Historical variability in Atlantic meridional baroclinic transport at 26.5°N from boundary dynamic height observations

The strength of the Atlantic Meridional Overturning Circulation (MOC) at 26.5°N may be inferred from the combination of Florida Straits transport (derived from cable measurements), Ekman transport estimated from wind stress climatologies and mid-ocean geostrophic shear (traditionally obtained from hydrographic sections) with application of mass balance to the section to yield the mid-ocean barotropic flow. The recent Rapid monitoring project has provided time series information for the mid-ocean geostrophic shear since 2004. This work presents methods to assemble a comparable dataset from CTD end stations and boundary mooring temperature and pressure time series to estimate the past variability from 1980 to 2005. Variability in the end station derived transport anomalies suggests that the MOC has fluctuated by more than 10 Sv, encompassing all MOC estimates reported in the literature. Interannual changes in MOC transport are masked by this variability and calculated trends in layer transports are not statistically significant. More extensive datasets of CTD casts and moored temperature records at the western boundary do show significant changes with warming in the thermocline and long-term freshening of the deep waters between the 1980s and 2005. These changes are associated with stronger southward flow in the upper waters and weaker southward flow in the deep waters, and suggest a decrease in the MOC strength of 2–4 Sv. Any such decrease, however, is masked by the scale of variability in layer transports derived from the historical database of CTD end stations.