Physical-Biogeochemical Interactions and CO₂ Air-Sea Exchange Along the U.S. West Coast

Gian-Kasper Plattner, Hartmut Frenzel, Nicolas Gruber, Anita Leinweber, James C. McWilliams, Patrick Marchesiello, Gernot E. Friederich, and Francisco P. Chavez
 

The processes controlling the variability and magnitude of the coastal carbon cycle are complex and not yet well understood. In particular, the contribution of the coastal oceans to the net flux of CO₂ between the ocean and atmosphere remains unclear.

We investigate the CO₂ air-sea gas exchange and its controlling processes for the U.S. West Coast on the basis of a coupled physical-biogeochemical model. We aim to quantify the relative contributions of ocean circulation, marine biological processes and sea surface temperature (SST) to the seasonal variations of the CO₂ gas exchange. The ocean model is based on the Regional Oceanic Modeling System (ROMS), which has been coupled to an NPDZ-type ecosystem model with a formulation of the carbon cycle, and is driven by climatological forcing. A special focus will be on the central region of the California Current System (CCS), extending about 500 km offshore from Point Conception to Cape Mendocino (34.5^oN-40.4^oN). The investigated region is dominated by intense coastal upwelling, highly turbulent flow, and high biological production.

We compare the modeling results with satellite observations of sea surface chlorophyll from SeaWiFS and measurements of total dissolved inorganic carbon concentration and surface ocean pCO₂ from in- and offshore Monterey Bay as well as from Santa Monica Bay.

Our analysis suggests that the air-sea flux of CO₂ constitutes only a small component of a very active and dynamic cycle in the euphotic zone. We find that the central region of the CCS is on average a source of CO₂ to the atmosphere of roughly 0.9 mol C m^-2 yr^-1 compared to e.g. photosynthesis that fixes approximately 5 mol C m^-2 yr^-1. A net CO₂ outgassing is modeled over most time of the year, with the maximum during summer (June-August), and only interrupted by CO₂ uptake at the end of winter and early spring (February-April). The seasonal cycle is most pronounced in the nearshore upwelling-dominated region, where in spring and summer upwelling-driven CO₂ outgassing occurs in a narrow (<20 km) band close to the coast with annual mean fluxes as high as 4.5 mol C m^-2 yr^-1. Further offshore, biologically-driven CO₂ uptake significantly influences the air-sea CO₂ fluxes, mainly within filaments originating at capes and other prominent topographical features along the coast. The seasonal variations of the open ocean CO₂ gas exchange are found to be largely determined by SSTs. Apart from the very nearshore region, the combined circulation/biological components act to oppose the SST component.

To analyze the influence of mesoscale processes on the upper ocean carbon cycle in the CCS, we have performed simulations with different horizontal model resolutions (20 km, 15 km, and 5 km). We find that the temporally and spatially integrated values of the modeled CO₂ air-sea flux, biological productivity and carbon export within the central CCS are very similar irrespective of the resolution, although instantaneous patterns show significant differences.
 
 

For citations, please use:

Plattner, G.-K., H. Frenzel, N. Gruber, A. Leinweber, J. C. McWilliams, P. Marchesiello, G. E. Friederich, and F. P. Chavez, Physical-Biogeochemical Interactions and CO₂ Air-Sea Exchange Along the U.S. West Coast, Eos Trans. AGU, 84(52), Ocean Sci. Meet. Suppl., Abstract OS51L-01, 2003.