Phytoplankton seasonal distribution from SeaWiFS data in the Agulhas Current system
The interocean conduit for warm Indian Ocean water into the Atlantic Ocean is the Agulhas system which plays an important role in maintaining the global thermohaline circulation. The frontal system formed by the Agulhas Return Current (ARC) and the Subtropical Convergence (STC) is also a region of intense mesoscale activity presenting enhanced levels of biological production and chlorophyll a. We jointly analyzed three different satellite data sets to relate the remotely sensed distribution of phytoplankton (SeaWiFS ocean color data) to the dynamical environment (Topex/Poséidon-ERS sea level anomalies-SLA- and sea-surface temperature-SST-) to gain insight into the seasonal behavior of the Agulhas Current system. We used a wavelet analysis to retrieve the characteristic wavelengths of the ARC and STC associated to their meandering. By meridionally averaging (between 15- 45E) the two-dimensional power Hovmöller of each signal (Chla, SLA, and SST), we obtained a seasonal average variance for Chla, SLA and SST as a function of latitude. Within the double frontal Agulhas and Subtropical frontal system, an extended temporal maximum in chlorophyll a concentration is observed in spring-summer-fall and a well-marked minimum occurs in winter, in phase opposition with the southwest Indian Ocean subtropical gyre north of the frontal system. Seasonal changes in strength of cross-frontal mixing with the subtropical gyre, in density strength of the juxtaposed fronts, and in mixed-layer depth and light availability seem the most likely explanations for the observed spatial and seasonal variability of the chlorophyll a distribution.
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Document Type: Research Article
Publication date: September 1, 2001
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