| Summary: | 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. Attribution 4.0 International (CC BY 4.0). ILB, FS, TCB, and JH gratefully acknowledge the US National Science Foundation (NSF): this work was supported by grants OCE-1258823, OCE-1756272, OCE-1948335, and OCE-2038481. JC gratefully acknowledges NSF support through grant OCE-1924354. HLJ was supported by the SNAP-DRAGON program (UK Natural Environment Research Council grant number NE/T013494/1). We gratefully acknowledge the many scientists and mariners who went to sea to collect the observational data, and Bob Pickart in particular. Data Availability. OSNAP mooring data used in this study are available at http://www.o-snap.org/observations/data/ under "US East Cape Farewell Slope Array". ERA5 reanalysis hourly data was downloaded from https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels. Hourly wind stress and heat flux fields were downloaded on June 18, 2020 and hourly sea level pressure data was downloaded on July 13,4472021. ERA5 monthly wind stress fields shown in Figure 1 were ...
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