Influence of Freshwater on the Vertical Structure of Tidal Currents: A Case Study of the Pearl River Estuary

Field observations in the Pearl River Estuary (PRE) indicate that vertical structures of tidal currents vary in different regions, which is associated with freshwater-induced stratification. A three-dimensional unstructured grid model (PRE-model) based on FVCOM is developed to study the mechanism of vertical structural changes in tidal currents. With the effect of freshwater, the amplitude of tidal currents increases in the upper layers and decreases in the bottom layers, and in regions with geometrical complexity, it is maximum in the middle depths. Momentum analysis indicates that vertical friction (VF), pressure gradient forces (PGF), and momentum advection (ADV) are the main factors leading to changes in tidal vertical structures. Variations of VF induced by stratification lead to an increase of tidal currents amplitude in the upper layers while a decrease in the bottom layers. Changes in PGF and ADV can also have significant impacts on tidal currents. Tidal frequency variations of density gradients result in pronounced tidal frequency baroclinic PGF in the bottom depths and enlarge total PGF. The amplitude of tidal currents increases at corresponding depths, which leads to subsurface maxima of tidal currents. ADV is special as its phase is variable relative to tidal currents, which can enhance or weaken tidal currents in different regions. The generation of tidal frequency baroclinic PGF is highly correlated with nonlinear processes, and the horizontal advection is considered the primary source of baroclinic PGF. The similar sources of ADV and baroclinic PGF also make them dominant in regions with geometrical complexity or large river plumes.