At 15°S a vast tongue of He-3 rich water extends over 2000 km to the west from the East Pacific Rise. Known as the He-3 plume, it has fascinated oceanographers ever since it was first reported (Lupton and Craig, 1981). The isotopic composition (rich in He-3 relative to He-4) indicated that it must be coming from the earth’s interior, presumably via hydrothermal vents near 15°S on the East Pacific Rise (EPR). The authors suggested that the plume’s orientation reflected a westward mean flow at the 2500 m mean depth of the plume. Other writers have suggested that the plume reflected a dynamical response to the injection of warm water from the vent site. Prof. Steve Riser at the University of Washington asked me if I might be interested in teaming up with him to take a closer look at the plume with a combined hydrographic and RAFOS float study - I said yes!
The plan was to conduct a systematic hydrographic survey of the He-3 plume with four north-south sections. Each hydrocast profiled temperature, salinity, oxygen, He-3 and various nutrients. These data were used in a major study by Hautala and Riser, 1993. Their paper is informative not only about the field program, but also about the beta-spiral literature and how they approached the problem. It is an excellent piece of work; I recommend it highly.
But this blog post is about the RAFOS float study. We deployed seven sound sources to provide the acoustic navigation, and along each of the sections RAFOS floats were deployed for total of 48 floats. All but a few were programmed to drift for 18 months. Most floats surfaced as planned, but, sadly, many failed to telemeter their data: acoustic arrival times, pressure and temperature. And several didn’t report at all – all we got was their surface position. Having enjoyed enormous success with RAFOS floats in the Gulf Stream, these were abysmal returns. I didn’t know what to do, and basically walked away from it all. Fortunately, Susan and Steve found the data useful in their beta-spiral study.
Last year (August 11, 2024) I wrote about conducting a study of mean flow in a quiet ocean. Then earlier this year I remembered the Helios float work which was also in a quiet part of the Pacific. Fortunately, Steve and Susan had a copy of all float data. The figure shows the mean velocities of the floats with the vectors anchored to their deployment sites. As Hautala and Riser noted, the vectors show a distinct pattern of topographically constrained flow north along the EPR and principally zonal flows west of the ridge. This suggests that larger/longer time scale flow patterns dominate the vectors and not the unresolved mesoscale eddy field.
The mean velocities are small (black line = 5 mm/s). The average and standard deviation of all 39 vectors = <-0.003 ± 0.0063, 0.0002 ± 0.0030 > m/s., resp. These are small numbers, but, crucially, they are measured, not inferred. The standard errors (0.001, 0.0005) indicate an ensemble average of 3±1 mm/s to the west. This suggests that by averaging a population of displacement vectors we can determine mean flows in the abyssal ocean to the mm/s or better accuracy.
The figure has a lot of information. Notice how floats over the EPR move north along, not across the ridge whereas floats away from the ridge exhibit zonal motion. East of the EPR two floats suggest a southward drift there. Floats peel off from the ridge to the west with floats north of 15°S showing a clear drift to the west. Also floats at ~22°S show a distinct westward drift. The few floats between 15 and 20°S suggest a region of weak zonal flow that may reflect the dynamics of the He-3 plume. I say this in light of the Hautala-Riser analysis of the float vectors. But there is another message here.
The ensemble of displacement vectors is like conducting a geochemical tracer study with the advantage that the tracer has an accurate clock built-in. With each float serving as a giant ‘radioactive molecule’, imagine the tracer studies clusters of drifters (no acoustics) would enable us to conduct. The ensemble drift gives us mean flow while the spread of the vectors informs us about eddy activity. More on this in the next post.
Lupton, J. and H. Craig, 1981. A major He-3 source at 15°S on the East Pacific Rise. Science, 214,13-18.
Hautala, S. and S. Riser, 1993. A non-conservative beta-spiral determination of the deep circulation in the eastern South Pacific. Journal of Physical Oceanography, 23, 1975-2000.