Identification and separation of baroclinic M4 current in estuaries

Abstract

Tidal current asymmetry is a challenging issue in estuaries and is generally quantified based on the relative amplitude and phase of M4 constituents in semi-diurnal regions. Previous studies have focused on the barotropic M4 current (BTC), whereas baroclinic overtides are often ignored. Using field data and numerical simulation results, we established a separation method of the baroclinic M4 current (BCC), clarified its typical features in estuaries of large tidal range, and explored the mechanism of BCC from the perspective of frontogenesis. The results showed that a 180° phase reversal in the vertical direction and significant enhancement of the normalised amplitude were fundamental features of BBC. Meanwhile, a decrease in currents around the salt intrusion head may not be inevitable owing to the strong BTC induced by nonlinearity, especially in meso- or macro-tidal estuaries. In contrast, near the river plume, BCC may develop and be stronger than BTC. Furthermore, BCC leads to zero longitudinal net transport of water mass, as it is treated as an ‘oscillatory’ full-depth lock-exchange flow. Our findings present a framework for BCC in estuarine zones.