The role of physical processes in the Southern Ocean CO2 sink

The absorption rate of CO₂ in the Southern Ocean significantly impacts predicted future global atmospheric CO₂ levels. Temporal and spatial variations of the surface water partial pressure of CO₂ (pCO₂) determine the amount of CO₂ that can be absorbed by the Southern Ocean. In CMIP5 Earth System Models (ESMs), the climate models have been coupled for the first time to ocean carbon models. These models provide sufficient space and time coverage to allow us to examine (1) the long-term trends of the surface water pCO₂ and its drivers (sea surface temperature, salinity, Dissolved Inorganic Carbon (DIC), and alkalinity), and (2) the relations among westerly winds, total upwelled DIC, and surface water pCO₂.

Our preliminary results show that most ESMs reproduce the observed annual mean pCO₂ values averaged over the Drake Passage region, however, these models show significantly different long-term trends from the observations. In the Pacific-Indian sectors (295E:0) of the Southern Ocean, only two ESMs reproduce the weakly decreasing sink from 1995 to 2008 found in the observational analysis. In the Atlantic sector (65W:0) of the Southern Ocean, none of the existing nine ESMs show little or no trend from 2001 to 2008 found in the observational analysis. However, our results show that the model trends in the Atlantic sector vary significantly with the selection latitudes and years. Moreover, we examined the relation between surface water pCO₂ and the total amount of DIC upwelled from deep ocean south of the Polar Front (PF). The amount of DIC was estimated by integrating DIC over the outcrop surface area of the corresponding water mass. Specifically, we examined the correlations among these variables over the upwelling region south of the PF and the subduction region north of the PF,where the Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) subduct DIC to the deeper ocean. These results emphasize that a realistic representation of physical circulation inthe Southern Ocean is critical to the prediction of future atmospheric CO₂ levels in CMIP5 ESMs.