Air-sea CO2 fluxes in the California Current: Impacts of model resolution and coastal topography

The present study uses a suite of coupled physical-biogeochemical model simulations at 1/3 degrees, 1/10 degrees, and 1/30 degrees to assess the impact of horizontal resolution on air-sea CO2 fluxes in the California Current System (CCS), a relevant issue for downscaling between coarser resolution global climate models and higher resolution regional models. The results demonstrate that horizontal resolution is important to reproduce the sharp transition between near-shore outgassing and offshore absorption, as well as to resolve regions of enhanced near-shore outgassing in the lee of capes. The width of the outgassing region is overestimated when horizontal resolution is not eddy resolving (i.e., 1/3 degrees) but becomes more dependent on shelf topography for eddy-resolving simulations (i.e., 1/10 degrees and 1/30 degrees). Enhanced near-shore outgassing is associated with a local increase in wind-driven upwelling in the lee of capes (i.e., expansion fans), meaning that sufficient horizontal resolution is needed both in the ocean circulation model and in the wind field forcing the model. From a global carbon budget perspective, the model indicates that biological production generates sufficient absorption within a few hundred kilometers of the coast to offset near-shore outgassing, which is consistent with the notion that midlatitude eastern boundary current upwelling systems act both as a sink and source for atmospheric CO2. Based on the 1/30 degrees solution, the CCS between 35 and 45N and out to 600km offshore is a net carbon sink of approximately 6 TgC yr(-1), with the 1/10 degrees solution underestimating this value by less than 10% and the 1/3 degrees solution by a factor of 3. Key Points <list list-type="bulleted" id="gbc20153-list-0001"> <list-item id="gbc20153-li-0001">Outgassing intensification linked to coastal topographic features <list-item id="gbc20153-li-0002">Near-shore outgassing balanced by offshore absorption <list-item id="gbc20153-li-0003">Carbon fluxes most sensitive to horizontal resolution for 35-40N