Recent evidence for a strengthening CO₂ sink in the Southern Ocean from carbonate system measurements in the Drake Passage (2002-2015)

We present a 13-year (2002-2015) semimonthly time series of the partial pressure of CO₂ in surface water (pCO_2surf) and other carbonate system parameters from the Drake Passage. This record shows a clear increase in the magnitude of the sea-air pCO₂ gradient, indicating strengthening of the CO₂ sink in agreement with recent large-scale analyses of the world oceans. The rate of increase in pCO_2surf north of the Antarctic Polar Front (APF) is similar to the atmospheric pCO₂ (pCO_2atm) trend, whereas the pCO_2surf increase south of the APF is slower than the pCO_2atm trend. The high-frequency surface observations indicate that an absence of a winter increase in total CO₂ (TCO₂) and cooling summer sea surface temperatures are largely responsible for increasing CO₂ uptake south of the APF. Muted winter trends in surface TCO₂ also provide temporary stability to the carbonate system that is already close to undersaturation with respect to aragonite.