What controls the variability of CO$_{2}$ fluxes in Eastern Boundary Upwelling Systems?

Abstract

Eastern Boundary Upwelling Systems (EBUS) are mediated by alongshore, equatorward winds that force cold, corrosive, and nutrient-enriched waters to the surface. These regions are highly productive biologically, compensating for the effect of upwelling on pCO$_{2}$. This leads to a variable mosaic of air-sea CO$_{2}$ fluxes (F$_{CO_{2}}$). Thus, variability in F$_{CO_{2}}$ in EBUS provides a constraint on the magnitude of the ocean carbon sink. This variability also dictates when we might expect the anthropogenic signal of ocean acidification to become emergent in each system. In this study, we diagnose the physical and biological mechanisms that control historical (1920-2015) F$_{CO_{2}}$ in the four major EBUS (California, Humboldt, Canary, and Benguela Currents). We utilize biogeochemical output from the CESM Large Ensemble, a global coupled 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 independent and unique representations of the natural climate system, allowing us to robustly assess variability in F$_{CO_{2}}$. We find that F$_{CO_{2}}$ in the Humboldt and Canary Currents is driven primarily by variability in ocean circulation and biology, rather than by variability in CO$_{2}$ solubility. The California Current is unique, as it exhibits a spatial dipole in its F$_{CO_{2}}$ sensitivity. We find that offshore and poleward coastal areas respond more to changes in CO$_{2}$ solubility, while equatorward coastal regions are most sensitive to variability in ocean circulation. A better understanding of the sensitivity of F$_{CO_{2}}$ in each system might lead to some short-term predictive skill in the ocean-atmosphere carbon cycle. For example, our understanding of the remote forcing and teleconnections associated with ENSO could allow us to anticipate the anomalous air-sea exchange of CO$_{2}$ in the major EBUS.

Date
Location
Portland, Oregon