Submesoscale horizontal stirring enhances seasonal enrichment of low-chlorophyll surface waters

The large seasonal increase in marine photosynthetic organisms - i.e., phytoplankton bloom - is a ubiquitous oceanic phenomenon that contributes to the removal of carbon dioxide from the atmosphere and that supports the growth and development of larger organisms throughout the marine ecosystem. The underlying mechanisms controlling the intensity and timing of these blooms have been proposed to be dominated by vertical transport and mixing processes that are enhanced at front and filament circulation patterns, commonly known as submesoscale currents. Here we show that the winter blooms characterizing oligotrophic waters, which are manifested by a seasonal increase in satellite-derived levels of surface chlorophyll, may also be intensified by horizontal stirring motions induced by submesoscale currents. Using ocean color remote sensing data and high-resolution numerical simulations in the Eastern Mediterranean Sea, we demonstrate that the commonly observed energization of submesoscale currents in winter can efficiently connect the coastal waters and the ultra-oligotrophic waters in the sea interior, thereby enriching the latter with chlorophyll-rich water. A comparison of climatological chlorophyll time series indicates that this submesoscale horizontal stirring mechanism is responsible for ∼ 24% of the seasonal surface chlorophyll increase in the region. These results shed new light on the processes governing phytoplankton bloom intensity and emphasize the important role of submesoscale horizontal stirring in regulating the marine ecosystem.