency variability in the winter over the North Atlantic sector is mainly associated with variations in the latitudinal positions of the North Atlantic eddy-driven jet stream. The Japanese Reanalysis JRA-55 data has been used to analyze the jet latitude statistics. The results reveal robust trimoclality of the North Atlantic jet reflecting the latitudinal (i.e., northern, central and southern) positions in agreement with other reanalysis products. 30 major Sudden Stratospheric Warming (SSW) events are analyzed in relation to the three modes or regimes of the eddy-driven jet. The frequency of occurrence of the eddy-driven jet to be in a specific latitudinal position is largely related to the wave amplitude. The stratospheric polar vortex experiences significant changes via upward wave propagation associated with the jet positions. It is found that when the jet is close to its central mode the wave propagation of zonal wave number 2 (WN2) from the troposphere to the stratosphere is significantly high. Eliassen-Palm (EP) fluxes from all waves and zonal wave number 1 (WN1) depict the deceleration of the stratospheric polar vortex for the eddy-driven jet with a latitudinal position close to the northern mode. Plumb wave activity variations originate mainly in the Atlantic sector depending on the North Atlantic eddy-driven jet states. These significant associations between preferred latitudinal positions of the North Atlantic eddy-driven jet and the stratospheric dynamics may be a source of predictability.
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| 16. |
- Iqbal (وحید اقبال ), Waheed, 1985-
(författare)
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On atmospheric low frequency variability, teleconnections and link to jet variability
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The atmosphere is a complex system with an infinite number of independent variables. The best approximations of the atmosphere are made using numerical models. The use of such models provides an invaluable tool for studying the atmospheric system. In the atmosphere, narrow bands of strong winds at upper levels, called jet streams, impact the underlying large-scale weather conditions. In this Ph.D. thesis, I have studied jet stream variability from reanalyses and climate models. The regional climate model RCA4 simulations over South Asia reveal a good agreement between model results and reanalysis for jet stream representation. Lateral boundary data sources are believed to contribute to discrepancies over the mountainous regions.Currently, the weather forecasts have an upper limit of around 10 days. The atmospheric variability between 10 to 40 days is known as low frequency variability (LFV). This Ph.D. thesis also examined the LFV from a non-linear perspective, which indicated the existence of multiple recurring atmospheric conditions. The North Atlantic eddy-driven jet, which explains a major part of the winter variability over the North Atlantic region, has three preferred latitudinal positions situated south, closest to, and north of its climatological mean position. These positions represent, respectively, Greenland blocking, a low-pressure system over the North Atlantic, and a high-pressure system over the North Atlantic. An improved representation of this jet is reported from CMIP5 GCMs. However, the existence of three preferred latitudinal positions remains a challenge for these models.The statistical properties of recurring atmospheric conditions can potentially enhance current weather and climate predictions. Techniques from dynamical system theory, like unstable periodic orbits, can be employed to reconstruct such statistical properties. This has been demonstrated, for the first time, in a three-level baroclinic model, of intermediate complexity, for the Northern Hemisphere winter.In the Northern Hemisphere winter, there are times when the stratosphere gets warmer due to upward propagation of heat fluxes from the troposphere. This type of situation triggers a major sudden stratospheric warming, resulting in the equatorward shift of the jet streams and yielding much colder than usual surface conditions over the extratropics. I have studied thirty such events from the Japanese reanalysis data in relation to the three preferred latitudinal positions of the North Atlantic eddy-driven jet. The probability of strong upward propagation from the troposphere is significantly higher for the central position of the North Atlantic eddy-driven jet. These findings can potentially improve the troposphere-stratosphere predictions.
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| 17. |
- Kamis, Ahmed Samy, et al.
(författare)
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Effect of reservoir models and climate change on flood analysis in arid regions
- 2020
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Ingår i: Arabian Journal of Geosciences. - : Springer Science and Business Media LLC. - 1866-7511 .- 1866-7538. ; 13:16
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Tidskriftsartikel (refereegranskat)abstract
- Dams are built in arid regions across watersheds for flood control among other purposes. Capacity-elevation (C-E) curves are vital for reservoir routing and dam operation. Different models are available for representing C-E relationships. Power and logarithmic laws are evaluated and tested for reservoir routing. The evaluation is based on the analysis of 136 reservoirs across different regions of Saudi Arabia (SA). The analysis revealed that 75.7% of the reservoirs are of flood plain foothill type. A case study on Al-Lith dam basin is utilized for application based on measured events. The resulting routed outflow hydrographs showed that the logarithmic law is better to represent the reservoir than the power law. With respect to the climate change effect, the results show that the predicted rainfall from Representative Concentration Pathways scenario (RCP4.5) increased by about 20 to 31.4% from 5 to 100 years return periods respectively with an average of 27%. While for scenario RCP8.5, the predicted rainfall increased by 42% to about 55% from 5 to 100 years return periods respectively with an average of 49%. For the RCP4.5 scenario, the peak flows,Q(p), and volumes,W, increased by an average of 69% and 67% respectively. While for the RCP8.5 scenario, the same parameters increased by an average of 139% and 134% respectively. The effect of transmission losses in the results seems to be minor with respect to climate change signal (for RCP4.5,Q(p)andWare lowered on average by 2% and 0.5% respectively, and for RCP8.5,Q(p)andWare lowered on average by 4.5% and 1.3% respectively). The results of this research recommend to use the logarithmic law and to take into account the effect of climate change on future dam projects in SA.
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| 18. |
- Lind, Petter
(författare)
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On the representation of precipitation in high-resolution regional climate models
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Weather and climate models applied with sufficiently fine mesh grids to enable a large part of atmospheric deep convection to be explicitly resolved have shown a significantly improved representation of local, short-duration and intense precipitation events compared to coarser scale models. In this thesis, two studies are presented aimed at exploring the dependence of horizontal resolution and of parameterization of convection on the simulation of precipitation. The first examined the ability of HARMONIE Climate (HCLIM) regional climate model to reproduce the recent climate in Europe with two different horizontal resolutions, 15 and 6.25 km. The latter is part of the ”grey-zone” resolution interval corresponding to approximately 3-10 km. Particular focus has been given to rainfall and its spatial and temporal variability and other characteristics, for example intensity distributions. The model configuration with the higher resolution is much better at simulating days of large accumulated precipitation amounts, most evident when the comparison is made against high-resolution observations. Otherwise, the two simulations show similar skill, including the representation of the spatial structure of individual rainfall areas of primarily convective origin. The results suggest a ”scale-awareness” in HCLIM, which supports a central feature of the model’s description of deep convection as it is designed to operate independently of the horizontal resolution. In the second study, summer season precipitation over the Alps region, as simulated by HCLIM at different resolutions, is investigated. Similar model configurations as in the previous study were used, but in addition a simulation at the ”convection-permitting” 2 km resolution has been made over Central Europe. The latter considerably increases the realism compared to the former regarding the distribution and intensities of precipitation, as well as other important characteristics including the duration of rain spells, particularly on sub-daily time scales and for extreme events. The simulations with cumulus parameterization active underestimate short-duration heavy rainfall, and rainspells with low peak intensities are too persistent. Furthermore, even though the 6.25 km simulation generally reduces the biases seen in the 15 km run, definitive conclusions of the benefit of ”grey-zone” resolution is difficult to establish in context of the increased requirement of computer resources for the higher-resolution simulation.
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| 19. |
- Messori, Gabriele, et al.
(författare)
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The impact of future atmospheric circulation changes over the Euro-Atlantic sector on urban PM2.5 concentrations
- 2018
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 70:1
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Tidskriftsartikel (refereegranskat)abstract
- Air quality management is strongly driven by legislative aspects related to the exceedance of air quality limit values. Here, we use the Norwegian Climate Centre's Earth System Model to assess the impact of a future scenario of maximum feasible aerosol emission abatement and increasing greenhouse gases (RCP4.5) on urban PM2.5 concentrations in Europe. Daily PM2.5 concentrations are assessed using a novel downscaling method which allows us to compute exceedances of current and planned air quality thresholds. For the latter, we assume that future ambitious emission reductions are likely to be accompanied by stricter air quality thresholds. The changes in PM2.5 concentrations are discussed in the context of the large-scale atmospheric changes observed relative to the present-day climate.Our results show a more positive North Atlantic Oscillation mean state in the future, combined with a large eastward shift of both North Atlantic sea-level pressure centres of action. This is associated with more frequent mid-latitude blocking and a northward shift of the jet stream. These changes favour higher than expected anthropogenic urban PM2.5 concentrations in Southern Europe, while they have the opposite effect on the northern half of the continent. In the future scenario, PM concentrations in substantial parts of Southern Europe are found to exceed the World Health Organisation Air Quality Guideline daily limit of 25g/m(3) on 25 to over 50days per year, and annual guidelines of 10 mu g/m(3) on more than 80% of the 30years analysed in our study. We conclude that alterations in atmospheric circulation in the future, induced by stringent maximum feasible air pollution mitigation as well as GHG emissions, will negatively influence the effectiveness of these emission abatements over large parts of Europe. This has important implications for future air quality policies.
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| 20. |
- Moon, Woosok, et al.
(författare)
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River Nile discharge, the Pacific Ocean and world climate - a seasonal synchronization perspective
- 2021
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 73:1, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- The Nile is the longest river in Africa stretching over around 6650 km through 11 countries. From the times of the ancient Egyptian Pharaonic civilization, the Nile is known to be a blessing, which provides major resources including water and fertile soil for agriculture, and facilitates transportations and international trades in nearby countries. Due to its invaluable importance to local economy and agriculture, it is undoubtedly of paramount importance to know how the variability of the Nile is controlled by local and global climate and its morphological characteristics. Here, we utilize a newly developed time-series analysis method applied to monthly Nile river inflow data to reveal various factors changing the river inflow from seasonal to inter-annual, decadal and beyond. On seasonal time-scales a positive feedback, associated mostly with river's morphological change driven by summer precipitation, is identified as a main mechanism for maximal variability in September leading to major flooding or drought. In particular, the positive feedback is quite similar in its mechanism to major climate feedbacks observed, e.g. with ice albedo and Bjerknes feedbacks. The slow time-evolution of the positive feedback explains human endeavour history to control nature, such as the control of the Nile annual flooding through dam construction. The analysis of climate association reveals clear link with large-scale and low-frequency forcing. Decadal and multi-decadal timescales of local precipitation and associated teleconnection with atmospheric and oceanic circulation can be traced back to the Pacific Ocean, and involve mostly the El-Nino Southern Oscillation and the Pacific Decadal Oscillation.
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| 21. |
- Mukhin, Dmitry, et al.
(författare)
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Revealing recurrent regimes of mid-latitude atmospheric variability using novel machine learning method
- 2022
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Ingår i: Chaos. - : AIP Publishing. - 1054-1500 .- 1089-7682. ; 32:11
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Tidskriftsartikel (refereegranskat)abstract
- The low-frequency variability of the extratropical atmosphere involves hemispheric-scale recurring, often persistent, states known as teleconnection patterns or regimes, which can have a profound impact on predictability on intra-seasonal and longer timescales. However, reliable data-driven identification and dynamical representation of such states are still challenging problems in modeling the dynamics of the atmosphere. We present a new method, which allows us both to detect recurring regimes of atmospheric variability and to obtain dynamical variables serving as an embedding for these regimes. The method combines two approaches from nonlinear data analysis: partitioning a network of recurrent states with studying its properties by the recurrence quantification analysis and the kernel principal component analysis. We apply the method to study teleconnection patterns in a quasi-geostrophical model of atmospheric circulation over the extratropical hemisphere as well as to reanalysis data of geopotential height anomalies in the mid-latitudes of the Northern Hemisphere atmosphere in the winter seasons from 1981 to the present. It is shown that the detected regimes as well as the obtained set of dynamical variables explain large-scale weather patterns, which are associated, in particular, with severe winters over Eurasia and North America. The method presented opens prospects for improving empirical modeling and long-term forecasting of large-scale atmospheric circulation regimes.
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| 22. |
- Pausata, Francesco S. R., et al.
(författare)
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High-latitude volcanic eruptions in the Norwegian Earth System Model : the effect of different initial conditions and of the ensemble size
- 2015
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 67
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Tidskriftsartikel (refereegranskat)abstract
- Large volcanic eruptions have strong impacts on both atmospheric and ocean dynamics that can last for decades. Numerical models have attempted to reproduce the effects of major volcanic eruptions on climate; however, there are remarkable inter-model disagreements related to both short-term dynamical response to volcanic forcing and long-term oceanic evolution. The lack of robust simulated behaviour is related to various aspects from model formulation to simulated background internal variability to the eruption details. Here, we use the Norwegian Earth System Model version 1 to calculate interactively the volcanic aerosol loading resulting from SO2 emissions of the second largest high-latitude volcanic eruption in historical time (the Laki eruption of 1783). We use two different approaches commonly used interchangeably in the literature to generate ensembles. The ensembles start from different background initial states, and we show that the two approaches are not identical on short-time scales (<1 yr) in discerning the volcanic effects on climate, depending on the background initial state in which the simulated eruption occurred. Our results also show that volcanic eruptions alter surface climate variability (in general increasing it) when aerosols are allowed to realistically interact with circulation: Simulations with fixed volcanic aerosol show no significant change in surface climate variability. Our simulations also highlight that the change in climate variability is not a linear function of the amount of the volcanic aerosol injected. We then provide a tentative estimation of the ensemble size needed to discern a given volcanic signal on surface temperature from the natural internal variability on regional scale: At least 20-25 members are necessary to significantly detect seasonally averaged anomalies of 0.5 degrees C; however, when focusing on North America and in winter, a higher number of ensemble members (35-40) is necessary.
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| 23. |
- Pires, Carlos A. L., et al.
(författare)
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Bispectral analysis of nonlinear interaction, predictability and stochastic modelling with application to ENSO
- 2021
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 73:1, s. 1-30
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Tidskriftsartikel (refereegranskat)abstract
- Non-Gaussianity and nonlinearity have been shown to be ubiquitous characteristics of El Nino Southern Oscillation (ENSO) with implication on predictability, modelling, and assessment of extremes. These topics are investigated through the analysis of third-order statistics of El Nino 3.4 index in the period 1870-2018, namely bicovariance and bispectrum. Likewise, the spectral decomposition of variance, the bispectrum provides a spectral decomposition of skewness. Positive and negative bispectral contributions identify modes contributing respectively to La Ninas and El Ninos, mostly in the period range 2-6 years. The ENSO bispectrum also shows statistically significant features associated with nonlinearity. The analysis of bicovariance reveals a nonlinear correlation between the Boreal Spring and following Winter, coming from an asymmetry of the persistence of El Nino, contributing hence to a reduction of Spring Predictability Barrier. The positive skewness and main features of the ENSO bicovariance and bispectrum are shown to be well reproduced by fitting a bilinear stochastic model. This model shows improved forecasts, with respect to benchmark linear models, especially of the amplitude of extreme El Ninos. This study is relevant, particularly in a changing climate, to better characterize and predict ENSO extremes coming from non-Gaussianity and nonlinearity.
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| 24. |
- Sura, Philip, et al.
(författare)
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Perspectives of Non-Gaussianity in Atmospheric Synoptic and Low-Frequency Variability
- 2015
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 28:13, s. 5091-5114
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Forskningsöversikt (refereegranskat)abstract
- Understanding non-Gaussian statistics of atmospheric synoptic and low-frequency variability has important consequences in the atmospheric sciences, not least because weather and climate risk assessment depends on knowing and understanding the exact shape of the system's probability density function. While there is no doubt that many atmospheric variables exhibit non-Gaussian statistics on many time (and spatial) scales, a full and complete understanding of this phenomenon remains a challenge. Various mechanisms behind the observed atmospheric non-Gaussian statistics have been proposed but remain, however, multifaceted and scattered in the literature: nonlinear dynamics, multiplicative noise, cross-frequency coupling, nonlinear boundary layer drag, and others. Given the importance of this subject for weather and climate research, and in an attempt to contribute to this topic, a thorough review and discussion of the different mechanisms that lead to non-Gaussian weather and climate variability are presented in this paper and an outlook is given.
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| 25. |
- Unkel, Steffen, et al.
(författare)
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Independent Component Analysis for Three-Way Data With an Application From Atmospheric Science
- 2011
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Ingår i: Journal of Agricultural Biological and Environmental Statistics. - : Springer Science and Business Media LLC. - 1085-7117 .- 1537-2693. ; 16:3, s. 319-338
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a new approach to independent component analysis (ICA) for three-way data is considered. The rotational freedom of the three-mode component analysis (Tucker3) model is exploited to implement ICA in one mode of the data. The performance of the proposed approach is evaluated by means of numerical experiments. An illustration with real data from atmospheric science is presented, where the first mode is spatial location, the second is time and the third is a set of different meteorological variables representing geopotential heights at various vertical pressure levels. The results show that the three-mode decomposition finds spatial patterns of climate anomalies which can be interpreted in a meteorological sense and as such gives an insightful low-dimensional representation of the data.
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| 26. |
- Wang, Tongmei, et al.
(författare)
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On the dynamics of the spring seasonal transition in the two hemispheric high-latitude stratosphere
- 2019
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 71:1
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Tidskriftsartikel (refereegranskat)abstract
- The seasonal transition is one of the main features of the atmospheric general circulation and is particularly manifest in the high-latitude stratosphere. To explore the dynamics of stratospheric seasonal transition in both hemispheres, the observational features of the annual cycle and seasonal transition in high-latitude stratosphere are investigated using the 38-year ERA-interim reanalysis. Climatological analysis shows that tropospheric planetary waves propagate to the stratosphere and affect significantly the winter-to-summer stratospheric seasonal transition over both hemispheres, but with a much stronger wave activity in austral spring than its boreal counterpart. The austral spring seasonal transition occurs first at the stratopause then propagates down to the lower stratosphere due to enhanced planetary wave breaking, weakening the westerlies. In boreal spring, the seasonal transition occurs simultaneously across the depth of the stratosphere, mainly due to the solar radiation and weaker planetary wave activity. Interannual variability analysis shows that the timing of stratospheric seasonal transition is closely linked to the intensity of upward propagation of planetary wave activity, i.e. the stronger the upward propagation of planetary wave activity in high-latitudes in spring the earlier the stratospheric seasonal transition. Transition indexes are defined and the probability distributions of the indexes show that there are two types of transition in both hemispheres: synchronous/asynchronous in the Northern Hemisphere (NH), and steep/moderate transitions in the Southern Hemisphere (SH). A composite analysis shows that before the transition, stronger wave activity leads to asynchronous rather than synchronous transition in the NH, which propagates downward from the stratopause. In the SH, a moderate rather than steep transition is obtained, which occurs earlier and takes longer to propagate from the upper to lower stratosphere.
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| 27. |
- Wang, Tongmei, et al.
(författare)
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Response of stratospheric water vapour to CO2 doubling in WACCM
- 2020
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 54, s. 4877-4889
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Tidskriftsartikel (refereegranskat)abstract
- Stratospheric water vapour (SWV), as a greenhouse gas, modulates the radiative energy budget of the climate system. It is sensitive to, and plays a significant role in the climate change. In this study, we investigate the SWV response to CO2 increase with the Whole Atmosphere Community Climate Model (WACCM). In addition, we study its possible feedback on stratospheric temperature and relevant mechanisms. In our model experiments, the CO2 concentration and sea surface temperature (SSTs) are changed at the same time, as well as separately, to enable separating the radiative-photochemical and dynamical response to CO2 doubling scenarios. The model results show that the response of SWV to CO2 doubling is dominated by the changes in the SSTs, with an increase of the SWV concentration by similar to 6 to 10% in most of the stratosphere and more than 10% in the lower stratosphere, except for winter pole in the lower stratosphere, where the CO2 doubling decreases water vapour. The increase of SWV is mostly due to a dynamical response to the warm SSTs. Doubled CO2 induces warm SSTs globally and further leads to moist troposphere and a warmer tropical and subtropical tropopause, resulting in more water vapour entering stratosphere from below. As a greenhouse gas, large increase of SWV in the lower stratosphere, in turn, affects the stratospheric temperature, resulting in a warming of the tropical and subtropical lower stratosphere, offsetting the cooling caused by CO2 doubling.
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| 28. |
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| 29. |
- Wang, Tongmei, 1973-
(författare)
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Seasonality and variability of stratospheric water vapour
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Stratospheric water vapour (SWV) plays a critical role in the climate system by modulating the radiation budget and influencing the stratospheric chemistry. Studying changes of SWV on global scale is helpful for our understanding of climate change. This thesis aims to gain an improved understanding of the stratospheric processes and dynamic mechanisms that determine the seasonality and variability of SWV. Water vapour is characterized by its compound, which leaves an isotopic fingerprint in relevant atmospheric and hydrologic processes. The thesis starts with analyzing the global features of three stable water isotopes (SWIs) in the stratosphere by using satellite retrievals from Odin/SMR. The spatial pattern of SWI indicates clear effects of methane oxidation in the upper stratosphere, dehydration at the tropopause and stratospheric transport via the Brewer-Dobson circulation (BDC). In addition to the tropical tape recorder in the lower stratosphere, a pronounced downward propagation of the seasonal signal from the upper to the lower stratosphere is observed in high-latitudes. These observed features are further compared to model outputs to identify possible causes of model deficiencies in reproducing the distribution of SWV.The downward propagation signal of zonal wind has been demonstrated in the high-latitude stratosphere in spring seasonal transition in the Southern Hemisphere, but not in the Northern Hemisphere. This inter-hemispheric difference is due to the stronger stratospheric planetary wave activity in austral spring than in boreal spring. With strong wave activity in spring, the transition is inclined to occur first at the stratopause followed by a downward propagation to the lower stratosphere. In particular, the stronger the upward propagation of planetary waves in high-latitudes in spring the earlier the stratospheric seasonal transition. The new generation reanalysis ERA5 represents climatological distribution and seasonal cycle of SWV better than its predecessor ERA-Interim by assimilating more satellite observations. The variability of SWV in ERA5 is highly consistent with SDI MIM observation. The interannual variability of water vapour in the lower stratosphere is found to be closely linked to the tropical Quasi-Biennial Oscillation (QBO) and QBO-induced residual circulation. On decadal scale, the deficit of SWV in boreal winter is associated with a warm sea surface temperature (SST) anomaly in the North Atlantic, which leads to stronger upward propagation of planetary waves, resulting in a warmer pole in the lower stratosphere, colder tropical tropopause and stronger BDC, hence less water vapour enters the stratosphere through the tropopause and the anomaly extends to the entire stratosphere. Sensitivity experiments for a CO2 doubling scenario are performed with the model WACCM to investigate the SWV response to climate change. The response of SWV is dominated by the warm SST, which is induced by CO2 doubling in a coupled ocean-atmosphere system. The enhanced SST leads to a moist troposphere and warmer tropical and subtropical tropopause, resulting in more water vapour entering the stratosphere from below. A large increase of SWV in the lower stratosphere, in turn, affects stratospheric temperature. It results in a warming in the tropical and subtropical lower stratosphere, offsetting the cooling caused by CO2 doubling in general.
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| 30. |
- Wang, Tongmei, et al.
(författare)
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Stable Water Isotopologues in the Stratosphere Retrieved from Odin/SMR Measurements
- 2018
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Ingår i: Remote Sensing. - : MDPI AG. - 2072-4292. ; 10:2
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Tidskriftsartikel (refereegranskat)abstract
- Stable Water Isotopologues (SWIs) are important diagnostic tracers for understanding processes in the atmosphere and the global hydrological cycle. Using eight years (2002-2009) of retrievals from Odin/SMR (Sub-Millimetre Radiometer), the global climatological features of three SWIs, (H2O)-O-16, HDO and (H2O)-O-18, the isotopic composition D and O-18 in the stratosphere are analysed for the first time. Spatially, SWIs are found to increase with altitude due to stratospheric methane oxidation. In the tropics, highly depleted SWIs in the lower stratosphere indicate the effect of dehydration when the air comes through the cold tropopause, while, at higher latitudes, more enriched SWIs in the upper stratosphere during summer are produced and transported to the other hemisphere via the Brewer-Dobson circulation. Furthermore, we found that more (H2O)-O-16 is produced over summer Northern Hemisphere and more HDO is produced over summer Southern Hemisphere. Temporally, a tape recorder in (H2O)-O-16 is observed in the lower tropical stratosphere, in addition to a pronounced downward propagating seasonal signal in SWIs from the upper to the lower stratosphere over the polar regions. These observed features in SWIs are further compared to SWI-enabled model outputs. This helped to identify possible causes of model deficiencies in reproducing main stratospheric features. For instance, choosing a better advection scheme and including methane oxidation process in a specific model immediately capture the main features of stratospheric water vapor. The representation of other features, such as the observed inter-hemispheric difference of isotopic component, is also discussed.
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| 31. |
- Wang, Tongmei, et al.
(författare)
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Tropical water vapour in the lower stratosphere and its relationship to tropical/extratropical dynamical processes in ERA5
- 2020
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 146:730, s. 2432-2449
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Tidskriftsartikel (refereegranskat)abstract
- Stratospheric water vapour (SWV), in spite of its low concentration in the stratosphere as compared to the troposphere, contributes significantly to the surface energy budget and can have an influence on the surface climate. This study investigates the dynamical processes that determine SWV on interannual to decadal time-scales. First, we evaluate two SWV reanalysis products and show that SWV is better represented in a new-generation reanalysis product, ERA5, than in its predecessor, ERA-Interim. In particular, it is shown that SWV in ERA5 is highly consistent with observational data obtained from the SPARC Data Initiative Multi-Instrument Mean (SDI MIM). Second, we investigate the variability of tropical SWV and its relationship to dynamical stratospheric variables. The analyses show that the interannual variability in the tropical lower-stratospheric water vapour is closely linked to the tropical Quasi-Biennial Oscillation (QBO). When westerlies occupy the middle stratosphere and easterlies the lower stratosphere, a decrease is observed in lower-stratospheric water vapour due to a colder tropical tropopause and a QBO-induced enhanced residual circulation. On decadal time-scales, the composite analysis of the boreal winter in two typical periods shows that less SWV is related to a warm anomaly in the North Atlantic sea-surface temperature, which leads to stronger upward propagation of planetary wave activity at high latitudes, a weaker polar vortex and an enhanced residual circulation. The opposite occurs during periods with higher concentrations of SWV.
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| 32. |
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| 33. |
- Zveryaev, Igor I., et al.
(författare)
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Interannual variability of Mediterranean evaporation and its relation to regional climate
- 2012
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 38:3-4, s. 495-512
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Tidskriftsartikel (refereegranskat)abstract
- Gridded monthly evaporation data for 1958-2006 from the Woods Hole Oceanographic Institution data set are used to investigate interannual variability of Mediterranean evaporation during cold and hot seasons and its relation to regional atmospheric dynamics, sea surface temperature and atmospheric elements of the hydrological cycle. The first EOF mode of Mediterranean evaporation, explaining more than 50% of its total variance, is characterized by the monopole pattern both in winter and summer. However, despite structural similarity, the EOF-1 of Mediterranean evaporation is affected by different climate signals in cold and hot seasons. During winter the EOF-1 is associated with the East Atlantic teleconnection pattern. In summer, there is indication of tropical influence on the EOF-1 of Mediterranean evaporation (presumably from Asian monsoon). Both in winter and summer, principal components of EOF-1 demonstrate clear interdecadal signals (with a stronger signature in summer) associated with large sea surface temperature anomalies. The results of a sensitivity analysis suggest that in winter both the meridional wind and the vertical gradient of saturation specific humidity (GSSH) near the sea surface contribute to the interdecadal evaporation signal. In summer, however, it is likely that the signal is more related to GSSH. Our analysis did not reveal significant links between the Mediterranean evaporation and the North Atlantic Oscillation in any season. The EOF-2 of evaporation accounts for 20% (11%) of its total variance in winter (in summer). Both in winter and summer the EOF-2 is characterized by a zonal dipole with opposite variations of evaporation in western and eastern parts of the Mediterranean Sea. This mode is associated presumably with smaller scale (i.e., local) effects of atmospheric dynamics. Seasonality of the leading modes of the Mediterranean evaporation is also clearly seen in the character of their links to atmospheric elements of the regional hydrological cycle. In particular, significant links to precipitation in some regions have been found in winter, but not in summer.
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| 34. |
- Zveryaev, Igor I., et al.
(författare)
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Interdecadal changes in the links between Mediterranean evaporation and regional atmospheric dynamics during extended cold season
- 2017
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Ingår i: International Journal of Climatology. - : Wiley. - 0899-8418 .- 1097-0088. ; 37:3, s. 1322-1340
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Tidskriftsartikel (refereegranskat)abstract
- Monthly evaporation data for 1958-2010 from the Woods Hole Oceanographic Institution dataset are used to investigate interdecadal changes in interannual variability of Mediterranean evaporation and its links to regional climate. Analysis performed for the two climate periods characterized by the downward (1958-1978) and upward (1979-2010) trends of evaporation revealed significant season-dependent interdecadal changes in its interannual variability. The largest changes in interannual variability have been revealed during autumn when the leading empirical orthogonal functions (EOFs) are characterized by a zonal dipole pattern (except in November 1979-2010). During winter and spring, the EOF-1 and EOF-2 are characterized, respectively, by a monopole pattern and a zonal dipole and are associated with the East Atlantic (EA) and the East Atlantic-West Russia (EAWR) teleconnections. It is shown that interdecadal changes in interannual variability of Mediterranean evaporation during cold season were determined by the EA transition from the strongly negative to more neutral phase that occurred in late 1970s. Seasonally dependent changes in the structure of the leading evaporation EOFs reflect changing roles of the EA and EAWR which impact near surface air temperature, specific humidity and wind.
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| 35. |
- Zveryaev, Igor I., et al.
(författare)
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Summertime variability of Mediterranean evaporation : competing impacts from the mid latitudes teleconnections and the South Asian monsoon
- 2021
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Ingår i: Journal of Theoretical and Applied Climatology. - : Springer Science and Business Media LLC. - 0177-798X .- 1434-4483. ; 144:1-2, s. 779-791
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Tidskriftsartikel (refereegranskat)abstract
- Interannual variability of Mediterranean evaporation and its links to regional climate during summer are investigated based on evaporation data from the Woods Hole Oceanographic Institution dotaset. An EOF (Empirical Orthogonal Function) analysis performed on the monthly means (i.e., separately for June, July, August, and September time series) revealed two leading modes of evaporation variability, characterized by the monopole (EOF-1) and zonal dipole (EOF-2) patterns. These modes explain altogether more than 60% of the total variability of Mediterranean evaporation for each month. In all summer months, the EOF-1 reflects an interdecadal change signal characterized by below normal evaporation in 1970-2000 and above normal evaporation before and after this period. This mode is associated with the Atlantic Multidecadal Oscillation. The EOF-2 pattern reflects interannual variations of Mediterranean evaporation that differ significantly from month to month. The reason for this difference is the changing roles of regional teleconnections, such as the summer North Atlantic Oscillation (SNAO), the Scandinavian and East Atlantic teleconnections, and the Asian monsoon. The impacts of these teleconnections on Mediterranean evaporation are highly variable both in space and time. The largest impact of the SNAO on Mediterranean evaporation is detected in early summer, but its impact weakens and disappears towards the end of the summer season. An opposite tendency is obtained with the Asian monsoon, having the strongest impact on evaporation in late summer. The study suggests that these teleconnections impact Mediterranean evaporation mostly through atmospheric dynamics (the SNAO) and thermodynamics (the Asian monsoon) in early and late summer respectively.
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| 36. |
- Zveryaev, Igor, et al.
(författare)
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Inter-Annual Variability of Mediterranean Evaporation and Its Drivers During Summer
- 2022
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Ingår i: New Prospects in Environmental Geosciences and Hydrogeosciences. - Cham : Springer. - 9783030725426 - 9783030725433 ; , s. 25-28
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Bokkapitel (refereegranskat)abstract
- Monthly evaporation data for 1958–2015 from the Woods Hole Oceanographic Institution dataset are used to investigate inter-annual variability of Mediterranean evaporation and its links to regional climate during the extended (June–September) summer season. An EOF (Empirical Orthogonal Functions) analysis performed on the monthly means (i.e., separately for June, July, August and September time series) revealed two leading modes of evaporation variability, characterized by the monopole (EOF-1) and zonal dipole (EOF-2) patterns. These modes explain together more than 50% of the total variability of Mediterranean evaporation. It is shown that the EOF-1 reflects interdecadal changes characterized by below normal evaporation during the period 1970–2000, and above normal evaporation before and after that period. The EOF-2 reflects inter-annual and decadal scale variations of Mediterranean evaporation. Analysis of correlations between the leading PCs (principal components) of evaporation and indexes of large-scale teleconnections suggests moderate, but statistically significant links between Mediterranean evaporation and the North Atlantic Oscillation, Scandinavian teleconnection, East Atlantic teleconnection, Atlantic Multi-decadal Oscillation and the Asian monsoon. It is revealed that the dynamic impact on the North Atlantic Oscillation on evaporation weakens toward the end of the summer season, whereas thermodynamic impact from the Asian monsoon increases.
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| 37. |
- Önskog, Thomas, et al.
(författare)
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Predictability and non-Gaussian Characteristics of the North Atlantic Oscillation
- 2018
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Ingår i: Journal of Climate. - : American Meteorological Society. - 0894-8755 .- 1520-0442. ; 31, s. 537-554
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Tidskriftsartikel (refereegranskat)abstract
- The North Atlantic Oscillation (NAO) is the dominant mode of climate variability over the North Atlantic basin and has a significant impact on seasonal climate and surface weather conditions. It is the result of complex and nonlinear interactions between many spatiotemporal scales. Here, the authors study the statistical properties of two time series of the daily NAO index. Previous NAO modeling attempts only considered Gaussian noise, which can be inconsistent with the system complexity. Here, it is found that an autoregressive model with non-Gaussian noise provides a better fit to the time series. This result holds also when considering time series for the four seasons separately. The usefulness of the proposed model is evaluated by means of an investigation of its forecast skill.
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