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.