Ocean fronts trigger high latitude phytoplankton blooms

Density fronts are ubiquitous features of the upper ocean. Here, numerical simulations show that restratification at fronts inhibits vertical mixing, triggering phytoplankton blooms in low‐light conditions. The stability of the water column at fronts is set by a competition between frontal instabilities, which restratify the upper ocean, and turbulent mixing, which acts to destroy this stratification. Recent studies have found that frontal instabilities can restratify the upper ocean, even in the presence of strong surface cooling and destabilizing winds. During winter at high latitudes, primary production by phytoplankton is generally limited by low ambient light levels and deep turbulent mixing. When the turbulent mixing, inhibited by frontal restratification, becomes smaller than a ‘critical turbulence’ threshold, a phytoplankton bloom can develop. The finding that fronts can trigger phytoplankton blooms by reducing mixing, provides an explanation for satellite observations of high chlorophyll concentrations at high latitude fronts. Key Points Oceanic fronts reduce turbulent mixing in the upper ocean Reduced turbulent mixing at fronts triggers high‐latitude phytoplankton blooms Fronts act as hotspots for primary productivity