Abstract.We quantify the CO 2 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 CO 2 (pCO 2 ) 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 CO 2 : 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 CO 2 fluxes vary substantially in time, both on seasonal and sub-seasonal timescales, largely driven by variations in surface ocean pCO 2 . Most of the variability in pCO 2 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 CO 2 fluxes dominate. The prevalence of eddy-driven variability in the nearshore 100 km leads to a complex spatiotemporal mosaic of surface ocean pCO 2 and air-sea CO 2 fluxes that require a substantial observational effort to determine the source/sink nature of this region reliably.

We reconstruct the evolution of ocean acidification in the California Current System (CalCS) from 1979 through 2012 using hindcast simulations with an eddy-resolving ocean biogeochemical model forced with observation-based variations of wind and fluxes of heat and freshwater. We find that domain-wide pH and arag W in the top 60 m of the water column decreased significantly over these three decades by about −0.02 decade −1 and −0.12 decade −1 , respectively. In the nearshore areas of northern California and Oregon, ocean acidification is reconstructed to have progressed much more rapidly, with rates up to 30% higher than the domain-wide trends. Furthermore, ocean acidification penetrated substantially into the thermocline, causing a significant domain-wide shoaling of the aragonite saturation depth of on average −33 m decade −1 and up to −50 m decade −1 in the nearshore area of northern California. This resulted in a coast-wide increase in nearly undersaturated waters and the appearance of waters with 1 arag W < , leading to a substantial reduction of habitat suitability.Averaged over the whole domain, the main driver of these trends is the oceanic uptake of anthropogenic CO 2 from the atmosphere. However, recent changes in the climatic forcing have substantially modulated these trends regionally. This is particularly evident in the nearshore regions, where the total trends in pH are up to 50% larger and trends in arag W and in the aragonite saturation depth are even twice to three times larger than the purely atmospheric CO 2 -driven trends. This modulation in the nearshore regions is a result of the recent marked increase in alongshore wind stress, which brought elevated levels of dissolved inorganic carbon to the surface via upwelling. Our results demonstrate that changes in the climatic forcing need to be taken into consideration in future projections of the progression of ocean acidification in coastal upwelling regions.

Abstract.We use an eddy-resolving, regional ocean biogeochemical model to investigate the main variables and processes responsible for the climatological spatio-temporal variability of pCO 2 and the air-sea CO 2 fluxes in the southwestern Atlantic Ocean. Overall, the region acts as a sink of atmospheric CO 2 south of 30 • S, and is close to equilibrium with the atmospheric CO 2 to the north. On the shelves, the ocean acts as a weak source of CO 2 , except for the mid/outer shelves of Patagonia, which act as sinks. In contrast, the inner shelves and the low latitude open ocean of the southwestern Atlantic represent source regions. Observed nearshoreto-offshore and meridional pCO 2 gradients are well represented by our simulation. A sensitivity analysis shows the importance of the counteracting effects of temperature and dissolved inorganic carbon (DIC) in controlling the seasonal variability of pCO 2 . Biological production and solubility are the main processes regulating pCO 2 , with biological production being particularly important on the shelves. The role of mixing/stratification in modulating DIC, and therefore surface pCO 2 , is shown in a vertical profile at the location of the Ocean Observatories Initiative (OOI) site in the Argentine Basin (42 • S, 42 • W).

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-ecosystem-biogeochemical ocean model on the basis of the Regional Oceanic Modeling System (ROMS). The model-simulated pCO2 agrees very well with in situ observations over the entire domain with virtually no bias, but the model overestimates pCO2 in the nearshore 100 km, and underestimates the observed temporal variability. 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 Tg C yr–1, corresponding to a very small average flux density of −0.05 mol C m–2 yr–1, with an uncertainty of the order of ±0.20 mol C m–2 yr–1. This near zero flux is a consequence of an almost complete regional compensation between the strong outgassing in the nearshore region (first 100 km), with flux densities of more than 3 mol C m–2 yr–1 and a weaker, but more widespread uptake flux in the offshore region with an average flux density of −0.17 mol C m–2 yr–1. This pattern is primarily a result of the interaction between upwelling in the nearshore that brings waters with high concentrations of dissolved inorganic carbon (DIC) to the surface, and an intense biological drawdown of this DIC, driven by the nutrients that are upwelled together with the DIC. The biological drawdown occurs too slowly to prevent the escape of a substantial amount of CO2 into the atmosphere, but this is compensated by the biological generation of undersaturated conditions offshore of 100 km, permitting the CalCS to take up most of the escaped CO2. Thus, the biological pump over the entire CalCS is essentially 100% efficient, making the preformed DIC and nutrient concentrations of the upwelled waters a primary determinant of the overall source/sink nature of the CalCS. The comparison of the standard simulation with one for preindustrial conditions show that the CalCS is taking up anthropogenic CO2 at a rate of about −1 mol C m–2 yr–1, implying that the region was a small source of CO2 to the atmosphere in preindustrial times. The air–sea CO2 fluxes vary substantially in time, both on seasonal and sub-seasonal timescales, largely driven by variations in surface ocean pCO2. There are important differences among the subregions. Notably, the total variance of the fluxes in the central nearshore CalCS is roughly 4–5 times larger than elsewhere. Most of the variability in pCO2 is associated with the seasonal cycle, except in the nearshore, 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 dominance 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.

Abstract. Coastal upwelling systems, such as the California Current System (CalCS), naturally experience a wide range of O 2 concentrations and pH values due to the seasonality of upwelling. Nonetheless, changes in the El Niño-Southern Oscillation (ENSO) have been shown to measurably affect the biogeochemical and physical properties of coastal upwelling regions. In this study, we use a novel, high-resolution global climate model (GFDL-ESM2.6) to investigate the influence of warm and cold ENSO events on variations in the O 2 concentration and the pH of the CalCS coastal waters. An assessment of the CalCS response to six El Niño and seven La Niña events in ESM2.6 reveals significant variations in the response between events. However, these variations overlay a consistent physical and biogeochemical (O 2 and pH) response in the composite mean. Focusing on the mean response, our results demonstrate that O 2 and pH are affected rather differently in the euphotic zone above ∼ 100 m. The strongest O 2 response reaches up to several hundreds of kilometers offshore, whereas the pH signal occurs only within a ∼ 100 km wide band along the coast. By splitting the changes in O 2 and pH into individual physical and biogeochemical components that are affected by ENSO variability, we found that O 2 variability in the surface ocean is primarily driven by changes in surface temperature that affect the O 2 solubility. In contrast, surface pH changes are predominantly driven by changes in dissolved inorganic carbon (DIC), which in turn is affected by upwelling, explaining the confined nature of the pH signal close to the coast. Below ∼ 100 m, we find conditions with anomalously low O 2 and pH, and by extension also anomalously low aragonite saturation, during La Niña. This result is consistent with findings from previous studies and highlights the stress that the CalCS ecosystem could periodically undergo in addition to impacts due to climate change.

Abstract. We use an eddy-resolving, regional ocean biogeochemical model to investigate the main variables and processes responsible for the climatological spatio-temporal variability of pCO2 and the air–sea CO2 fluxes in the southwestern Atlantic Ocean. Overall, the region acts as sink of atmospheric CO2 south of 30° S, and is close to equilibrium with the atmospheric CO2 to the north. On the shelves, the ocean acts as a weak source of CO2, except for the mid/outer shelves of Patagonia, which act as sinks. In contrast, the inner shelves and the low latitude open ocean of the southwestern Atlantic represent source regions. Observed nearshore-to-offshore and meridional pCO2 gradients are well represented by our simulation. A sensitivity analysis shows the importance of the counteracting effects of temperature and dissolved inorganic carbon (DIC) in controlling the seasonal variability of pCO2. Biological production and solubility are the main processes regulating pCO2, with biological production being particularly important on the shelf regions. The role of mixing/stratification in modulating DIC, and therefore surface pCO2 is shown in a vertical profile at the location of the Ocean Observatories Initiative (OOI) site in the Argentine Basin (42° S, 42° W).

Abstract. We use a novel, high-resolution global climate model (GFDL-ESM2.6) to investigate the influence of warm and cold El Niño/Southern Oscillation (ENSO) events on the physics and biogeochemistry of the California Current System (CalCS). We focus on the effect of ENSO on variations in the O2 concentration and the pH of the coastal waters of the CalCS. An assessment of the CalCS response to six El Niño and seven La Niña events in ESM2.6 reveals significant variations in the response between events. However, these variations overlay a consistent physical and biogeochemical (O2 and pH) response in the composite mean. Focusing on the mean response, our results demonstrate that O2 and pH are affected rather differently in the euphotic zone above ~100 m. The strongest O2 response reaches up to several 100 km offshore, whereas the pH signal occurs only within a ~100 km-wide band along the coast. By splitting the changes in O2 and pH into individual physical and biogeochemical components that are affected by ENSO variability, we found that O2 variability in the surface ocean is primarily driven by changes in surface temperature that affect the O2 solubility. In contrast, surface pH changes are predominantly driven by changes in dissolved inorganic carbon (DIC), which in turn is affected by upwelling, explaining the confined nature of the pH signal close to the coast. Below ~100 m, we find conditions with anomalously low O2 and pH, and by extension also anomalously low aragonite saturation, during La Niña. This result is consistent with findings from previous studies and highlights the stress that the CalCS ecosystem could periodically undergo in addition to impacts due to climate change.

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