Tidal, density and wind-driven subtidal circulation in a hypersaline coastal lagoon

Abstract

Subtidal circulation patterns were studied along an elongated hypersaline system. Month-long data were collected using three acoustic Doppler current profilers. These profilers were moored at depths of around 15 m in three locations: 10 km inland from the mouth of the lagoon, within the central region, and near the head of the lagoon. The variability of temporal and spatial subtidal circulation was explored using Empirical Orthogonal Functions analysis, which explained 93% of the variability through the first three modes. The first mode was influenced by tidal advection, resulting in a tidally-driven residual circulation for elongated channels, with inflows occurring over the shoals and outflows in the deeper channel sections. During the neap-tide cycle, inverse density gradients became relevant, coinciding with the peaks of mode 2 amplitude that accounted for 15% of the total variability. The gravitational circulation exhibited its highest intensity near the head, characterized by a vertically sheared flow with inflow occurring near the surface and outflow underneath. Predominant onshore wind stresses had a significant impact on the subtidal circulation and displayed a strong correlation with the mode 3 amplitude, explaining 11% of the total variability. Wind-driven circulation exhibited a vertically sheared profile, characterized by downwind near-surface flow and upwind flow underneath in the lagoon mouth and head, and a homogeneous downwind profile in the central part of the lagoon. It is concluded that tidal stresses governed the subtidal circulation, followed by density gradients, and wind stresses modified spatial circulation patterns induced by tides and density gradients. This research provides new observational evidence on tidal residuals in long channels and contributes to the understanding of the interactions between the main subtidal circulation drivers in hypersaline lagoons.