Abstract

Some semienclosed bays in the eastern boundary upwelling systems worldwide display a strong surface temperature gradient between cold water upwelled outside and warmer water inside these bays. This anomalous coastal temperature pattern is known as upwelling shadow (US). Research on this topic has mainly focused on identifying spatiotemporal patterns of US events, but the influence of the atmospheric synoptic variability on the temporal evolution of US events remains less well documented. This study uses observational, satellite-derived, and reanalysis data to propose a mechanism that relates synoptic-scale atmospheric phenomena with the development of US events in the Gulf of Arauco (GA), a highly productive and the largest semienclosed bay in central Chile (around 37 degrees S). The mechanism associates the passage of migratory anticyclones over the study area with two key processes. The first process is the formation of a coastal low-level wind jet, which intensifies the upwelling of cold subsurface waters outside the GA. We strongly suggest that the second one is the development of a coastal low pressure, which usually implies clear skies (increasing the solar radiation input) and weak downwelling favorable winds along the coast, factors that could largely explain the observed surface warming inside this embayment during these events. The presence of cold water upwelled offshore and the enhanced surface warming inside the GA, possibly also modulated by a weakened circulation, results in a strong thermal gradient with average temperature differences of more than 3 degrees C, generating the US.