The Ross Sea

The Ross Sea sector is the region hosting most (13.7 ± 3.6 103 km2 yr−1) of the overall sea-ice extent increase around Antarctica (17.5 ± 4.1 103 km2 yr−1) over the last three decades1. Here, we study the changes in Antarctic sea-ice coverage using both the OSISAF21 sea-ice concentration satellite retrievals over 1979–2014 and sea-ice fields from a simulation performed with the global ocean–sea-ice model NEMO-LIM3.622, 23, driven by atmospheric reanalyses (over 1979–2013, see Methods section). Except for the outer Weddell and inner Amundsen Seas, this simulation reproduces reasonably well the observed sea-ice concentration trends (Fig. 1) and, in particular, exhibits the largest positive trends in the Ross Sea. Fully coupled climate models fail in general to simulate Antarctic sea-ice trends correctly18, 24, 25, but the Ross Sea is the region where the mechanisms behind sea-ice trends are the most poorly understood and where the disagreements between climate simulations and observations are the largest26. Hereafter, we focus on the Ross Sea (our main diagnostic for the other regions is presented in Supplementary Figs. 1 and 2), as understanding the processes driving sea-ice changes in this region appears key to determining the origin of the slight circumpolar ice extent increase.

Signature of the ice-coverage–ocean-heat-storage feedback

The role of oceanic vertical heat fluxes is investigated based on observational ocean temperature profiles from the BLUELink Ocean Archive27 (1979–2014), and on ocean outputs from the model simulation. Compared to in situ observations, model results have the advantage of providing a complete spatial and temporal coverage. We evaluate the spatial correlation between trends in ice concentration and trends in ocean temperature at various depths (Fig. 2). All trends are evaluated from annual mean data over the full period length (see Methods section). A map showing the spatial distribution of the ocean temperature profiles used to compute the observation-based correlation is given in Supplementary Fig. 1. As expected, ice concentration and sea surface temperature trends are negatively correlated, meaning that an increase (decrease) in ice concentration is associated with a surface cooling (warming). Nonetheless, the correlation becomes positive as depth increases and reaches a maximum at a depth of nearly 120 m in observations and 150 m in the model: this entails that an increase (decrease) in ice concentration is also associated with heat gain (loss) at depth. Both peaks in positive correlation (from observational and model data sets) are significant at the 95% confidence level, despite weaker correlation values in the observations. In addition, those peaks occur at comparable depths, which in the model corresponds approximately to the depth of the winter mixed layer, averaged over all the seasonally ice-covered grid cells of the Ross Sea and over 1979–2013. Supplementary Figures 1 and 2 display these same correlations between ice concentration trends and ocean temperature trends applied to all sectors of the Southern Ocean. The significance of trends and subsequently of correlations is mainly conditioned by the number of records available. Correlations are thus more robust for model results than for observations. Over the last three decades, only the Ross Sea exhibits a distinct positive maximum in correlation at depth.

Fig. 2 Correlation coefficients between ice concentration and depth-dependent ocean temperature trends in the Ross Sea. Observation-based correlations a are computed from the OSISAF21 sea-ice concentration satellite product and ocean temperature profiles from the BLUELink Ocean Archive27. The same diagnostic is applied to a simulation performed with the ocean–sea-ice model NEMO-LIM3.6 (b, see Methods section). c depicts the mean depth-dependent ocean temperature trends (model-based) over Ross Sea areas, where ice concentration trends are positive and considered significant, i.e., larger than 3% dec−1, the 95% confidence interval for ice concentration trends in the model, and observations. The regions associated with b and c are thus slightly different, with b including areas with deeper MLD. At each depth, the horizontal bar represents the standard deviation around the spatially averaged trends. Trends are computed over 1979–2013 and 1979–2014 for the model and observations, respectively. Gray areas show the depths at which the correlations are significant at the 95% confidence level. Below 220 m, a and c show negative correlations and trends that are probably due to processes unrelated to sea-ice trends, hence not further discussed in this study Full size image

We argue that this positive correlation at depth is the signature of the ice-coverage–ocean-heat-storage feedback whereby increased sea-ice coverage conduces to, and is amplified by, a salinity-related strengthening of the stratification and a mixed layer shoaling. Indeed, the model simulation shows that, in regions of significant positive sea-ice concentration trends (Fig. 3d), increased stability (Fig. 3b) and reduced MLDs (Fig. 3a) act to weaken the upward oceanic heat transfer, causing heat gain at and below the base of the mixed layer (Figs. 2c and 3c). Although the number of observational temperature profiles available in the seasonally ice-covered Ross Sea is too limited to highlight a clear geographical match between strong trends in MLD, subsurface temperature and ice concentration, trend correlations at the scale of the Ross Sea concur with the proposed mechanism. Indeed, the vertical structure of the correlations (Fig. 2a) shows strong similarity with the simulated profile of temperature trends in the region of large sea-ice increase (Fig. 2c), which is also the region where the observational coverage is densest (Supplementary Fig. 1). Further, Fig. 4 shows that on average over the region where positive ice concentration trends are the most significant, a simultaneous shoaling of the mixed layer, increase in surface stratification, and warming at depth seem to occur in observations too. Though these trends are highly uncertain, they do not invalidate our hypothesis and suggest a behavior broadly consistent with that simulated.

Fig. 3 1979–2013 model trends in upper ocean properties in the Ross Sea. 1979–2013 model trends in a mixed layer depth, b near-surface vertical salinity gradient, c ocean temperature at depth, and d sea-ice concentration in the sea-ice-covered region of the Ross Sea. In c, temperature trends are shown at the depth at which they are maximum, within the upper 300 m. Except in the southernmost Ross Sea where MLD increases a, the depth of this maximum is always greater than the local mean MLD (Supplementary Fig. 3). This depth is also used as the lower boundary of the column over which salinity gradients of b are computed Full size image

Fig. 4 1979–2014 observed annual time series and trends in upper ocean properties in the Ross Sea. 1979–2014 annual time series and trends (from in situ observations27) in a 100–160 m mean ocean temperature, b mixed layer depth, and c salinity gradient across the top 130 m, on average over the Ross Sea areas where observed ice concentration trends are larger than 3% dec−1 (typical interval of confidence for ice concentration trends). Annual means (± one standard error) are shown in gray. Linear trends are shown as red lines and given with their 95% confidence interval at the bottom of each panel Full size image

A two-way process not specific to recent decades

The correlation pattern observed for the Ross Sea only over the period 1979–2013 is found in all but one sector and for the whole Southern Ocean in model results over 1952–1978 (Supplementary Fig. 5). The relationship between trends in sea-ice concentration and ocean temperature is consequently not specific to the 1979–2013 period and thus does not seem to be driven by the climate change of the past three decades28. The Weddell, Indian, and Bellingshausen–Amundsen sectors, in particular, present a correlation profile over 1952–1978 analogous to that found for the Ross Sea during 1979–2013. However, the sign of the trends is reversed. Supplementary Figure 6 shows a deepening of the mixed layer and cooling of subsurface waters co-occuring with negative sea-ice concentration trends and stratification weakening over much of the Antarctic sea-ice zone (note that the situation is different at the centre of the Ross gyre due to a shallow MLD anomaly, blurring the feedback signature when averaging over the Ross sector over that period). A cooling at depth is thus associated with a decay of the sea-ice cover. This indicates that the reorganization of energy within the ocean–sea-ice system may occur in both ways, provided that an initial perturbation enables its establishment over a longer, multi-decadal time scale.

Relationship with seasonality

The ice-coverage–ocean-heat-storage positive feedback is intrinsically related to the seasonal cycle of the mixed layer but it applies to decadal trends of annual mean ice concentration. It favors the production of ice and inhibits its melting when heat is stored at depth; conversely, it inhibits production and favors melting when heat stored at depth is released. Hence, it cannot explain alone opposite trends across seasons. In the Ross Sea, 1979–2013 sea-ice trends are coherent across seasons (Supplementary Fig. 4), leading to strong annual mean trends and a clear feedback signature. In other sectors, the trends over 1979–2013 vary between seasons, leading to smaller trends in annual average, and suggesting that other processes with out-of-phase impacts across seasons are dominant29. Furthermore, the insignificant positive correlations from 0 to 50 m in the Bellingshausen–Amundsen sector in observations (Supplementary Fig. 1) may also be explained by seasonality. If winter and summer trends in ice concentration have opposite signs, the correlations computed from annual values of sea-ice concentration are difficult to interpret and may even become meaningless. This is why a statistically significant maximum in positive correlation between ice concentration and ocean subsurface temperature trends is found only in the Ross Sea, where summer and winter trends are consistent. Note that although changes are expected to have the same sign all year round if the ice-coverage–heat-storage feedback is the dominant process, they need not have the same magnitude in all seasons. Furthermore, the feedback may be triggered by processes occurring during a specific season, and is thus compatible with the suggestion that sea-ice positive trends in the Ross Sea are initiated primarily in the melt season30. In short, each single season may have a specific influence on the feedback intensity and its signature, but as it establishes over long-time scales and is the result of the interplay between mechanisms happening in different seasons, the role of each season cannot be isolated.

Quantifying the vertical heat redistribution