Subtidal Hydrodynamics in a Tropical Lagoon During the Transition from Dry to Wet Seasons

This study examined the subtidal hydrodynamics in a tropical costal lagoon determined by the tides and density gradients competition. A dimensional analysis was motivated by observations in Chelem lagoon, a tropical system in the Yucatan peninsula. The subtidal hydrodynamics was associated with transition from dry to wet season, which modifies the evaporation, precipitation and groundwater system, thus the Chelem lagoon exhibited seasonal and spatial density variations. To diagnose the relative importance of the dominant subtidal driving forces, tidal forcing was compared to baroclinic forcing through the ratio of tidal stress to longitudinal baroclinic pressure gradient, known as the tidal Froude number (Fr₀). Subtidal flows were driven mainly by advective accelerations during spring tides (log(Fr₀)>>0), and by baroclinicity during neap tides (log(Fr₀)<<0). The maximum value of log(Fr₀) was around the fortnightly declinational period (13.6 days). Chelem lagoon had 3 different zones: main channel, west channel and east channel. Each zone exhibited a different subtidal response to the ratio of tidal stress and to the longitudinal baroclinic pressure gradient, which depended on the zone’s spring-neap cycle tide, the location in the system and the freshwater inflows into it. In addition, wind stress was compared to baroclinicity, through the Wedderburn number. Furthermore, a combination of the Froude and Wedderburn numbers was explored.