Strong winter cooling over the Irminger Sea in winter 2014–2015, exceptional deep convection, and the emergence of anomalously low SST

摘要

Deep convection is presumed to be vital for the North Atlantic Meridional Overturning Circulation,even though observational evidence for the link remains inconclusive. Modeling studies have suggested thatconvection will weaken as a result of enhanced freshwater input. The emergence of anomalously low seasurface temperature in the subpolar North Atlantic has led to speculation that this process is already at work.Hereweshow that strong atmospheric forcing in the winter of 2014–2015, associated with a high North AtlanticOscillation (NAO) index, produced record mixed layer depths in the Irminger Sea. Local mixing removed thestratification of the upper 1400mand ventilated the basin to middepths resembling a state similar to themid-1990s when a positive NAO also prevailed. We show that the strong local atmospheric forcing ispredominantly responsible for the negative sea surface temperature anomalies observed in the subpolar NorthAtlantic in 2015 and that there is no evidence of permanently weakened deep convection.