Abstract

Today, we tend to think of the Meridional Overturning Circulation (MOC) of the ocean as a layered circulation with multiple surface sources of dense water that sinks and flows along density surfaces (isopycnals) until it approaches a boundary, where it is mixed vertically and transits to a different level of the overturning. In this context, oceanographers like to talk about the existence of an “ocean mixing problem”, in which two different schools of thought propose the dominance of either mixing along isopycnals by mesoscale eddies or mixing across isopycnals by breaking internal gravity waves in closing the MOC by returning deep water to the upper ocean. These two models result in similar-looking oceans (in terms of their temperature and salinity distributions) but exhibit very different energetics and sensitivities to changing climatic forcing. Further, either of the two generally implies that water parcels have long journeys and residence times in the deep ocean, a result that shapes our view of how the ocean regulates the Earth’s climate and biogeochemical cycles over climatic time scales.

In this talk, I will discuss a collection of circumstantial evidence from the Southern Ocean to argue for an alternative zeroth-order description of the MOC . This new model proposes that the two classical paradigms of ocean mixing are strongly coupled and cannot be considered in isolation. It suggests that regions where mesoscale eddies are dissipated (in particular, the Antarctic Circumpolar Current) may host MOC short circuits, or ‘hot spots’ of rapid flow along and across density surfaces where water parcels can short-circuit their circulation through the deep global ocean. Motivated by this and other uncomfortable evidence, I will conclude by outlining a BAS -partnered NERC consortium project that seeks to test and redefine the present paradigm of Southern Ocean mixing.