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

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 37°S with low (high) biomass south (north) of 37°S 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 37°S induced by the fresh Río de la Plata discharge. High variability in CSAT between 47 and 51°S in the inner and outer shelves could be related to the southern Patagonian fronts. On interannual time scales the influence of El Niño-Southern Oscillation on CSAT during spring was estimated and related to wind stress, vertical velocities and Río de la Plata discharge. During El Niño events the continental shelf north of 45°S is characterized by high CSAT values (anomalies>0.5 mg m−3) while low values are found to the south (anomalies<−0.5 mg m−3), except for positive anomalies near to the Malvinas Islands. The opposite pattern occurred in La Niña years. Conversely, the Brazil–Malvinas confluence has a lower CSAT in El Niño years in comparison with La Niña years. The higher chlorophyll-a of some areas over the shelf north of 45°S during El Niño was supported by increased Río de la Plata discharges, northerly winds anomalies and upwelling generated in the shelf between 33 and 39°S. The winds tend to retain the patch of high chlorophyll-a off Río 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 45°S and in the Malvinas–Brazil confluence.