Impact of islands on the cross-shelf transport among the Pearl River Estuary, its adjacent coast and inner shelf

Cross-shelf transport is crucial for material exchange in the estuary-coast-shelf continuum. This study employs the Coupled Ocean-Atmospheric-Wave-Sediment Transport (COAWST) modeling system to quantify the cross-shelf transport and investigate the influence of islands on the cross-shelf transport between the Pearl River Estuary (PRE), the adjacent coast and the inner shelf. A budget-based method is applied to calculate the cross-shelf volume transport across key interfaces: the estuary-coast interface (the exit between Lantau Island and Macau at the PRE mouth) and the coast-inner shelf interface (25 m isobath). The results show that the Lantau-Macau exit serves as a key transport gateway for estuary-coast exchange, with a net offshore transport of 1.97 × 10³ m³ s⁻¹ in the dry season and 2.61 × 10³ m³ s⁻¹ in the wet season, respectively. The dynamical analysis shows that, at the estuary-coast interface, the islands strengthen the onshore horizontal advection, increasing the net onshore transport by 41.37 % in the dry season, and augment the offshore barotropic gradient, increasing the net offshore transport by 422 % in the wet season. At the coast-inner shelf interface, the cross-shelf transport is onshore at 11.35 × 10³ m³ s⁻¹ during the dry season and offshore at 0.74 × 10³ m³ s⁻¹ during the wet season. During the dry season, the islands enhance both the onshore and offshore transport through increased bottom pressure torque (BPT) and nonlinear advection, respectively, with the two effects nearly balancing each other. However, in the wet season, the islands strengthen the Joint Effect of Baroclinity And Relief (JEBAR), counteracting the advection and making BPT-driven onshore transport to become dominant, thereby enhancing onshore transport by 63 % at this interface. This study has implications for land-ocean interaction research and effective coastal management.