With the advent of the ALACE (Autonomous LAgrangian Circulation Explorer) float and its role as a non-acoustic RAFOS float, came the realization that it could also be used to profile water column temperature during its ascent to the surface - what became the PALACE or Profiling ALACE float. This development led to the genesis of the enormously successful Argo program, an international cooperative to monitor the state of the upper ocean. i.e., the temperature and salinity fields in all ocean basins. It’s kind of curious how what started as an effort to scope out the velocity field at depth evolved into a very powerful hydrographic tool. We are, however, still poorly equipped with respect to sampling the velocity field.
We profile upper ocean currents on a systematic basis with acoustic Doppler current profilers (ADCP) along select vessel routes. Thanks to which we can obtain detailed information about eddy activity and accurate estimates of mass transport. Coupled with temperature and salinity these sections can give us transports of heat and salt. For example, Chafik and Rossby (2019) took velocity data from two ADCP-equipped MM-vessels to estimate fluxes and their divergences in the subpolar gyre. At present we are largely limited to the top km of the ocean. The widely used 75-kHz ADCP reaches to 600-700 m depths. The less widely used 38-kHz ADCP can reach to 1200-1500 m depths (actual depths dependent upon backscatter density).
We need to reach far deeper. At present we can’t even profile density(!), limited as we are to XBTs to 900 m. Thus, a pressing challenge is to develop T/S probes with 2000 m reach (see blog-post April 22, 2024). These would be an attractive addition to the Argo community since they can be deployed according to prearranged routes and schedules. I have zero doubt interest in the MM would be far greater had we this capability today.
Scanning currents is the other major challenge. In one important way we do currents better than hydrography: the ADCPs give us high resolution in the horizontal. But we need to reach below the main thermocline if we ever want to get a handle on the circulation of the deep ocean. We can’t use altimetry and sparse density (Argo profiles) to resolve the deep pressure field due to course resolution of the latter.
We know topography plays a powerful if not dominant role in shaping currents at depth – think flows through passages, around sea mounts, or along the margins of ocean basins. Conversely, mean flow over abyssal plains is probably very weak, perhaps so weak that water properties there may be maintained through stirring and mixing processes, rather than through advection. We could address these questions if we could scan ocean currents to greater depths. We don’t know that we can’t do this (see my post June 29, 2024), we just haven’t made it a pressing issue.
Suppose the ocean instrumentation community could be invited to address these needs. Years ago, ONR challenged industry to come up with a modern solution to the mechanical bathythermograph. This led to the successful development of the XBT, still in widespread use 60 years later! If the astronomical community can develop the Webb telescope to explore the cosmos, if the physics community can build giant accelerators to dissect the infinitesimal, then surely we oceanographers can team up to develop the resources to scope out the deep ocean!?
Chafik, L. and T. Rossby (2019). Volume, heat, and freshwater divergences in the subpolar North Atlantic suggest the Nordic Seas as key to the state of the meridional overturning circulation. Geophys. Res. Lett.,46. https://doi.org/10.1029/2019GL082110