Comment on cp-2023-4

<strong class="journal-contentHeaderColor">Abstract.</strong> Over the past three decades, discordant trends in sea ice extent have been observed between the Arctic and Antarctic regions. Arctic sea ice extent has been characterised by a rapid decline, whereas Antarctic sea ice extent, while highly variable inter-annually, has tended to increase. Climate models have so far failed to capture these trends. Coupled with the limited pre-1970 sea ice dataset, this poses a significant challenge to quantifying the mechanisms responsible for driving such trends. However, historical records from early Antarctic expeditions contain a wealth of information regarding the nature and concentration of sea ice. Such records have been under-utilised, and their analysis may enhance our understanding of recent Antarctic sea ice variability. For the purpose of this study, 9 records from 8 Antarctic expeditions have been examined. Summer sea ice positions recorded during 1820&ndash;1843 have been compared to satellite observations from 1987&ndash;2017, as well as historical data for the period 1897&ndash;1917. Through analysis of these three time series, estimations for summer sea ice extent in the Weddell Sea, during the early nineteenth century have been produced. The key findings of this study indicate a nineteenth century average core summer northernmost sea ice latitude in much of the Weddell Sea that was further north than during the modern era, with nineteenth century February having significantly more sea ice by all measures. However, late summer sea ice was most extensive in the early years of the twentieth century.

&amp;lt;p&amp;gt;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.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;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.&amp;lt;/p&amp;gt;

The year 2024 has become the warmest year on record globally. As key indicators of global warming, the Antarctic and Arctic have shown heightened sensitivity and experienced more frequent climate anomalies. In this study, using various observations, reanalysis CMA-RA and remote sensing data, we reported detailed 2024 polar climate changes, including air and sea surface warming, sea ice, atmospheric composition and extreme events. In 2024, the air temperature of the Antarctic continent (–31.79 °C) from CMA-RA was slightly higher than the 1991–2020 average, with strong differences between the eastern and western regions and distinct seasonal variations. A persistent warm event occurred in Queen Maud Land, while Wilkes Land remained continuously cold. The inland Dome and Victoria Land showed a ‘cold summers and warm winters’ pattern. The average winter temperature anomaly at Vostok station (+4.34 °C) set a new historical record. The Arctic region continued its warming trend (–6.89 °C, on annual average), with the Canadian Arctic experiencing persistent extreme warmth in CMA-RA. CMA-RA shows that the warming rate in the Arctic (0.52°C per decade, p < 0.01) was 2.9 times the global average since 1979. In August 2024, the Barents Sea surface temperature reached a record high, while the Chukchi Sea reached a record low. The Antarctic sea ice extent remained historically low in 2024, and both the Antarctic and Arctic set new records for sea ice melt. The minimum extent of Antarctic sea ice was 1.97 × 10 6 km 2 (on 20 February), the third lowest on record. The sea ice extent in November set a 46-year low for that month since 1979. The minimum extent (4.21 × 10 6 km 2 ) of Arctic sea ice in 2024 ranked seventh lowest in history. The development of the Antarctic ozone hole in 2024 was relatively stable, showing some alleviation in the previous four years. In the Arctic region, the weakened polar vortex led to an abnormal increase in ozone. The concentrations of greenhouse gases in the polar atmosphere show a similar trend to global changes, all showing a stable upward trend. This investigation and summary of the 2024 polar climate status will help people better understand global and polar climate change.

Recent changes in global climate patterns have triggered the accelerated melting of polar sea ice, especially in Arctic regions. A much faster rate of decrease in the sea ice extent (SIE) is observed at than previously expected. The Antarctic region, on the other hand, has shown a stable sea ice pattern throughout the last few decades. However, the southern polar region is not completely unaffected. Recent studies of the Bellingshausen and Amundsen Seas have shown a downward trend in sea ice. The SIE is crucial for regulating global climate patterns, ocean circulation, and human activities, including shipping and fishing. Hence, forecasting sea ice extent is vital for global economic planning and climatological studies. In this proposed study, time-series forecasting of five Antarctic and Arctic regions are evaluated using a hybrid convolutional long short-term memory (ConvLSTM) and a bidirectional long short-term memory (BiLSTM) and compared with a standalone long short-term memory (LSTM). This study uses regional sea ice extent data rather than considering the extent across entire hemispheres. With lower and stable RMSE scores across all lead times, the proposed hybrid BiLSTM model shows better performance in regional sea ice forecasting than does the standalone and ConvLSTM. The study also indicated that the climatic conditions of a particular region play a crucial role in forecasting efficiency, especially at longer lead times.

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.

Climate warming has enabled the Arctic region to achieve seasonal navigation, and sea ice concentration is an important factor affecting the navigation of the Arctic waterways. This article uses the Arctic sea ice concentration data of the three highest temperatures in 2016, 2019, and 2020, combined with the Arctic summer sea level pressure, wind field, temperature, temperature anomaly, ice age, and sea ice movement data to analyze the spatial and temporal variation of sea ice and connectivity in the Northeast Passage (NEP) of the Arctic in Summer in three hot years, and summarizes the causes of sea ice anomalies. The results show that: (1) the summer Arctic sea ice extent in 2016, 2019 and 2020 were all lower than the multi-year average sea ice extent, and the summer sea ice extent in 2020 had the largest change trend; (2) in October of these three years, the sea ice was all negative anomalies, extending the opening time of the NEP; (3) when the sea ice concentration was 30% as the threshold, the navigation period of the NEP in 2016 was from mid-August to late October, 2019 was from the beginning of August to mid-October, 2020 was from the end of July to the end of October, and 2020 was the longest year since the opening of the NEP; (4) when the sea ice concentration was 10% as the threshold, the navigation period of the NEP in 2016 was from the end of August to the end of October, 2019 was from early August to mid-October, and 2020 was from the beginning of August to the end of October; (5) the key navigable areas of the NEP in the past three years were the central waters of the East Siberian Sea, the New Siberian Islands and the Vilkitsky Strait; (6) the navigation period of the NEP in 2016, 2019 and 2020 was longer. The main reasons were that the temperature of the NEP in the past 3 years was relatively high, the wind was weak, the sea ice movement had little effect, and the sea ice age in the key navigable areas was first year ice, which was easy to melt, which greatly promoted the opening of the NEP.

Antarctic sea ice extent (SIE) has experienced unprecedented variability in recent decades, with record expansion through 2015, followed by an abrupt transition to sustained decline. Using over two decades of under-ice Argo float observations, we show that changes in ocean heat ventilation have modulated these extreme sea ice variations on interannual timescales. Between 2007 and 2015, the ocean thermocline warmed and shoaled within the Weddell Sea and off East Antarctica, with the former accounting for most of the interannual variability in Antarctic SIE. After 2016, as Antarctic SIE declined, surface salinity increased, enhancing exchange between the sharpened thermocline and surface waters. Idealized modeling of the Weddell Sea indicates that these upper ocean trends were due to concurrent variations in wind-driven Ekman upwelling and precipitation. During the sea ice expansion phase, increased precipitation enhanced ocean stratification, suppressing the upward flux of subsurface heat while promoting sea ice growth. However, between 2014 and 2016, a nearly three-fold increase in upwelling rates weakened the upper ocean stratification, releasing the accumulated subsurface heat. Though a similar sequence of events occurred along the East Antarctic margin, distinct upper-ocean trends and surface forcing in the Pacific sector of the Southern Ocean imply alternative drivers of recent sea ice loss in that region. Nevertheless, these results suggest that future multiyear Antarctic SIE variability will depend on the competing influences of wind-driven upwelling and surface freshwater fluxes.

Arctic sea ice has declined rapidly. A record low of sea ice extent occurred in summer 2012 as observed over three decades of satellite data. The regime of the Arctic ice cover has shifted to a new state dominated by seasonal sea ice, as perennial sea ice has diminished drastically. Changes in the distribution of sea ice classes can precondition summer melt, alter the surface albedo, upset the Bowen ratio, and impact atmospheric photochemical processes. To investigate Arctic sea ice, environmental, and chemical changes, we conducted the Bromine, Ozone, and Mercury Experiment (BROMEX) around Barrow, Alaska, in March-April 2012, together with continuous measurements through the present. We used satellite active microwave scatterometer (AMS) and passive microwave radiometer (PMR) to observe the change in sea ice extent (SIE). A problem for sea ice observations in summer is that surface melt can impact PMR signatures of sea ice, causing large uncertainties in the measurement of SIE. We developed an AMS algorithm to determine SIE, primarily based on a quadruplet combination of relative AMS values rather than the absolute magnitudes of the signature and thus improving SIE accuracy. Then we took the SIE product from AMS in 2000–2009 to calibrate PMR results and reconstruct a SIE time series over three decades (1979–2013). To monitor sea surface temperature change caused by river discharge, we obtain results from two different algorithms using data acquired from the Moderate-resolution Imaging Spectroradiometer (MODIS) instruments that are currently in orbit on both the Terra and Aqua satellites, launched in 1999 and 2002, respectively. We present results from spring-summer 2012 over the Beaufort Sea where warm waters from the Mackenzie River surged onto the sea surface layer thereby accelerating sea ice melt. Additionally, we estimated the total annual heating power carried by 72 rivers into the Arctic Ocean. We found that Arctic rivers could provide an enormous amount of energy to melt sea ice. With their immense watersheds (several million square kilometers), the Arctic rivers are effective conduits that transport heat from the warm continents in summer to the Arctic Ocean, thus contributing to sea ice melt, and enhancing the sensitivity of Arctic sea ice to climate change. In stark contrast, Antarctica is a frozen continent without any warm rivers to affect sea ice in the Southern Ocean. Using satellite observations, we directly address the question: Why has Arctic sea ice decreased precipitously while Antarctic sea ice has been relatively stable? This topic draws tremendous attention from the cryospheric science community, and has been a challenging riddle in climate change research. The influence of river discharge into the Arctic Ocean helps to answer that puzzle.

Changes in deep-ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide concentrations. It has been recently proposed that such changes are associated with variations in Antarctic sea ice through two possible mechanisms: an increased latitudinal extent of Antarctic sea ice and an increased rate of Antarctic sea ice formation. Both mechanisms lead to an upward shift of the Atlantic meridional overturning circulation (AMOC) above depths where diapycnal mixing is strong (above 2000 m), thus decoupling the AMOC from the abyssal overturning circulation. Here, these two hypotheses are tested using a series of idealized two-basin ocean simulations. To investigate independently the effect of an increased latitudinal ice extent from the effect of an increased ice formation rate, sea ice is parameterized as a latitude strip over which the buoyancy flux is negative. The results suggest that both mechanisms can effectively decouple the two cells of the meridional overturning circulation (MOC), and that their effects are additive. To illustrate the role of Antarctic sea ice in decoupling the AMOC and the abyssal overturning cell, the age of deep-water masses is estimated. An increase in both the sea ice extent and its formation rate yields a dramatic “aging” of deep-water masses if the sea ice is thick and acts as a lid, suppressing air–sea fluxes. The key role of vertical mixing is highlighted by comparing results using different profiles of vertical diffusivity. The implications of an increase in water mass ages for storing carbon in the deep ocean are discussed.

Changes in snow cover and sea ice extents associated with temperature changes in the Northern and Southern Hemispheres for the period 1979-2020 are analysed using monthly-mean satellite and reanalysis data. Quantitative estimates of the relationship between the Antarctic and Arctic sea ice and changes in the surface air temperature were obtained. Overall increase of the Antarctic sea ice extent is associated with the regional manifestation of natural multidecadal climate modes with periods of up to several decades (against the background of global warming and a rapid decrease in the extent of sea ice in the Arctic). The results of correlation and cross-wavelet analyses show significant coherence and negative correlation of the surface air temperature in both Arctic and Antarctic with the respective sea ice extent in recent decades. Seasonal and regional features of the snow cover sensitivity to changes in the temperature regime in the Northern Hemisphere for the past four decades are noted. The features of snow cover variability in Eurasia and North America are presented.

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.

The annual minimum Antarctic sea ice extent (SIE) in February 2022 hits a record low in the satellite era, with less than 2 million square kilometres observed on 25 February 2022, contrasting with the slightly positive trend in the Antarctic SIE prior to 2014. However, the preceding Amundsen Sea Low (ASL) in austral spring 2021 was the deepest since 1950. According to a linear regression model, the very low ASL contributed about 60% to the record low SIE in 2022. This study further investigates the underlying mechanism. The investigation of the lagged impact of the ASL on Antarctic SIE is based on observational data and state-of-the-art simulations. We found that (a) the deepened ASL associated with strengthened southerly winds accelerates the sea ice export away from the western Antarctic continent in spring, leading to the expansion of coastal polynyas (open water areas); (b) through the positive ice-ocean albedo feedback, the lack of the sea ice off the coastline enhances solar heating in the upper ocean and further sea ice melting in summer can occur. Specifically, in spring 2021, the deepest ASL is accompanied by a large sea-ice area flux of about 17.6 × 103 km2 across 70° S over the Ross Sea in October and November, contributing to a significant increase in net surface radiation of 20–30 W m−2 and upper ocean warming of about 0.5 K in summer. Therefore, the deepened ASL in spring 2021 plays a crucial role for the record low Antarctic SIE in February 2022. In addition, it is found that both the La Niña conditions and the strong stratospheric polar vortex contributed significantly to the very strong ASL in 2021. Currently, nearly 2/3 of Earth system models in the Coupled Model Intercomparison Project Phase 6 have difficulties capturing the relationship between the ASL and the Antarctic SIE.

Abstract. The Arctic has featured the strongest surface warming over the globe during the recent decades, and the temperature increase has been accompanied by a rapid decline in sea ice extent. However, little is known about Arctic sea ice change during the early twentieth century warming (ETCW) during 1920–1940, also a period of a strong surface warming, both globally and in the Arctic. Here, we investigate the sensitivity of Arctic winter surface air temperature (SAT) to sea ice during 1875–2008 by means of simulations with an atmospheric general circulation model (AGCM) forced by estimates of the observed sea surface temperature (SST) and sea ice concentration. The Arctic warming trend since the 1960s is very well reproduced by the model. In contrast, ETCW in the Arctic is hardly captured. This is consistent with the fact that the sea ice extent in the forcing data does not strongly vary during ETCW. AGCM simulations with observed SST but fixed sea ice reveal a strong dependence of winter SAT on sea ice extent. In particular, the warming during the recent decades is strongly underestimated by the model, if the sea ice extent does not decline and varies only seasonally. This suggests that a significant reduction of winter Arctic sea ice extent may have also accompanied the early twentieth century warming, pointing toward an important link between anomalous sea ice extent and Arctic surface temperature variability.

Mechanisms associated with the rapid decline in sea ice cover around a stranded ship in the Lazarev Sea, Antarctica

Arctic sea ice variation in the Northwest Passage in 1979–2017 and its response to surface thermodynamics factors