Seasonal variability of the coastal upwelling off Concepción (37ºS), Chile

s process. The region near Concepción (37ºS, Chile) shows one of the strongest upwelling of the Chilean coast. The high strength of the upwelling there has large consequences for the biological productivity of this zone, particularly for its fisheries. In order to characterize the seasonal and inter-annual variability of the upwelling around Concepción, Bakun upwelling indices from 25 years of daily local winds and temperature gradients from around 18 years of weekly sea surface temperatures have been analyzed. Results show that upwelling off Concepción occurs on average between middle September until middle April. During the downwelling period (middle April until middle September) a relative maximum of the Bakun index is observed between middle May until middle June, indicating a weakening of the downwelling during this period, and suggesting the possibility of upwelling events during the winter. Upwelling events, defined as periods with at least three consecutive days with daily Bakun indices larger than 20, have rather similar Bakun index monthly means between September and February. The frequency of occurrence of such events diminishes rapidly as the length of the events increases, from a hundred of three-days events to a ten of six-days events in 25 years. The surface temperature gradient between the coast and offshore, that should be correlated with the strength of the upwelling, shows a strong seasonal behavior. On the average, the maximal temperature difference, around 0.7ºC per degree of longitude, occurs during the second fortnight of March. On the other hand, the minimum of the difference, near 0ºC, occurs around middle October. On an inter-annual scale, the effect of El Niño/La Niña events is clearly recognized as perturbations on the normal values of the upwelling strength, strengthening (La Niña) or weakening (El Niño) th