Eddydriven stratification initiates North Atlantic spring phytoplankton blooms

Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming. Springtime phytoplankton blooms in high-and mid-latitude oceans contribute sub-stantially to global photosynthetic fixation and export of carbon from the surface ocean, with the North Atlantic alone accounting for 20 % of net global CO2 uptake (1). During winter, strong surface cooling and winds drive turbulent mix-ing that repeatedly carries phytoplankton to depth