Abstract
Abstract. We quantify the CO2 source/sink nature of the California Current System (CalCS) and determine the drivers and processes behind the mean and spatiotemporal variability of the partial pressure of CO2 (pCO2) in the surface ocean. To this end, we analyze eddy-resolving, climatological simulations of a coupled physical–biogeochemical oceanic model on the basis of the Regional Oceanic Modeling System (ROMS). In the annual mean, the entire CalCS within 800 km of the coast and from ∼33° N to 46° N is essentially neutral with regard to atmospheric CO2: the model simulates an integrated uptake flux of −0.9 ± 3.6 Tg C yr−1, corresponding to an average flux density of −0.05 ± 0.20 mol C m−2 yr−1. This near zero flux is a consequence of an almost complete regional compensation between (i) strong outgassing in the nearshore region (first 100 km) that brings waters with high concentrations of dissolved inorganic carbon (DIC) to the surface and (ii) and a weaker, but more widespread uptake flux in the offshore region due to an intense biological reduction of this DIC, driven by the nutrients that are upwelled together with the DIC. The air–sea CO2 fluxes vary substantially in time, both on seasonal and sub-seasonal timescales, largely driven by variations in surface ocean pCO2. Most of the variability in pCO2 is associated with the seasonal cycle, with the exception of the nearshore region, where sub-seasonal variations driven by mesoscale processes dominate. In the regions offshore of 100 km, changes in surface temperature are the main driver, while in the nearshore region, changes in surface temperature, as well as anomalies in DIC and alkalinity (Alk) owing to changes in circulation, biological productivity and air–sea CO2 fluxes dominate. The prevalence of eddy-driven variability in the nearshore 100 km leads to a complex spatiotemporal mosaic of surface ocean pCO2 and air–sea CO2 fluxes that require a substantial observational effort to determine the source/sink nature of this region reliably.
Highlights
The coastal ocean often has not been appropriately taken into account in global carbon budget estimates, despite the fact that the associated carbon fluxes are disproportionately large with respect to the small fraction of the global ocean area that coastal oceans occupy (e.g., Liu et al, 2000; Borges et al, 2005; Chavez et al, 2007; Liu et al, 2010; Regnier et al, 2013)
Does this near complete spatial compensation occur by chance, or are there some underlying mechanisms at play? Second, if such underlying mechanisms exist, how might they control the air–sea CO2 balance under future climate change? Third, what is the contribution of the oceanic uptake of anthropogenic CO2 to the overall source/sink balance? Fourth, how do the air–sea CO2 fluxes within the California Current System (CalCS) compare to fluxes elsewhere, and in particular, how do these results fit into the global picture?
We used a series of eddy-resolving simulations of the CalCS (i) to assess the climatological mean air–sea CO2 fluxes and their spatiotemporal variability and (ii) to determine the drivers and processes behind the variability of these fluxes and surface ocean pressure of CO2 (pCO2)
Summary
The coastal ocean often has not been appropriately taken into account in global carbon budget estimates, despite the fact that the associated carbon fluxes are disproportionately large with respect to the small fraction of the global ocean area that coastal oceans occupy (e.g., Liu et al, 2000; Borges et al, 2005; Chavez et al, 2007; Liu et al, 2010; Regnier et al, 2013). Global ocean models tend to be too coarse to resolve important coastal processes and observational data are often limited in space and time (e.g., Laruelle et al, 2010). Coastal air–sea CO2 fluxes are currently still relatively poorly quantified, with considerable regional and global uncertainties. Coastal upwelling regions are dynamic in terms of carbon cycling as they experience extreme temporal and spatial variability in carbon fluxes (e.g., Friederich et al, 2002; Cai et al, 2006; Leinweber et al, 2009; Evans et al, 2011), further adding to the uncertainty in the coastal carbon budget. The upwelled nutrients stimulate phytoplankton productivity, which supports a large
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