Tuesday, 25 November 2008

Interesting results!

Having now surveyed the entire Mascarene Plateau we have a better understanding of its influence on the surrounding ocean environment. What have we learnt..

It appears that the SEC is displaced southwards from its mean position of 10°-16°S by the obstruction caused by the shallow bathymetry of the Mascarene Plateau. Upstream of this Plateau the SEC exists as a broad (~650 km in width) shallow (~1000 m in depth) current with speeds averaging 0.30 ms-1. On approaching the Mascarene Plateau the SEC splits into separate cores centered near 18°, 12° and 8°S. Once passed the Plateau it seems likely that these cores continue westwards towards the Madagascar coast at 50°E and there form the North East and South East Madagascar Currents.

Deep channels separating individual banks (see past blog) act as choke points funneling the flow of the SEC from east to west.

Atmospheric circulation as a result of the SE trade wind field results in a gradual shoaling of water masses between 15°-5°S. The thermocline depth changed from 250 m close to Mauritius to just under 30 m on the Seychelles Bank. Since nutrients increase with depth, it would be expected that nutrient levels would gradually increase with distance north thus influencing the biological productivity of the surrounding region. This probably explains improved fish catches as we moved closer to the Seychelles!

Finally, the presence of an eastward flow between 6° - 2°S (blue box) can be related to the position of the eastward flowing South Equatorial Counter Current. This current lies north of the SEC and flows in the opposite direction. This proved to be extremely interesting with salty warm water being swept into the Seychelles region from as far away as the Arabian sea!
Image captions:
Top Image: General circulation over the Mascarene Plateau (The 1000 m isobaths is shown in red). Note the influence of the deeper channels speeding up the surface flow downstream of the plateau (highlighted by the green boxes)– exactly what we had hoped to find! The blue box shows the prevailing eastward flow associated with the South Equatorial Counter Current.
Bottom Inage: After 55 days this is what the drifter data looks like! See how they all race westwards through the main gaps with the South Equatorial Current. The really interesting pattern is actually just south of the seychelles where two drifters seem to be caught up in the eastward flowing South Equatorial Counter Current.
Text by Isabelle Ansorge

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