The response of large outflows to wind forcing

A numerical model is used to gauge the impact of winds on the evolution of coastal plumes generated by a variety of inlet outflows. The analysis is summarized by a conceptual model that accounts for the formation of surface and bottom mixed layers and tilting of the plume front. It also provides the basis for a two parameter classification of upwelling. The first parameter indicates when a wind event is capable of fully exporting plume waters offshore. The second determines when winds can overcome the plume buoyancy-driven flow. These indices help to explain why larger outflows tend to be less susceptible to upwelling. During an upwelling event, large plumes tend to maintain their structure, while smaller systems are commonly detached and dispersed offshore. The onset of downwelling events often reorganizes large plumes, thus promoting their net downshelf displacement. In contrast smaller systems frequently restart their formation, consequently limiting their downshelf penetration. The addition of long-term fluctuations, superimposed to the synoptic wind forcing, suggests a mechanism for typical seasonal to interannual variability commonly observed for large discharges.