The influence of mesoscale processes on the dynamics of the upper ocean carbon cycle along the U.S. West Coast

Gernot E. Friederich, Francisco P. Chavez
Monterey Bay Aquarium Research Institute
 
 

The processes controlling the spatial and temporal variability of the coastal carbon cycle are not well understood. For example, the impact of meso- and submesoscale phenomena on the surface ocean carbon cycle and the air-sea exchange of CO₂ are virtually unknown. Meso- and submesoscale phenomena such as eddies, fronts and filaments are common features in the coastal oceans and have been shown to represent key processes controlling the input of nutrients into the euphotic zone and consequently biological productivity. Therefore, they can be expected to be of particular importance for the upper ocean carbon cycle as well.

Here we investigate the dynamics of the upper ocean carbon cycle and its controlling processes for the U.S. West Coast on the basis of a coupled physical-biogeochemical model, with special focus on the influence of mesoscale phenomena. The investigated region is dominated by intense coastal upwelling, highly turbulent flow, and high biological production. 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. We use three model versions with varying horizontal resolutions (20km, 15km, and 5km) to study the influence of the grid resolution on the modeled upper ocean carbon cycle, in particular the CO₂ air-sea gas exchange. The models are forced using the COADS climatology. We compare the modeling results with satellite observations of sea surface chlorophyll from SeaWiFS and measurements of total dissolved inorganic carbon concentration and air-sea partial pressure difference of CO₂ from in- and offshore Monterey Bay as well as from Santa Monica Bay.

Our analysis suggests that in general upwelling-driven CO₂ outgassing occurs in a narrow (>40km) band close to the coast, whereas biologically-driven CO₂ uptake dominates the air-sea CO₂ fluxes further offshore. The open ocean CO₂ gas exchange is found to be determined by the CO₂ solubility in seawater, mainly driven by sea surface temperatures. This overall picture appears to hold irrespective of the model's horizontal resolution. However, with increasing model resolution the representation of mesoscale phenomena is significantly improved, and fine structures mainly within a few hundred of kilometers of the coast become dominant features. In the 5km model, the upwelling-dominated region is confined to a very narrow (>10km) strip along the coast, and biologically-driven CO₂ uptake occurs in filaments originating mostly at capes and other prominent topographical features along the coast. It is interesting to note that although instantaneous patterns of the modeled biological productivity and carbon export largely differ depending on the horizontal resolution, the temporally and spatially integrated values within the Central California Current System are still very similar.