Starting in Fall 1992 we have been operating an acoustic Doppler current profiler (ADCP) on the freighter Oleander, a container vessel that operates on a weekly schedule between Hamilton, Bermuda and Port Elisabeth, NJ. The key objective of this project has been to monitor the strength of the Gulf Stream, how it varies over time including whether there is any evidence of a long-term trend. This matters because the Gulf Stream is the only place where water is moving poleward, everywhere else across the Atlantic and at depth water is moving equatorward. What makes the Oleander route so effective it that besides spanning the transport of warm water by the Gulf Stream, it also traverses the westward flow of cold water the Slope Sea and shelf waters to its north, and the westward flow of Sargasso Sea water between the Gulf Stream and Bermuda.
Scanning ocean currents from a vessel in regular traffic allows us to profile the horizontal and vertical structure of currents in detail – repeatedly This is the only tool we oceanographers have for mapping the spatial structure of ocean currents. So, in addition to resolving how the Gulf Stream varies over time, we can examine its structural stability including details like variations in peak velocity, width, position, and lateral shear. With weekly sampling at best, we obviously can’t resolve short-term variability, but through appropriate averaging we can resolve the seasonal cycle and variations from year-to-year quite well.
Early on we introduced an operational definition of the Gulf Stream that takes into account that it is constantly meandering. The width of the current is defined by where the surface velocity in the downstream direction first changes sign to either side of the maximum. The velocity maximum defines where the current is at that time, and the zero crossings define the edge of the stream. Transport is thus the integral of downstream velocity from side-to-side. Since the ADCP that was first used had limited vertical reach (~150-300 m) we decided to focus on the horizontal: currents at 50 m depth. Thus, transport is layer transport at 50 m depth (with dimensions m2s-1). This might seem like a serious limitation, but bear in mind that working in the horizontal opened a whole new dimension for study for the first time. Not just of the Gulf Stream, but of other phenomena such as the ubiquitous Gulf Stream rings, the Slope Jet, and patterns of eddy variability across the entire domain. The Oleander data have been the basis for numerous studies. Many of these are summarized or referenced here: https://tos.org/oceanography/article/oleander-is-more-than-a-flower-twenty-five-years-of-oceanography-aboard-a-m
Perhaps the question most people ask is whether there is any evidence of a change in Gulf Stream transport over the last 25 years. The answer is no. We do see significant variations in transport from year-to-year on the order of a few percent. The cause of these has yet to be determined, but most likely they have a regional extent reflecting variations in wind patterns. A nice finding from the Oleander project was that layer transport between the continental slope and Bermuda goes to zero at about 1000 m depth. This level of zero transport corresponds to the maximum in the overturning of the AMOC. This depth is also below the upper ocean wind-driven circulation. Knowing the depth of this level of zero transport allowed us to use the hydrographic record that had been built up over the years to extend our estimate of AMOC and wind-driven transport back another 70+ years to the 1930s-1950s. We found at modest confidence evidence for a weak slow-down (2.0±0.8 Sv) of the Gulf Stream, of which 0.4±0.6 Sv we attribute to the AMOC. These are probably the best estimates to date of Gulf Stream and AMOC trends over this period of time. The reason is that they are based on the direct measurement of velocity, the definition of a level of zero transport, and a large pool of high-quality hydrographic data. See https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL099173