In the previous blog I discussed the frustrations we had trying to realize the ‘one-shot’ float concept, a subsurface instrument that would give us only its end point after a long period of drifting at depth. A single vector may not be very illuminating, but a large cluster of vectors would tell us about mean drift and dispersion, both simple statistics of great interest. Clusters deployed from merchant marine vessels in regular traffic would keep operating costs to a minimum. Repeat deployment of clusters would give us a window into very low-frequency (sub-annual) oceanic variability, something we know very little about. We often imagine the deep ocean circulation as slow and not very energetic, but we also know that the kinetic energy spectrum is red meaning that there is variability at ever lower frequencies.
In the ten years since those earlier efforts a host of new technologies gave new life to the deep-drifter concept. In this note I describe our approach: a glass pipe float drifts wherever the currents take it, and that after a prescribed time surfaces and reports its position. It measures pressure and temperature along the way (easy to do), but it is not tracked along the way. We used it to tag a Mediterranean salt lens (meddy) so we know where to find it when the drifter surfaces at a later time.
Following the discovery of the meddy off the Bahamas and the subsequent findings of meddies in the eastern Atlantic, several of us (Larry Armi, myself, Phil Richardson, Neil Oakey, Barry Ruddick) decided on a program to study one in detail. My contribution was to use Pegasus velocity profiler to resolve the velocity and vorticity structure of a lens. Armi and Richardson would use SOFAR floats tracked by autonomous listening stations to determine the drift and location of the lens a year later so we could study its change over time. That worked very well. In fact, a float orbited around the center of the meddy for two years! In parallel with the SOFAR float effort, I proposed using deep drifters, essentially RAFOS floats without the acoustics to do the same. We built six, 3 to surface after 3 months and 3 after six months; 2 in the first group and all in the second group surfaced as planned. From the stable temperature record we could tell that they remained in the meddy the entire time although one in the latter group showed fluctuations in temperature indicating that it was at the edge of the meddy. Although the longest mission was only 6 months, but the point was made that they all remained trapped in the lens. In future studies one could use deep drifters to tag eddies and lenses for repeat visits to examine how they age. For example, we know that meddy-like vortices in the South Pacific decade far more slowly than the meddies of the eastern Atlantic, why is this? The point is that the deep drifter is a simple tool to tag lenses of water so that they can be revisited at a later time. But there are so many questions that can be addressed with this kind of tool. Glass pipes are extremely tough and stable allowing them to be deployed for multi-year missions at any depth.
Example 1. Clusters of deep drifters can be used as ‘geochemical tracers’ like freons or radioactive decay species, but without the constraints of available species. One could target a region of interest precisely and with large clusters get useful statistics of mean flow and spread. An example of this might be the 3He-plume in the South Pacific.
Example 2: Does roughness of the bottom act to suppress mean flow? There are broad expanses especially in the North Pacific where the bottom is deep and smooth whereas much of the South Pacific pockmarked with submarine volcanoes. How do the dispersion characteristics differ between these two regions? Does a rough bottom impose a drag on the overlying water column that you wouldn’t see over a smooth flat bottom. This could be explored at very modest cost with clusters of deep drifters.
Example 3: Trapping over seamounts. As indicated in the previous example, we know changes in bathymetry impose strong constraints on the overlying water column. We might deploy floats over seamounts to determine how effective trapping might be – for example as a function of height above the mount. Of course, the effectiveness of trapping will not only depend upon the local dynamics but also on the strength of the larger scale flow around the seamount.
Example 4: Meridional transport. The AMOC has its maximum close to 1000 m depth, we know this from direct observation between the US and Bermuda, and we think this should be true between Bermuda and Africa as well. But now we have a tool with which to address this with considerable accuracy. One could design a program to deploy drifters along a trans-oceanic section for 1-year drifts, say, one for every degree of longitude or roughly 50 floats per deployment. Maintaining a program like this annually would be a cost-effective approach to address questions about variability on annual to decadal timescales. If deemed important one could at added cost use RAFOS to resolve their trajectories. One can imagine any number of studies of this type to learn more about the long variability of circulation patterns at various depths. This knowledge can be used to better understand and quantify what drives the deep ocean – it is not at rest!