Adjustment of river plume fronts during downwelling-favorable wind events

Abstract

Downwelling-favorable winds modify the front structure of river plumes and are important for cross-shelf mass transport. To understand the adjustment process of river plume fronts in response to downwelling-favorable wind events, a series of generic numerical experiments were conducted. The adjustment of river plume fronts underwent three stages: destratification and restratification during the wind event and relaxation after the wind event. Ekman pumping drove a downwelling cross-shelf circulation that steepened the isohalines, leading to a destratification stage. As the wind continued blowing, instabilities occurred within the plume. The symmetric instability (SI) dominated and restratified the front during the remainder of the wind event, resulting in a restratification stage. After the wind event, baroclinic instabilities became dominant and further stratified the front. In the development of SI, the along-shelf freshwater transport was enhanced by the suppression of the bulge due to the down-front wind and provided a negative potential vorticity source that triggered and sustained the SI, together with the Ekman buoyancy flux. The result confirmed the importance of along-shelf processes in plume front adjustment. The timescale of the destratification process was related to the time taken by Ekman transport to reduce the width of the front. If the steepening time exceeds the duration of the wind event, the wind would be too weak to produce a well-mixed front, but instabilities would still develop and stratify the front during the restratification and relaxation stages.