Physical-biological interactions to the west of Hawaiian Islands: impact of submesoscale dynamics on biological productivity

Abstract. Abundant energetic eddies and frontal processes occur frequently to the west of Hawaiian Islands. Their impacts on biological productivity, however, are ambiguous because satellite-measured surface chlorophyll often shows a completely different pattern to eddy kinetic energy field. Our study suggests a new mechanism of how those oceanic eddies and frontal processes affect phytoplankton dynamics by changing their physiological conditions. Due to eddy–eddy or eddy–front interactions, high eddy activity creates regions with enhanced shear and straining that leads to rapid upper ocean restratification and submesoscale vertical motions. The restratification process deceases mixed layer depth that increases the mean exposure of the phytoplankton cells to light, thus resulting in enhanced photosynthetic carbon-based production. In contrast, increased light in the surface layer could either decrease phytoplankton chlorophyll due to the photoacclimation effect or increase chlorophyll when light is a limiting factor for phytoplankton growth. Combined with another two competing processes for vertical nutrient flux, ocean restratification and submesoscale upward motions, it introduces different responses and uncertainties of observed chlorophyll-based production to eddy activity and frontal processes.