Comment on cp-2022-15

Antarctic sea ice plays a crucial role in buttressing ice shelves, enhancing their stability, and protecting them from potential catastrophic collapse – a significance underscored by recent calving events along the Antarctic Peninsula. Presence or absence of sea ice in the Southern Ocean, and details of its distribution patterns, therefore have relevance far beyond the realm of high latitudes of the Southern Hemisphere. Investigating past sea-ice conditions in proximity to ice shelves, and changes in sea ice distribution over time, particularly across glacial-interglacial cycles, is therefore essential. We may gain insights into the sea-ice’s response to a changing climate, and address gaps in our understanding of ocean-sea ice-ice shelf interactions and dynamics. In our study, we adopt a multiproxy approach to explore glacial-interglacial environmental variability since the Last Penultimate Glacial close to the Antarctic continental margin in the Weddell Sea. We analyze the novel sea-ice biomarker IPSO25 (a di-unsaturated highly branched isoprenoid (HBI)), open-water biomarkers z-/e-trienes (tri-unsaturated HBI), diatom assemblages and primary productivity proxies in a marine sediment core (PS118_63-1) retrieved from Powell Basin in the northwestern Weddell Sea. These biomarkers are reliable proxies for reconstructing near-coastal sea-ice conditions in the Southern Ocean, where the use of sea ice-related diatoms may be subject to bias due to silica dissolution. We present the first continuous record of ice-proximal Antarctic sea ice since the Last Penultimate Glacial. Our results unveil a highly dynamic environment, characterized by significant shifts from a climate with perennial (sea) ice cover to more seasonal sea-ice cover and open ocean conditions, over the last approximately 145 kyrs. Furthermore, to gain a better understanding of the spatial heterogeneity of sea-ice distribution and sea ice-ice shelf system dynamics in the Southern Ocean, we use numerical climate modeling to expand our view across the Southern Ocean, while comparing data from marine cores PS67/219-1 (southern Scotia Sea) and PS128_14 (eastern Weddell Sea) helps track latitudinal sea-ice changes and identify common forces driving sea ice-ice shelf system dynamics along continental margin, respectively.

The sea ice concentration dataset (covering the period 1992-2008) used in this study is a new dataset based on the Sea Ice Climate Change Initiative (SICCI) algorithm. We investigate whether the SICCI dataset is on a par with other datasets for studying sea ice cover changes in the Southern Ocean. We then examine spatiotemporal variations in sea ice derived from the SICCI dataset over the Southern Ocean, and analyse relationships of sea ice with sea surface temperature (SST). The results indicate that there is no significant difference between the SICCI dataset and the NASA Team dataset, and therefore the former can also be used for studying sea ice changes. Both sea ice extent (SIE) and sea ice area (SIA) derived from the SICCI dataset over the Southern Ocean increased slightly from 1992 to 2008, at rates of (17.75 ± 11.50) × 10^3 and (17.37 ± 9.51) × 10^3 km^2 yr^(–1), respectively. Antarctic sea ice has significant seasonal variations; all seasonally averaged SIE and SIA show an increase, with spring showing the largest positive changing rate. The Weddell Sea, Ross Sea, and Indian Ocean have positive yearly changing rates in SIE and SIA, while the Bellingshausen/Amundsen seas and western Pacific Ocean have negative yearly changing rates. However, overall sea ice over the Southern Ocean has a slight positive trend, which is the same as the sea ice change pattern derived from the NASA Team dataset. This indicates that the contributions to the change in sea ice over the whole Southern Ocean due to the Weddell Sea, Ross Sea, and Indian Ocean dominate over those by the Bellingshausen/Amundsen seas and western Pacific Ocean. Further analysis shows that both SIE and SIA are negatively correlated with SST in the Southern Ocean or each of the 5 longitudinal sectors, and sea ice is more sensitive to SST in spring and autumn.

Geophysical constraints on the Antarctic sea ice cover

Despite the overwhelming warming over most oceans under global warming, cooling in the surface of the Southern Ocean between Australia and South America has been previously reported. The surface cooling is due to wind-driven sea-ice transport and its subsequent melting. However, Antarctic sea ice which has a steady increasing trend during several decades, experienced an abrupt decline to a record low in the mid-2010s. Since then, the interannual variability of Antarctic sea ice has been further intensified, showed another record low in 2022 and 2023 consecutively. Accordingly, the Pacific sector of the Southern Ocean has experienced warming during the period of rapid sea ice decline.Nevertheless, we find that a coherent cooling pattern is observed in a very specific area, near the Ross Sea gyre. Here, to investigate the mechanisms associated with the cooling pattern, we examined how the rapid decline in Antarctic sea ice in recent years has contributed to the strengthening of the cooling trend near the Ross Sea gyre, using satellite observations of sea ice, as well as oceanic and atmospheric reanalysis data. Our result shows that the cooling trend has strengthened despite the rapid decrease in Antarctic sea ice in recent years. The significant cooling trend has been attributed to record atmospheric low pressure systems over the Pacific sector of the Southern Ocean. We demonstrate that the cooling near the Ross Sea gyre is likely influenced by the local and remote large-scale atmospheric variabilities that lead to substantial sea ice anomalies. Since the mid-2010s, local anomalous winds and surface heat flux associated with strong events of natural climate oscillations like the Zonal Wavenumber 3, El Niño–Southern Oscillation, and Southern Annular Mode in the Southern Hemisphere that drive the recent change in Antarctic sea ice, induce strong meridional flows led to enhanced sea ice drifts and melting, and consequently to the strong cooling trend.    

Abstract. This paper examines the ability of coupled global climate models to predict decadal variability of Arctic and Antarctic sea ice. We analyze decadal hindcasts/predictions of 11 Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Decadal hindcasts exhibit a large multi-model spread in the simulated sea ice extent, with some models deviating significantly from the observations as the predicted ice extent quickly drifts away from the initial constraint. The anomaly correlation analysis between the decadal hindcast and observed sea ice suggests that in the Arctic, for most models, the areas showing significant predictive skill become broader associated with increasing lead times. This area expansion is largely because nearly all the models are capable of predicting the observed decreasing Arctic sea ice cover. Sea ice extent in the North Pacific has better predictive skill than that in the North Atlantic (particularly at a lead time of 3–7 years), but there is a re-emerging predictive skill in the North Atlantic at a lead time of 6–8 years. In contrast to the Arctic, Antarctic sea ice decadal hindcasts do not show broad predictive skill at any timescales, and there is no obvious improvement linking the areal extent of significant predictive skill to lead time increase. This might be because nearly all the models predict a retreating Antarctic sea ice cover, opposite to the observations. For the Arctic, the predictive skill of the multi-model ensemble mean outperforms most models and the persistence prediction at longer timescales, which is not the case for the Antarctic. Overall, for the Arctic, initialized decadal hindcasts show improved predictive skill compared to uninitialized simulations, although this improvement is not present in the Antarctic.

Antarctic sea ice cover experienced an abrupt decline in 2016, transitioning from a record maximum state to a record minimum state. However, the drivers of this rapid retreat are currently not well understood. Therefore, it is difficult to determine whether this signals the start of a long term melting trend, as has been long anticipated by climate models, or is an isolated episode of internal climate variability. In this study, we utilise the CMIP6 archive to understand if internal climate variability could be responsible for this Antarctic sea ice anomaly, and if so what the primary atmospheric and oceanic drivers are. This involves examining composites of the tropical teleconnections, subsurface ocean heat content, and high latitude atmospheric variability preceding extreme Antarctic sea ice anomalies in CMIP6 simulations. The primary objective is to elucidate the multifaceted factors influencing these extreme events, specifically addressing the 2016-2017 sea ice retreat, with lessons for 2023’s extreme Antarctic sea ice state. Initial results indicate that such events are possible in the absence of anthropogenic emissions in some climate models, although the occurrences are considered rare. We also show that that using the limited observed record alone will underestimate the interannual variability of the Antarctic sea ice cover and therefore overestimate how rare such an anomaly would be. In fact, if we extend the observed record further back using statistical reconstructions, rapid declines of sea ice extent occurred in the early and mid 20th century.  Our results highlight the importance of internal climate variability in the Southern high latitudes and advance our understanding of the drivers and predictability of Antarctic sea ice changes. We discuss the implications of this work for 2023’s record Antarctic sea ice anomaly.

Relationship between Antarctic sea ice and the Southern Annular Mode and tropical climate variability has been widely studied. Focusing on the midlatitude oceanic frontal zones, this study identifies a fingerprint of South Atlantic (SA) variability in the Antarctic summer sea ice. The interaction between spring SA sea surface temperature (SST) and summer storm‐track activity modulates the hemispheric atmospheric circulation, which in turn alters Antarctic summer sea ice concentration (SIC) through thermal and wind‐driven forcing. Specifically, warm SST anomalies in the western SA frontal zone, corresponding to the strengthened SST front, cause the increased SIC in the eastern Ross Sea via anomalous cold air advection and offshore drift of sea ice. The opposite effect (i.e., warm air advection and onshore ice drift) results in the decreased SIC in the northwestern Weddell Sea. The findings also imply a potential impact of the midlatitude SA variability to the South Pole air temperature variability.

Polar regions have a profound impact on global climate change, because polar regions are one of the world’s largest sources of cold. Sea ice is the main influencing factor in the polar regions, which has a great influence on the heat transfer, which in turn affects the global climate system. In the past two decades, Arctic sea ice has decreased year by year, while Antarctic sea ice has increased. The use of passive satellite remote sensing data for long-term observation and analysis of polar sea ice will provide an important theoretical basis for studying global climate change and an important data source for polar sea ice research. This study uses the data of the Antarctic and Arctic sea ice density data from 1998 to 2018 to compare and analyze the changes in time and space to find out the changes of the sea ice in the south and Arctic. Using the SSM / I data of the US National Defense Meteorological Satellite and the NASA TEAM algorithm, the sea ice data from 1998 to 2018 was compared and analyzed. The specific process is to use the NASA TEAM algorithm to invert the Arctic and Arctic sea ice, obtain multi-year ice, one-year ice, and overall sea ice density data, and then determine the sea ice area. Through comparative analysis, it is found that the area of Antarctic sea ice is significantly larger than that of the North Pole, and the change trend of the two is similar. The downward trend is greater than that of the South Pole; the total annual ice volume of the South Pole is greater than that of the North Pole, both of which have an upward trend, and the Arctic has a greater increase; the overall sea ice has fallen, and the multi-year ice has decreased while the annual ice has increased. Sea ice has a tendency to transform from multi-year ice to one-year ice; Antarctic multi-year ice is mainly distributed in the Weddell Sea, with stable areas accounting for 60% of the multi-year ice area; Antarctica is dominated by one-year ice, and one-year ice stable area accounts for one 80.1% of the total annual ice area; Arctic multi-year ice mainly exists in the central area of the Arctic Ocean. The Arctic multi-year ice stable area only accounts for 35.06%; the Arctic one-year ice is mainly distributed around the multi-year ice periphery, and the stable existence area accounts for the total annual ice area 48.5%.

In the tropics, deep convection triggers upper-level quasi-stationary Rossby waves that propagate to higher latitudes and influence local climate patterns. This study examines the teleconnection between the Madden-Julian Oscillation (MJO), the dominant mode of tropical intraseasonal variability, and Antarctica, using daily gridded observational datasets (precipitation from CMAP, 250-hPa geopotential height, 2 m air temperature, and 10 m winds from ERA5, and sea ice concentration from NSIDC) and the Linear Response Theory Method (LRTM) across all southern seasons during 1979-2014. Our results reveal that MJO-driven variations in surface temperature and winds substantially affect Antarctic sea ice concentration throughout the year. In austral summer and autumn, significant sea ice responses are evident in both the eastern (Lazarev Sea to Somov Sea) and western Antarctic sectors (Ross Sea, Amundsen Sea, and Weddell Sea). During summer, the most notable sea ice changes occur in MJO phases 1 and 5, while in autumn, the most potent responses are associated with phases 1–3. Conversely, in winter and spring, the sea ice responses are primarily restricted to the western Antarctic sectors (Ross Sea, Amundsen Sea, Bellingshausen Sea, and Weddell Sea). All MJO phases exert a pronounced influence on sea ice in winter, whereas in spring, phases 1 and 5 dominate. The LRTM effectively elucidates the mechanisms underlying these changes, attributing the observed sea ice variability to wind-driven forcing, thermal advection, or their combined effects.

<p>Quantitative estimates of the relationship between the interannual variability of Antarctic and Arctic sea ice and changes in the surface temperature in the Northern and Southern Hemispheres using satellitedata, observational data and reanalysis data for the last four decades (1980-2019) are obtained. The previously noted general increase in the Antarctic sea ice extent (up to 2016) (according to satellite data available only since the late 1970s), happening simultaneously with global warming and rapid decrease in the Arctic sea ice extent, is associated with the regional manifestation of natural climate fluctuations with periods of up to several decades. The results of correlation and crosswavelet analysis indicate significant coherence and negative correlation of hemispheric surface temperature with not only Arctic,but also Antarctic sea ice extent in recent decades.</p><p>Seasonal and regional peculiarities of snow cover sensitivity to temperature regime changes in the Northern Hemisphere are noted with an assessment of changes in recent decades. Peculiarities of snow cover variability in Eurasia and North America are presented. In particular, the peculiarities of changes in snow cover during the autumn seasons are noted.</p>

A review of sea ice proxy information from polar ice cores

Following over 3 decades of gradual but uneven increases in sea ice coverage, the yearly average Antarctic sea ice extents reached a record high of 12.8 × 106 km2 in 2014, followed by a decline so precipitous that they reached their lowest value in the 40-y 1979-2018 satellite multichannel passive-microwave record, 10.7 × 106 km2, in 2017. In contrast, it took the Arctic sea ice cover a full 3 decades to register a loss that great in yearly average ice extents. Still, when considering the 40-y record as a whole, the Antarctic sea ice continues to have a positive overall trend in yearly average ice extents, although at 11,300 ± 5,300 km2⋅y-1, this trend is only 50% of the trend for 1979-2014, before the precipitous decline. Four of the 5 sectors into which the Antarctic sea ice cover is divided all also have 40-y positive trends that are well reduced from their 2014-2017 values. The one anomalous sector in this regard, the Bellingshausen/Amundsen Seas, has a 40-y negative trend, with the yearly average ice extents decreasing overall in the first 3 decades, reaching a minimum in 2007, and exhibiting an overall upward trend since 2007 (i.e., reflecting a reversal in the opposite direction from the other 4 sectors and the Antarctic sea ice cover as a whole).

Chapter 7 - Antarctic sea ice changes and their implications

In the Antarctic, sea ice algae use the highly dynamic sea ice as a platform for growth. Antarctic sea ice extent has recently been highly variable, first showing a slight increase and then a record decrease starting in 2016. We investigated the response of Antarctic ice algal habitat to variations in sea ice and other environmental forcings during 2004–2019. Combining an ice growth model, remote sensing and reanalysis data, and a radiative transfer model, we assessed whether light penetration to the bottom ice was sufficient for ice algal growth. Trends in the inputs over the 16 years were relatively small: there were no changes in ice thickness or bottom ice melt date, a 6.4% decrease in snow depth, a 1.2% decrease in incident light, and a 0.8°C decrease in air temperatures. Eighty‐one percent of the sea ice cover was habitable by ice algae for ≥14 days each year. The Antarctic has a larger extent and duration of potential ice algal habitat than the Arctic. Over time, the spatially averaged seasonal duration of habitat increased because a higher proportion of each pixel became habitable on average, compensating for the 2016–2019 reduction in sea ice extent. The spatial variability in potential habitat was strikingly high, even within geographic sectors. Bottom ice melt date (bloom termination) far surpassed other environmental factors in explaining variation (45%) in ice algal habitat on the 25 km scale. Because melt date depends on the ice‐atmosphere heat balance, Antarctic ice algal habitat may be highly sensitive to future climate changes.

This study investigates the response of the Southern Ocean sea surface temperature (SST) and Antarctic sea ice to stratospheric ozone recovery, focusing on the time scale and seasonality of the response. The response is quantified by contrasting two twenty-first-century ensemble simulations conducted with the Goddard Earth Observing System Chemistry–Climate Model: one with decreasing ozone-depleting substances (ODSs) and the other with fixed 2005 levels of ODSs. In our simulations, the response to ozone recovery has large seasonal variations, but it does not show a two-time-scale behavior. Ozone recovery causes Southern Ocean SST warming in austral summer and cooling in other seasons. Ozone recovery mitigates Antarctic sea ice decrease in the twenty-first century in austral spring, fall, and winter. However, the summer Antarctic sea ice extent is not affected by ozone recovery despite strong surface warming, because the warming occurs north of the sea ice edge. The absence of summer sea ice response likely results from the model bias of underestimating summer sea ice climatology. The summer surface warming response is associated with cooling directly below the mixed layer. Temperature tendency budget analysis shows that reduced vertical mixing plays a critical role in driving this vertical dipole temperature response. We also find that the impact of the vertical temperature advection on SST depends not only on changes in upwelling but also on changes in vertical temperature gradient. Significance Statement We study the climate impact of the projected stratospheric ozone recovery in the twenty-first century on Southern Ocean sea surface temperature and Antarctic sea ice using coupled atmosphere–ocean–chemistry model simulations. The model results show that ozone recovery causes weakening of surface winds over the Southern Ocean, which leads to warming of the Southern Ocean surface in summer and cooling in other seasons. Ozone recovery also reduces Antarctic sea ice loss in the twenty-first century. We quantify the relative importance of meridional and vertical advection and vertical mixing in determining the upper Southern Ocean temperature response to stratospheric ozone recovery. These results improve our understanding of how changes in surface winds affect the Southern Ocean temperature, circulation, and Antarctic sea ice.