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Climate feedback Climate change does not occur uniformly around the world: instead, in a process called polar amplification, the Arctic warms more rapidly than the tropics or mid-latitudes. Recent work published in Nature suggested that upper-atmospheric transport processes accounted for much of the recent polar amplification, but this conclusion proved controversial. Using updated reanalysis data from the past two decades, James Screen and Ian Simmonds now show that reductions in sea ice cover and thickness, rather than upper atmosphere processes, are responsible for most of the recent polar amplification. These findings reinforce suggestions that strong positive ice–temperature feedbacks are at work in the Arctic, and suggest that rapid warming and sea ice melting are likely to continue in the near future. Climate change does not occur symmetrically; instead, in a process called polar amplification, polar areas warm faster than the tropics. Recent work indicated that transport processes in the upper atmosphere account for much of the recent polar amplification, but this conclusion proved controversial. Here, updated reanalysis data have been used to show that reductions in sea ice are instead responsible. The rise in Arctic near-surface air temperatures has been almost twice as large as the global average in recent decades 1 , 2 , 3 —a feature known as ‘Arctic amplification’. Increased concentrations of atmospheric greenhouse gases have driven Arctic and global average warming 1 , 4 ; however, the underlying causes of Arctic amplification remain uncertain. The roles of reductions in snow and sea ice cover 5 , 6 , 7 and changes in atmospheric and oceanic circulation 8 , 9 , 10 , cloud cover and water vapour 11 , 12 are still matters of debate. A better understanding of the processes responsible for the recent amplified warming is essential for assessing the likelihood, and impacts, of future rapid Arctic warming and sea ice loss 13 , 14 . Here we show that the Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover. Changes in cloud cover, in contrast, have not contributed strongly to recent warming. Increases in atmospheric water vapour content, partly in response to reduced sea ice cover, may have enhanced warming in the lower part of the atmosphere during summer and early autumn. We conclude that diminishing sea ice has had a leading role in recent Arctic temperature amplification. The findings reinforce suggestions that strong positive ice–temperature feedbacks have emerged in the Arctic 15 , increasing the chances of further rapid warming and sea ice loss, and will probably affect polar ecosystems, ice-sheet mass balance and human activities in the Arctic 2 .

On 2 August 2012 a dramatic storm formed over Siberia, moved into the Arctic, and died in the Canadian Arctic Archipelago on 14 August. During its lifetime its central pressure dropped to 966 hPa, leading it to be dubbed ‘The Great Arctic Cyclone of August 2012’. This cyclone occurred during a period when the sea ice extent was on the way to reaching a new satellite‐era low, and its intense behavior was related to baroclinicity and a tropopause polar vortex. The pressure of the storm was the lowest of all Arctic August storms over our record starting in 1979, and the system was also the most extreme when a combination of key cyclone properties was considered. Even though, climatologically, summer is a ‘quiet’ time in the Arctic, when compared withall Arctic storms across the period it came in as the 13th most extreme storm, warranting the attribution of ‘Great’. Key Points Analysis and diagnosis is performed on the dramatic Arctic storm of August 2012 Storm's evolution and longevity tied to baroclinicity and a tropopause vortex Storm is the most intense Arctic August system in the record (since 1979)

We investigate linear trends in Antarctic skin temperatures (temperatures from about the top millimeter of the surface) over the four seasons using ERA5 ensemble mean reanalysis data. During 1950–2020, statistically significant warming occurred over East and West Antarctica in spring, autumn and winter, and over the Antarctic Peninsula in autumn and winter. A surface energy budget analysis revealed that increases in downward longwave radiation related to increases in air temperature and total column integrated cloud had a key role in Antarctic surface warming. There were negative sea level pressure trends around the periphery of Antarctica throughout the year, and the associated circulation contributed to warm advection from the middle latitudes to West Antarctica and the Antarctic Peninsula. Over the interior of East Antarctica, increase in moisture advection from lower latitudes enhanced the low-level cloud cover. A two-dimensional parameter diagram showed that skin temperature trends for time segments longer than 30 years starting before 1960 exhibited statistically significant warming in autumn and winter in East and West Antarctica and the Antarctic Peninsula. In spring, West Antarctica also showed statistically significant warming for long segments. In summer, the Antarctic Peninsula had statistically significant warming trends for long segments and cooling trends for segments less than 30 years. For all the studied time intervals, when skin temperatures had statistically significant positive trends, increases in downward longwave radiation contributed more than 70% of the warming and vice versa. This result demonstrates that on all time and space scales, changes in downward longwave radiation associated with variations in air temperature and atmospheric moisture loading play a dominant role controlling skin temperatures.

The Pacific–South American (PSA) pattern is an important mode of climate variability in the mid-to-high southern latitudes. It is widely recognized as the primary mechanism by which El Niño–Southern Oscillation (ENSO) influences the southeast Pacific and southwest Atlantic and in recent years has also been suggested as a mechanism by which longer-term tropical sea surface temperature trends can influence the Antarctic climate. This study presents a novel methodology for objectively identifying the PSA pattern. By rotating the global coordinate system such that the equator (a great circle) traces the approximate path of the pattern, the identification algorithm utilizes Fourier analysis as opposed to a traditional empirical orthogonal function approach. The climatology arising from the application of this method to ERA-Interim reanalysis data reveals that the PSA pattern has a strong influence on temperature and precipitation variability over West Antarctica and the Antarctic Peninsula and on sea ice variability in the adjacent Amundsen, Bellingshausen, and Weddell Seas. Identified seasonal trends toward the negative phase of the PSA pattern are consistent with warming observed over the Antarctic Peninsula during autumn, but are inconsistent with observed winter warming over West Antarctica. Only a weak relationship is identified between the PSA pattern and ENSO, which suggests that the pattern might be better conceptualized as a preferred regional atmospheric response to various external (and internal) forcings.

A winter Eurasian cooling trend and a large decline of winter sea ice concentration (SIC) in the Barents–Kara Seas (BKS) are striking features of recent climate changes. The question arises as to what extent these phenomena are related. A mechanism is presented that establishes a link between recent winter SIC decline and midlatitude cold extremes. Such potential weather linkages are mediated by whether there is a weak north–south gradient of background tropospheric potential vorticity (PV). A strong background PV gradient, which usually occurs in North Atlantic and Pacific Ocean midlatitudes, acts as a barrier that inhibits atmospheric blocking and southward cold air intrusion. Conversely, atmospheric blocking is more persistent in weakened PV gradient regions over Eurasia, Greenland, and northwestern North America because of weakened energy dispersion and intensified nonlinearity. The small climatological PV gradients over mid- to high-latitude Eurasia have become weaker in recent decades as BKS air temperatures show positive trends due to SIC loss, and this has led to more persistent high-latitude Ural-region blocking. These factors contribute to increased cold winter trend in East Asia. It is found, however, that in years when the winter PV gradient is small the East Asian cold extremes can even occur in the absence of large negative SIC anomalies. Thus, the magnitude of background PV gradient is an important controller of Arctic–midlatitude weather linkages, but it plays no role if Ural blocking is not present. Thus, the “PV barrier” concept presents a critical insight into the mechanism producing cold Eurasian extremes and is hypothesized to set up such Arctic–midlatitude linkages in other locations.

Arctic sea ice is declining at an increasing rate with potentially important repercussions. To understand better the atmospheric changes that may have occurred in response to Arctic sea ice loss, this study presents results from atmospheric general circulation model (AGCM) experiments in which the only time-varying forcings prescribed were observed variations in Arctic sea ice and accompanying changes in Arctic sea surface temperatures from 1979 to 2009. Two independent AGCMs are utilized in order to assess the robustness of the response across different models. The results suggest that the atmospheric impacts of Arctic sea ice loss have been manifested most strongly within the maritime and coastal Arctic and in the lowermost atmosphere. Sea ice loss has driven increased energy transfer from the ocean to the atmosphere, enhanced warming and moistening of the lower troposphere, decreased the strength of the surface temperature inversion, and increased lower-tropospheric thickness; all of these changes are most pronounced in autumn and early winter (September–December). The early winter (November–December) atmospheric circulation response resembles the negative phase of the North Atlantic Oscillation (NAO); however, the NAO-type response is quite weak and is often masked by intrinsic (unforced) atmospheric variability. Some evidence of a late winter (March–April) polar stratospheric cooling response to sea ice loss is also found, which may have important implications for polar stratospheric ozone concentrations. The attribution and quantification of other aspects of the possible atmospheric response are hindered by model sensitivities and large intrinsic variability. The potential remote responses to Arctic sea ice change are currently hard to confirm and remain uncertain.

The impact of winter atmospheric blocking over the Ural Mountains region (UB) coincident with different phases of the North Atlantic Oscillation (NAO) on the sea ice variability over the Barents and Kara Seas (BKS) in winter is investigated. It is found that the UB in conjunction with the positive phase of the NAO (NAO+) leads to the strongest sea ice decline. During this phase composites and trajectory analyses reveal an efficient moisture pathway to the BKS from the mid-latitude North Atlantic near the Gulf Stream Extension region where water vapor is abundant due to high sea surface temperatures. The NAO+-UB combination is an optimal circulation pattern that significantly increases the BKS water vapor that plays a major role in the BKS warming and sea ice reduction, while the increased sensible and latent heat fluxes play secondary roles. By contrast, much fewer dramatic impacts on the BKS are observed when the UB coincides with the neutral or negative phases of the NAO. Our results present new insights into the complex processes involved with Arctic sea ice reduction and warming. The mechanisms highlighted here potentially offer a perspective into the mechanisms behind Arctic multi-decadal climate variability.

In this paper, the effects of Eurasian circulation patterns such as high-latitude European blocking (HEB) and Ural blocking (UB) events on winter sea-ice concentration (SIC) in the Barents–Kara seas (BKS) and Eurasian cooling is examined to differentiate the different roles of HEB and UB in association with positive North Atlantic Oscillation (NAO + ) events. A particular focus is on the SIC variability resulting from the effect of sea surface temperature (SST) near the Gulf Stream Extension (GSE) region through to the position change of Eurasian blocking. It is found that the SST shows a dipole pattern with a positive (negative) anomaly to the south (north) of the GSE, while the high SST in BKS plays a major role in the BKS SIC decline. The strengthening of North Atlantic westerly winds associated with the SST dipole tends to promote long-lived UB and HEB events associated with NAO + to further reduce the BKS SIC, while HEB and UB depend on the prior BKS warming and UB requires stronger North Atlantic westerly winds than HEB. During UB, warm moist air from the GSE can reach the BKS to enhance downward infrared radiation (IR) via increased northward transport produced by the NAO + -UB relay. The downward IR is weak during HEB as the moisture is transported mainly into the western part of BKS, even though the NAO + -HEB relay still operates. Thus, UB leads to more pronounced BKS sea-ice declines than under HEB, although the latter still significantly contributes to the SIC loss. It is also found that the central-eastern Asian cooling occurring during UB is related to an intense, widespread SIC decline in BKS prior to the UB onset, whereas the European cooling during HEB is linked to a small SIC decline in the western part of BKS.

The Antarctic Peninsula of West Antarctica was one of the most rapidly warming regions on the Earth during the second half of the 20th century. Changes in the atmospheric circulation associated with remote tropical climate variabilities have been considered as leading drivers of the change in surface conditions in the region. However, the impacts of climate variabilities over the mid-latitudes of the Southern Hemisphere on this Antarctic warming have yet to be quantified. Here, through observation analysis and model experiments, we reveal that increases in winter sea surface temperature (SST) in the Tasman Sea modify Southern Ocean storm tracks. This, in turn, induces warming over the Antarctic Peninsula via planetary waves triggered in the Tasman Sea. We show that atmospheric response to SST warming over the Tasman Sea, even in the absence of anomalous tropical SST forcing, deepens the Amundsen Sea Low, leading to warm advection over the Antarctic Peninsula. The Antarctic Peninsula sees some of the strongest warming of the whole continent over the last decades, the drivers of which are not well known. Here, the authors show that winter sea surface temperature increases in the Tasman sea lead to changes in Southern Ocean storm tracks that in turn warm the Antarctic Peninsula.