Seasonal spreading and transport of buoyant plumes in the shelf off Kochi, South west coast of India- A modeling approach

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

doi:10.1038/s41598-019-56103-9

G Seena, C Revichandran + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-56103-9

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Abstract

We investigated the seasonal spreading and transport of buoyant plume in the shelf off Kochi using Finite Volume Community Ocean Model (FVCOM). The modelled river plume typically consisted of an offshore bulge and a coastal current. The spreading of the bulge extended up to a distance of 19 km from inlet during the summer monsoon to <10 km in the spring inter-monsoon. The Kelvin number varied between 0.1 and 0.9 which revealed that the plume exhibited both the features of small and large scale plumes, resulting in a highly complicated plume pattern. During the southwest monsoon the plume fringe twisted towards the south, while during the northeast monsoon it twisted towards north according to the reversal of monsoonal winds. The fresh water transport with respect to coastal currents varied in accordance with seasonal river discharge such that the value peaked in the wet season and dropped in the dry season. During the non-realistic (no wind) condition the plume initiated barotropic and baroclinic flow, after which it was acted upon by earth’s rotation so that the plume propagated in the direction of Coriolis force (towards north), as geostrophic currents. The model run ‘with wind’ and ‘without wind’ condition revealed that in the shelf off Kochi the plume is transported in accordance with monsoonal winds/currents by nullifying the effect of earth’s rotation. The categorization of plume influenced area and realization of the direction of plume transport can be used for interpreting the dynamically and potentially active zones in the shelf off Kochi.

Highlights

The fresh water influx and the wave action make the coastal ocean a very dynamic and enriched ecosystem
Fresh water is directly injected into the shelf region due to heavy river discharge, while in estuarine plumes the fresh water gets mixed with saline water within the estuary and as a result a modified buoyant plume protrudes out into the shelf
The plume can be classified into small scale plume and large scale plume

Summary

Introduction

The fresh water influx and the wave action make the coastal ocean a very dynamic and enriched ecosystem. Since the fresh water influx has a strong dependence on seasonal rainfall, it drastically affects spatial extension and vertical gradients of the plume This has several effects on coastal zone properties such as reduction of salinity, increase in stratification and distribution of parameters like dissolved matters, pollutants, nutrients etc.[3,4,5,6,7,8]. Since the estuarine volume of 521 × 106 m3 cannot hold the large monsoonal riverine influx, it is further transported as a plume to the shelf- through two major openings: Fort Kochi and Munambam These out flows generate buoyant structure in the coastal waters and play a key role in the marine ecosystem by influencing stratification, frontogenesis, internal waves, nutrient enrichment and enhanced phytoplankton growth. As the study region is influenced by the summer monsoon, fall inter-monsoon, winter monsoon and spring inter-monsoon with strong and directionally varying coastal currents, it is imperative to know whether the plume transport is influenced by the coastal current or the earth’s rotation

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