Seasonal and Decadal variability of Antarctic Sea Ice driven by the Southern Annular Mode

We show from analysis of data that the response of Antarctic sea ice extent (SIE) to a positive summertime Southern Annular Mode (SAM) opposes that of the seasonal cycle leading to a diminution in its amplitude: an increase in SIE in autumn and a decrease in the following late spring. To investigate the physical causes of these observed signals we study SAM-induced sea ice variability using a fully coupled climate model from GISS. In response to strengthened summertime zonal winds, the surface cools and the mixed layer deepens on moving from autumn to the following spring. Enhanced vertical mixing triggered by the wind and draws heat down from the surface, warming the subsurface ocean. This warmth re-emerges later in the season leading to diminution of wintertime ice.

Guided by residual-mean theory, we go to examine how zonal winds result in decadal modulations in SIE. The essentially white-noise forcing by SAM results in high-frequency fluctuations of the ocean’s overturning circulation. The eddy-driven component, instead, responds on dedadal timescales – set by eddy-compensation dynamics - leading to decadal modulation in meridional heat transport into the seasonal ice zone and decadal variations in SIE. A simple Hasselmann-like stochastic model is developed and guides analysis of the GISS coupled climate model. This provides a null-hypothesis from which to contemplate the dramatic decline in Antarctic sea ice observed in recent years: - it may be part of natural variability and not necessarily associated with anthryeaopogenic climate change.