Abstract
River plumes transport and mix land‐based tracers into the ocean. In tidally pulsed river plumes, wind effects have long been considered negligible in modulating interfacial mixing in the energetic nearfield region. This research tests the influence of variable, realistic winds on mixing in the interior plume. A numerical model of the Merrimack River plume‐shelf system is utilized, with an application of the salinity variance approach employed to identify spatial and temporal variation in advection, straining, and dissipation (mixing) of vertical salinity variance (stratification). Results indicate that moderate wind stresses (∼0.5 Pa) with a northward component countering the downcoast rotation of the plume are most effective at decreasing stratification in the domain relative to other wind conditions. Northward winds advect plume and ambient shelf stratification offshore, allowing shelf water salinity to increase in the nearshore, which strengthens the density gradient at the plume base. Straining in the plume increases with winds enhancing offshore‐directed surface velocities, leading to increased shear at the plume base. Increased straining and larger density gradients at the plume base enhance variance dissipation in the near‐ and midfield plume, and dissipation remains enhanced if the shelf is clear of residual stratification. The smaller spatial and temporal scales of the Merrimack plume allow the mechanisms to occur at tidal time scales in direct response to instantaneous winds. This is the first study to show tidal time scale wind‐induced straining and advection as controlling factors on near‐ and midfield mixing rates in river plumes under realistic winds.
Plain Language Summary
Rivers transport pollutants, nutrients, and sediments into the ocean. Often, fresh water from rivers spread out over the denser, salty ocean water, creating a river plume. How these plumes mix and move under different environmental conditions is important to understand for accurate tracing of river‐borne materials into the ocean. In river plumes which form anew on each ebbing, seaward‐directed tide (called tidally pulsed), winds have often been considered unimportant to mixing relative to tidal mixing. In this work, a numerical model of the tidally pulsed Merrimack River plume is used to test that assumption. Mixing in the energetic plume interior can, in fact, be enhanced during specific wind conditions, when the plume and coastal ocean surface waters are pushed offshore, the plume moves faster than normal, and the salinity difference from surface to bottom increases. Essentially, wind can control differences in density under the plume and how quickly the plume moves, both of which influence vertical mixing. The results presented are particularly applicable for smaller plumes over shallower water which are not influenced by larger scale ocean dynamics.
Key Points
Wind opposing the direction of Kelvin wave propagation can enhance near‐ and midfield mixing in small‐ to midsized tidal plumes
Advection of plume and ambient stratification offshore and enhanced wind straining contribute to larger mixing rates
The largest mixing events are linked to shelf stratification, while straining and advection are linked to instantaneous wind direction