Tuesday, 16 December 2008

SCRATCHING THE SURFACE

What is happening below the surface?

Our first task was to define the eddies using satellite tracked drifters (see previous blog page). The next step was to answer the questions; how can we characterize an eddie when looking downward at its hydrological properties and: how can we investigate its potential impact on the living realm? Our sampling strategy provided a partial but quasi systematic survey of part of the eddy field. The survey consisted of hydrographic stations (black dots on the SLA map below) along a north – south transect. At each station, temperature, salinity, dissolved oxygen and fluorescence were recorded continuously from surface to 1000 m - the nominal maximum depth chosen for this study - focusing in on the biological signature of the eddies (sections plotted to 250 m). Sea water samples were collected at various depths. Analyses (at the laboratory) of nutrients, total chlorophyll, primary production, particulate organic matter, pigments and absorption will produce valuable information to relate the physics to the biology of the ocean.


For the moment, let’s have a look at what the “physics” say and let’s comment on what the vertical sections tell us. Keep in mind that this transect crossed two cylonic (clockwise) and one anti-cyclonic (anti-clockwise) eddies. Theoretically, a cyclonic eddy is an area of divergence at the surface (basically, due to the earth rotation and the so-called “Coriolis force”), while convergence occurs at the centre of an anticyclone. Surface water missing at the centre of a cyclone has to be replaced from the subsurface layers. This upwelling brings colder water towards the surface, which is clearly shown by the “doming” of the isotherms in the first diagram (see at 150 m depth and 620 km distance). The same signature is also found in the oxygen distribution (water is oxygen depleted relative to the surface at depth) and - interestingly for biology - in the fluorescence distribution. This means that the whole water column (or at least the upper 250 m) upwells toward the surface. This is important in terms of biological productivity. Indeed, deeper water is also richer in nutrients (nutrients are exhausted at the surface due to consumption by the primary production). Upwelling brings nutrients into the upper euphotic zone (i.e. where light is available), so primary production can occur and the whole trophic chain can potentially take benefit from the fertilization! Note that the fluorescence maximum values are higher and shallower in the centre of the two cyclones. Contrary to the above scenario, the convergence at the centre of an anticyclone pushes down the lower layers (downwelling). Again, the temperature, oxygen - and even fluorescence - vertical distribution perfectly illustrate this downward movement (middle of the transect at 400 km). In terms of its biological signature, the anticyclone will tend to suppress productivity, resulting in fewer catches when trawling within a cyclone! Last but not least – at least for an oceanographer – the salinity distribution also illustrates these up and down vertical displacements. Essentially, downwelling in the centre of the anticyclone vertically expands the intermediate layer which is characterized in this part of the Mozambique Channel by a salinity subsurface minimum. In contrast, cyclonic eddy upwelling “compresses” the intermediate layer, which expels the low salinity water around the eddy. Both processes result in a beautiful “bubble” of low salinity water, right in the middle of the anticyclone!written by: Jean-Francois Ternon and Tammy Morris


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