Ocean carbon cycling and CO2 air-sea exchange along the U.S. West Coast

G.-K. Plattner (1), T. Nagai (2), N. Gruber (1), H. Frenzel (3), and
J. C. McWilliams (3,4)

(1) Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland

(2) Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Tokyo, Japan

(3) Institute of Geophysics and Planetary Physics, UCLA, Los Angeles CA, USA

(4) Department of Atmospheric and Oceanic Sciences, UCLA, Los Angeles CA, USA

(gian-kasper.plattner@env.ethz.ch / Fax: +41-44-632-1691)

We investigate the coastal ocean carbon budget for the U.S. West Coast on the basis of a coupled physical-biogeochemical model, with a focus on the central upwelling region off California. This region is dominated by intense coastal upwelling, highly turbulent flow, and a high biological production. The ocean model is based on the Regional Oceanic Modeling System (ROMS), coupled to an NPZD-type ecosystem model including a formulation of the ocean carbon cycle, which is driven by climatological mean forcing. Our analyses suggest that the air-sea flux of CO2 constitutes only a small component of a very active and dynamic carbon cycle in the euphotic zone. The central California upwelling region is nearly balanced with regard to CO2 air-sea gas exchange, overall constituting a weak source of CO2 to the atmosphere of roughly 0.2 mol C m-2 yr-1. This is the consequence of upwelling-driven CO2 outgassing nearshore and biologically-driven CO2 uptake offshore, often associated with filaments originating at capes and other prominent topographical features along the coast. The net CO2 flux is small compared to e.g. photosynthesis that fixes approximately 8.8 mol C m-2 yr-1. New production amounts to 3.3 mol C m-2 yr-1 and thus accounts for 37% of total production. Spatially averaged export production nearly equals averaged new production, but locally new and export production are substantially decoupled in this dynamic ocean region. This is due to lateral transport associated with mean horizontal fluxes induced by persistent meso- and submesoscale circulation structures and to lesser degree by the mean lateral offshore transport induced by Ekman transport. We will report on these budgets as well as ongoing efforts to estimate the magnitude of the lateral transport of carbon and nutrients between the coastal and open ocean, which we expect to be substantial.