Shutdown of convection triggers increase of surface chlorophyll

The long-standing explanation of the triggering cause of the surface increase of phytoplankton visible in spring satellite images argues that phytoplankton biomass accumulation begins once the mixed layer depths become shallower than a ‘critical depth’. However, a series of recent studies have found evidence for phytoplankton increase in deep mixed layers, and several hypotheses have been proposed to explain this early increase. In this manuscript it is suggested that the surface concentration of phytoplankton increases rapidly in a ‘surface bloom’ when atmospheric cooling of the ocean turns into an atmospheric heating at the end of winter. The hypothesis is supported by analysis of satellite observations of chlorophyll and of heat fluxes from atmospheric reanalysis from the North Atlantic.