The Role of Source Water Carbonate Chemistry in Setting Outgassing in the Southern Ocean in CMIP6 Models

Abstract

The Southern Ocean is a critical region of carbon dioxide exchange between the atmosphere and the ocean's interior. With historically sparse observations, much of our understanding of the Southern Ocean and its role in the global carbon budget comes from climate models, which continue to show great variation and uncertainty in physical and biogeochemical parameters. The limited understanding we glean from climate models makes future projections of the Southern Ocean's role in taking up anthropogenic carbon and mitigating warming uncertain. The purpose of this project is to evaluate the representation of the Southern Ocean's carbon cycle across a suite of models from the sixth generation of the Coupled Model Intercomparison Project (CMIP6), specifically analyzing the carbonate chemistry and outgas potential of the source waters in the region where upwelled deep waters release carbon from the deep ocean. We determine outgas potential by calculating potential pCO2--the partial pressure of carbon dioxide a water parcel would have if brought adiabatically to the surface. There is a consistent spatial relationship between subsurface potential pCO2 and surface CO2 fluxes across the models: CO2 outgassing begins where high potential pCO2 upwells to the mixed layer, and continues equatorward as potential pCO2 declines. Moreover, the magnitude of potential pCO2 below the winter mixed layer is positively correlated with outgas intensity, yet has no relationship with total outgassing. Thus, we hypothesize that models respond differently to their subsurface potential pCO2, on account of intermodel variability in transport and surface processes that affect CO2 fluxes.