| Popis: |
The atolls of the Tuamotu archipelago (South Tropical Central Pacific) are located in the central part of the great anticyclonic gyre where air subsidence leads to an arid climate with high evaporation. The bi-modal system of trade winds created by the existence of dual subtropical high pressure centres (Easter Island and Kermadec Island) drives a global westward drift, the South Equatorial Current, with a speed of 10 cm.s(-1). The zone where the two south trade winds converge (SPCZ) has a strong seasonal variability in position and intensity and controls the eastward counter circulation. During abnormal ENSO events, the counter-circulation is dominant between the equator and 15 degrees S, and the elevated heat content favours active cyclogenesis in French Polynesia. The average thermohaline field is characteristic of a two-layered system : (i) a warm surface mixing layer (0-150 m), with a high salinity core, (> 36.3), South Tropical Water (STW); and (ii) Antarctic Intermediate Water (AIW) with a salinity minimum (34.5). They are separated by a permanent pycnocline. In the central convergent part of the gyre (12 degrees S-30 degrees S), the surface mixing layer is depleted in nutrients; they occur below 200 m with strong vertical gradients in the vicinity of the nutrient-rich AIW. The strong oligotrophy of the euphotic layer is preserved even close to atolls, due to the great depth of the pycnocline-nutricline barriers, which impedes any diapycnal exchange from AIW toward surface layers. Oceanic waters surrounding the Tuamotu atolls constitute a buffered system with very low interannual variability and extreme oligotrophy and clarity. |
