Tidal Mixing Sustains a Bottom‐Trapped River Plume and Buoyant Coastal Current on an Energetic Continental Shelf. Issue 11 (8th November 2018)

• Record Type: Journal Article
• Authors: Wu, Tianning; Wu, Hui
• Is Part Of: Journal of geophysical research. Volume 123:Issue 11(2018)
• Page Start: 8026
• Page End: 8051
• Publication Date: 2018-11-08
• Subjects: bottom‐trapped river plume -- coastal current -- tidal mixing front -- Changjiang River; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2018JC014105
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Conventional wisdoms on river plume dynamics suggest that a down‐shelf buoyant coastal current will ultimately be trapped at a specific depth, that is, the trapping depth, as constrained by riverine outflow and offshore bottom Ekman transport. Theoretically, a prerequisite down‐shelf current is necessary to form a stable bottom‐trapped river plume. In this study an alternative is described by carrying out a modeling study on the Zhe‐Min Coastal Current (ZMCC). Buoyant water from the Changjiang River is a major factor driving the ZMCC, as is common in bottom‐trapped river plumes; however, the trapping depth is more determined by tidal mixing. When the plume water comes to the sloping topography, strong tidal mixing induces a mixing front, shoreward of which the bottom Ekman layer occupies the entire water column. Such a tidal‐induced front maintains a down‐shelf frontal current, which is intensified both at the surface due to the thermal wind balance and on the top of bottom boundary layer due to the tidal rectification. Direct wind‐induced transport only covers a small fraction of the ZMCC; however, it redistributes the plume water and, thus, affects the coastal current. The tide‐induced frontal trapping depth varies much less between seasons than that predicted by previous plume theories. Instead, it fluctuates strongly in the spring‐neap cycle. Even in summer when upwelling‐favorable winds prevail, the mixing front still sustains a down‐shelf coastal current. Intense tidal mixing exists in many coastal waters, which might be an alternative mechanism in forming bottom‐trapped river plumes and their associated buoyant coastal current. Plain Language Summary: Large rivers, such as the Amazon, Mississippi, and Changjiang, export a huge amount of terrestrial materials to the receiving waterbodies. Typically, the lighter riverine freshwater mixes with the denser ambient seawater and then propagates rightward along the coast (in the Northern Hemisphere), which is known as the buoyant coastal current. Large river‐induced buoyant coastal current can propagate hundreds of kilometers. It frequently causes harmful algal blooms, hypoxia, and other environmental problems. Therefore, understanding the formation mechanisms of buoyant coastal currents is very important. Conventional theories on buoyant coastal currents are very successful; however, they rarely consider the effect of tidal mixing, which exists ubiquitously in coastal waters. Here in this study, we investigated this issue by using the Changjiang River‐induced buoyant coastal current as an example, through a series of well‐designed numerical experiments. The results showed that tidal mixing plays an essential role in maintaining a stable buoyant coastal current under unfavorable wind conditions. Without the tidal mixing, the stable buoyant coastal current can hardly exist. The finding of this study can promote our understandings on the coastal dynamic processes and other relevant processes. Key Points: Plume trapping depth can be determined by the tidal mixing Buoyancy drives the coastal current, while wind redistributes the buoyancy Tidal mixing front sustains a down‐shelf buoyant coastal current under upwelling‐favorable winds