Recent variability in the heat and carbon inventory of the deep ocean

The bottom limb of the Meridional Overturning Circulation (MOC) plays a significant role in the global carbon budget. Fed by deep convection at high latitudes, it accumulates carbon as it circulates throughout the abyssal ocean on a time scale of 1000s of years before eventually upwelling in the Southern Ocean along the Antarctic Circumpolar Current (ACC). Here we present an assessment of recent variability in the strength of the bottom limb of the MOC through temperature analysis of deep (below 2000m) CTD data collected along repeat hydrographic sections around the globe between 1990 and 2015 by the international World Ocean Circulation Experiment (WOCE) Hydrographic Program and the international Global Ocean Ship Based Hydrographic Investigations Program (GO-SHIP). Since the 1990s the abyssal ocean has warmed significantly around the globe, albeit with large spatial and temporal variability. The strongest warming was observed between the 1990s and 2000s, with a smaller but still significant warming during the 2000 to 2015 period. Over both time periods, the strongest warming was seen near the source regions of abyssal waters in the Southern Ocean, however, less so in the later period. This warming is mostly attributable to a decrease in volume of the cold dense waters formed through deep convection in the Southern Ocean, indicating a slowdown in the ventilation and circulation rates of the deep ocean. We assess the impact of these changes in deep circulation on the global deep ocean heat content and carbon inventory.