Variability of chlorophyll-a in the Southwestern Atlantic from satellite images: Seasonal cycle and ENSO influences

Abstract

Seasonal and interannual satellite chlorophyll-a variability (CSAT) was assessed in the Southwestern Atlantic based on over 11 years (1997–2008) of Sea-Viewing Wide Field-of-View Sensor data. An Empirical Orthogonal Function analysis of the seasonal CSAT cycle showed strong variability and the spatial structure of the leading pattern revealed an opposite behavior over the continental shelf north and south of 371S with low (high) biomass south (north) of 371S during wintertime. This distribution is related to the lack of stratification of the water column in the southernmost region during winter due to heat loss to the atmosphere as well as wind induced and convective mixing, in contrast to a vertically stable water column north of 371S induced by the fresh Rı´o de la Plata discharge. High variability in CSAT between 47 and 511S in the inner and outer shelves could be related to the southern Patagonian fronts. On interannual time scales the influence of El Nin˜ o-Southern Oscillation on CSAT during spring was estimated and related to wind stress, vertical velocities and Rı´o de la Plata discharge. During El Nin˜ o events the continental shelf north of 451S is characterized by high CSAT values (anom- alies 4 0.5 mg m - 3) while low values are found to the south (anomalies o - 0.5 mg m - 3), except for positive anomalies near to the Malvinas Islands. The opposite pattern occurred in La Nin˜ a years. Conversely, the Brazil–Malvinas confluence has a lower CSAT in El Nin˜ o years in comparison with La Nin˜ a years. The higher chlorophyll-a of some areas over the shelf north of 451S during El Nin˜ o was supported by increased Rı´o de la Plata discharges, northerly winds anomalies and upwelling generated in the shelf between 33 and 391S. The winds tend to retain the patch of high chlorophyll-a off Rı´o de la Plata in spring, but advect it toward the Brazilian coast in summer. This result indicates the extreme importance of wind variability for the spreading or retention of phytoplankton in this area. No support was found for a mechanism linking Ekman pumping and CSAT variability over the continental shelf south of 451S and in the Malvinas–Brazil confluence