Subtidal flow and its variability at the entrance to a subtropical lagoon

Spatial and temporal variability of the subtidal exchange flow at West Pass, an inlet at the entrance to a subtropical lagoon (St. Andrew Bay, Florida), was studied using moored and towed current velocity profiles and hydrographic data. Towed and hydrographic measurements were captured over one diurnal tidal cycle to determine intratidal and spatial changes in flow. Hydrographic profiles over the tidal cycle showed that tidal straining modified density stratification asymmetrically, thus setting up the observed mean flow within the inlet. During the towed survey, the inlet's mean flow had a two-layer exchange structure that was moderately frictional and weakly influenced by Coriolis accelerations. Moored current profiles revealed the additional contribution to the dynamics from centrifugal accelerations. Along channel residual flows changed between unidirectional and exchange flow, depending on the forcing from the along-estuary wind stress and, to a lesser extent, the spring–neap tidal cycle. Increases in vertical shear in the along channel subtidal flow coincided with neap tides and rain pulses. Lateral subtidal flows showed the influence on the dynamics of centrifugal accelerations through a well-developed two-layer structure modulated in magnitude by the spring–neap tidal cycle.