Coastal upwelling and seasonal cycles that influence the water column over the continental shelf off central Chile

The COPAS research center (Universidad de Concepción, Chile) initiated a time series station on the continental shelf off central Chile that is used for multi-disciplinary oceanographic research. In this manuscript, we analyze the temporal evolution (2002–2006) of hydrographic variables at this station related to annual cycles of surface winds, sea surface temperatures, net heat flux, and the local freshwater balance in terms of their coupling with coastal upwelling. Off central Chile, coastal upwelling is forced by seasonal variable winds along the coast, competing with two other seasonal processes that also modify the vertical density stratification of the water column. These processes are the mixed layer’s heat balance, which is dominated by solar radiation with maxima in January, and the fresh water balance, which is dominated by river discharge and precipitation with maxima in June and July. On a seasonal time scale, the temperature of the first 10 m of the water column closely follows the annual cycle of net surface heat flux, resulting in a shallow, warm mixed layer from December to April. The temperature below 15 m depth is controlled by seasonal coastal upwelling, which is favored by southerly alongshore winds in austral spring and summer and suppressed by mainly northerly winds in winter. Salinity is also determined on a seasonal scale, in the first 20 m of the water column, by the annual cycle of river discharge and precipitation and, below 25 m depth, by the seasonal upwelling pattern. Considering oxygen during spring and summer (the upwelling season), the 0.5 ml l−1 isoline rises – in extreme cases up to 20 m – thus, exposing the euphotic water column to hypoxic conditions.