Antarctic sea-ice expansion and Southern Ocean cooling linked to tropical variability

Satellite observations show slight increases in Antarctic sea-ice extent, yet climate models predict declines. Here sea-ice expansion is shown to occur when the Southern Ocean surface cools from natural climate variability, primarily linked via teleconnections with the tropical Pacific Ocean. A variety of hypotheses, involving sub-ice-shelf melting, stratospheric ozone depletion and tropical teleconnections, have been proposed to explain the observed Antarctic sea-ice expansion over the period of continuous satellite monitoring and corresponding model-observation discrepancy, but the issue remains unresolved. Here, by comparing multiple large ensembles of model simulations with available observations, we show that Antarctic sea ice has expanded due to ocean surface cooling associated with multidecadal variability in the Southern Ocean that temporarily outweighs the opposing forced response. In both observations and model simulations, Southern Ocean multidecadal variability is closely linked to internal variability in the tropics, especially in the Pacific, via atmospheric teleconnections. The linkages are, however, distinctly weaker in simulations than in observations, accompanied by a marked model-observation mismatch in global warming resulting from potential model bias in the forced response and observed tropical variability. Thus, the forced response dominates in simulations, resulting in apparent model-observation discrepancy.

Automated summary

Satellite observations show slight increases in Antarctic sea-ice extent, contradicting climate model predictions. This study finds that the expansion of sea ice is due to cooling of the Southern Ocean surface, which is primarily linked to teleconnections with the tropical Pacific Ocean. The discrepancy between models and observations may be caused by model bias in the forced response and observed tropical variability.