Abstract
Several processes have been hypothesized to explain the slight overall expansion of Antarctic sea ice over the satellite observation era, including externally forced changes in local winds or in the Southern Ocean’s hydrological cycle, as well as internal climate variability. Here, we show the critical influence of an ocean–sea-ice feedback. Once initiated by an external perturbation, it may be sufficient to sustain the observed sea-ice expansion in the Ross Sea, the region with the largest and most significant expansion. We quantify the heat trapped at the base of the ocean mixed layer and demonstrate that it is of the same order of magnitude as the latent heat storage due to the long-term changes in sea-ice volume. The evidence thus suggests that the recent ice coverage increase in the Ross Sea could have been achieved through a reorganization of energy within the near-surface ice-ocean system.
Highlights
Several processes have been hypothesized to explain the slight overall expansion of Antarctic sea ice over the satellite observation era, including externally forced changes in local winds or in the Southern Ocean’s hydrological cycle, as well as internal climate variability
As the Antarctic sea-ice zone is characterized by the presence of Circumpolar Deep Water (CDW) under the mixed layer, which is warm compared to the surface water, an increased stability due to surface freshening leads to smaller vertical heat fluxes toward the surface layer
We show the signature of this feedback and quantify the heat trapped at the base of the ocean mixed layer in the Ross Sea, where long-term positive sea-ice trends are observed and simulated by a model
Summary
Several processes have been hypothesized to explain the slight overall expansion of Antarctic sea ice over the satellite observation era, including externally forced changes in local winds or in the Southern Ocean’s hydrological cycle, as well as internal climate variability. If ice formation is large during one or a few years, the brine released can be transported downward to deeper layers and not incorporated back into the mixed layer of the subsequent winters This leads to a decrease in surface salinity, a stronger stratification, a shallower mixed layer and reduced vertical oceanic heat fluxes. We demonstrate that the heat gain is of the same order of magnitude as the latent heat storage due to the associated changes in sea-ice volume, indicating that the trends may be sustained via a vertical redistribution of energy in the near-surface ice-ocean system This implies that enough energy is available at depth to melt the extra ice formed during the last few decades, should the mechanism be reversed
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