Is the enhanced primary production on the Mascarene Plateau caused by a mini-monsoon? A Satellite perspective

摘要

The Mascarene Plateau of the southwest Indian Ocean is studied for its primary production. The study is also aimed at demonstrating the feasibility of using remotely sensed satellite observation to characterise the chlorophyll 'a' distribution around the plateau and to depict any seasonal variation. The influence of other oceanographic parameters on primary production, like sea surface temperature and sea surface height anomaly are discussed. The sensors used are respectively, SeaWiFS, ATSR 2, and Topex/Poseidon during the period 1998-2000. The results show a seasonal variation with enhanced primary production occurring between the months of May and September. Although the Southern Indian Ocean is mainly oligotrophic, enhanced chlorophyll biomass around the Mascarene plateau was found with a maximum 0.3 mg/m~3 during June 2000. The minimum production of around 0.1 mg/m~3 was registered during March 1998. A general increase in primary production is observed from south to north and east to west of the Plateau with peaks corresponding to the shallow banks. The postulated explanation about a divergence zone on the western part of the Mascarene Plateau is substantiated by the detection of fronts, eddies and a relatively cooler sea surface temperature. Nutrients are thus upwelled closer to the euphotic zone enhancing primary production. The presence of a seasonal high-pressure centre corresponding to the Southwest monsoon is shown and an analogy is drawn to a mini-monsoon where both the South Equatorial Current and the Southeast Trade wind strengthened.