The annual cycle of phytoplankton growth in many parts of the ocean is dominated by a rapid, intense population explosion in late winter and early spring. High levels of phytoplankton production during the spring bloom and the subsequent sinking of organic material contributes significantly to the carbon flux to the deep ocean.

The onset of phytoplankton blooms has been traditionally associated with the rising of the mixed layer above a critical depth at the end of winter. However, we find that the onset of the spring bloom is most likely triggered by the reduction in air-sea heat loss at the end of winter. We will illustrate our results with a combination of theory, numerical simulations, and remote sensing data.