Physical oceanography has come a long way over the last 50 years. Starting in the 1970s we’ve deployed moorings with current meters across the world ocean. Thanks to these efforts we have far better ideas about mean flows, time scales of variability, and eddy activity in the ocean. Similarly, Lagrangian floats and drifters have mapped out pathways of fluid motion. They have given us much information about meso-scale activity, dispersive processes, and they’ve shown us the enormous role bathymetry can play in steering flow at depth. Numerous floats have been trapped in coherent vortices for varying periods of time, indicating that these can persist for long periods of time – even measured in years. In more recent years we’ve come to rely upon the global Argo float array to give us valuable information on the hydrographic state of the ocean. But just as with moored instruments and floats, Argo does not have the capability resolve currents and fluxes and how they vary.
The story is different at the surface. There, satellites routinely map out a wide range of surface properties, from surface temperature, salinity and ocean color to waves, tides, and sea surface height. From the latter we can make strong inferences about ocean circulation at the surface. The challenge has always been how to extend that mapping skill to below the surface. The short answer is we can’t. No matter how well we resolve the surface, we cannot extrapolate to depth. But we can take vertical slices through the ocean, in other words map out properties along a line and as a function of depth. These include temperature and currents. This is exactly what a few of us do on a regular basis: measure currents and transports and their variability along select lines. For example, a single ship – the Oleander, operating on a weekly schedule between New Jersey and Bermuda – monitors all warm water flowing north in the Atlantic.
Thanks to this operation, which started over 30 years ago, we have a good sense of how much the Gulf Stream and the Atlantic meridional overturning circulation (AMOC) vary over time in this region. Thanks to two other ships, the Nuka Arctica and the Norröna, we also got a pretty good handle on the strength of these flows farther north in the North Atlantic. These operations also discovered previously unrecognized currents both at the surface and at depth.
Seeing what we have learned from these merchant marine operations, it continues to baffle me why there is so little interest in exploring this approach to ocean observation. It’s true that we can’t map out subsurface currents, but high spatial resolution of currents along select routes impose severe constraints on allowable transport pathways. But more importantly, we can measure fluxes quite accurately. Today we are limited to scanning the top kilometer of the ocean, but we have the know-how and skills to develop the tools that would enable us to scan deep ocean currents and transports at high accuracy. I discuss these in my January 15, 2024 blog. There’s so much we could be doing.