Submesoscale Frontal Heterogeneity Enhances Phytoplankton Chlorophyll in the North Pacific Subtropical Gyre

Subtropical gyres contribute significantly to global ocean productivity. As the ocean warms, the strength of these gyres as a biological carbon pump is predicted to diminish due to increased stratification and depleted surface nutrients. However, these predictions do not incorporate changes in fine-scale dynamics (<10 km), which may play an important role in determining large-scale ocean biogeochemistry. We present results suggesting that the impact of submesoscale physics on phytoplankton in the oligotrophic ocean is substantial and may compensate the weakening of the carbon pump. A new statistical tool, the Heterogeneity Index, was developed to quantify surface patchiness from 1-km resolution satellite observations of sea surface temperature. We demonstrate that chlorophyll concentrations in the North Pacific Subtropical Gyre are enhanced by submesoscale frontal heterogeneity. The greatest impact was found during the late winter, with an average chlorophyll increase of 39% (max. 83%) in highly patchy regions. Data from biogeochemical Argo floats suggest that strong convection and shoaling of the nutricline during this period may facilitate access to deep nutrients by episodic, submesoscale frontal features. On average, the magnitude of annual chlorophyll enhancement due to fine-scale physical processes is comparable to the observed decline due to a warming of ~1.1°C. This suggests that future changes in fine-scale dynamics (e.g. the intensification of episodic, submesoscale features) have the potential to enhance carbon pump thereby compensating the impact of climate warming. Our results highlight the need for an understanding of variability in short-term, fine-scale physics in order to accurately model the response of marine ecosystems to projected climate changes.