Mean, Seasonal Cycle, Trends, and Storage of the Southern Ocean carbon cycle in the RECCAP2 assessment (1985-2018)

Abstract

The Southern Ocean has long been known to be an important region for ocean CO2 uptake, and one which is especially sensitive to changes in the overlying climate. Here we assess the Southern Ocean CO2 uptake (1985–2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2 (RECCAP2). These include global ocean biogeochemical models (GOBMs), surface ocean pCO2-products, data-assimilated models, and interior ocean biogeochemical observations. Over this period the Southern Ocean acted as a sink for CO2, with magnitudes which are roughly half of those reported by RECCAP1 for the same region and timeframe. Close agreement is found between GOBMs and pCO2-products, partly due to some compensation of seasonal and regional differences. Seasonal analyses revealed agreement in driving processes in winter (with uncertainty in the magnitude of outgassing), whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the balance of non-thermal processes of biology and mixing/circulation. The data sets emphasize strong latitudinal variations in the mean and seasonality of the CO2 flux and asymmetries in the mean and amplitude of the CO2 flux between Atlantic, Pacific and Indian sectors. The present-day net uptake is to first order a response to rising atmospheric CO2. This drives large amounts of anthropogenic CO2 (Cant) into the ocean, thereby overcompensating the loss of natural CO2 to the atmosphere driven by the changing climate. The GOBMs show, however, a 20% spread and an overall underestimate of Cant storage in the ocean interior. An apparent knowledge gap is the increase of the sink since 2000, with pCO2-products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs. This is despite nearly identical pCO2 trends in GOBMs and pCO2-products when both products are compared only at the locations where pCO2 was measured.