Diahaline Mixing and Exchange Flow in A Large Multi-outlet Estuary with Islands

Abstract

<p>The relationship between the diahaline mixing, the diffusive salt transport, and the diahaline exchange flow is examined using salinity coordinates. The diahaline inflow and outflow volume transports are defined in this study as the integral of positive and negative values of the diahaline velocity. A numerical model of the Pearl River Estuary (PRE) shows that this diahaline exchange flow is analogous to the classical concept of estuarine exchange flow with inflow in the bottom layers and outflow at the surface. The inflow and outflow magnitudes increase with salinity, while the net transport equals the freshwater discharge <em>Q<sub>r</sub></em> after sufficiently long temporal averaging. In summer, intensified diahaline mixing mainly occurs in the surface layers and around the islands. The patchy distribution of intensified diahaline velocity suggests that the water exchange through an isohaline surface can be highly variable in space. In winter, the zones of intensification of diahaline mixing occur mainly in deep channels. Apart from the impact of freshwater transport from rivers, the transient isohaline mixing is also controlled by an unsteadiness term due to estuarine storage of salt and water volume. In the PRE, the diahaline mixing and exchange flow show substantial spring-neap variation, while the universal law of estuarine mixing <em>m=2SQ<sub>r</sub></em> (with <em>m</em> being the sum of physical and numerical mixing per salinity class <em>S</em>) holds over longer averaging period (spring-neap cycle). The correlation between the patterns of surface mixing, the vorticity, and the salinity gradients indicates a substantial influence of islands on estuarine mixing in the PRE. </p>

Spatial composition of the diahaline overturning circulation in a fjord-type, non-tidal estuarine system

“…The two contrasting Merrimack River and Hudson River estuaries were investigated by Chen et al (2012), the Salish Sea by Sutherland et al (2011), the Wadden Sea in the southeastern North Sea by Gräwe et al (2016) and Purkiani et al (2016) and the Baltic Sea by Burchard et al (2018). The study by MacCready et al (2021) used the framework to inspect water quality problems in the Salish Sea, and Li et al (2022) investigated the very intricate Pearl River Estuary in South China. Analyses of dynamics in estuaries and river plumes using isohaline surfaces as reference have been carried out by MacCready and Geyer (2001), MacCready et al (2002), andHetland (2005).…”

“…The diahaline transport (referred to as entrainment there) was in upward direction (towards lower salinity classes) below the interface and in downward direction (towards higher salinity classes) above. Similarly, the diahaline exchange flow was studied for the Pearl River Estuary (Li et al, 2022). They find that the diahaline exchange flow shows a classical exchange flow consisting of inflow in the deeper and outflow in the shallower water layers.…”

“…All the methods presented up to this point are important diagnostic tools to study estuarine dynamics. In particular, maps of diahaline velocity and diffusive salt fluxes, as seen in Wang et al (2017) and Li et al (2022), allow for studying the link between mixing and diahaline exchange flow. Recently, Klingbeil and Henell (2023) derived a local Water Mass Transformation framework and provided exact definitions for local diasurface quantities.…”

“…Recently, Klingbeil and Henell (2023) derived a local Water Mass Transformation framework and provided exact definitions for local diasurface quantities. Moreover, they derived relations between these quantities, thereby generalizing the formulations by Li et al (2022). In particular, they provided an equation that directly formulates how mixing, i.e.…”

Temporal and spatial characteristics of submesoscale motions in the Bohai Sea

“…The averaged depth of the Bohai Sea is about 18 m (Zhao and Wei, 2005), so the water movement is inevitably affected by bottom friction. The complex coastlines and islands may induce horizontal shear of velocity, which could trigger submesoscale fronts (Dauhajre et al, 2017;Li et al, 2022). It also has a typical monsoon climate characteristic, including the seasonal variations of heat flux and winds (Wei et al, 2020).…”