The Variable Southern Ocean Carbon Sink

The CO2 uptake by the Southern Ocean (< 35 degrees 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 degrees 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.