Dynamics of freshwater plumes: observations and numerical modeling of the wind-forced response and alongshore freshwater transport by

A plume often results when a river discharges fresh water into the nearshore waters. This thesis combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The alongshore transport of freshwater is studied using a numerical model. In the absence of ambient currents, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" in the vicinity of the river mouth. The presence of an ambient current augments the freshwater transport in the coastal current so that it matches the freshwater source. The thesis also studies the wind-induced advection and mixing of a river plume, using data collected in the western Gulf of Maine (GOM) and idealized numerical modeling experiments. The cross-shore structure of the GOM plume varies as function of fluctuations in alongshore wind forcing. Estimates of the alongshore momentum balance indicate an approximate Ekman balance. The model simulations of a plume forced by upwelling winds demonstrate that the plume thins and is advected offshore by the cross-shore Ekman transport. The thinned plume is susceptible to significant mixing because of the sheared horizontal currents. Thesis co-supervisor: Dr. W. Rockwell Geyer Title: Associate Scientist, Woods Hole Oceanographic Institution Thesis co-supervisor: Dr. David C. Chapman Title: Senior Scientist, Woods Hole Oceanographic Institution