Large-scale impacts of submesoscale dynamics on phytoplankton: Local and remote effects

The sensitivity of an idealized bio-physical model of seasonally varying subtropical and subpolar gyres to increased horizontal resolution is presented. Switching from mesoscale-resolving (1/9°) to submesoscale-resolving (1/54°) allows the emergence of a denser and more energetic vortex population sustained by submesoscale physics. The experiments display a global decrease in phytoplankton abundance of ~10-20% as the resolution is refined. This result contrasts with previous studies, which suggested that eddy-driven vertical fluxes of nutrients locally boost phytoplankton growth at mid-latitudes in the open ocean. The explanation is that the long-term (50. years) modification of the large-scale, or basin-scale, circulation and distribution of nutrients by submesoscale processes (remote effects), not taken into account by previous studies because of their much shorter time integration, balance the local, small-scale effects. More precisely, dynamical remote effects (involving only the physics) affect the location of the boundary between the two gyres as well as the large-scale mixed-layer depth (MLD) and thermocline depth. Biological remote effects result from the dynamical remote effects that strongly modify the physical-biological interactions at all scales, including at large scales. These biological remote effects involve changes of the nutricline depth. In the mid-latitude subpolar gyre, phytoplankton decrease in abundance at higher resolution is due to the shallower MLD and nutricline, which promote zooplankton grazing; in the subtropical gyre, it is due to deeper MLD and nutricline, which diminishes regenerated production. In addition, remote effects modulate the dynamical supplies of nutrients to the euphotic layer through a combination of changes in mean advection, eddy advection and vertical mixing.