The impact of cold-air outbreaks and oceanic lateral fluxes on dense-water formation in the Greenland Sea from a ten-year moored record (1999–2009)

Abstract The Greenland Sea produces a significant portion of the dense water from the Nordic Seas that supplies the lower limb of the Atlantic Meridional Overturning Circulation. Here, we use a continuous 10-year hydrographic record from moored profilers to examine dense-water formation in the central Greenland Sea between 1999 and 2009. Of primary importance for dense-water formation is air-sea heat exchange, and 60–80% of the heat lost to the atmosphere during winter occurs during intense, short-lived events called cold-air outbreaks (CAOs). The long duration and high temporal resolution of the moored record has for the first time facilitated a statistical quantification of the direct impact of CAOs on the wintertime mixed layer in the Greenland Sea. The mixed-layer development can be divided into two phases: a cooling phase and a deepening phase. During the cooling phase (typically between November and January), CAOs cooled the mixed layer by up to 0.08 K per day, depending on the intensity of the events, while the mixed-layer depth remained nearly constant. Later in winter (February–April), heat fluxes during CAOs primarily led to mixed-layer deepening, of up to 38 m per day. Considerable variability was observed in the mixed-layer response, indicating that lateral fluxes of heat and salt were also important. The magnitude and vertical distributions of these fluxes were quantified, and idealized mixed-layer simulations suggest that their combined effect is a reduction in the mixed-layer depth at the end of winter of up to several hundred meters.