Pacific sub-decadal sea surface temperature variations contributed to recent Antarctic Sea ice decline trend

Domain: earth_science
Published: 2025-04-10
Editorial outcome: major_revision

Abstract
Total sea ice extent (SIE) across the Southern Ocean increased from 1979-2014, but declined rapidly after 2016. Significant sea ice decline has emerged since the peak of SIE in 2014, coincident with Pacific sub-decadal sea surface temperature (SST) trends resembling a strong La Niña-like cold condition and the negative phase of the interdecadal Pacific oscillation (IPO). Previous studies suggest that the warm subsurface Southern Ocean was an important driver of the low sea ice in spring 2016 and the sustained low sea ice state since. Here we show that the observed atmospheric circulation changes near Antarctica during the period from June 2013-May 2023 are conducive to increasing surface temperature via warm advection from north and reducing Antarctic SIE, involving a deepening of the Amundsen Sea Low and anomalous high pressures over the Weddell Sea and West Pacific sectors. Through coupled pacemaker experiments, we demonstrate that Pacific sub-decadal SST trends have dominantly driven these atmospheric circulation changes through tropical–polar teleconnections and also induced significant Southern Ocean subsurface warming in the recent decade. The consequent decreasing SIE has enhanced the Southern Ocean subsurface warming effect and significantly contributed to the rapid Antarctic SIE decline.

Introduction
Satellite observation shows total Antarctic sea ice extent (SIE) had a small positive linear trend during 1979-2014, with large regional variability1. There is evidence that internal decadal variability of the climate system could be responsible for trends of Antarctic sea ice1–6. In particular, decadal variability from the tropical Pacific7–14 or tropical Atlantic15–17 could play an important role. Some studies also proposed that Southern Ocean freshening and sea surface temperature (SST) cooling trend might have contributed to the observed positive trend before 201414,18–22. After the peak of 2014, Antarctic SIE started to decrease and dramatically dropped to a record low in late 201623–25, after which the unexpected low sea ice state persisted for several years, and the SIE reached a new record low in the mid-2023, and remained at, or near, record-low values during ensuing Austral winter and spring26,27. Nearly all regions have experienced significant sea ice loss, particularly in the Weddell Sea (Figs. 1a and S1).

These prompt the critical question: What factors have caused the tremendous Antarctic SIE decline since 2014? Recent studies indicate that the decadal-long subsurface Southern Ocean warming associated with the Southern Annular Mode (SAM) and the negative phase of IPO contributed to the sudden Antarctic SIE decline of 2016 and sustained a persistent sea ice decrease since12,26,28,29, and may have caused a regime shift in Antarctic sea ice26. Through numerical simulations, Zhang et al.28 demonstrate that the negative SIE in 2016–2021 corresponds to an overall warm and salty upper ocean and that the SIE declines are preceded by a heat buildup in the subsurface ocean (depths of roughly 100–500m) and a salinification in the surface ocean before 2016, which tended to destabilize the upper ocean stratification and trigger vertical instabilities. But the subsurface ocean status before 2016 only sustains the overall low states of sea ice, and the tremendous decline of sea ice after 2016 is still largely impacted by the simultaneous atmospheric circulation and ocean variabilities12,28. This study mainly examines the relative roles of the Pacific sub-decadal SST variations, originating from the evolution of ENSO variations and the shift of the IPO from the positive phase in 2014–2016 to the negative phase after 2016, on Southern Hemisphere (SH) atmospheric changes and Antarctic SIE decline. During 2014–2016, the tropical Pacific experienced persistent warm conditions that included a weak El Niño in 2014 and an extreme El Niño in 2015–2016, while the prolonged 2020–2023 La Niña conditions developed and decayed in autumn 2023. During June 2013–May 2023, the observed SST trend patterns over the Pacific (Figs. 2a, S2a–d) resemble the typical La Niña conditions and the negative phase of the IPO characterized by strong cold conditions in the central-eastern tropical Pacific and large warming in the west-central midlatitude Pacific. The Nino34 SST and raw unfiltered IPO indices show significant downward trends of −1.32 and −1.76°C/decade from June 2013 to May 2023, respectively (Fig. 2d). The raw IPO has an extremely negative value below −1.5°C in 2022, indicating that the IPO has very likely transited to negative from a short-lived positive phase during 2015-1630. In each season, significant and persistent SST warming is found over the tropical Atlantic and the high-latitude North and South Atlantic (Fig. S2a–d). Here, we use the Community Earth System Model (CESM1.2) to perform two sets of 100-year coupled pacemaker simulations in which observed trends of SST anomalies over the Pacific and global ocean between 60°S to 60°N from June 2013 to May 2023 are added to the model-simulated SST climatology, named the PACTrend and GLTrend experiments, respectively (see “Methods” section). These coupled pacemaker experiments only assimilate prescribed observed SST trend anomalies into the ocean component of the coupled model, and the sea ice component fully responds to atmospheric and ocean variations.

Results
Observed atmospheric forcing in SIE decline
The atmosphere is an important driver of Antarctic sea ice variability and changes1,6,8,31–35. Figure 1 indicates that changes in atmospheric circulation might have governed the contemporary 10-year trend in SAT and sea ice concentration (SIC) over the Southern Ocean in the chosen time period from June 2013 to May 2023. There is a striking coherence of the trends in SAT and SIC over most areas, where the negative SIC trends are collocated with warming temperature trends, suggesting that the SAT trend largely contributed to sea ice cover trends35. In each season, a deepening of the Amundsen Sea Low (ASL) and high sea level pressure (SLP) anomalies over the Weddell Sea and Western South Pacific are observed, which induce warm SAT trends over the Southern Ocean through warm advection from the north (Fig. 1f–m). Maximum SLP trend values reach −10.0 (6.0), −15.0 (3.0), −9.0 (6.0), and −8.0 (3.0) hPa/decade associated with the deepening ASL (anomalous anticyclonic over the Weddell Sea) in JJA, SON, DJF, and MAM, respectively. Circulation around the ASL promotes southerly flow on its western side and northerly flow on the eastern side, and the intensity and location of the ASL strongly influence surface temperature and sea ice extent in the Southern Ocean31–34. Strong negative SIC trends over the Weddell–Bellingshausen Seas are consistent with large positive surface temperature trends and warm northwesterly flow associated with enhanced ASL and positive SLP anomalies in the Weddell Sea sector in Austral spring (September–October–November, SON) and summer (December–January–February, DJF) (Fig. 1c, d, g, h), along with warm northerly flow and marked poleward heat flux as a result of a couple to flow and high SLP anomalies that were centered on 90°W and 0° in autumn (March–April–May, MAM) or 140°W and 60°W in winter (June–July–August, JJA) (Fig. 1f, i), respectively. Similarly, sea ice loss and positive temperature trends over the West Pacific and Ross Sea sectors are consistent with warm northwesterly flow associated with high (low)-pressure anomalies over the Western South Pacific (the Antarctic) in Austral summer and spring. Meanwhile, the mechanical effect of the northerly winds would push ice southward, decreasing the SIE and vice versa. In each season, anomalous positive (negative) v-component 850hPa winds are strongly connected with greater offshore (onshore) flow and sea ice gain (loss)11, but part of the Amundsen-Bellingshausen Sea (ABS) and eastern Ross Sea experienced strong southerly wind and only weak increase in sea ice. This contrasts dramatic sea ice loss over other regions, particularly in the Weddell Sea. During Austral spring (summer), the SLP trend pattern is characterized by strongly (weakly) positive SAM but with considerable zonal asymmetry, which is important in contributing to the sea ice decline31–34. The greatest warming and SIE loss are observed in the Weddell Sea sector, accounting for over 41% of the total Southern Ocean SIE trend (−1.84×106km2/decade). Figure 1 also shows the concurrence of cold or weak warm SAT and large SIC decline over the eastern Weddell Sea in JJA and vast areas from the Weddell Sea to Ross Sea in DJF, indicating that subsurface warming might have driven the sea ice decline, as will be discussed later.

In all seasons, the 300-hPa geopotential height (Z300) changes over the South Pacific are associated with a negative Pacific South America (PSA)-like Rossby wave train that originates in the tropical Pacific, and the corresponding wave activity flux (WAF) exhibits a distinctive arc-shaped trajectory, extending from the central tropical Pacific toward the ABS region and continuing to the South Atlantic (Fig. S2e–h). Coincident with the trends of the strengthened ASL and negative PSA patterns in the anomalous circulation, tropical Pacific rainfall trends show the La Niña-related dipole-like patterns with strong negative rainfall over the western to central equatorial Pacific in all seasons, and positive rainfall over the Maritime Continent in JJA and SON and northwestern tropical Pacific and the South Pacific convergence zone (SPCZ) in DJF and MAM (Fig. S3). A La Niña condition typically generates a negative PSA teleconnection pattern and deepens the ASL36–38, and could also drive a SAM teleconnection in Austral summer17,39,40, while the IPO can affect Antarctic atmospheric circulation through PSA-like Rossby wave trains and SAM teleconnection, with anomalies persisting for decades10,11. Over the tropical Atlantic, there are warming SST and increased precipitation trends, but the precipitation magnitudes are much smaller compared with the equatorial western Pacific or the SPCZ, suggesting that teleconnections from Pacific precipitation anomalies are primarily responsible for observed circulation changes in Figs. 1 and S2.

Modeling impacts of Pacific and global sub-decadal SST trends on SIE decline
We first examine the forcing of Pacific sub-decadal SST trends in atmospheric circulation changes and the rapid decline of Antarctic SIE in the recent decade. In the PACTrend experiment, significant ensemble-mean responses in Fig. 3e–h reproduce major features of observed SLP and UV850 trends, especially the enhanced ASL and positive SLP anomalies over the Weddell Sea and Western South Pacific in all seasons and positive SAM-like teleconnections in SON and DJF. The most noticeable disagreement is that the strengthening of JJA ASL in the observation and simulation are centered at about 135°W and 90°W, respectively. The pattern correlations between observed SLP trend and forced SLP response between 50°S and 90°S are 0.01, 0.68, 0.89, and 0.34 in JJA, SON, DJF, and MAM, respectively. In all seasons, the magnitudes of the ensemble-mean SLP are less than observed trends, mainly due to the ensemble averaging process. Figures S4 and 5a–d further indicate that the responses of precipitation, Z300, and associated WAF responses to Pacific SST trends strikingly resemble the corresponding observed trends in Figs. S2 and S3, except for the longitude phase shift (about 45°) between observed and forced Z300 in JJA as in the SLP field. Sub-decadal negative SST anomalies in the tropical Pacific drive enhanced Walker circulation over the tropical Pacific with weakened (enhanced) vertical motion and reduced (increased) convection and rainfall over the equatorial central-eastern Pacific (equatorial western Pacific and SPCZ) in all four seasons (Fig. S4a–d). There is a pair of symmetric cyclones over the Pacific from ~30°N–30°S responding to the cooling tropical Pacific SST in each season, which appears to be quantitively consistent with the simple model solution exhibited by Gill41 (Fig. S5a–d). This convection produces anomalous diabatic heating in the mid-troposphere and accelerates the local Hadley circulation over the western tropical Pacific, forming anomalous convergent flows at the descending branch. There is a large-scale zonally oriented dipole pattern in the tropics with anomalous upper-level divergence (convergence) over the western tropical Pacific and Indian Ocean (the central-eastern Pacific and tropical Atlantic Ocean) in each season (Fig. S6a–d). The resultant subtropical upper-level convergent/divergent anomalies perturb the subtropical jet and generate anomalous Rossby wave sources (RWS)42, which subsequently excite patterns of stationary Rossby waves and influence the SH extratropics15,17,40. Significant positive RWS is found over the subtropical areas from South Indian Ocean and Australia to the western South Pacific in all seasons. The propagation of WAF as indicated by arrows shows a wavetrain originating from these areas, and a negative PSA teleconnection is clearly seen (Fig. S5a–d), leading to the significant anomalous low over the ASL region (maximum anomaly −3.5, −4.5, −3.5, and −1.5 hPa in JJA, SON, DJF and MAM, respectively) and significant anomalous high over the northern Weddell Sea (maximum anomaly 1.5-2.0 hPa in SON, DJF and MAM, respectively) (Fig. 3e–h).

Overall, above atmospheric responses to Pacific sub-decadal SST variations are similar to those to the La Niña SST anomalies36–40 and negative IPO SST anomalies10,11, and the associated tropical–polar teleconnections are consistent with the understanding of tropical teleconnections to the SH extratropics arising from the tropical Pacific variability associated with ENSO and IPO reviewed in Li et al.17. In Fig. 3, sea ice increase and cold SAT over the ABS and Ross Sea are simulated in all four seasons mainly due to the southerly wind anomalies associated with the deepening of the ASL, which assists in bringing more cold air from Antarctica, driving sea ice northward and enhancing sea ice expension1,8,11,33. In cold seasons, JJA and SON, the simulations generally reproduce observed sea ice decline and warm SAT trends over the Weddell and Indian Ocean Seas and West South Pacific when tropical–polar teleconnections are strongest (Fig. 3a, b, i, j). Although the PACTrend experiment captures spatial patterns of observed SLP trends in DJF and MAM, the sea ice decline from the Bellingshausen-Weddell Seas and Western South Pacific are much weaker than the corresponding observed trends, indicating important contributions to SIC decline by other forcings in these two warm seasons. In Fig. 3g, h, strengthened westerlies likely drive cooling and sea ice increase over the southern Weddell Sea in DJF and MAM by advecting cold and surface mixed-layer waters northward by the Ekman effect8,25, and reduce sea ice loss over the northern Weddell Sea and Indian Ocean.

Second, the global SST trends are added to the model-simulated SST in the global ocean between 60°S and 60°N in the GLTrend experiment. As shown in Figs. 2a and S2, significant and persistent SST warming is found over the tropical Atlantic and the high-latitude North and South Atlantic, and weak cooling occurs over the Indian Ocean in each season. The contributions of the global SST trend to the SIC and atmospheric changes are shown in Figs. 4 and S4–S5. Key circulation responses (e.g., the deepening of the ASL and positive SLP anomalies in the Western South Pacific and PSA Rossby wave patterns in all seasons, and DJF positive SAM) to global SST trends remarkably resemble those to Pacific SST trends. Compared to those responses in Fig. 3, the strengthened ASL responses are slightly enhanced in JJA, DJF and MAM with the maximum anomalies of −4.0, −4.0, and −2.5 hPa, respectively, and the anticyclonic responses over the Weddell and Ross Seas are reduced in JJA, SON and MAM (Fig. 4). The model atmospheric responses of SLP and Z300 to the global SST trend (Figs. 4e–h, S5e–h) reproduce the major features of the observed circulation changes in all seasons, except similar eastward shifts of the strengthening JJA ASL and PSA teleconnection as in the PACTrend experiment. The pattern correlations between observed SLP trend and forced SLP response between 50°S and 90°S are −0.08, 0.39, 0.92, and 0.69 in JJA, SON, DJF, and MAM, respectively. Two sets of pacemakers run thus better reproduce the observed SLP trends in the other three seasons but simulate a deepening of the ASL with a 45° longitude phase shift from observations in Austral winter. This indicates that Southern Ocean atmospheric circulation changes are mainly induced by Pacific sub-decadal SST variations in the other three seasons in the recent decade but also by other factors in winter, such as atmospheric internal variability.

The relative contributions of Atlantic and Indian Ocean SST trends are shown in Figs. S7–S9 by subtracting responses in the PACTrend simulation from those in the GLTrend simulation. Tropical Atlantic SST warming and Indian Ocean cooling can perturb the zonal Walker circulation and change the local Hadley cell15–17, creating anomalous convergent (divergent) flow across the upper troposphere in the central-eastern Pacific and eastern Indian Ocean (tropical Atlantic and western Indian Ocean) (Fig. S7a–d), and leading to significant positive precipitation responses over tropical Atlantic-South America with a strengthened and southward shifted SACZ and increased (reduced) precipitation over tropical Africa and the western tropical Indian Ocean (the eastern tropical Indian Ocean) in all four seasons (Fig. S8a–d). As demonstrated in previous studies15–17, the interaction between the anomalous convergence (divergence) over the subtropic Atlantic (eastern Indian Ocean) and mean vorticity produces positive (negative) RWS in the South Atlantic through to the western South Indian Ocean and also over the western South Pacific (eastern South Indian Ocean and South Pacific) (Fig. S7e–h). These RWS subsequently excite two wave trains that teleconnect the region of the tropical Atlantic and Indian Ocean, forcing to the extratropic, one from the tropical Atlantic to the Amundsen-Bellingshausen-Weddell seas and another wave train from the subtropical South Indian Ocean and Australia toward the western South Pacific and South Atlantic (Fig. S8e–h). These two wave trains appear to constructively increase the strength of both the trough and the ridge that develops over the Southern Ocean significantly in the JJA, SON, and DJF seasons.

Overall, SLP and UV850 responses to Atlantic and Indian Ocean SST trends are much weaker than those to Pacific SST forcing (Figs. S9e–h, 3e–h) but cause significant warm SAT and sea ice reduction over most areas of the ABS and Weddell Sea, and Western South Pacific. In DJF, Atlantic and Indian Ocean SST trends induce a strong positive (negative) Z300 response over the Weddell Sea (ABS) (Fig. S8g), and a weak but significant SLP and anticyclonic wind response over the Weddell Sea and warm northerly flow, leading to local SAT warming and significant sea ice loss over the Weddell Sea and ABS (Fig. S9c, g, k). This explains why a large SIC decline over the Weddell Sea and less SIC increase over the ABS is simulated in the GLTrend experiment (Fig. 4c, k). Similar warm air advected from the north and northerly onshore flow effect on sea ice loss associated with responses of 850hPa winds are also simulated in other seasons, significantly enhancing (reducing) the effects of SAT warming (cooling) and sea ice decline (gain) by the Pacific SST forcing over the Weddell Sea and Western South Pacific (the ABS). The GLTrend experiment thus simulates much stronger signals of warming SAT and sea ice decline over the Southern Ocean than the PACTrend experiment (Figs. 3 and 4). The spatial and seasonal patterns of Antarctic sea ice concentration decline, particularly the significant ice loss in the Bellingshausen-Weddell Sea and Western South Pacific, are better captured in the GLTrend experiment. Seasonal Antarctic SIE decreases −0.70 (−0.39), −0.68 (−0.47), −1.21 (−0.25) and −0.53 (0.09) *106km2 from JJA to MAM in the GLTrend (PACTrend) experiment, respectively (Table S1). The Pacific and global SST-induced annual Antarctic SIE decreases are −0.40 and −0.72 *106km2, accounting for about 22% and 40% of the observed SIE trend (−1.84 *106km2/decade), respectively.

Third, the PACTrend experiment reproduces observed North and South Atlantic SST warming and tropical to subtropical (mid-latitude) Indian Ocean cooling (warming) trends (the left panels), but simulates a cooling tropical Atlantic SST response, in contrast to observed warming trend there (Fig. 5a–d and left panels in Fig. S10). Consistent with warming SST responses over the South Atlantic and Indian Oceans, significant upper ocean warming responses over these areas are also simulated in each season (right panels in Fig. S10, Fig. 5e and S11). Amplitudes of 0–50m annual mean temperature responses averaged over 50°–65°S in the South Atlantic and Indian Ocean sectors are about 0.37°C and 0.18°C, respectively. Figure 6 shows a strong (weak) annual warming trend of 0.51°C/decade (0.13°C/decade) from the Argo float data in the South Atlantic (South Indian Ocean). Numerous studies have demonstrated that Pacific SST variability can excite other modes of climate variability in the Atlantic and Indian Oceans by altering the general circulation of the atmosphere on interannual and decadal timescales, as reviewed by Wang et al.43 and Li et al.17. When an El Nino event is developed and matured, a basin-side warm Indian Ocean and a warm tropical North Atlantic Ocean occur in boreal winter or in the following spring43. On decadal timescales, there is a same-sign SST response in the tropical Atlantic when observed SSTs are specified in historical tropical Pacific pacemaker experiments, and processes in the Pacific and Atlantic are sequentially interactive through the atmospheric Walker circulation along with contributions from midlatitude teleconnections for the Atlantic response to the Pacific44. In fact, seasonal SST responses over the Atlantic and Indian Ocean in the PACTrend experiment in Fig. S10a–d are similar to, but stronger than those to the IPO forcing in Fig. 2c (also Fig. 1a in Meehl et al.44). The monthly mixed-layer heat budget in the Southern Ocean reveal that the SST and top layer warming over the Weddell Sea and Indian Ocean sector is driven by both oceanic heat advection and surface heat fluxes received by the ocean in the PACTrend experiment (Fig. S12). This is consistent with the previous findings that mixed-layer temperature variations averaged over the latitude band of 50°–65°S are primarily governed by heating due to air-sea fluxes and northward Ekman transport and wind-driven entrainment29,45. Therefore, the impacts of Indian Ocean and high-latitude Atlantic sub-decadal SST variations on atmospheric circulation and the consequent rapid decline in Figs. S7–S9 are likely relevant to that of the contemporary Pacific sub-decadal variations in Fig. 2a and S2. Above results suggest the dominant forcing of La Niña-like Pacific sub-decadal SST variations on the deepening of the ASL and positive SLP trends over the Weddell Sea, Western South Pacific and Indian Ocean sectors in all seasons, and the joint forcing of Pacific and tropical Atlantic sub-decadal SST variations on Southern Ocean surface warming and SIE decline in the recent decade.

Meanwhile, the temperature profiles of the upper 1000m of the ocean in Figs. 5e and S11 and spatial patterns in Fig. S10e–h reveal that Pacific sub-decadal SST variations have induced 100–500m subsurface warming in the Weddell Sea and Indian Ocean and the whole Southern Ocean throughout the year. Surface warming can penetrate into the subsurface through lateral advection, absorption of solar energy to warm the ocean mixed layer, and Ekman pumping12,29. Although a cold top-layer temperature is nudged over the ABS and Ross Sea in the PACTrend experiment, 100–500m subsurface warming is also simulated there, which is contributed by the oceanic poleward heat advection and Ekman pumping effect bringing warmer water upward as discussed in the next section. In the GLTrend experiments, significant subsurface warming is also simulated in the Weddell Sea and Indian Ocean, as well as the whole Southern Ocean (Fig. S13).

Compounding Pacific and subsurface forcing of SIE decline
A strengthened ASL induces strong southwesterly flow across the Amundsen-Ross Seas and is supposed to increase SIE as simulated in both experiments (Figs. 3 and 4), which is in contrast to a decrease in SON and weak sea ice increase there in other seasons in observations (Fig. 1). As noted earlier, Fig. 1 also shows concurrent cold or weak warm SAT and large SIC decline over the eastern Weddell Sea in DJF and wide areas from the Weddell Sea to Ross Sea in JJA. The Southern Ocean subsurface warming (Fig. 6a) is suggested to link to the current low sea ice state afterward25,26,28. Five Antarctic sectors show similar subsurface ocean features (Fig. 6b–f), but with the largest magnitude and increased trend over the Weddell Sea (Figs. 6d and S14). The expected sea ice gain over the Amundsen and eastern Ross Seas due to the strengthened ASL resulting from the La Niña Pacific sub-decadal SST trends (Fig. 3a–h) is offset by the subsurface ocean warming effect, explaining why a weak sea ice trend is observed there. In contrast, the atmospheric changes and consequent decreasing SIE over the Weddell–Bellingshausen Seas and Indian Ocean and Western Pacific Ocean by Pacific and tropical Atlantic SST forcing are enhanced by the Southern Ocean subsurface warming effect. Significant and largest warming trends of 0–50m mean temperature (about 0.76°C/decade) are found over Weddell Sea in DJF and MAM (Fig. S14c, d), which is consistent with the maximum sea ice decline there in these two seasons (Fig. 1d, e). Even when the observed SST trends are prescribed in the top mixed layer, the seasonal and annual mean responses of 0–50m mean temperature over the whole Southern Ocean and five sectors to global sub-decadal SST trends are less than the corresponding warming trends (Fig. S13). A weak SIC decline in the GLTrend experiment could be part of due to smaller upper ocean warming forcing compared to observations.

Meehl et al.12 suggest that the negative trend in wind stress curl (WSC) during 2000-2014 associated with negative IPO/positive SAM produces upward advection of warmer subsurface water through Ekman pumping. During 2014–2023 Austral summer, the negative trends in WSC over the southeast Weddell Sea, Western Pacific Ocean and Ross Sea (Fig. 7c), associated with anomalous westerlies for the Pacific-forced positive SAM in Fig. 1h, would also result in more upwelling of warm subsurface water around Antarctica and contribute to sea ice decline in these areas. This explains why a large sea ice decline but weak SAT warming occurred in those areas to some extent (Fig. 1d, l). Similar but weaker negative WSC responses over the Weddell Sea in the Austral summer are also simulated in PACTrend and GLTrend experiments (Fig. 7g, k). In all four seasons, significant negative trends in WSC are observed over most areas in the ABS and Ross Sea (Fig. 7a–d), and the association upwelling Ekman pumping effect greatly reduces surface cooling and sea ice gain over the Amundsen and Ross Seas induced by the strengthened ASL as analyzed in last two sections. Therefore, weak sea ice and SAT trends are observed over the Amundsen and eastern Ross Seas from SON to MAM (Fig. 1c–e, k–m). Spatial and seasonal patterns of negative WSC trends over the ABS and Ross Sea are well reproduced but underestimated in two pacemaker experiments (Fig. 7e–l). Although still contributing to 100–500m subsurface warming over the ABS and the Ross Sea in simulations, upwelling Ekman pumping effects would weakly offset the large sea ice gain and cooling SAT due to the enhanced ASL, explaining why significant cold SAT and large sea ice are simulated over these areas in Figs. 3 and 4. In JJA, a negative trend in WSC is observed over the northeastern Weddell Sea (Fig. 7a) and likely contributed to 100–500m subsurface warming and sea ice decline there (Figs. S14a and 1b), but such upwelling Ekman pumping is not simulated in two experiments. The underestimated atmospheric and ocean signals in two pacemaker experiments jointly provide an interpretation as to why the forced SIE declines are weaker than the observed SIE trend.

The radiative forcing is fixed at the preindustrial level in both PACTrend and GLTrend experiments. Contributions of radiative forcing resulting from anthropogenic-induced factors (e.g., rising well mixed greenhouse gas and changes of stratospheric and tropospheric ozone, aerosol and biomass Burning Emissions, and land uses) and natural factors (solar and volcanic forcing) to Antarctic SIE and associated circulation changes are assessed by examining the output from the seventeen models covering the observed period of June 2013 to May 2023 under the Coupled Model Intercomparison Project Phase 6 (CMIP6) historical and the SSP3-7.0 future radiative forcing scenarios. Figure S15 indicates that there is a circumpolar decrease in SIC and warming SAT over the large areas of the Southern Ocean in all four seasons, with larger forced signals in MAM and JJA. The SLP and UV850 trends are much weaker than those in the PACTrend experiment, with positive SAM-like patterns in MAM and JJA and a maximum trend of −0.75 (−0.50) hPa/decade over Weddell Sea in JJA (Antarctica in MAM). The seasonal SIE trends range from −0.16 to −0.26 *106km2/decade, and the annual mean trend is −0.22 *106km2/decade, about 12% of the observed annual SIE trend (Table S1).

Methods
Observational datasets and analysis
The six observational datasets used here are as follows: (1) The monthly SIC data are the newly updated homogeneous monthly high-resolution data derived from passive microwave satellite data with the bootstrap algorithm35. (2) The monthly SLP, 850hPa u- and v-components of wind (UV850), and 300hPa geopotential height (Z300) and SAT are from the ERA5 reanalysis56. (3) SST dataset is the Extended Reconstructed Sea Surface Temperature (ERSSTv5) dataset derived from the International Comprehensive Ocean-Atmosphere Dataset (ICOADS)57. (4) Ocean subsurface temperature data from January 2004 to December 2023 are derived from the extension of the gridded Argo data product58. (5) The IPO and Nino34 SST indices are from the Earth System Research Laboratory (ESRL) of the National Oceanic and Atmospheric Administration (NOAA) and calculated from the ERSSTv5 dataset. The IPO index is the Tripole Index (TPI)59, based on the difference between the SST anomaly averaged over the central equatorial Pacific (10°S–10°N, 170°E–90°W) and the average of the SST anomalies in the Northwest (25°N–45°N, 140°E–145°W) and Southwest Pacific (50°S–15°S, 150°E–160°W): SSTA2 − (SSTA1 + SSTA3)/2, which is a measure of interdecadal variability in the Pacific. (6) Precipitation data is from CMAP Precipitation60.

Antarctic sea ice has dropped rapidly since a record high in 2014. Linear trends are calculated for seasonal mean of Antarctic SIC anomalies in three successive months of June–July–August (JJA), to September–October–November (SON), December–January–February (JFM), March–April–May (MAM) in the period of June 2013 to May 2023. The time period June 2013 to May 2023 was chosen to calculate a 10-year linear trend for each season, avoiding the influence of the ongoing El Niño event starting from June 2023. When a strong El Niño condition developed during the spring of 2023, 10-year linear SST trends in the central-eastern Pacific in Austral winter or spring during 2014–2023 significantly weakened. The mechanisms of the continuing Antarctic SIE decline in the 2023 Austral winter and spring are mainly due to atmospheric forcing and subsurface warming, as discussed in the main text. Similarly, linear trends of seasonal mean ERA5 SAT, UV850, SLP, and Z300 anomalies are calculated in the above period. We examine SIE for the Southern Ocean as a whole and for five sectors that have been considered in a number of earlier studies (e.g., Turner et al.23). These are the Ross Sea (160°E–130°W), Amundsen-Bellingshausen Sea (ABS) (130°W–60°W), Weddell Sea (60°W–20°E), Indian Ocean (20°E–90°E), and western Pacific Ocean (90°E–160°E).

We analyze the responses of Rossby wave source and Rossby wave flux (WAF) on 300hPa. The upper-level component of this vorticity source, denoted the Rossby wave source (RWS)42, is defined as:

RWS= −\zeta D − V\chi \cdot \nabla\zeta (1)

Where \zeta is the absolute vorticity and D is the horizontal wind divergence, V\chi is the divergent component of the horizontal wind vector and \nabla represents the horizontal gradient operator. WAF is used to study the propagation of Rossby waves to study the propagation mechanism of the atmospheric circulation to the SST forcing. We use the formula proposed by Takaya and Nakamura61 to obtain the horizontal component of WAF. The horizontal component of the WAF in the form of 300hPa geostrophic wind can be written as:

W= 1/2d [U\psi0x^2 + V\psi0y^2] (2)

Where U = ~u,~v is the horizontal climatological geostrophic wind field, \psi0 is a stream function anomaly derived from the 300hPa geopotential height by the quasi-geostrophic approximation method, and x and y represent the longitude and latitude directions respectively. The upper horizontal line represent the time average.

Following Meeth et al.12, we estimate the magnitude of Ekman pumping by calculating the vertical component of the curl of the wind stress (WSC):

WSC= 1/\rho0 f [\partial\tau y/\partial x - \partial\tau x/\partial y] (3)

Where constant \rho0 is the density of seawater (1025 kg m−3), f is the Coriolis parameter, and \tau x and \tau y are eastward and northward surface stress, respectively. Negative trend or response values of WSC denote areas of Ekman suction that would move water upward in the column and cause sea ice decline.

Coupled Pacific and global Pacemaker experiments
We perform a 100-year fully coupled control simulation (CTR) with the preindustrial radiative forcings and two sets of coupled pacemaker simulations using Community Earth System Model 1.2 (CESM1.2)62 to examine the effects of Pacific and global sub-decadal SST trends from June 2013 to May 2023. CESM1.2 ensemble pacemaker experiments with a partial assimilation approach in the Pacific and global oceans are similar to the idealized pacemaker experiment in Meehl et al.44. In these experiments, the SST anomaly that amounts to observed ERSSTv5 SST trends from June 2013 to May 2023 at each grid box is added to the model-simulated SST climatology over the Pacific or global region from 60°S to 60°N, named the PACTrend and GLTrend experiments, respectively, with preindustrial radiative forcings. In other words, the model SST is only restored to the observed trend anomalies superimposed on the model’s own climatology over the specified region of the Pacific or global oceans, and the model evolves freely outside of the restoring region, allowing a full response of the climate system. The SST restoration is performed using a restoring timescale of 2 days. Data in sea ice regions are not assimilated, so temperature anomalies in the sea ice region are not constrained by observations and sea ice variations are completely controlled by the internal dynamics. Each set of pacemaker simulations begins from the control run and runs for 100 years, and the prescribed SST forcing repeats annually. Each year is considered to be an ensemble member. The climate response of any variable is the difference between the seasonal means from the last 80 years of each coupled pacemaker run and the 80 years of the coupled control run, allowing a 20-year spin-up of the Parallel Ocean Program (POP) ocean model. The statistical significance of grid-box responses is assessed by a standard two-tailed difference of means Student’s t-test.

To examine responses of top-layer ocean temperature to Pacific SST trends, we utilize monthly output to calculate the mixed-layer heat budget over the Southern Ocean. The equation for the mixed-layer heat budget can be expressed as follows29,46:

\partial T/\partial t = -V \cdot \nabla T + Qnet/(\rho0 Cp H) + Residual (4)

Here, T represents the mixed-layer temperature and H is the mixed-layer depth; ~V signifies horizontal and vertical velocity and \nabla represents the gradient operator; Qnet denotes the surface net heat flux into the ocean as the sum of shortwave radiation, longwave radiation, latent heat flux, and sensible heat flux. Constant Cp (3996 J kg−1 K−1) represents specific heat capacity. Residual includes the effects of entrainment and horizontal/vertical diffusion. The CESM1.2 pacemaker experiments are used to examine the response to the shift of IPO from a short-lived positive phase during 2015-1630,45 to the negative phase. There are two sets of ten ensemble idealized Pacific pacemaker simulations in which one standard deviation of SST anomalies is added to the model-simulated SST in the Pacific between 40°S to 60°N: positive Pacific Decadal Variability (PDV) and negative PDV in Fig. 2c. The IPO and the Pacific Decadal Oscillation (PDO) are highly correlated both spatially and temporally, and they are referred to collectively as the PDV mode. Each ensemble selected 10 starting years with varying PDV states from the CESM1 large-ensemble preindustrial control run: negative PDV (years 926, 1331, 1366); positive PDV (years 606, 1011, 1251); neutral PDV but trending toward negative (years 1321, 1356). Each ensemble commences on January 1 and continues for a duration of 10 years, during which the specified seasonal cycle of SST anomalies in the respective areas was kept constant in time while the rest of the model was fully coupled45. Annual responses of SST in Fig. 2c and seasonal responses of SIC, SLP, and UV850 to PDV-SST forcing in Fig. S16 are the ensemble-mean difference between the PDV- simulation and PDV+ simulation. Global responses of SST and SLP to the PDV forcing were examined by Meehl et al.44.

CMIP6 simulation analysis
To estimate the forced SIC and atmospheric responses to radiative forcing, we use the multimodel ensemble mean from phase 6 of the Coupled Model Intercomparison Project (CMIP6)63. To match observations, CMIP6 data from the historical and SSP3-7.0 experiments are concatenated from June 2013 to May 2023. We include 17 CMIP6 models that have at least three ensembles for both historical and SSP3-7.0 simulations to reduce the impacts of large internal variability on the 10-year SIC trends (Table S2). Similar amplitudes of Antarctic SIE trends in Table S1 and SIC trends in Figure S15 are obtained if SSP5-8.5 experiments are used. For significance testing, we compute the standard deviation (\sigma) of 10-year trends in any region or grid box using the 304 superensemble members from these 17 models, with all members assumed to be independent. We apply a two-sided local significance test (|trend| \ge 2.0\sigma/ \sqrt{Ne}) to determine whether the superensemble mean simulated trend is significantly different from zero at the 5% significance level, where Ne = 304. The estimated seasonal and annual mean 10-year SIE trends range from −0.16 to −0.26 *106km2/decade (Table S1). When estimated as the average of ensemble-mean trends of 17 models, similar 10-year SIE trends are found (Table S1).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is a timely and interesting study that addresses some of the questions raised recently in the literature concerning the decline in Antarctic sea ice extent over the last few years. The study provides insights into the role of different ocean basins as the source of teleconnections that modify atmospheric circulation and ultimately sea ice extent around Antarctica. The authors follow a standard modelling method for assessing the role of teleconnections and conclude that sea surface temperature variability in the Pacific Ocean, with a smaller contribution from the Atlantic Ocean have forced changes to sea level pressure and therefore the wind regime that have ultimately tended towards less sea ice in recent years. The pattern of changes in the tropical ocean reflects the negative phase of the IPO and a typical La Niña condition. The major influence of the teleconnection on the Antarctic atmosphere over the last decade is a deepening of the ASL and increased pressure over the Weddell Sea and West Pacific region.\n\nThe introduction gives a concise but well-researched description of Antarctic sea ice variability. I didn't notice any missing studies and have discovered a couple of studies that I wasn't aware of. Overall, I find this to be an interesting piece of work that adds to the ongoing discussions regarding the drivers of Antarctic sea ice variability. The suggestion that the negative IPO is a driver of Antarctic sea ice change is not new, but this study applies a modelling study to demonstrate how the trends in Pacific SSTs link to the patterns in Antarctic sea ice extent change, which gives some extra weight to the suggestion that the IPO is a major drive of sea ice variability when this is a climate mode of decadal variability that is challenging to compare to the short Antarctic sea ice record. I outline a couple of major concerns below but would be interested to see this article published if the authors address these. I also list some minor comments below.\n\nMajor Concerns\n----------------\nOther recent work has highlighted the potential role of the ocean in driving recent sea ice change, e.g. Hobbs et al., 2024 and Purich and Doddridge, 2023 (see references listed below), but so far there is a lack of evidence as to how and why. This study concludes that that teleconnections from the Pacific and Atlantic have increased subsurface warming (lines 206-208). However, the only evidence I see for this link is that these mechanisms have decreased sea ice and the inference is that therefore there would be ocean warming as there is less sea ice. I do not see how this definitively shows why the ocean has warmed. During spring, an early retreat, or more extensive retreat allows more solar energy to warm the ocean mixed layer. But, subsequent sea ice melt forms a well-stratified surface layer, which does would have to be overcome to lead to increased melting of sea ice. This study does not give evidence of any mechanism for overcoming the increased stratification. Either the statement should be revised or more evidence provided for the role of the teleconnections in driving the ocean warming.\n\nI don't know much about the CESM1.2 model. The authors do not show the modelled sea ice extent, perhaps because the model does not adequately reproduce the sea ice extent. This may not be important as all models struggle to reproduce Antarctic sea ice extent. However, it would be helpful to show that the atmosphere is well represented, either through a direct comparison with ERA5 or with reference to articles that support the use of the model in this way.\n\nIs there a particular reason that you chose 2013-2023 for your analysis. It may be perfectly reasonably to pick any time period you like, but I ask because Purich and Doddridge, 2023 found a transition in Antarctic sea ice state in 2007, which could be an interesting break point.\n\nMinor comments\n---------------\nLine 40: suggest replacing \"middle\" with \"mid'\"\nLine 155: refers to \"divergence\" but on line 152, the text talks about anomalous convergence, not divergence.\nLine 173: suggest to add \"activity\" after \"anticyclonic\"\nLine 174: Why does the fact that the GLTrend experiment is similar to the PACTrend experiment mean that the model captures the observed atmospheric circulation trend features remarkably well?\nLine 189: suggest adding Hobbs et al., 2024 to the references 27 and 28.\nLine 194: The similarities are broad scale, It is interesting that the Amundsen-Bellingshausen Seas have an overall decreasing trend while showing such hotspots of warming during 2006, 2016 and 2020. The decadal trend masks a large amount of interannual variability. How much data is there for each ocean basin? I.e., are there many data point between 50 and 65 S in the Amundsen Sea?\n\nReferences\n-----------\nHobbs, W., Spence, P., Meyer, A., Schroeter, S., Fraser, A. D., Reid, P., Tian, T. R., Wang, Z., Liniger, G., Doddridge, E. W., & Boyd, P. W. (2024). Observational evidence for a regime shift in summer Antarctic sea ice. Journal of Climate 37, 2263–2275. https://doi.org/10.1175/JCLI-D-23-0479.1\nPurich, A., and E. W. Doddridge, 2023: Record low Antarctic sea ice coverage indicates a new sea ice state. Communications Earth & Environment, 4, 314, 10.1038/s43247-023-00961-9.", "reviewer_2": "This study focuses on the observed decline in Antarctic sea ice extent (SIE) in the recent decade (2013-2023). The authors propose that the recent SIE decline has been significantly influenced by atmospheric circulation changes near Antarctica. Using a set of prescribed SST simulations with CESM1, the authors further conclude that the observed circulation changes, including a deepening of the Amundsen Sea Low and associated northerly winds that are conducive to reducing SIE, were mainly forced by the sub-decadal Pacific SST changes (notably the negative phase of IPO), with additional contributions from Atlantic SST changes. The atmospheric circulation impact on SIE has enhanced the Southern Ocean subsurface warming effect, altogether contributing to the total observed Antarctic climate changes.\n\nOverall, I find the paper to be interesting and the topic to be important. However, I have several major concerns about the foundation of the main argument and the experiments used to support the story, which I will elaborate on below. These concerns led me to worry that the current version of the manuscript is not suitable for publication.\n\nMajor concern 1.\nRegarding the recent decline of Antarctic SIE, previous studies have proposed the role of Southern Ocean subsurface warming (e.g., Zhang et al. 2022). Here the authors propose that the decadal-long SIE decline can be additionally caused by atmospheric circulation changes, which are forced mainly by Pacific SST variations. The authors have done a great job establishing the link between Antarctic circulation changes and their corresponding SST forcing. Although I appreciate the thorough discussion in the paper, I don’t think this atmospheric bridge mechanism is a new piece. There have been many papers showing how tropical Pacific SST anomalies remotely modulate Antarctic circulation (e.g., Ding et al. 2011, Meehl et al. 2016 for the early SIE expansion period). On the contrary, the other part of the proposed causal connection - atmospheric circulation changes drive SIE changes – is not well discussed and validated. The authors put forward the hypothesis that “the forcings of atmospheric circulation changes have played a key role in driving the rapid decline of Antarctic SIE in the recent decades” (line 71). However, this main argument seems built simply on a qualitative speculation that the SLP-associated wind anomalies induce anomalous advection, resulting in SIE changes (Fig. 1). The simulations and analysis in the rest of the paper, awkwardly, fail to verify this hypothesis. That is, the prescribed SST (presumably also fixed SIE) simulations are unable to provide the actual SIE response to the SST-driven circulation forcing by construction. Thus, I find that the inconsistency between the modeling evidence and the conclusion makes the paper less convincing.\n\nFurthermore, the conclusion that atmospheric circulation plays a significant role in driving SIE changes is weakened by the finding that the SIE decline appears to be indeed associated with Southern Ocean subsurface warming, most pronounced in the Weddell Sea (Fig. 4), as previously proposed. This raises a key question for this paper: to what extent does the proposed atmospheric circulation effect contribute to the recent SIE decline? Is it a major contributor or only a secondary player compared to the subsurface warming effect? In my opinion, the argument would have been more novel and impactful if the authors had quantified the relative roles of atmospheric circulation and ocean subsurface warming effects on the recent SIE decline.\n\nMajor concern 2.\nRelated to the above, the limitation of this work seems to in part arise from the caveat that the prescribed-SST simulations do not allow one to estimate the SIE response. The authors also invoke results from a set of fully-coupled pacemaker simulations with PDV-associated SST anomalies in the Pacific, although the SIE decline response in the Weddell Sea appears to be much weaker and the SIE increase in the Amundsen Sea and Ross Sea appears to be too strong than those observed. I wonder why the authors don’t use more realistic pacemaker simulations, with SSTs in the Pacific/Atlantic sectors nudged towards observed transient anomalies over the targeted period (2013-2023). This way, the authors could isolate and quantify the impacts of realistic SST variations on Antarctic SIE in the coupled system. These are likely to be better than the AGCM runs (which don’t allow dynamic SIE response) and the idealized PDV-SST pacemaker runs (which only provide the SIE response to PDV phase transitions rather than the realistic full SST variations).\n\nMinor comments:\nLine 111: when introducing the “pacemaker” simulations, it would be helpful to highlight that these are “coupled” simulations, distinct from the above AGCM simulations.\nLine 117: I would suggest adding pattern correlation here to quantify the level of resemblance between the observed circulation pattern and the SST-driven circulation pattern.\nLine 184: northwesterly -> southerly?\nLine 225 – 226: I do not follow why this suggests the SIE decline results from internal variability. The IPO pacemaker simulations show a weaker SIE response compared to observations and the PACTtrend simulation. Doesn’t this suggest the observed SST forcing on SIE may have involved some forced response beyond just the IPO SST forcing?\nLine 288: how is SIE handled in these AGCM runs? Is it fixed to the observed climatology?\nLine 291-292: I would suggest adding a plot to show the regions of the SST forcing specified in AGCM runs (if the authors were to continue using these simulations).\n\nReference:\nDing, Q., Steig, E. J., Battisti, D. S., & Küttel, M. (2011). Winter warming in West Antarctica caused by central tropical Pacific warming. Nature Geoscience, 4(6), 398-403.\nMeehl, G. A., Arblaster, J. M., Bitz, C. M., Chung, C. T., & Teng, H. (2016). Antarctic sea-ice expansion between 2000 and 2014 driven by tropical Pacific decadal climate variability. Nature Geoscience, 9(8), 590-595.\nZhang, L., Delworth, T. L., Yang, X., Zeng, F., Lu, F., Morioka, Y., & Bushuk, M. (2022). The relative role of the subsurface Southern Ocean in driving negative Antarctic Sea ice extent anomalies in 2016–2021. Communications Earth & Environment, 3(1), 302.", "reviewer_3": "There have been several papers written on the 2023 sea ice anomaly, which have come into press after the writing of this paper, but now cannot be ignored. However, there is still a requirement for a paper on the remote drivers of the changes. Unfortunately, such drivers can be quite complicated (similar to what the drivers are of PDO). Attempting to do this through a trend analysis is, I feel, not the correct method for the reasons given below.\n\n39 & 44: The low of 2016 does not persist but recovers such that the ice is back at the climatic mean by 2021 (Fig 1). This confirms a subsurface ocean heat source being depleted. Why do the authors now plot a trend line through the 2013-2023 when clearly this is not a sustained trend? To do so is an oversimplification. The decadal trend theme, rather than intermittent events, as most other papers on the subject describe, possibly associated with different mechanisms, is a weakness of this paper. It requires a complex sequence of drivers, as evidenced by the number of supplementary plots, none of which is convincing. As a consequence, the paper is confusingly written, attempting to describe too much in words when a schematic diagram such as Fig3 in Scott Power’s paper Decadal climate variability in the tropical Pacific: Characteristics, causes, predictability, and prospects | Science would greatly simplify and clarify the process. The intermittency of the events would also make the use of causal networks (were they to be used) to solve the remote forcing very difficult.\n\nBreaking up the description into smaller paragraphs and making a distinct connection in each paragraph would help clarify the argument. As it stands the argument is a mess, and very hard to follow.\n\n51-69: Monica Ionita (Frontiers | Large-scale drivers of the exceptionally low winter Antarctic sea ice extent in 2023 (frontiersin.org)) makes a compelling case that the event of 2023 was associate with a wave mode 3 pattern. It is possible that a deepening and shift of the Bellingshausen Sea Low is associate with this pattern, but this would need to be demonstrated. Your description here is rater messy and would be better summarized by the wave mode 3 analogy. Of course, the driver of the wave mode 3 is not discussed in Ionita’s paper and needs investigation.\n\nAnother characteristic on the sea ice changes, as discussed by Hobbs et al (2024) Observational Evidence for a Regime Shift in Summer Antarctic Sea Ice in: Journal of Climate Volume 37 Issue 7 (2024) (ametsoc.org) is that the Antarctic sea ice has recently become predicable season by season. This would need to be explained within a remote forcing context."}, "author_response": "We thank three reviewers for their insightful and helpful comments. We have considered all comments and significantly revised the manuscript. We perform two sets of 100-year coupled pacemaker experiments and are able to simulate realistic atmospheric and sea ice responses to Pacific and global sub-decadal SST variations. All results of previous AGCM simulations are deleted. All changes and discussions of results from the coupled Pacemaker experiments in the manuscript text file are shown with color highlighting.\n\nReviewer #1 (Remarks to the Author):\n[...]\nWe thank the reviewer for her/his support. All issues are addressed in the answers (and associated modifications of the manuscript) to the specific comments below. In the revised manuscript, we perform two sets of 100-year coupled pacemaker experiments and simulate similar atmospheric responses as that in the AGCM simulations of the old manuscript, but realistic sea ice responses to Pacific and global sub-decadal SST trends.\n\nIn the old manuscript, we performed AGCM simulations and demonstrated that Pacific and Atlantic sub-decadal SST variations contributed to the observed deepening of the Amundsen Sea Low and anomalous high pressure over the Weddell Sea and West Pacific sectors, which substantially induced Southern Ocean surface air temperature warming (not the ocean warming), leading to Antarctic SIE decline. Therefore, Pacific and Atlantic sub-decadal SST variations and subsurface warming have acted in concert to drive the observed sea ice loss in the recent decade.\n\nIn the revised manuscript, we perform the coupled pacemaker experiments and demonstrate that La Niña-like Pacific sub-decadal SST variations have dominantly driven Southern Ocean atmospheric circulations through tropical–polar teleconnections and significantly contributed to Southern Ocean surface (SST) and subsurface warming and Antarctic SIE decline in the recent decade. There is a warmer upper ocean column from the surface down to 500 m depth in the Southern Ocean and all five sectors that contributed to the sustained retreat of Antarctic sea ice (Figs. 5 and S11).\n\nIt is true that sea ice melt forms a well-stratified surface layer, which suppresses vertical instabilities and upwelling of subsurface warm water to the surface, potentially reducing sea ice loss. However, such negative effects of enhanced stratification and weakened deep convection may be small in the sub-decadal time scales. Responses of these warmer Southern Ocean surface air temperatures and upper ocean temperatures to La Niña-like Pacific and global SST trends, combined with the direct effects of surface wind forcing on the sea ice, have produced the rapid decrease of Antarctic sea ice extent in the recent decade.\n\n[...]\n\nReviewer #2 (Remarks to the Author):\n[...]\nWe thank the reviewer for her/his support. We carefully considered all your comments and significantly modified the manuscript accordingly. In the revised manuscript, we perform two sets of 100-year coupled pacemaker experiments and simulate similar atmospheric responses as that in the AGCM simulations of the old manuscript, but realistic sea ice responses to Pacific and global sub-decadal SST trends.\n\n[...]\n\nReviewer #3 (Remarks to the Author):\n[...]\nWe thank the reviewer for her/his insightful comments and have significantly modified the manuscript accordingly. The above concerns are addressed as follows (and associated modifications of the manuscript).\n\nFirst, we agree that the abrupt 2016 spring decline and 2023 all-year record lows are intermittencies of the extreme SIE events, which contribute to strong interannual variability of Antarctic SIE and also the downward trend. Several recent studies have examined the roles of atmospheric forcing and subsurface warming in the 2016 rapid loss and the 2023 record low, as we discuss in the last section of our revised manuscript. However, as shown in Figure 1 in the next figure, the seasonal mean Antarctic SIE has clearly exhibited significant downward trends in all four seasons since the peak in 2014 (Fig. S1). The evolutions of observed Antarctic SIE anomalies include components of both sub-decadal and strong interannual variations. We also agree that the mechanisms for the decadal trend theme and intermittent events are different.\n\n[...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have reviewed the revised manuscript, which has been substantially improved to address the concerns raised in my initial review. The manuscript investigates the causes of the significant decline in Antarctic sea ice extent from June 2013 to May 2023, focusing on the roles of Pacific sub-decadal sea surface temperature trends, atmospheric circulation changes, and the influence of tropical polar teleconnections. This is an interesting manuscript, and the topic is important given the need to understand the drivers of the recent low sea ice cover around Antarctica.\n\nIn particular, the methodology has been strengthened by switching from AGCM simulations to coupled pacemaker experiments, providing more robust evidence for the proposed mechanisms. The authors employ a multi-method approach. They analysed satellite-derived sea ice concentrations and atmospheric variables (ERA5 reanalysis) to demonstrate that the decline in Antarctic sea ice is closely linked to a strengthened ASL and associated wind anomalies. They use a heat budget analysis to evaluate the mixed layer temperature in the Southern Ocean and demonstrate the role of subsurface ocean heat flux in driving the decline of Antarctic sea ice, with wind-driven upwelling and Ekman transport as key mechanisms. The revised manuscript now includes coupled ensemble pacemaker simulations to impose Pacific and global SST anomalies over the 10-year study period (June 2013 to May 2023). The PACTrend experiments impose observed SST from the Pacific Ocean, while the GLTrend experiment imposes global SST anomalies. The pacemaker experiments provide robust evidence of the link between Pacific SST trends and Antarctic sea ice variability by demonstrating a clear mechanistic connection between tropical SST anomalies and the decline of Antarctic sea ice through tropical-polar teleconnections. By controlling these external forcing patterns, the experiment can simulate the atmospheric and oceanic responses and isolate regional contributions. They subsequently examine the responses to radiative forcing under historical and SSP370 scenarios from 17 CMIP6 models to independently validate their findings by confirming that natural variability, particularly driven by Pacific SST anomalies, is the dominant factor behind the recent decline in Antarctic sea ice.\n\nTheir key findings reveal that Pacific sub-decadal sea surface temperature (SST) trends were the primary driver of changes in Southern Ocean atmospheric circulation and subsurface ocean warming and that these SSTs, through tropical-polar teleconnections, significantly contributed to the decline of Antarctic sea ice. Approximately 40% of the observed decline in Antarctic sea ice extent (SIE) is attributed to variations in Pacific and Atlantic SSTs. Radiative forcing contributed about 12% to the overall reduction in SIE, emphasising the dominant influence of teleconnections.\n\nMy main concern with the previous version of the manuscript was the lack of robust evidence for the teleconnection in driving ocean warming. The revised (and strengthened) approach to using pacemaker experiments has addressed this concern with the PACTrend simulation reproducing warming in the Southern Ocean subsurface layers (100-500 m depth) in response to tropical Pacific SST anomalies. The authors acknowledge my point about stratification, and I accept that these effects may be small at the timescales this work is looking at (sub-decadal). The model agreement with observations strengthens the evidence for this mechanism. Warmer Pacific SSTs strengthen the IPO in its negative phase, creating a La Nina-like state. These anomalies intensify the ASL, enhancing cyclonic wind patterns over the Southern Ocean. Strengthened winds drive Ekman transport, upwelling warm subsurface water that melts sea ice.\n\nI also queried how well the model represented the atmosphere, and the revised manuscript demonstrates that the atmospheric and sea ice responses in the new coupled pacemaker experiments align well with the reanalysis data and satellite observations. The coupled experiments capture the spatial and seasonal patterns of Antarctic sea ice concentration decline, particularly the significant ice loss in the Amundsen-Bellingshausen Sea and the Weddell Sea, especially in austral spring and summer when tropical–polar teleconnections are strongest. The simulations also reproduce the strengthening of the ASL in all seasons, positive sea-level pressure anomalies over the Weddell Sea and the western South Pacific, and PSA-like Rossby wave patterns. The model reproduces the spatial and seasonal patterns of subsurface warming seen in the Argo observations with high fidelity but underestimates the magnitude of warming in certain regions. The agreement between observed and simulated warming trends provides strong support for the proposed mechanisms linking tropical Pacific SST anomalies, Southern Ocean circulation changes, and Antarctic sea ice decline.\n\nThe choice of study period has been justified with clear reasoning related to ENSO evolution and IPO phase shifts.\n\nI recommend this manuscript for publication. The authors have successfully addressed all major concerns and significantly improved the scientific quality of their work. The revised analysis presents a comprehensive and well-supported understanding of ocean-atmosphere-sea ice interactions in Antarctica.", "reviewer_2": "I appreciate that the authors considered my suggestion and added the new set of fully-coupled pacemaker runs. These new simulations have greatly strengthened the paper and provided valuable modeling evidence. Overall, I find the paper in much better shape.\n\nRegarding the results of these new simulations, I have some additional concerns. As listed below, some of them may be critical for interpreting the modeling results. I may have some misunderstandings here, in which case, clarifications would be helpful (potentially also for the broader audience).\n\nLine 473 – 483: The method section describing the coupled pacemaker runs would benefit from further clarification. For example, what SSTs are exactly restored in the model? The SST climatology + linearly ramping anomalies that amount to the observed trend at each grid box? If so, why are the simulations 100-years long while the targeted observational SST trends span only 10 year (2013-2023)? In other words, do the pacemaker runs maintain the same trend (K/dec) as the observations but extend it over a longer period, making the total imposed change in the model 10 times larger than in reality?\n\nLine 99: This doesn’t seem to hold for JJA and DJF.\n\nLine158: what is the pattern correlation level for SLP in JJA? Any thoughts on why the pacemaker runs better reproduce the observed patterns in other seasons but not JJA?\n\nLine193: But this *seasonally robust* simulated sea-ice feature is not in observation.\n\nLine 265: Does this imply that the Atlantic SST effect, derived from these two sets of coupled pacemaker runs, is biased?\n\nLine288-291: I am not sure I follow this… Isn’t the Atlantic/Indian Ocean effect obtained by subtracting the Pacific effect from the global SST simulation? If so, why is it still related to Pacific SST variations?\n\nFig. 298 Fig. S10.\n\nFig. 6: The ocean temperatures shown are averaged over 50-65°S. However, the simulations nudge SSTs all the way to 60°S. Doesn’t this mean that more than half of these “responses” are prescribed forcing rather than the actual response?\n\nLine 353-355: How should we interpret the remarkable differences between the simulation forced by observed decadal SST trends (Fig. 3) and the simulation forced by PDV phase change (Fig. S16)? What does it imply for the observed SST trends? The authors mention the ensemble average. However, I assume that the ensemble averaging primarily removes random atmospheric variability and can still yield a robust “forced response” to the imposed PDV SST variation. Therefore, I wonder if other dominant factors are at play. For example, it could suggest that the observed SST trends are not solely driven by a negative PDV or that random atmospheric variability also plays a major role here.", "reviewer_3": "Most concerns have been addressed except that some Results/Analysis could be split up by sub-heading to make it easier to read and refer to figures. For example 'observations' and 'model'\n\nOtherwise I suggest accepting the paper.\n\nI am uneasy about the use of sea ice extent when ice area is a better diagnostic. The ice edge might retreat but the ice concentration increase. However, I do not expect the authors to change the assessment method at this point.\n\nThe authors refer to the difference in longitude of the ASL in models and observations, which can alter the poleward heat transport into the Weddell Sea. However, it is worth noting that the 2023 low extent was associated with a real shift in the ASL towards 90W and hence large heat transports and loss of sea ice in the Weddell. As well as the depth of the ASL has there been a drift in its longitude? Again, not essential that this be investigated in this paper."}, "author_response": "We thank three reviewers for their support! We have considered all comments and carefully revised the manuscript. In the revised manuscript, we add observed trends of wind stress curl and the corresponding responses to Pacific and global SST trends in Fig. 7. All changes in the manuscript text file are shown with red color highlighting.\n\nReviewer #1 (Remarks to the Author):\n[...]\n\nReviewer #2 (Remarks to the Author):\n[...]\nWe thank the reviewer for his/her support and insightful comments. We really appreciate it.\n\n[...]\n\nReviewer #3 (Remarks to the Author):\n[...]\nWe thank the reviewer for his/her support and suggestions. We really appreciate it.\n\n[...]"}, {"round": 3, "reviewer_comments": {"reviewer_2": "Thanks for the response and the revision. I am pleased to recommend publication.", "reviewer_3": "The paper is mush improved and results more robust in this latest iteration due to the inclusion of the pacemaker simulations. I recommend accepting the submission.\n\nI have only two issues\n1. the use of the term 'Ekman suction' it is normally referred to as 'Ekman pumping'.\n2. There are no DOI in the references."}, "author_response": "We thank three reviewers for their support! We have considered all comments and carefully revised the manuscript. In the revised manuscript, we add observed trends of wind stress curl and the corresponding responses to Pacific and global SST trends in Fig. 7. All changes in the manuscript text file are shown with red color highlighting.\n\nReviewer #2 (Remarks to the Author):\nWe thank the reviewer #2 for his/her encouragement and support.\n\nReviewer #3 (Remarks to the Author):\nWe thank the reviewer #3 for his/her comments and support. We really appreciate it.\n\nI have only two issues\n1. the use of the term 'Ekman suction' it is normally referred to as 'Ekman pumping'.\n“Ekman suction” is changed to “Ekman pumping” in the revised manuscript. Ekman suction is mentioned in the method section as follows: “Negative trend or response values of WSC denote areas of Ekman suction that would move water upward in the column and cause sea ice decline”. Thanks for pointing it out!\n2. There are no DOI in the references.\nWe follow the format of the recent papers published in Nature Communications and do not add DOI in the references."}]

Decision letter