Time-dependent Plume Front Positioning and Its Dynamics Coupled With Seasonal River Efflux

Abstract The time-dependent plume front fluctuation concerning different tidal phases and its dynamics coupled with seasonal river efflux in the shelf off Kochi, south west coast of India, were investigated using Finite Volume Community Ocean Model (FVCOM). The region is linked with a monsoonal estuary, featured by mixed semi-diurnal tide (1 m) and exhibited a highly complicated plume pattern. The rivalry between river efflux with tidal phases create plume fronts in the shelf, whose gradients fortified or weakened by mixing dynamics. Eventhough the incessant river efflux in the summer monsoon impart significant momentum in the shelf, the range of frontal fluctuation was curtailed to 2 km by strong monsoon currents. During transient phase of the season (fall inter-monsoon), the tidal forcings on plume positioning overwhelm the shelf currents, such that the plume front fluctuate between 6-17 km (range increased to ~11 km) from the inlet. In low tides, the region near to the inlets was almost homogenized (Rd<1). While, it gets more stratified in high tides due to the transport of high saline ambient water towards the inlet and also by the decreasing kinetic energy (Rd>1). The location of frontal zones suitable for the propagation of nonlinear waves (F≤1)will change in respect to the competition between river efflux and tide-topography interaction. The strong stratified plume front regions with increased Brunt Vaisala Frequency (BVF) in summer monsoon behave as active zones of non linear wave propagation only when the plume front decelerates from supercritical to subcritical. During dry season, the F≤1 was satisfied at limited locations, but the absence of BVFmax zone (frequency >0.3 s-1) revealed that the amplitude of such nonlinear waves would be considerably small. The study divulge that tidally pulsating plume front fluctuates between 3-18 km from inlet and also highlights that the propagation of nonlinear waves with considerable amplitude will depend on both the plume front velocity and the Brunt Vaisala Frequency of the water column.