How Are Phytoplankton Sustained in Subpolar North Atlantic during the Winter?

The subpolar North Atlantic is a biologically active region of the ocean that persistently acts as a sink of the atmospheric carbon. A key question is that how phytoplankton productivity is sustained in the subpolar North Atlantic during the winter when the light is scarce, the mixed layer is considerably deep and the ocean surface is under influence of very episodic intense air-sea fluxes. The severe winter conditions lead to turbulent processes in the upper ocean which are known to modulate the light and nutrients in the ocean and hence have substantial effect on the productivity of phytoplankton. Understanding these processes can enhance our ability for assessing the biology of the ocean. To this end, in this study we use highly-resolved three dimensional numerical simulations and investigate the upper ocean’s response to hourly varying air-sea fluxes in the North Atlantic provided by the MERRA2 reanalysis during the winter. We will show that the following factors can contribute to the sustenance of phytoplankton in the winter time: 1) initiation and evolution of instabilities, 2) coupling of subsurface mesoscale and surface submesoscale eddies, 3) short time scale processes and transient restratification in the upper ocean.