Wind influence on residual circulation in Patagonian channels and fjords

Abstract

Circulation in estuaries is typically considered to be driven by horizontal density gradients, called gravitational circulation, but can be modified by other forcing such as the wind. Studies have shown that wind influence can either augment or counteract density-driven residual flows in estuaries, but the temporal and spatial extent of this influence remains unknown, particularly in deep and highly stratified fjords. This study aims to fill this gap by investigating the seasonal variability in residual circulation, wind forcing, and stratification in an extensive fjord and channel system in Chilean Patagonia. Observations of current velocity and wind are supplemented by atmospheric and hydrodynamic circulation models in the Moraleda Channel, a system that connects the fjord system of Northern Patagonia to the Pacific Ocean. Results show a marked seasonality, given that the surface outflow is reversed by wind forcing during the winter, a season characterized by strong southward (up-channel) winds. This produces a near-surface flow directed up-channel in the upper ∼40 m of the water column, in opposition to the horizontal pressure gradient. Cross-channel transects revealed that the vertical structure of the along-channel residual velocity was three-layer during the winter. The near-surface (<50 m) along-channel current on the eastern side of the channel followed the wind direction (southward). Between 50 and 100 m depth the flow was out-channel and below 100 m depth the flow was again in-channel. On the western side, the along-channel residual flow was two-layer, with near surface flow directed northward (against the wind) with a return flow below (deeper than 50 m). During the spring, northward (out-channel) winds dominated and the vertical structure of the along-channel residual velocity was two-layer, augmenting the horizontal pressure gradient, which stabilized the water column, enhancing stratification, inhibiting mixing and elevated gravitational circulation.