What Can Hydrography Between the New England Slope, Bermuda and Africa Tell us About the Strength of the AMOC Over the Last 90 years?

The Gulf Stream is the only pathway in the subtropical North Atlantic by which warm water flows poleward. This transport of warm water and return of cold water at depth is called the Atlantic Meridional Overturning Circulation (AMOC). The dynamic method is applied to hydrocasts collected since the 1930s to estimate upper‐ocean transport (0–1,000 m) between the U.S. Continental Slope and Bermuda and separately to Africa with focus on the longest directly observable timescale. Calculating transport between the Slope and Bermuda eliminates the Gulf Stream's northern and southern recirculation gyres, while calculations between the Slope and Africa remove all other recirculating geostrophic flow. The net Slope‐Bermuda upper‐ocean transport is estimated to be 41.1 ± 0.4 Sv, decreasing by 2.0 ± 0.8 Sv between 1930 and 2020. The AMOC contribution is 18.4 ± 0.6 Sv, decreasing by 0.4 ± 0.6 Sv between 1930 and 2020. Plain Language Summary: The Gulf Stream is the only pathway by which the ocean transports warm water to high latitudes in the North Atlantic and Nordic Seas where it cools, sinks and flows back south at intermediate depths from the subpolar North Atlantic and at greater depths from the Nordic Seas. This circulation pattern, the Atlantic Meridional Overturning Circulation (AMOC), plays an important role in Earth's climate. But the Gulf Stream is also the wester boundary current to the horizontal circulation driven by the trade winds and westerlies at low and mid‐latitudes. The Gulf Stream also drives large recirculations to its south and north. In this study we use hydrographic data taken since the 1930s to calculate the poleward flow in the Gulf Stream, while canceling out the recirculation and wind‐driven transports to estimate the strength of the north‐flowing portion of the AMOC. There is evidence for a 2.0 Sv Gulf Stream slow‐down between 1930 and 2020. Whether and to what extent this reflects a slowdown of the AMOC or wind‐driven circulation cannot be established with certainty. Our estimate of a 0.4 Sv AMOC decrease is reported with low confidence. Key Points: Direct measurements along the Oleander line (∼37°N) show that the depth of the Atlantic Meridional Overturning Circulation (AMOC) maximum is close to 1,000 mNet northward transport between North America and Bermuda equals 41.1 ± 0.4 Sv of which the AMOC accounts for about 18.4 ± 0.6 SvNet northward transport NW of Bermuda slowed by 2 ± 0.8 Sv between 1930 and 2020 with 0.4 ± 0.6 Sv attributed to an AMOC decline [ABSTRACT FROM AUTHOR]