| 1. |
- Chen, Hans, 1988, et al.
(author)
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A robust mode of climate variability in the Arctic: The Barents Oscillation
- 2013
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In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 40:11, s. 2856-2861
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Journal article (peer-reviewed)abstract
- The Barents Oscillation (BO) is an anomalous wintertime atmospheric circulation pattern in the Northern Hemisphere that has been linked to the meridional flow over the Nordic Seas. There are speculations that the BO has important implications for the Arctic climate; however, it has also been suggested that the pattern is an artifact of Empirical Orthogonal Function (EOF) analysis due to an eastward shift of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). In this study, EOF analyses are performed to show that a robust pattern resembling the BO can be found during different time periods, even when the AO/NAO is relatively stationary. This BO has a high and stable temporal correlation with the geostrophic zonal wind over the Barents Sea, while the contribution from the AO/NAO is small. The surface air temperature anomalies over the Barents Sea are closely associated with this mode of climate variability.
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| 2. |
- Axelsson, Josefine, et al.
(author)
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A Precipitation Isotopic Response in 2014-2015 to Moisture Transport Changes in the Central Himalayas
- 2023
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In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 128:13
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Journal article (peer-reviewed)abstract
- The impact of moisture transport and sources on precipitation stable isotopes (d(18)O and d-excess) in the central Himalayas are crucial to understanding the climatic archives. However, this is still unclear due to the lack of in-situ observations. Here we present measurements of stable isotopes in precipitation at two stations (Yadong and Pali) in the central Himalayas during 2014-2015. Combined with simulations from the dispersion model FLEXPART, we investigate effects on precipitation stable isotopes related to changes in moisture sources and convections in the region, and possible influence by El Nino. Our results suggest that the moisture supplies related to evaporation over northeastern India and moisture losses related to convective activities over the Bay of Bengal (BoB) and Bangladesh region play important roles in changes in d(18)O and d-excess in precipitation in the Yadong Valley. Outgoing longwave radiation and moisture flux divergence analysis further confirm that the contribution from continental evaporation dominates the moisture supply in the central Himalayas with a lesser contribution from convection over the BoB during the 2015 monsoon season compared with 2014. A change in the altitude effect is observed in 2015, which is more significant than the temperature and precipitation amount effect during the observation period. These findings provide valuable insights into climatic interpretations of paleo-isotopic archives with an isotopic response to changes in moisture transport to the central Himalayas.
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| 3. |
- Chen, Deliang, 1961, et al.
(author)
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Summary of a workshop on extreme weather events in a warming world organized by the Royal Swedish Academy of Sciences
- 2020
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In: Tellus Series B-Chemical and Physical Meteorology. - : Stockholm University Press. - 1600-0889 .- 0280-6509. ; 72:1
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Journal article (peer-reviewed)abstract
- Climate change is not only about changes in means of climatic variables such as temperature, precipitation and wind, but also their extreme values which are of critical importance to human society and ecosystems. To inspire the Swedish climate research community and to promote assessments of international research on past and future changes in extreme weather events against the global climate change background, the Earth Science Class of the Royal Swedish Academy of Sciences organized a workshop entitled 'Extreme weather events in a warming world' in 2019. This article summarizes and synthesizes the key points from the presentations and discussions of the workshop on changes in floods, droughts, heat waves, as well as on tropical cyclones and extratropical storms. In addition to reviewing past achievements in these research fields and identifying research gaps with a focus on Sweden, future challenges and opportunities for the Swedish climate research community are highlighted.
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| 4. |
- Lu, Zhengyao, et al.
(author)
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Natural decadal variability of global vegetation growth in relation to major decadal climate modes
- 2023
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In: Environmental Research Letters. - : IOP Publishing. - 1748-9326 .- 1748-9318. ; 18:1
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Journal article (peer-reviewed)abstract
- The ongoing climate change can modulate the behavior of global vegetation and influence the terrestrial biosphere carbon sink. Past observation-based studies have mainly focused on the linear trend or interannual variability of the vegetation greenness, but could not explicitly deal with the effect of natural decadal variability due to the short length of observations. Here we put the variabilities revealed by remote sensing-based global leaf area index (LAI) from 1982 to 2015 into a long-term perspective with the help of ensemble Earth system model simulations of the historical period 1850-2014, with a focus on the low-frequency variability in the global LAI during the growing season. Robust decadal variability in the observed and modelled LAI was revealed across global terrestrial ecosystems, and it became stronger toward higher latitudes, accounting for over 50% of the total variability north of 40 degrees N. The linkage of LAI decadal variability to major natural decadal climate modes, such as the El Nino-Southern Oscillation decadal variability (ENSO-d), the Pacific decadal oscillation (PDO), and the Atlantic multidecadal oscillation (AMO), was analyzed. ENSO-d affects LAI by altering precipitation over large parts of tropical land. The PDO exerts opposite impacts on LAI in the tropics and extra-tropics due to the compensation between the effects of temperature and growing season length. The AMO effects are mainly associated with anomalous precipitation in North America and Europe but are mixed with long-term climate change impacts due to the coincident phase shift of the AMO which also induces North Atlantic basin warming. Our results suggest that the natural decadal variability of LAI can be largely explained by these decadal climate modes (on average 20% of the variance, comparable to linear changes, and over 40% in some ecosystems) which also can be potentially important in inducing the greening of the Earth of the past decades.
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| 5. |
- Sun, Weiyi, et al.
(author)
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Middle east climate response to the saharan vegetation collapse during the mid-holocene
- 2021
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In: Journal of Climate. - 0894-8755 .- 1520-0442. ; 34:1, s. 229-242
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Journal article (peer-reviewed)abstract
- © 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Understanding climate change in the Middle East (ME) is crucial because people's living environment depends on rain-fed crop systems. It remains unclear whether the ME climate would be affected by the Saharan vegetation collapse at the end of the mid-Holocene (MH). Proxy data suggest a transition from humid to more arid ME conditions during the period of 6.5-5 kyr BP. Using a set of idealized sensitivity experiments with an Earth system model (EC-Earth), we infer that the shift of Saharan vegetation plays a role in this wet-to-dry transition over the ME. The experimental results show that the Saharan greening can significantly increase the late winter and early spring precipitation over the ME. The reason is that the vegetation decreases the surface albedo, which induces a warming in North Africa and generation of an anomalous low-level cyclonic flow, which transports moisture from tropical North Africa and the Red Sea to the ME. The moisture also flows from the Mediterranean Sea region to the ME through the enhanced mid- to upper-level westerlies. The enhanced moisture carried by westerly and southwesterly flows is lifted upon reaching Mesopotamia and the Zagros Mountains, substantially increasing the precipitation there. When the Sahara greening is removed, a drier condition happens in the ME. The crop model simulation further shows a substantial decrease in wheat yield in Mesopotamia with the reduction of Saharan vegetation, which is consistent with paleoclimatic reconstructions. These results imply that future changes in Saharan land cover may have climatic and agricultural impacts in the Middle East.
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| 6. |
- Sun, Weiyi, et al.
(author)
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Northern Hemisphere Land Monsoon Precipitation Increased by the Green Sahara During Middle Holocene
- 2019
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:16, s. 9870-9879
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Journal article (peer-reviewed)abstract
- Changes in land cover and dust emission may significantly influence the Northern Hemisphere land monsoon precipitation (NHLMP), but observations are too short to fully evaluate their impacts. The Green Sahara during the mid-Holocene (6,000 years BP) provides an opportunity to unravel these mechanisms. Here we show that during the mid-Holocene, most of the NHLMP changes revealed by proxy data are reproduced by the Earth System model results when the Saharan vegetation cover and dust reduction are taken into consideration. The simulated NHLMP significantly increases by 33.10% under the effect of the Green Sahara. The North African monsoon precipitation increases most significantly. Additionally, the Saharan vegetation (dust reduction under vegetated Sahara) alone remotely intensifies the Asian (North American) monsoon precipitation through large-scale atmospheric circulation changes. These findings imply that future variations in land cover and dust emissions may appreciably influence the NHLMP. Plain Language Summary Northern Hemisphere land monsoon precipitation (NHLMP) provides water resources for about two thirds of the world's population, which is vital for infrastructure planning, disaster mitigation, food security, and economic development. Changes in land cover and dust emissions may significantly influence the NHLMP, but observations are too short to understand the mechanisms. The Sahara Desert was once covered by vegetation and dust emission was substantially reduced during the mid-Holocene (6,000 years BP), which provides an opportunity to test the models' capability and unravel these mechanisms. Here we use an Earth System model and find that when the Saharan vegetation and dust reduction are taken into consideration, the simulated annual mean precipitation over most of the NHLM regions shows a closer agreement with proxy records. The sensitivity experiments show that the North African monsoon precipitation increases most significantly under the regional effects of Green Sahara. The Saharan vegetation (dust reduction under vegetated Sahara) alone also remotely increases the Asian (North American) monsoon precipitation through large-scale atmospheric circulation changes. The knowledge gained from this study is critical for improved understanding of the potential impacts of the land cover and dust changes on the projected future monsoon change.
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| 7. |
- Zhang, Peng, et al.
(author)
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How similar are annual and summer temperature variability in central Sweden?
- 2015
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In: Advances in Climate Change Research. - : Elsevier BV. - 1674-9278 .- 2524-1761. ; 6, s. 159-170
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Journal article (peer-reviewed)abstract
- Tree-ring based temperature reconstructions have successfully inferred the past inter-annual to millennium scales summer temperaturevariability. A clear relationship between annual and summer temperatures can provide insights into the variability of past annual mean temperaturefrom the reconstructed summer temperature. However, how similar are summer and annual temperatures is to a large extent stillunknown. This study aims at investigating the relationship between annual and summer temperatures at different timescales in central Swedenduring the last millennium. The temperature variability in central Sweden can represent large parts of Scandinavia which has been a key regionfor dendroclimatological research. The observed annual and summer temperatures during 1901e2005 were firstly decomposed into differentfrequency bands using ensemble empirical mode decomposition (EEMD) method, and then the scale-dependent relationship was quantifiedusing Pearson correlation coefficients. The relationship between the observed annual and summer temperatures determined by the instrumentaldata was subsequently used to evaluate 7 climate models. The model with the best performance was used to infer the relationship for the lastmillennium. The results show that the relationship between the observed annual and summer temperatures becomes stronger as the timescaleincreases, except for the 4e16 years timescales at which it does not show any relationship. The summer temperature variability at shorttimescales (2e4 years) shows much higher variance than the annual variability, while the annual temperature variability at long timescales (>32years) has a much higher variance than the summer one. During the last millennium, the simulated summer temperature also shows highervariance at the short timescales (2e4 years) and lower variance at the long timescales (>1024 years) than those of the annual temperature. Therelationship between the two temperatures is generally close at the long timescales, and weak at the short timescales. Overall the summertemperature variability cannot well reflect the annual mean temperature variability for the study region during both the 20th century and the lastmillennium. Furthermore, all the climate models examined overestimate the annual mean temperature variance at the 2e4 years timescales,which indicates that the overestimate could be one of reasons why the volcanic eruption induced cooling is larger in climate models than inproxy data.
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