Sack Lunch Seminar (SLS)

Abstract: In 1974, newly available satellite observations unveiled the presence of a giant ice-free area, or polynya, within the Antarctic ice pack of the Weddell Sea, which persisted during the two following winters. Subsequent research showed that deep convective overturning had opened a conduit between the surface and the abyssal ocean, and had maintained the polynya through the massive release of heat from the deep sea. Since 1976, the presence of a fresh surface layer has prevented the recurrence of deep convection there. Observations show that surface freshening of the southern polar ocean since the 1950s has considerably enhanced the salinity stratification. Meanwhile, among the present generation of global climate models, deep convection is common in the Southern Ocean under pre-industrial conditions, but weakens and ceases under a climate change scenario owing to surface freshening. Thus, we hypothesize that Southern Ocean deep convection may have been more active in the past, and is being suppressed by anthropogenic climate change. A decline of open-ocean convection would reduce the production rate of Antarctic Bottom Waters, with important implications for ocean heat and carbon storage. We also show how variability and change of the Southern Ocean deep convection influences ocean carbon storage on centennial time scales in a climate model, and explore the implications for uncertainty in IPCC projections of future ocean carbon storage.