Exploring Accumulation Dynamics in the Pearl River Estuary from Lagrangian Perspective

Abstract. Utilizing Lagrangian methods and Markov Chains, we examined the water accumulation dynamics in the Pearl River Estuary (PRE), which are essential for understanding estuarine mass distribution and ecosystem management. Influenced by plume fronts and velocity convergence, we observed significant bottom-layer accumulations in the western estuary and Hong Kong waters during summer, while accumulation region shifts to upper estuary and Macau waters in winter. Generally, there is notable correlation between the horizontal velocity divergence (∇_hV^→_h) and the accumulation probability, where local negative ∇_hV^→_h fosters the accumulation. However, the net accumulation depends on the cumulative effects of convergence along the trajectories, not just its Eulerian pattern. The plume fronts also obstructed the particles transport and played an important role in the accumulation. Combined with trajectories, subregions with distinct accumulation patterns and interconnection among them were identified. During summer, western estuary and Macau waters with substantial net negative ∇_hV^→_h and strong front were major accumulation targets, which attracted particles from the whole estuary. Conversely, the eastern estuary and Hong Kong waters exhibit significant westward motions, influencing the western side. In winter, particles were more likely to accumulate in their original regions. The upper estuary becomes a major accumulation area due to the obstructive density front, while Hong Kong waters show a tendency to diverge throughout the estuary. Sensitivity experiments illustrated that tides mainly promoted the accumulation in the western and upper estuary regions during winter and in Macau and HK waters during summer. Additionally, larger river discharges were conducive to the seaside transport in the upper and western estuary during summer and in the HK regions during winter.