Evolution of the winter mixed layer observed during MOSAiC

Abstract:

The Arctic Ocean mixed layer (ML) regulates the transport of oceanic heat to the sea ice, takes up heat from downwelling radiation, and transfers momentum taken up from the moving sea ice to the stratified ocean below. However, observational data are sparse, in particular during winter, and many properties and processes associated with the winter ML remain to be characterized. We recorded a 6.5 months-long time series of wintertime ML properties at unprecedented resolution underneath an ice floe drifting through the central Arctic Ocean during the first three legs of the MOSAiC expedition. We found that the mixed layer deepened from 20 m at the beginning of the MOSAiC drift in late October 2019 to up to 120 m in mid-May 2020. Particularly strong changes in ML depth occurred during February and April 2020. ML salinity showed a decrease by 1 kg m^-3 between early November 2019 and mid-January 2020, followed by a pronounced increase of 2.5 kg m^-3during February and March 2020 (marking the coldest period of the drift). The partly decoupled variability in ML depth and ML salinity points to different origins, such as brine release by cooling-induced sea ice formation, the transit of the floe through geographical gradients in water mass properties and mechanical turbulence-induced mixing during the passage of storms. Analyzing time series of upper ocean vertical shear in horizontal velocity, dissipation of turbulent kinetic energy, drift speed of the MOSAIC floe, surface air temperature and historical hydrographic data, we determine the relative impacts of those processes on the wintertime evolution of the mixed layer.

The Arctic Ocean mixed layer (ML) regulates the transport of oceanic heat to the sea ice, takes up heat from downwelling radiation, and transfers momentum taken up from the moving sea ice to the stratified ocean below. However, observational data are sparse, in particular during winter, and many properties and processes associated with the winter ML remain to be characterized. We recorded a 6.5 months-long time series of wintertime ML properties at unprecedented resolution underneath an ice floe drifting through the central Arctic Ocean during the first three legs of the MOSAiC expedition. We found that the mixed layer deepened from 20 m at the beginning of the MOSAiC drift in late October 2019 to up to 120 m in mid-May 2020. Particularly strong changes in ML depth occurred during February and April 2020. ML salinity showed a decrease by 1 kg m^-3 between early November 2019 and mid-January 2020, followed by a pronounced increase of 2.5 kg m^-3during February and March 2020 (marking the coldest period of the drift). The partly decoupled variability in ML depth and ML salinity points to different origins, such as brine release by cooling-induced sea ice formation, the transit of the floe through geographical gradients in water mass properties and mechanical turbulence-induced mixing during the passage of storms. Analyzing time series of upper ocean vertical shear in horizontal velocity, dissipation of turbulent kinetic energy, drift speed of the MOSAIC floe, surface air temperature and historical hydrographic data, we determine the relative impacts of those processes on the wintertime evolution of the mixed layer.