EAPS

The southern limb of the global meridional overturning circulation has a critical impact on the climate system by regulating the air-sea balance of carbon dioxide (CO2) and shaping biological production throughout the tropics and subtropics. However, observations of this region have historically been limited in both space and time. Here we present estimates of the seasonal cycle of air-sea CO 2 flux in the Southern Ocean during 2014-2017 computed from measurements collected by new autonomous biogeochemical profiling floats. Significant outgassing of CO2 from the ocean to the atmosphere is observed in the subpolar region during wintertime resulting in a much greater annual net flux than is found in previous observational estimates and most global climate models. Potential drivers of this surprisingly large outgassing signal are discussed and recent work investigating the upwelling of deep waters in the Southern Ocean is used to highlight the role of the three-dimensional overturning circulation. The implications of this finding for climate modeling and for our understanding of the global carbon cycle are considered..