The Atlantic meridional overturning circulation at 35N from deep moorings, floats, and satellite altimeter

Abstract

From 2004 to 2014, the Line W moorings measured a 0.7 Sv/yr slowing of the Deep Western Boundary Current (DWBC) offshore of Cape Cod, Massachusetts. Here, we combine these deep mooring observations with float and satellite altimeter data and find that this DWBC change corresponded to a slowing of the cross-basin Atlantic Meridional Overturning Circulation (AMOC) of about 0.3 Sv/yr. Our AMOC transport time series corresponds well with the ECCO state estimate, particularly when the Line W mooring data influences our reconstruction of upper ocean volume fluxes. We compare our 35N time series with a similar time series at 41N as well as with the 26N RAPID AMOC, and find AMOC declines across datasets during this time period. The relative magnitudes of these declines are consistent with interdecadal variability originating in the Labrador Sea. We find that though our integrated overturning estimate agrees well with ECCO, the structure of the deep flow differs substantially. While we cannot rule out a decreasing AMOC trend during the 20th century, we find that natural variability is too large to detect a net AMOC decrease in direct observations or the ECCO ocean model since 2004.