The subpolar North Atlantic is a site of significant carbon dioxide,
oxygen, and heat exchange with the atmosphere. This exchange, which
regulates transient climate change and prevents large-scale hypoxia
throughout the North Atlantic, is thought to be mediated by vertical
mixing in the ocean's surface mixed layer. Here we present observational
evidence that waters deeper than the conventionally defined mixed layer
are affected directly by atmospheric forcing. When northerly winds blow
along the Irminger Sea's western boundary current, the Ekman response
pushes denser water over lighter water and triggers slantwise
convection. We estimate that this down-front wind forcing is four times
stronger than air--sea heat flux buoyancy forcing and can mix waters to
several times the conventionally defined mixed layer depth. Slantwise
convection is not included in most large-scale ocean models, which
likely limits their ability to accurately represent subpolar water mass
transformations and deep ocean ventilation.