On the role of eddies for coastal productivity and carbon cycling

H. Frenzel (1), N. Gruber (2), G.-K. Plattner (2), J. C. McWilliams (1),
T. Nagai (3), and Z. Lachkar (2)

(1) Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, USA,
(2) Institute of Biogeochemistry and Pollutant Dynamics, ETH Zuerich,
Zuerich, Switzerland,
(3) Department of Ocean Sciences, Tokyo University of Marine Science
and Technology, Tokyo, Japan.

(hfrenzel@igpp.ucla.edu)

The impact of meso- and submesoscale processes on ocean productivity
and carbon cycling are not well understood. While eddies and other
mesoscale variations have been shown to represent key processes
enhancing nutrient supply and biological productivity in open ocean
conditions, little is known for the coastal ocean. We will demonstrate
the impact of mesoscale eddies on coastal ocean productivity and
biogeochemistry using an eddy-resolving coupled
physical-ecosystem-biogeochemical model of the California Current
System. Our results suggest that in coastal upwelling systems
mesoscale processes tend to reduce biological productivity and the
downward export of carbon, opposite to the effect reported for open
ocean waters. In contrast, their integrated impact on coastal air-sea
fluxes of CO2 is relatively small. The reduction in biological
productivity is caused by a lateral eddy-induced transport that brings
warm, nutrient depleted waters toward the shore, thereby suppressing
the effect of Ekman-transport induced upwelling. This mechanism could
explain the substantially lower productivity of the California Current
System in comparison to the Canary Current or Benguela Current
upwelling systems, since the latter two have a substantially lower
eddy activity, despite similar upwelling strengths. In the California
Current System, strong lateral transport of organic carbon associated
with persistent mesoscale circulation structures leads to substantial
spatial decoupling of export from new production. Westward propagating
eddies represent the main vehicle for transporting organic carbon from
the nearshore region to the offshore, thereby enhancing heterotrophic
consumption in the open ocean. In summary, meso- and submesoscale
processes are of fundamental importance in shaping coastal
biogeochemistry and carbon balances.