Thursday, 11 December 2008

CHASING EDDIES IN THE MOÇAMBIQUE CHANNEL

As mentioned previously, the aim of Leg 4 of the ASCLME / EAF Nansen research cruise is to survey eddies in the Moçambique Channel. In order to do so, their location needs to be identified first, and near-real time satellite observations help in providing an idea of what is going on in this very chaotic, or even turbulent part of the ocean.

The problem with near-real time satellite observations is just that, they are near-real time. Sea surface height measurements from altimeters are processed by AVISO (www.aviso.oceanobs.com), and in order to provide a map of the currents at any particular day (see figure below), data from a host of satellites needs to be incorporated and interpolated. This takes time, generally the data is made available with a 7 day delay. So, in effect, we have to guess what the eddy is going to do, based on 7 day old information, adjust our sampling strategy accordingly, and hope not much has changed in the last week!!

To help us make a more informed guess as to the whereabouts of the eddies, we deployed surface drifters during the first (north-south) transect of our cruise. Surface drifters are essentially buoys attached to a 5 m long sock (for more information go to www.oceanafrica.com/drogues/drogues.html). These drogues drift in the ocean following the currents and transmit their longitude and latitude positions to satellites, which are relayed to our support team in Cape Town and then sent to us. Below right is an image of the near-real time geostophic current velocities derived from sea surface height measurements from altimetry with the successive positions of the surface drifters overlaid. The numbers represent the I.D. of the drifters and the positions where they were thrown overboard. We have lovingly given them nicknames such as “Bob” or “Amper Vergeeten” (in this case as the name states).



Comparing their drift patterns to the week old geostrophic currents (see above), one can see that in some cases these follow the current patter described by the delayed satellite observations, but in many cases they do not. This highlights the difficulty in surveying Moçambique Channel eddies accurately given the tools at our disposal! On one particular day we arrived on station expecting westward currents, but the ship board current meters were showing eastward currents!!

The drifter tracks show that the current system itself is indeed very chaotic and turbulent, with drifter tracks criss-crossing, over-lapping and splitting from each other. Modern thinking tends toward describing the flow dynamics in the Moçambique Channel as “eddy-driven”, and our drifters very neatly show this to be the case.


Written by: Bjorn Backeberg and Tammy Morris




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