How polar-midlatitude atmospheric teleconnections depend on regional sea ice fraction and global warming level

In recent years, the Southern Ocean has experienced extremely low sea ice cover in multiple summers. These low events were preceded by a multidecadal positive trend that culminated in record high ice coverage in 2014. This abrupt transition has led some authors to suggest that Antarctic sea ice has undergone a regime shift. In this study we analyze the satellite sea ice record and atmospheric reanalyses to assess the evidence for such a shift. We find that the standard deviation of the summer sea ice record has doubled from 0.31 million km2 in 1979–2006 to 0.76 million km2 for 2007–22. This increased variance is accompanied by a longer season-to-season sea ice memory. The atmosphere is the primary driver of Antarctic sea ice variability, but using a linear predictive model we show that sea ice changes cannot be explained by the atmosphere alone. Identifying whether a regime shift has occurred is difficult without a complete understanding of the physical mechanism of change. However, the statistical changes that we demonstrate (i.e., increased variance and autocorrelation, and a changed response to atmospheric forcing), as well as the increased spatial coherence noted by previous research, are indicators based on dynamical systems theory of an abrupt critical transition. Thus, our analysis is further evidence in support of a changed Antarctic sea ice system. Significance Statement In recent years, there have been several summers with extremely low Antarctic sea ice cover, including consecutive record lows in February 2022 and February 2023. Since then, the 2023 winter has seen a remarkably low sea ice growth with an anomaly far below expected climatology. This has led researchers to question whether there has been a regime shift, and we assess the observational evidence for such a shift. In the last decade or so, the variability of summer sea ice has almost doubled, accompanied by a much longer sea ice memory from season to season. These statistical changes, as well an increased spatial coherence noted by other researchers, are consistent with theoretical indicators of a critical transition, or regime shift.

During the austral autumn/winter of 2023 Antarctic sea ice exhibited a pan-Antarctic wide delay in refreezing of roughly a month (peaked in July 2023, hereafter referred to as W23 event). As such it is unprecedented over the satellite era and may point to a start of a transitioning to a new state of Antarctic sea ice. However, the relatively short observational record obscures our understanding how natural variability in Antarctic sea ice can act together with anthropogenic climate change in creating favorable conditions for extreme Antarctic sea ice changes. Here we show that an anomalous atmospheric circulation pattern prior to W23 (May-to-July 2023) is part of a longer-term (1979-2022) trend in observed mid-to-upper tropospheric winds around the Antarctic continent towards a wavier manner that favors anomalous moisture transport to the Weddell Sea. We further show that this circulation pattern is associated with winter sea ice anomalies on both year-to-year and interdecadal timescales in preindustrial control simulations of CMIP6 climate models as well as in future projections of large ensemble simulations under greenhouse gas emission scenarios. By conducting standalone simulations with the global ocean-sea ice model NEMO4-SI3 (forced by the atmospheric reanalysis ERA5) at two horizontal resolutions (1º & 0.25º), we also study the influence of the recently observed acceleration of ocean warming around the Antarctic continent and the effect of model horizontal resolution on the simulation of sea ice extremes. Our results overall suggest that internal atmospheric-sea ice coupling could be an important contributor to future winter Antarctic sea ice changes, enhancing the forced Antarctic sea ice changes that are primarily driven by ocean warming.

The year 2023 marked a turning point for the Antarctic region, as the Southern Hemisphere experienced a significant reduction in its sea ice cover, with a record-breaking sea ice minimum in July 2023 of ~2.4 million square kilometers below the long-term mean. This study investigates the drivers behind this exceptional event, by combining observational, satellite and reanalysis data. Throughout the year, the Antarctic Sea ice extent broke record after record, ranking as the lowest sea ice on record from January to September, with the exception of March and April. The exceptionally low sea ice extent from May to August was mainly driven by the prevalence of a zonal wave number 3 pattern, with alternating surface high- and low-pressure systems, which favored the advection of heat and moisture, especially over the Ross Sea (RS), Weddell Sea (WS), and Indian Ocean (IO). From May 2023 to August 2023, record-breaking low sea ice extent and high temperatures were recorded, and the most affected regions were RS, WS, and IO. Over the Weddell Sea, temperature anomalies of up to 10°C have been observed from May to July, whereas over the Ross Sea, temperature anomalies of up to 10°C have been observed, especially in July and August. A regime shift in the Antarctic Sea ice, as well as in the average mean air temperature and subsurface ocean temperature over the Weddell Sea, was detected around 2015. The analysis revealed complex interactions between atmospheric circulation patterns, oceanic processes, and their implications for variability and change in Antarctic Sea ice. Understanding the underlying mechanisms of these extreme events provides crucial insights into the changing dynamics of Antarctic Sea ice and its broader climatic significance.

Comparison of Arctic and Antarctic sea ice spatial–temporal changes during 1979–2018

How are polar-to-midlatitude teleconnections represented in recent large ensembles of coupled climate model simulations? And how do they evolve with global warming? Using the rich information on internal variability available from large ensembles, we investigate the relationship between sea ice amount and atmospheric circulation for both Arctic and Antarctic sea ice variability in CESM2 and ACCESS-ESM1-5, using a composite analysis. We find that the links between sea ice and sea level pressure (SLP), the midlatitude jet stream and temperature depend on the region in which sea ice varies, for instance with low Barents-Kara sea ice in January being associated with a positive North Atlantic Oscillation SLP pattern and high pressure over Northern Eurasia. These circulation patterns persist with increased levels of global warming, until around 3 or 4°C when they start to evolve in some cases, as sea ice starts to disappear. Surface air temperatures are anomalously high around the region of sea ice retreat with varying patterns of remote cooling elsewhere. Lagged analysis shows that sea-ice circulation relationships when the atmosphere leads sea ice are very similar to the instantaneous relationships, suggesting that the latter largely reflects the atmospheric patterns leading to reduced sea ice. For positive lags (sea ice leading the atmosphere), for some regions the SLP teleconnections persist in a weakened state for subsequent months, whilst for others they evolve, e.g. into a negative Arctic Oscillation response for Barents-Kara sea ice reduction. However, results for positive lags differ between the two models examined. SLP relationships with Antarctic sea ice are model dependent, but feature a negative Southern Annular Mode pattern in ACCESS-ESM1-5. In CESM2, we find a less zonally symmetric pattern which also consists of high pressure over the pole in Autumn and Winter.

Abstract. Antarctic sea ice has exhibited significant variability over the satellite record, including a period of prolonged and gradual expansion, as well as a period of sudden decline. A number of mechanisms have been proposed to explain this variability, but how each mechanism manifests spatially and temporally remains poorly understood. Here, we use a statistical method called low-frequency component analysis to analyze the spatiotemporal structure of observed Antarctic sea ice concentration variability. The identified patterns reveal distinct modes of low-frequency sea ice variability. The leading mode, which accounts for the large-scale, gradual expansion of sea ice, is associated with the Interdecadal Pacific Oscillation and resembles the observed sea surface temperature trend pattern that climate models have trouble reproducing. The second mode is associated with the central Pacific El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode and accounts for most of the sea ice variability in the Ross Sea. The third mode is associated with the eastern Pacific ENSO and Amundsen Sea Low and accounts for most of the pan-Antarctic sea ice variability and almost all of the sea ice variability in the Weddell Sea. The third mode is also related to periods of abrupt Antarctic sea ice decline that are associated with a weakening of the circumpolar westerlies, which favors surface warming through a shoaling of the ocean mixed layer and decreased northward Ekman heat transport. Broadly, these results suggest that climate model biases in long-term Antarctic sea ice and large-scale sea surface temperature trends are related to each other and that eastern Pacific ENSO variability is a key ingredient for abrupt Antarctic sea ice changes.

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 54:69-84 (2012) - DOI: https://doi.org/10.3354/cr01101 Response of clouds and surface energy fluxes to changes in sea-ice cover over the Laptev Sea (Arctic Ocean) Neil P. Barton1,*, Dana E. Veron2 1Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, California 94551, USA 2College of Earth, Ocean, and Environment, University of Delaware, Newark, Delaware 19716, USA *Email: barton30@llnl.gov ABSTRACT: The response of Arctic clouds to changes in sea ice extent is examined using the Polar version of the Weather Research and Forecasting (WRF) regional model over the Laptev Sea. Polar WRF output provides detailed information on cloud properties, such as liquid water path, ice water content, cloud height, and cloud radiative forcing during periods of low and high sea ice extent. The Polar WRF is run for 8 Septembers and Octobers selected for anomalously low and high sea ice cover, and analyzes differences in cloud properties, cloud radiative forcing (CRF), temperature, and the surface radiative and heat budgets. Clouds were more frequent and had larger liquid water paths during low than during high sea ice cover years. Increased surface longwave CRF during the low sea ice years only occurred in September. In September, the averaged cloud liquid water path during high sea ice years resulted in a cloud emitting close to its maximum longwave radiation, and increases in cloud liquid water path during the low sea ice years did not increase the surface longwave cloud radiative effect. In October, the averaged cloud liquid water path during high sea ice years did not result in a cloud emitting its maximum longwave radiation, and cloud liquid water path increases that occurred in the low sea ice years affected the surface cloud radiative effect. Clouds warmed the surface during periods of low sea ice cover in October. KEY WORDS: Sea ice · Clouds · Arctic · Polar Weather Research and Forecasting · Laptev Sea Full text in pdf format PreviousNextCite this article as: Barton NP, Veron DE (2012) Response of clouds and surface energy fluxes to changes in sea-ice cover over the Laptev Sea (Arctic Ocean). Clim Res 54:69-84. https://doi.org/10.3354/cr01101 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 54, No. 1. Online publication date: August 22, 2012 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2012 Inter-Research.

Previous findings show that large-scale atmospheric circulation plays an important role in driving Arctic sea ice variability from synoptic to seasonal time scales. While some circulation patterns responsible for Barents–Kara sea ice changes have been identified in previous works, the most important patterns and the role of their persistence remain unclear. Our study uses self-organizing maps to identify nine high-latitude circulation patterns responsible for day-to-day Barents–Kara sea ice changes. Circulation patterns with a high pressure center over the Urals (Scandinavia) and a low pressure center over Iceland (Greenland) are found to be the most important for Barents–Kara sea ice loss. Their opposite-phase counterparts are found to be the most important for sea ice growth. The persistence of these circulation patterns helps explain sea ice variability from synoptic to seasonal time scales. We further use sea ice models forced by observed atmospheric fields (including the surface circulation and temperature) to reproduce observed sea ice variability and diagnose the role of atmosphere-driven thermodynamic and dynamic processes. Results show that thermodynamic and dynamic processes similarly contribute to Barents–Kara sea ice concentration changes on synoptic time scales via circulation. On seasonal time scales, thermodynamic processes seem to play a stronger role than dynamic processes. Overall, our study highlights the importance of large-scale atmospheric circulation, its persistence, and varying physical processes in shaping sea ice variability across multiple time scales, which has implications for seasonal sea ice prediction. Significance Statement Understanding what processes lead to Arctic sea ice changes is important due to their significant impacts on the ecosystem, weather, and shipping, and hence our society. A well-known process that causes sea ice changes is atmospheric circulation variability. We further pin down what circulation patterns and underlying mechanisms matter. We identify multiple circulation patterns responsible for sea ice loss and growth to different extents. We find that the circulation can cause sea ice loss by mechanically pushing sea ice northward and bringing warm and moist air to melt sea ice. The two processes are similarly important. Our study advances understanding of the Arctic sea ice variability with important implications for Arctic sea ice prediction.

With an increasing political focus on limiting global warming to less than 2 � C above pre-industrial levels it is vital to understand the consequences of these targets on key parts of the climate system. Here, we focus on changes in sea level and sea ice, comparing twenty-first century pro- jections with increased greenhouse gas concentrations (using the mid-range IPCC A1B emissions scenario) with those under a mitigation scenario with large reductions in emis- sions (the E1 scenario). At the end of the twenty-first century, the global mean steric sea level rise is reduced by about a third in the mitigation scenario compared with the A1B scenario. Changes in surface air temperature are found to be poorly correlated with steric sea level changes. While the projected decreases in sea ice extent during the first half of the twenty-first century are independent of the season or scenario, especially in the Arctic, the seasonal cycle of sea ice extent is amplified. By the end of the century the Arctic becomes sea ice free in September in the A1B scenario in most models. In the mitigation scenario the ice does not disappear in the majority of models, but is reduced by 42 % of the present September extent. Results for Antarctic sea ice changes reveal large initial biases in the models and a sig- nificant correlation between projected changes and the initial extent. This latter result highlights the necessity for further refinements in Antarctic sea ice modelling for more reliable projections of future sea ice.

Synoptic mode of Antarctic summer sea ice superimposed on interannual and decadal variability

Southern Hemisphere sea ice area (SH SIA) exhibited weak increases from the early 1980s until 2015 when it abruptly dropped, setting record low values in 2017, 2022, and 2023. The reasons for the rapid declines in SH SIA remain open to debate, with potential explanations ranging from changes in tropical Pacific climate, warming of the high latitude subsurface ocean, and contemporaneous variations in the extratropical atmospheric circulation. Here we provide novel insights into the role of the extratropical atmospheric circulation in driving year-to-year and long-term changes in Antarctic sea ice, with a focus on the influence of the Southern annular mode (SAM) on recent trends in SH sea ice area. The influence of the SAM on SH SIA exhibits a more pronounced seasonal variation than that indicated in previous work: during the annual sea ice minimum, anomalous circumpolar westerlies associated with the positive polarity of the SAM lead to increases in SH SIA that persistent for several months. In contrast, during the annual sea ice maximum, anomalous circumpolar westerlies associated with the positive polarity of the SAM lead to pronounced decreases in Antarctic sea ice that persist for up to a year. In terms of annual-mean SH SIA, by far the largest impacts arise from variations in the atmospheric circulation during the sea ice maximum. As a result, changes in the SAM during the sea ice maximum have had a marked impact on long-term changes in SH SIA. These linkages are robust in both observationally constrained data products and modeled data, with additional results exploring how this relationship changes as the mean state of the climate changes under global warming.

Both the Southern Annular Mode (SAM) and El Niño–Southern Oscillation (ENSO) are critical factors contributing to Antarctic sea ice variability on interannual time scales. However, their joint effects on sea ice are complex and remain unclear for each austral season. In this study, satellite sea ice concentration (SIC) observations and atmospheric reanalysis data are utilized to assess the impacts of combined SAM and ENSO on seasonal Antarctic sea ice changes. The joint SAM–ENSO impacts on southern high latitudes are principally controlled by the strength and position of the wave activity and associated atmospheric circulation anomalies affected by their interactions. In-phase events (La Niña/positive SAM and El Niño/negative SAM) are characterized with an SIC dipole located in the Weddell/Bellingshausen Seas and Amundsen/Ross Seas, while out-of-phase events (El Niño/positive SAM and La Niña/negative SAM) experience significant SIC anomalies in the Indian Ocean and western Pacific Ocean. Sea ice budget analyses are conducted to separate the dynamic and thermodynamic contributions inducing the sea ice intensification anomalies. The results show that in-phase intensification anomalies also display a pattern similar to the SIC dipole and are mainly driven by the direct thermodynamic forcing at the ice edge and thermodynamic responses to meridional sea ice drift in the inner pack, especially in autumn and winter. Dynamic processes caused by zonal sea ice drift also play an important role during out-of-phase conditions in addition to the same mechanisms during in-phase conditions.

Observed changes in Antarctic sea ice are poorly understood, in part due to the complexity of its interactions with the atmosphere and ocean. A highly simplified, coupled sea ice–ocean mixed layer model has been developed to investigate the importance of sea ice–ocean feedbacks on the evolution of sea ice and the ocean mixed layer in two contrasting regions of the Antarctic continental shelf ocean: the Amundsen Sea, which has warm shelf waters, and the Weddell Sea, which has cold and saline shelf waters. Modeling studies where we deny the feedback response to surface air temperature perturbations show the importance of feedbacks on the mixed layer and ice cover in the Weddell Sea to be smaller than the sensitivity to surface atmospheric conditions. In the Amundsen Sea the effect of surface air temperature perturbations on the sea ice are opposed by changes in the entrainment of warm deep waters into the mixed layer. The net impact depends on the relative balance between changes in sea ice growth driven by surface perturbations and basal-driven melting. The changes in the entrainment of warm water in the Amundsen Sea were found to have a much larger impact on the ice volume than perturbations in the surface energy budget. This creates a net negative ice albedo feedback in the Amundsen Sea, reversing the sign of this typically positive feedback mechanism.

In polar regions, positive feedback of snow and ice albedo can intensify global warming. While recent significant decreases in Arctic surface ice albedo have drawn considerable attention, Antarctic surface albedo variability remains underexplored. Here, satellite albedo product CLARA-A2.1-SAL is first validated and then used to investigate spatial and temporal trends in the summer albedo over the Antarctic from 1982 to 2018, along with their association with Antarctic sea ice changes. The SAL product matches well surface albedo observations from eight stations, suggesting its robust performance in Antarctica. Summer surface albedo averaged over the entire ice sheet shows a downward trend since 1982, albeit not statistically significant. In contrast, a significant upward trend is observed in the sea ice region. Spatially, for ice sheet surface albedo, positive trends occur in the eastern Antarctica Peninsula and the margins of East Antarctica, whereas other regions exhibit negative trends, most prominently in the Ross and Ronne ice shelves. For sea ice albedo, positive trends are observed in the Ross Sea and the Weddell Sea, but negative trends are observed in the Bellingshausen and the Amundsen Seas. Between 2016 and 2018, an unusual decrease in the sea ice extent significantly affected both sea ice and Antarctic ice sheet (AIS) surface albedo changes. However, for the 1982–2015 period, while the effect of sea ice on its own albedo is significant, its impact on ice sheet albedo is less apparent. Air temperature and snow depth also contribute much to sea ice albedo changes. However, on ice sheet surface albedo, the influence of temperature and snow accumulation appears limited.

<p>Over the last decades the change in the Arctic climate resulted in related sea-ice retreat and a much faster warming of the Arctic compared to the global average (Arctic amplification). These changes in sea ice can affect the large-scale atmospheric circulation over the mid-latitudes, in particular atmospheric blocking, and – mediated by the changes in blocking – the frequency and severity of related extreme events. As a step towards a better understanding of changes in weather and climate extremes over Central Europe (C-EU) associated with Arctic climate change, we study the linkage between periods of low and high Arctic sea ice area and the frequency of winter cold days in C-EU. Since frequency of winter cold days in C-EU is associated with atmospheric blocking, especially over the Ural and Scandinavian region, we investigate frequency changes of cold days with respect to the occurrence of blocking in different Euro-Atlantic regions by composite analysis based on ERA5 reanalysis data. </p><p>To separate the resulting changes from the global warming trend and associated Arctic sea ice loss, monthly sea ice area data is first detrended and then divided by the median into two parts representing either low or high sea ice periods. The frequency of occurrence of cold days with respect to both sea ice periods is then calculated and compared. The same procedure is applied to cold days occurring during a blocked day in certain regions to analyze the change of linkage between atmospheric blocking and cold days induced by different sea ice area periods. Preliminary results indicate an increased occurrence of cold days in Central Europe during low sea ice periods, which is enhanced during the occurrence of Ural Blocking.</p>