Flow separation, dipole formation, and water exchange through tidal straits

Ole Anders Nøst,Eli Børve

doi:10.5194/os-17-1403-2021

Abstract

Abstract. We investigate the formation and evolution of dipole vortices and their contribution to water exchange through idealized tidal straits. Self-propagating dipoles are important for transporting and exchanging water properties through straits and inlets in coastal regions. In order to obtain a robust dataset to evaluate flow separation, dipole formation and evolution, and the effect on water exchange, we conduct 164 numerical simulations, varying the width and length of the straits as well as the tidal forcing. We show that dipoles form and start propagating at the time of flow separation, and their vorticity originates in the velocity front formed by the separation. We find that the dipole propagation velocity is proportional to the tidal velocity amplitude and twice as large as the dipole velocity derived for a dipole consisting of two point vortices. We analyze the processes creating a net water exchange through the straits and derive a kinematic model dependent on dimensionless parameters representing strait length, dipole travel distance, and dipole size. The net tracer transport resulting from the kinematic model agrees closely with the numerical simulations and provides an understanding of the processes controlling net water exchange.

Highlights

Knowledge of coastal ocean transport processes is vital for predicting human impact on the coastal marine environment
It is important to investigate whether our conclusions regarding tracer transport, dipole propagation velocity, and separation time are affected by the model resolution
We have performed a total of 164 numerical simulations of an ideal tidal strait, investigating flow separation, dipole formation, and water exchange for different widths and lengths of the strait

Summary

Introduction

Knowledge of coastal ocean transport processes is vital for predicting human impact on the coastal marine environment. Coastal industry discharges pollutants and nutrients into the ocean. In order to understand the impact on the environment, we need coastal ocean circulation models to calculate concentrations and pathways of spreading. Setting up such models for a complex coastline requires a high level of understanding of nearshore transport processes in order to realistically represent these in the models. In shallow coastal regions with complex topography, tides are often a dominant driver of the ocean circulation and transport. We investigate the exchange process of tidal pumping through narrow tidal straits

Objectives

Discussion

Conclusion