| 1. |
- Jiang, Hui, et al.
(författare)
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Solar forcing of Holocene summer sea-surface temperatures in the northern North Atlantic
- 2015
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Ingår i: Geology. - 0091-7613. ; 43:3, s. 203-206
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Tidskriftsartikel (refereegranskat)abstract
- Mounting evidence from proxy records suggests that variations in solar activity have played a significant role in triggering past climate changes. However, the mechanisms for sun-climate links remain a topic of debate. Here we present a high-resolution summer sea-surface temperature (SST) record covering the past 9300 yr from a site located at the present-day boundary between polar and Atlantic surface-water masses. The record is age constrained via the identification of 15 independently dated tephra markers from terrestrial archives, circumventing marine reservoir age variability problems. Our results indicate a close link between solar activity and SSTs in the northern North Atlantic during the past 4000 yr; they suggest that the climate system in this area is more susceptible to the influence of solar variations during cool periods with less vigorous ocean circulation. Furthermore, the high-resolution SST record indicates that climate in the North Atlantic regions follows solar activity variations on multidecadal to centennial time scales.
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| 2. |
- Aemisegger, Franziska, et al.
(författare)
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A climatology of strong large-scale ocean evaporation events. Part II : Relevance for the deuterium excess signature of the evaporation flux
- 2018
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Ingår i: Journal of Climate. - 0894-8755. ; 31:18, s. 7313-7336
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Tidskriftsartikel (refereegranskat)abstract
- This paper discusses the relevance of transient events of strong large-scale ocean evaporation (SLOE) for the deuteriumexcess of marine boundary layer vapor d using a theoretical framework that invokes the closure assumption. We argue that during SLOE events, d is essentially determined by the evaporation flux signature. Distinct high d during SLOE with global-mean values in the range of 12‰-23‰ depending on the nonequilibrium fractionation factor αk result from the large air-sea humidity gradients reflected in low relative humidity with respect to sea surface temperature (hs 5 53% ± 9%) that characterize these events. Extratropical cyclones are highlighted as an important driver for the variability of d. On the one hand, they are themselves associated with high hs and low d, especially in areas of cloud formation and precipitation in the warm sector. On the other hand, cyclones are the main driver inducing SLOE events with high d in regions of cold-air advection upstream of their path. The sensitivity of d to its direct climate controls (hs and SST) is analyzed during SLOE for different αk formulations and found to be coherent with d-hs and d-SST slopes determined from available observations. The d-hs relationship exhibits a robust negative correlation as opposed to the d-SST relationship, which shows regional and time-scale-dependent variations in strength and sign that are induced by indirect hs-SST cross-correlation effects. The dynamical features involved in SLOE generation appear to exert a key control on the moisture source properties relevant for d in the extratropics.
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| 3. |
- Björck, Svante, et al.
(författare)
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A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial
- 2019
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 15:6, s. 1939-1958
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Tidskriftsartikel (refereegranskat)abstract
- Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4-18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37ĝF S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3-4 ĝC, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas..
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| 4. |
- Edwards, Thomas W. D., et al.
(författare)
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Seasonal variability in Northern Hemisphere atmospheric circulation during the Medieval Climate Anomaly and the Little Ice Age
- 2017
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 165, s. 102-110
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Tidskriftsartikel (refereegranskat)abstract
- Here we report new reconstructions of winter temperature and summer moisture during the past millennium in southeastern Sweden, based on stable-isotope data from a composite tree-ring sequence, that further enhances our knowledge and understanding of seasonal climate variability in the Northern Hemisphere over the past millennium. Key features of these new climate proxy records include evidence for distinctive fluctuations in winter temperature in SE Sweden, superimposed upon the general pattern of cooling between the so-called Medieval Climate Anomaly (MCA) of the early millennium and the Little Ice Age (LIA) of the late millennium, as well as evidence for sustained summer wetness during the MCA, followed by drier and less variable conditions during the LIA. We also explore these new records within a circumpolar spatial context by employing self-organizing map analysis of meteorological reanalysis data to identify potential modern analogues of mid-tropospheric synoptic circulation types in the Northern Hemisphere extratropics that can reconcile varying seasonal climate states during the MCA and LIA in SE Sweden with less variable conditions in southwestern Canada, as portrayed by paleoclimate records developed in the same manner in an earlier study.
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| 5. |
- Faber, Anne Katrine, et al.
(författare)
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How does sea ice influence δ18O of Arctic precipitation?
- 2017
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 17:9, s. 5865-5876
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates how variations in Arctic sea ice and sea surface conditions influence δ18O of present-day Arctic precipitation. This is done using the model isoCAM3, an isotope-equipped version of the National Center for Atmospheric Research Community Atmosphere Model version 3. Four sensitivity experiments and one control simulation are performed with prescribed sea surface temperature (SST) and sea ice. Each of the four experiments simulates the atmospheric and isotopic response to Arctic oceanic conditions for selected years after the beginning of the satellite era in 1979. Changes in sea ice extent and SSTs have different impacts in Greenland and the rest of the Arctic. The simulated changes in central Arctic sea ice do not influence δ18O of Greenland precipitation, only anomalies of Baffin Bay sea ice. However, this does not exclude the fact that simulations based on other sea ice and sea surface temperature distributions might yield changes in the δ18O of precipitation in Greenland. For the Arctic, δ18O of precipitation and water vapour is sensitive to local changes in sea ice and sea surface temperature and the changes in water vapour are surface based. Reduced sea ice extent yields more enriched isotope values, whereas increased sea ice extent yields more depleted isotope values. The distribution of the sea ice and sea surface conditions is found to be essential for the spatial distribution of the simulated changes in δ18O.
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| 6. |
- Fischer, Hubertus, et al.
(författare)
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Palaeoclimate constraints on the impact of 2 °C anthropogenic warming and beyond
- 2018
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Ingår i: Nature Geoscience. - : Springer Science and Business Media LLC. - 1752-0894 .- 1752-0908. ; 11:7, s. 474-485
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Tidskriftsartikel (refereegranskat)abstract
- Over the past 3.5 million years, there have been several intervals when climate conditions were warmer than during the pre-industrial Holocene. Although past intervals of warming were forced differently than future anthropogenic change, such periods can provide insights into potential future climate impacts and ecosystem feedbacks, especially over centennial-to-millennial timescales that are often not covered by climate model simulations. Our observation-based synthesis of the understanding of past intervals with temperatures within the range of projected future warming suggests that there is a low risk of runaway greenhouse gas feedbacks for global warming of no more than 2 °C. However, substantial regional environmental impacts can occur. A global average warming of 1–2 °C with strong polar amplification has, in the past, been accompanied by significant shifts in climate zones and the spatial distribution of land and ocean ecosystems. Sustained warming at this level has also led to substantial reductions of the Greenland and Antarctic ice sheets, with sea-level increases of at least several metres on millennial timescales. Comparison of palaeo observations with climate model results suggests that, due to the lack of certain feedback processes, model-based climate projections may underestimate long-term warming in response to future radiative forcing by as much as a factor of two, and thus may also underestimate centennial-to-millennial-scale sea-level rise.
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| 7. |
- Furukawa, Ryoto, et al.
(författare)
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Seasonal-Scale Dating of a Shallow Ice Core From Greenland Using Oxygen Isotope Matching Between Data and Simulation
- 2017
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996. ; 122:20, s. 10-10
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Tidskriftsartikel (refereegranskat)abstract
- A precise age scale based on annual layer counting is essential for investigating past environmental changes from ice core records. However, subannual scale dating is hampered by the irregular intraannual variabilities of oxygen isotope (δ18O) records. Here we propose a dating method based on matching the δ18O variations between ice core records and records simulated by isotope-enabled climate models. We applied this method to a new δ18O record from an ice core obtained from a dome site in southeast Greenland. The close similarity between the δ18O records from the ice core and models enables correlation and the production of a precise age scale, with an accuracy of a few months. A missing δ18O minimum in the 1995/1996 winter is an example of an indistinct δ18O seasonal cycle. Our analysis suggests that the missing δ18O minimum is likely caused by a combination of warm air temperature, weak moisture transport, and cool ocean temperature. Based on the age scale, the average accumulation rate from 1960 to 2014 is reconstructed as 1.02 m yr-1 in water equivalent. The annual accumulation rate shows an increasing trend with a slope of 3.6 mm yr-1, which is mainly caused by the increase in the autumn accumulation rate of 2.6 mm yr-1. This increase is likely linked to the enhanced hydrological cycle caused by the decrease in Arctic sea ice area. Unlike the strong seasonality of precipitation amount in the ERA reanalysis data in the southeast dome region, our reconstructed accumulation rate suggests a weak seasonality.
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| 8. |
- GuðlaugsdÓttir, Hera, et al.
(författare)
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North Atlantic weather regimes in δ18O of winter precipitation : isotopic fingerprint of the response in the atmospheric circulation after volcanic eruptions
- 2019
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Ingår i: Tellus. Series B: Chemical and Physical Meteorology. - : Stockholm University Press. - 1600-0889 .- 0280-6509. ; 71:1, s. 1-19
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Tidskriftsartikel (refereegranskat)abstract
- Equatorial volcanic eruptions are known to impact the atmospheric circulation on seasonal time scales through a strengthening of the stratospheric zonal winds followed by dynamic ocean-atmosphere coupling. This emerges as the positive phase of the North Atlantic Oscillation in the first 5 years after an eruption. In the North Atlantic, other modes of atmospheric circulation contribute to the climate variability but their response to volcanic eruptions has been less studied. We address this by retrieving the stable water isotopic fingerprint of the four major atmospheric circulation modes over the North Atlantic (Atlantic Ridge, Scandinavian Blocking and the negative and positive phases of the North Atlantic Oscillation (NAO − and NAO+)) by using monthly precipitation data from Global Network of Isotopes in Precipitation (GNIP) and 500 mb geo-potential height from the 20th Century Reanalysis. The simulated stable isotopic pattern of each atmospheric circulation mode is further used to assess the retrieved pattern. We test if changes in the atmospheric circulation as well as moisture source conditions as a result of volcanic eruptions can be identified by analyzing the winter climate response after both equatorial and high-latitude North Hemispheric volcanic eruptions in data, reanalysis and simulations. We report of an NAO + mode in the first two years after equatorial eruptions followed by NAO − in year 3 due to a decrease in the meridional temperature gradient as a result of volcanic surface cooling. This emerges in both GNIP data as well as reanalysis. Although the detected response is stronger after equatorial eruptions compared to high latitude eruptions, our results show that the response after high latitude eruptions tend to emerge as NAO − in year 2 followed by NAO + in year 3–4.
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| 9. |
- Klein, François, et al.
(författare)
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Assessing the robustness of Antarctic temperature reconstructions over the past 2 millennia using pseudoproxy and data assimilation experiments
- 2019
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 15:2, s. 661-684
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Tidskriftsartikel (refereegranskat)abstract
- The Antarctic temperature changes over the past millennia remain more uncertain than in many other continental regions. This has several origins: (1) the number of high-resolution ice cores is small, in particular on the East Antarctic plateau and in some coastal areas in East Antarctica; (2) the short and spatially sparse instrumental records limit the calibration period for reconstructions and the assessment of the methodologies; (3) the link between isotope records from ice cores and local climate is usually complex and dependent on the spatial scales and timescales investigated. Here, we use climate model results, pseudoproxy experiments and data assimilation experiments to assess the potential for reconstructing the Antarctic temperature over the last 2 millennia based on a new database of stable oxygen isotopes in ice cores compiled in the framework of Antarctica2k (Stenni et al.,). The well-known covariance between δ 18 O and temperature is reproduced in the two isotope-enabled models used (ECHAM5/MPI-OM and ECHAM5-wiso), but is generally weak over the different Antarctic regions, limiting the skill of the reconstructions. Furthermore, the strength of the link displays large variations over the past millennium, further affecting the potential skill of temperature reconstructions based on statistical methods which rely on the assumption that the last decades are a good estimate for longer temperature reconstructions. Using a data assimilation technique allows, in theory, for changes in the δ 18 O-temperature link through time and space to be taken into account. Pseudoproxy experiments confirm the benefits of using data assimilation methods instead of statistical methods that provide reconstructions with unrealistic variances in some Antarctic subregions. They also confirm that the relatively weak link between both variables leads to a limited potential for reconstructing temperature based on δ 18 O. However, the reconstruction skill is higher and more uniform among reconstruction methods when the reconstruction target is the Antarctic as a whole rather than smaller Antarctic subregions. This consistency between the methods at the large scale is also observed when reconstructing temperature based on the real δ 18 O regional composites of Stenni et al. (2017). In this case, temperature reconstructions based on data assimilation confirm the long-term cooling over Antarctica during the last millennium, and the later onset of anthropogenic warming compared with the simulations without data assimilation, which is especially visible in West Antarctica. Data assimilation also allows for models and direct observations to be reconciled by reproducing the east-west contrast in the recent temperature trends. This recent warming pattern is likely mostly driven by internal variability given the large spread of individual Paleoclimate Modelling Intercomparison Project (PMIP)/Coupled Model Intercomparison Project (CMIP) model realizations in simulating it. As in the pseudoproxy framework, the reconstruction methods perform differently at the subregional scale, especially in terms of the variance of the time series produced. While the potential benefits of using a data assimilation method instead of a statistical method have been highlighted in a pseudoproxy framework, the instrumental series are too short to confirm this in a realistic setup.
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| 10. |
- Le, Thanh, et al.
(författare)
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The influence of external forcing on subdecadal variability of regional surface temperature in CMIP5 simulations of the last millennium
- 2016
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996. ; 121:4, s. 1671-1682
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Tidskriftsartikel (refereegranskat)abstract
- We use Granger causality to investigate the influences of external forcings on subdecadal variability of regional near-surface air temperature (SAT) in past millennium simulations (period 850–1850 A.D.) of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Our results strengthen the conclusion for robust influence of volcanic forcing on SAT during preindustrial times of the last millennium. The SAT response to solar variations is detected in tropical and subtropical regions. In other regions, this response is weak. The impact of greenhouse gases (GHGs) radiative forcing to regional SAT is weak and uncertain. This is most probably due to the low amplitude of the variations in GHGs and hence weak GHGs forcing during the preindustrial millennium. The low agreement between models in simulating the impacts of solar variations on SAT in several regions suggests the different dynamical responses in these models, possibly associated with inaccurate parameterization of the processes related to solar forcing. Our analysis suggests that internal climate variability played a more significant role than external forcings in short-term SAT variability in the regions of the North Atlantic, the North Pacific, the Arctic, the Antarctic Peninsula, and its surrounding oceans. The possibility of long-term impacts of external forcings on SAT and the uncertainties that might be contained due to effects of internal climate modes other than El Niño–Southern Oscillation underscore the necessity for a more detailed understanding of the dynamical response of SAT to external forcings.
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