Abstract

Eastern Boundary Upwelling Systems (EBUS) are characterized by wind-driven circulation on seasonal and synoptic time scales. Most studies of shelf dynamics in these systems have focused on inter-annual or seasonal variability, whereas fewer have examined synoptic and diurnal processes that shape inner-shelf circulation. This study presents the first assessment of circulation and hydrographic variability in Algarrobo Bay, a small north-facing bay in central Chile, based on two months of in situ current, temperature, and wind measurements combined with satellite observations during spring-summer 2021-2022. Time series revealed strong variability at both synoptic (>= 40 h) and diurnal scales. Upwelling-favourable winds predominated, producing cold, stratified waters, and weak northeastward currents, whereas short episodes of downwelling-favourable winds generated a warmer, mixed water column, and strong southwestward flows. Tidal contributions to current variability were minor (<= 13%). At diurnal scales, the mean wind cycle drove a two-layer current response that lagged the winds. These diurnal patterns were further modified during upwelling and downwelling events, shifting circulation from a two-layer to either barotropic or three-layer structures. These findings provide the first characterization of physical variability in Algarrobo Bay and suggest that local topography and wind forcing may create conditions resembling an upwelling shadow. These dynamics alternate between offshore exchange during downwelling and local retention during upwelling, with implications for the transport and retention of larvae, nutrients, and pollutants. In the context of climate-driven upwelling intensification, understanding such local dynamics becomes increasingly relevant for this and other upwelling-influenced bays.