Coupling between sea-ice and atmospheric variability on weekly timescales

Coupling between the atmosphere and sea-ice is of considerable importance when determining their variability. For example, changes in sea-ice forced by large scale climate patterns can feedback onto these same patterns, resulting in a noticeable non-local response in atmospheric circulation. The timescales of the typical extratropical atmospheric modes of variability are short, requiring that we consider weekly averaged data. Using Empirical Orthogonal Functions (EOF) to identify large scale patterns of variability in observed atmospheric geopotential and sea-ice concentration, we use correlation analysis and a stochastically forced vector auto-regressive (VAR) model to diagnose directions of causality between various climate modes in the sea-ice/atmosphere system. The ability of the VAR model to separate directions of causality is a key feature given that atmospheric response to sea-ice feedback can be easily masked by atmospheric forcing conditions.

As a demonstration we analyze forcing and feedback between Pacific sector sea-ice and the Western Pacific pattern on weekly timescales. Correlation analysis shows that changes in Bering Sea-ice concentration are forced by the Western Pacific pattern. In addition, VAR model analysis of weekly indices measuring the strength of the Western Pacific pattern (WP) and sea-ice concentration variability in the Bering Sea (BER) indicates that there is significant feedback from sea-ice. Specifically, WP forced changes in BER have a positive feedback effect, sustaining the WP forcing conditions. This relationship is in the opposite sense to that in the North Atlantic, where the VAR model indicates a negative feedback mechanism between Barents Sea-ice concentrations and the strength of the North Atlantic Oscillation. We will also briefly comment on ongoing research applying these techniques to atmosphere/sea-ice interactions in the Southern hemisphere.