Decadal oscillations of the air‐ice‐ocean system in the Northern Hemisphere

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Abstract
Feedback mechanisms among decadal oscillations of Northern Hemisphere atmospheric circulation, Arctic‐subarctic ice cover and ocean circulation are studied. Decadal oscillations of the ice cover have been observed since 1950, most significantly in the Barents Sea and on the Labrador Shelf. The extreme southward extent of Labrador ice is explained by local weather conditions, such as low air temperatures and strong northwesterly winds. This weather pattern is attributed to an atmospheric circulation with low sea‐level pressure anomalies over the Eurasian Basin in the Arctic. Barents Sea ice is also highly (but inversely) correlated with this atmospheric circulation, i.e. Barents Sea ice is inversely correlated with Labrador ice. Barents Sea ice is weakly correlated with local air temperature. The following air‐ice‐ocean coupling is suggested for Barents Sea ice and atmospheric circulation: the increased cyclonic wind‐stress curl associated with the atmospheric circulation enhances the exchange of cold Arctic water, including sea ice, and warm Atlantic water, and reduces the amount of Barents Sea ice. The reduced ice cover encourages heat flux from the Barents Sea to the atmosphere, tending to reinforce the low pressure in the Arctic. This positive feedback amplifies the oscillations of the air‐ice‐ocean system driven by external forcing, such as solar activity with fairly weak variability. A mechanistic, 2‐level ocean model, which is driven by prescribed buoyancy flux and wind stresses, suggests that the Arctic‐Atlantic system resonates with the variable wind stresses at a decadal time‐scale. A conceptual air‐ice‐ocean model that includes the fundamental oceanic response found by the mechanistic ocean model as well as positive feedback between air and ice‐ocean systems shows that weak decadal variations in external forcing can be amplified in the air‐ice‐ocean system.