Presenter: H. Thomas Rossby (University of Rhode Island)

Description:
Starting in late 1992 upper ocean currents in the Gulf Stream and adjacent waters, the Slope and Sargasso Seas to its north and south respectively have been measured on a regular basis by the commercial freighter Oleander on its weekly transects between New Jersey and Bermuda. In several earlier publications it had been reported that apart from interannual variations of a few percent Gulf Stream transport has been quite stable with no hint of either a decrease or increase over the 25 years the program has been in effect. A stable Gulf Stream suggests that the upper limb of the Atlantic meridional overturning circulation (AMOC) must also be stable since it is embedded in the Gulf Stream, the only place in the North Atlantic where warm water flows north. Earlier hydrographic studies suggest this stability applies on longer time scales as well.  Here we update and expand on those efforts by examining the Fofonoff potential energy anomaly (PEA), an integrated form of dynamic height between New Jersey and Bermuda, thereby removing all recirculating waters which are known to be restricted to north of Bermuda. We find that the difference in PEA between Bermuda and the Slope Sea has been quite stable over at least the last 70 years and accounts for the upper limb AMOC transport and Sverdrup gyre flow quite well. What is more interesting is that the upper ocean has warmed well over 1°C during this time. Assuming a 1°C warming of a net New Jersey-Bermuda 20 Sv transport in the top 300 m only results in an O(80) TW heat transport or roughly 6% increase in the total at this latitude. We conclude that the Gulf Stream, which includes the embedded AMOC, is not only stable on decadal to last century time scales, it is also advecting more heat north, not less as would be the case for a weakening Gulf Stream.