Time-dependent plume front positioning and its dynamics coupled with seasonal river efflux

G Seena,S Abdul Azeez,C Revichandran,Sebin John,K R Muraleedharan,Ravikumar C Nair

doi:10.1007/s10236-022-01499-8

Abstract

The time-dependent plume front positioning with respect to different tidal phases and its dynamics coupled with seasonal river efflux on the shelf off Kochi, southwest coast of India, was investigated using Finite Volume Community Ocean Model (FVCOM). This region is linked with a monsoonal estuary, characterised by a mixed semidiurnal tide (1 m) and exhibited features of small and large-scale plumes. The interaction between river efflux and tidal phases modulates plume fronts on the shelf, where the density gradients are fortified or weakened by mixing dynamics. Even though the heavy river efflux in the summer monsoon imparts significant momentum on the shelf, the range of frontal fluctuation was curtailed to 2 km by strong monsoon currents. During the transient phase of the season (fall inter-monsoon), the tidal forcings on plume positioning overwhelm the shelf currents, such that the plume front fluctuates between 5 and 17 km from the inlet (range increasing to ~12 km). During low tides, the region near the inlets was almost homogenised (Rd<1), while during high tides, the region became more stratified due to the transport of high saline coastal water towards the inlet and also by the decreasing kinetic energy (Richardson number, Rd>1). The location of frontal zones suitable for the propagation of internal waves (Froude number, F≤ 1) changes as a result of the competition between river efflux and tide-topography interaction. Strong stratified plume frontal regions with high Brunt Vaisala Frequency (N) could be active zones of internal wave generation when the flow decelerates from supercritical to subcritical during the summer monsoon. The release of accumulated potential energy during the transition from high tide to low tide generates the hydraulic jump. This disturbance in the Nmax zone together with F ≤ 1 condition (supercritical flow changed to subcritical flow) favours the generation and propagation of plume-induced internal waves on the shelf. Satellite imageries demonstrate such propagation of plume-generated internal waves on the shelf off Kochi.

Highlights

IntroductionThe horizontal and vertical extension of such plumes in the shelf creates plume fronts, the locus of which depend on the terrestrial efflux and to the varying tidal amplitude
Freshwater/estuarine plumes are one of the momentous sources of nearshore dynamics
The fall inter-monsoon (FIM) was featured by a decreased monthly average river discharge of about 1312 m3/s; we considered the 30 psu contour to isolate the plume front

Summary

Introduction

The horizontal and vertical extension of such plumes in the shelf creates plume fronts, the locus of which depend on the terrestrial efflux and to the varying tidal amplitude. The relaxation of the gravity currents in the shelf region may radiate nonlinear internal waves such that the amplitude of which is comparable with that of waves generated from tide-topography interactions[2]. Previous studies[3] classified plumes in to surface advected, and bottom advected plumes depending on the interaction with bottom topography. These classifications have a strong correlation in determining the locus and offshore spreading of the plume front. The converging surface fronts[5] in the shelf due to the plumes are often places of high phytoplankton productivity, such that they serve as an important feeding and reproductive habitat for higher trophic organisms 6

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Discussion

Conclusion