The Variable Southern Ocean Carbon Sink

Abstract

The CO₂ uptake by the Southern Ocean (<35°S) varies substantially on all timescales and is a major determinant of the variations of the global ocean carbon sink. Particularly strong are the decadal changes characterized by a weakening period of the Southern Ocean carbon sink in the 1990s and a rebound after 2000. The weakening in the 1990s resulted primarily from a southward shift of the westerlies that enhanced the upwelling and outgassing of respired (i.e., natural) CO₂. The concurrent reduction in the storage rate of anthropogenic CO₂ in the mode and intermediate waters south of 35°S suggests that this shift also decreased the uptake of anthropogenic CO₂. The rebound and the subsequent strong, decade-long reinvigoration of the carbon sink appear to have been driven by cooling in the Pacific Ocean, enhanced stratification in the Atlantic and Indian Ocean sectors, and a reduced overturning. Current-generation ocean models generally do not reproduce these variations and are poorly skilled at making decadal predictions in this region.

Keywords: Southern Ocean; anthropogenic carbon; decadal variability; natural carbon; ocean carbon sink.