Subtidal variability of exchange flows produced by river pulses, wind stress and fortnightly tides in a subtropical stratified estuary

Abstract

Time series of current velocity profiles were obtained at the mouth and head of a Brazilian subtropical estuary, the Araranguá, to study its dry-period response to fortnightly tidal forcing, river pulses and wind forcing. The Araranguá exemplifies a series of river-dominated estuaries along southern Brazil with widths <200 m. A special feature of the Araranguá, as it is in dozens of subtropical estuaries in the world, is that depth increases landward. Its lower stretch runs parallel to a wind- and wave-dominated sandbar. Hydrographic data showed highly stratified mid and upper reaches, even during the dry period, and variable stratification in the lower reaches. Moored data were recorded at 7 and 30 km from the mouth during 70 days. Two pulses of moderate river discharge (>30 m3/s but <200 m3/s) occurred during the observation span. The estuary responded to these pulses by enhancing estuarine exchange. Discharge >200 m3/s appeared in two other occasions late in the observation period and caused seaward flushing of the entire system. Overall, the estuarine circulation resembled that of a fjord when river discharge was moderate and weaker: an active upper layer driven by river discharge moving over a semi-stagnant lower layer. Strongest subtidal exchange developed ∼2–3 days after neap tides, in contrast to other studies where fortnightly modulation is determined by vertical mixing from tides. In the lower estuary, subtidal flows seemed to result from a combination of vertical mixing, from tides and wind, and from tidal advection. Moreover, the lower estuary was much more sensitive to wind forcing than the upper estuary. The upper estuary responded preferentially to river discharge and tidal forcing.