Remote and Local Processes Controlling Decadal Sea Ice Variability in the Weddell Sea

Sea ice in the Weddell Sea plays important roles in the albedo, air‐sea heat, freshwater, and gas exchanges and the formation of the Antarctic Bottom Water on the continental shelf that regulates global climate through deep thermohaline circulation. Sea ice extent (SIE) in the Weddell Sea does not show any significant trend in the four‐decade satellite observation period but does experience large interannual‐to‐decadal variability. Physical processes underlying this variability are difficult to examine because of a paucity of prolonged observations. Using a coupled general circulation model, we corroborate the observational finding that decadal increases in SIE are preceded by weakening of the mean westerly winds linked to the Southern Annular Mode. Weakening of the westerly winds over the Weddell Sea reduces the northward Ekman current and suppresses upwelling of warm water from the subsurface ocean; this lowers the upper‐ocean temperature, causing an increase in sea ice. In a sensitivity experiment where the interannual sea surface temperature variations are suppressed outside the South Atlantic and the Weddell Sea, decadal increase in the SIE over the Weddell Sea is induced by a decrease in the upper‐ocean temperature owing to enhanced horizontal advection by the Weddell Gyre. The strengthening of the Weddell Gyre increases the upper‐ocean salinity as a result of enhanced evaporation and brine rejection associated with the sea ice increase. These results demonstrate that both the remote atmospheric forcing and local ice‐ocean interaction are crucial for generation of decadal sea ice variability in the Weddell Sea.