Salinity Mixing and Diahaline Exchange Flow in a Large Multi-Outlet Estuary with Islands

Abstract

The relationship between the salinity 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 Qr after sufficiently long temporal averaging. In summer, intensified salinity 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 salinity mixing occur mainly in deep channels. Apart from the impact of freshwater transport from rivers, the transient mixing is also controlled by an unsteadiness term due to estuarine storage of salt and water volume. In the PRE, the salinity mixing and exchange flow show substantial spring-neap variation, while the universal law of estuarine mixing m = 2SQr (with m being the sum of physical and numerical mixing per salinity class S) 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.

Spatial Composition of the Diahaline Overturning Circulation in a Fjord–Type, Non–Tidal Estuarine System

“…The strongest diahaline exchange flow with maximum in-and outflowing transports occurs across the isohaline with the largest area (salinity 7 g/kg, see Figure 4a). This is different to many tidal estuaries where the strength of the exchange flow increases toward the highest salt classes, situated at the outlet of the estuary (see e.g., Figures 4c and 4d in Li et al (2022) and Reese et al (2023)).…”

“…Recently, Klingbeil and Henell (2023) derived a local WMT 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 small-scale mixing, that is, local dissipation of tracer variance, is associated with the local dia-surface exchange flow.…”

“…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 WMT framework and provided exact definitions for local diasurface quantities.…”

“…(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 WMT framework and provided exact definitions for local diasurface quantities.…”

“…The magnitude of the inflowing and outflowing diahaline transport is found to increase with salinity, while the net diahaline transport stays constant, amounting to Q r after long enough temporal averaging. The progress made by Li et al (2022) is that horizontal maps were shown of the diahaline velocity and the local mixing per salinity class (their Figure 6). They found that high diahaline velocity correlated with strong mixing.…”