What Controls the Variability of CO2 fluxes in Eastern Boundary Upwelling Systems?
Riley Brady, University of Colorado
Eastern boundary upwelling systems (EBUS) are governed by alongshore, equator-ward winds that force cold, corrosive and nutrient-enriched waters to the surface. These regions are biologically productive, compensating for the effect of upwelling of carbon-rich waters on pCO2. This leads to variable air-sea CO2 fluxes with some of the highest flux density globally. In this study, we diagnose the physical and biological mechanisms that control historical (1920-2015) CO2 fluxes in the four major EBUS: the California (CalCS), Humboldt (HumCS), Canary (CanCS) and Benguela (BenCS) currents. We utilize biogeochemical output from the CESM Large Ensemble, a global climate model ensemble that is forced under historical and RCP8.5 radiative forcing. Differences between simulations can be attributed entirely to internal climate variability, as simulations are generated by introducing round-off perturbations to the initial atmospheric temperature. This experimental setup provides us with 34 unique representations of the natural climate system, allowing us to robustly assess variability in CO2 fluxes. We find that anomalous CO2 flux in the CalCS and CanCS is most associated with oscillations in subtropical gyres: the North Pacific Gyre Oscillation (NPGO) and the North Atlantic Oscillation (NAO), respectively. The CalCS (CanCS) has anomalous uptake (outgassing) of carbon during the positive phase of the NPGO (NAO). The HumCS responds mainly to El Nino Southern Oscillation (ENSO), with anomalous uptake of CO2 during an El Nino event. Variations in dissolved inorganic carbon (DIC) and sea surface temperatures (SST) are the major contributors to anomalous fluxes and are generally driven by changes to upwelling, mixed-layer depth and biology. A better understanding of the sensitivity of CO2 fluxes in EBUS to internal climate variability might lead to some short-term predictive skill in the ocean-atmosphere carbon cycle. Skillful prediction would be particularly useful in managing the onset of ocean acidification in systems that have a naturally low pH and carbonate ion concentration.
Abstract Author(s): Riley X. Brady, Nicole S. Lovenduski, Michael A. Alexander, Michael Jacox, Nicolas Gruber