The Role of Fronts in Horizontal Transports of the Changjiang River Plume in Summer and the Implications for Phytoplankton Blooms. Issue 8 (8th August 2022)

• Record Type: Journal Article
• Authors: Li, Shuangzhao; Zhang, Zhaoru; Zhou, Meng; Wang, Chuning; Wu, Hui; Zhong, Yisen
• Is Part Of: Journal of geophysical research. Volume 127:Issue 8(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-08
• Subjects: Changjiang River plume -- transport -- frontal convergence -- Lagrangian Coherent Structures; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2022JC018541
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Frontal processes play a significant role in estuarine and coastal ecosystem dynamics. In this study, a hydrodynamic model of the Changjiang River plume was developed based on the Regional Ocean Modeling System, and Lagrangian Coherent Structures (LCSs) were applied to the model simulations to analyze the horizontal transport characteristics of the river plume with the surface plume front, the tidal front, and the far‐field plume front in summer. The results show that the plume currents during the ebb tide were supercritical upstream of the near‐field surface plume front and tidal front and subcritical downstream of the fronts. In the LCSs results, transport barriers existing in the frontal regions indicated that the materials hardly crossed the fronts and tended to accumulate. The water parcels near the surface plume front and the tidal front in the near‐field entirely originated from the Changjiang River Estuary. The surface East China Sea shelf water could not influence the near‐field region due to the transport barrier formed by the far‐field plume front. Momentum balance analysis results reveal that the plume currents were in geostrophic balance around the near‐field fronts, rendering the flows mainly in the along‐front direction and significantly weakening the cross‐shore flows. These physical processes provided enough residence time of the plume water and growth time for phytoplankton in the frontal region and thus favored algal blooming with sufficient nutrients. Even though the frontal convergence induced by transport barriers also appeared near the far‐field plume front, insufficient supply of nutrients limited the phytoplankton growth. Plain Language Summary: The physical processes around boundaries (fronts) between two different water masses play a significant role in material transports in estuarine and coastal ecosystem dynamics. In the Changjiang River Estuary and adjacent shelf regions, which receive the greatest amount of freshwater and terrigenous materials among the China marginal seas, the transport characteristics associated with fronts are not fully understood. In this study, transport analysis methods were applied to numerical simulations of surface currents to reveal the horizontal transport characteristics associated with the fronts in different regions (nearshore and offshore) in summer. The results show that on the seaward side of the nearshore fronts, the momentum of currents decreased and the direction of currents changed from east‐westward to north‐southward. The fronts acted as transport barriers, and materials hardly crossed them and tended to accumulate. These retarded currents also increased the time for phytoplankton to stay around the fronts and grow up and thus were favorable for phytoplankton blooms with sufficient nutrients for the nearshore fronts. Near the offshore front, insufficient supply of nutrients limited the phytoplankton growth. The transport barriers prevented the East China Sea shelf water to reach and influence the estuarine water in the nearshore region. Key Points: The plume is supercritical upstream of the near‐field fronts and subcritical downstream, providing residence time for phytoplankton The plume currents are in geostrophic balance near the near‐field fronts, weakening the cross‐shore flows and leading to mass convergence Transport barriers always exist in the frontal regions either including or excluding tidal influence