Effect of eddy transport on the nutrient supply into the euphotic zone simulated in an eddy-permitting ocean ecosystem model

Hindcast experiment with an eddy-permitting ocean ecosystem model was performed in order to investigate the process of nutrient supply into the euphotic zone by vertical and horizontal fluxes with multiple time scales. The model reasonably reproduced the basin-scale spatial pattern of biological production and its seasonal cycles with eddy fields, statistically consistent with those observed in the satellite images. The model results illustrated the nutrient cycle in the euphotic zone with mesoscale eddies, in which the vertical advection with mesoscale spatial variation and convection with relatively large spatial variation supply nutrients into the euphotic zone, and the horizontal advection redistributes them within the euphotic zone, thereby fueling biological production. The vertical advection associated with mean flow supplies a substantial part of nutrients from the aphotic zone, and contributions from time-varied vertical fluxes are limited within the tropical region and regions where strong meandering currents exist. Horizontal advection due to seasonal and eddy fluctuations plays an important role in the redistribution process, in addition to that effected by temporal-mean advection. The relative importance of these fluctuations strongly depends on the horizontal scales of vertical nutrient supply, in contrast to the relatively small effects eddy fields have on heat or fresh-water transport in the global ocean.