River Plume Rooted on the Sea-Floor: Seasonal and Spring-Neap Variability of the Pearl River Plume Front

Abstract

The buoyant river plume front exhibits substantial variability in the sea surface under energetic external forcing. Although highly dynamic, the river plume is often “rooted” at specific locations through the bottom river plume front. In this study, we addressed this mechanism using the Pearl River plume as an example based on a well-validated numerical model. With this model, we described the spatiotemporal characteristics of the Pearl River salinity front. It was found that, although the surface Pearl River plume features bimodal extension in summer and winter seasons, owing to the reversal of the seasonal monsoon wind, there is a relatively stable bottom front extending from the river mouth to the downstream region (i.e., in the direction of propagation of Kelvin Wave). The occurrence probability of the bottom front showed that the front location varies only slightly and fixes at ∼8-m isobath. The Empirical Orthogonal Function (EOF) analysis demonstrated that runoff, wind, and tide are major regulating factors. These three factors jointly control the strength and position of the bottom front. In particular, the bottom front moves offshore during the spring tide but onshore during the neap tide, respectively, indicating a different mechanism from the classic frontal trapping theory. The sensitivity experiment without tide indicated that the bottom plume front shrinks significantly, and the river plume becomes more dynamic since it is no longer rooted on the seafloor.

Millennial-scale societal shifts drive the widespread loss of a marine ecosystem

“…Since the maximum known subtidal water depth limit of the locally found Magallana species (M. gigas, M. ariakensis and M. hongkongensis) in PRD range to 10-13 m (Meng et al 2018;Quan et al 2013;Lam & Morton 2004), we infer the habitable areas for historical oyster reef would have existed from the intertidal (0 m) to subtidal area up to 12 m between Tsing Yi in Hong Kong to Sunning in western PRD. This 12 m lower depth boundary also coincides with the salinity regime up to ~30 psu in the PRD (Zhi et al, 2022), which is a threshold that is tolerable for M. gigas, M. ariakensis and M. hongkongensis (Huo et al, 2014;Qin et al, 2020;Zhao et al, 2012). High resolution, point-based water depth information of the PRD was extracted from Zhang et al (2021).…”

Modelling the interannual variability of Maipo and Rapel river plumes off central Chile

“…Considering their relatively steep slopes and small watersheds, compared to larger rivers, Maipo and Rapel rivers are similar to systems that exhibit a rapid response to episodes of increase in freshwater, generating river plumes with a strong stratification within 1 km of the river mouth (Warrick et al, 2004). An interesting feature observed in river plumes is the occurrence of "rooted" plumes in shallow areas (Zhi et al, 2022). It is possible that this process can also occur in central Chile, especially, under the influence of northwesterly winds in winter.…”

Modeling the interannual variability in Maipo and Rapel river plumes off central Chile

“…Considering their relatively steep slopes and small watersheds, compared to larger rivers, the Maipo and Rapel rivers are similar to systems of small mountainous rivers that exhibit a rapid response to episodes of increase in freshwater, generating river plumes with strong stratification near the river mouth (Warrick et al, 2004). An interesting feature ob- served in river plumes is the occurrence of "rooted" plumes in shallow areas (Zhi et al, 2022). It is possible that this process can also occur off central Chile in winter when there is a predominance of northwesterly winds.…”