Synoptic Cross-Shore Movements of Outer Salinity Fronts in China’s Changjiang River Plume

Abstract Remote sensing observations reveal strong differences in winter cross-shore movements of the outer salinity fronts between the midfield and far-field regions outside China’s Changjiang River plume. However, the characteristics, mechanisms, and associated water exchanges of these outer fronts remain unclear. Based on 7-yr buoy salinity observations and a high-resolution model, we investigate periods of synoptic (∼9–13 days) variation and the spring–neap tidal cycle for outer fronts in the midfield and far field. Their cross-shore movements manifest as oscillations in the midfield and an alternation of single and double fronts in the far field. In the midfield, the cross-shore frontal oscillations occur with in-phase salinity variations on opposite sides of the front, indicating a balance between brackish plume water and offshore saline water. The replenishment of low-salinity water from the near field to this region induces significant stratification of seawater, making the entire surface layer highly buoyant and susceptible to modulation by tidal and wind-driven advection. In the far field, the alternations of single and double fronts are accompanied by antiphase salinity variations on opposite sides of the front. During strong downwelling winds, a single front occurs when coastal salinity decreases due to the southward expansion of plume, and offshore salinity increases due to wind-driven onshore transport. During weak downwelling winds, a double front is generated by the combined action of advection and vertical mixing. The salinity in the coastal branch of the double front increases at this time, providing evidence that saline Taiwan warm water can intrude into the coastal area when the offshore branch spreads further offshore. Significance Statement The Changjiang River plume extends to the whole coastal area of the East China Sea due to the wintertime downwelling wind, resulting in a widespread outer salinity front zone of more than 300 km from the midfield to far field. Such outer fronts’ natural movements and synoptical variation in their locations signal the transport of water masses and nutrients, which is crucial for shelf ecology. In this study, buoy data and a high-resolution model show that midfield and far-field regions differ strongly in the characteristics and mechanisms of front movement, as well as the associated water exchanges. Accordingly, we divide the outer fronts into an advective front in midfield mainly driven by advection and a mixing front in the far field additionally driven by mixing.