River plumes are generated by the flow of buoyant river water into the coastal ocean, where they significantly influence water properties and circulation. They comprise dynamically distinct regions spanning a large range of spatial and temporal scales, each contributing to the dilution and transport of freshwater as it is carried away from the source. River plume structure varies greatly among different plume systems, depending on the forcing and geometry of each system. Individual systems may also exhibit markedly different characteristics under varied forcing conditions. Research over the past decade, including a series of major observational efforts, has significantly improved our understanding of the dynamics and mixing processes in these regions. Although these studies have clarified many individual processes, a holistic description of the interaction and relative importance of different mixing and transport processes in river plumes has not yet been realized.

The dynamics of buoyant water entering a rotating basin are studied using a series of laboratory experiments designed to elucidate the alongshore transport mechanisms in river plumes. Inflowing water, which is discharged perpendicular to the tank wall, is observed to form a growing anticyclonic bulge and a coastal current downstream of the bulge. Detailed simultaneous measurements of the velocity and buoyancy fields in the plume confirm that the bulge momentum is in a gradient-wind balance and the coastal current is geostrophic. The growth of the bulge and accumulation of fluid within it coincides with a reduction in coastal current transport to approximately 50 % of the inflow discharge. The bulge is characterized by a depth scale, h, which is proportional to the geostrophic depth, h g , and two time-dependent horizontal length scales, y c , the displacement of the bulge centre from the wall, and r b , the effective radius of the bulge. These two length scales are proportional to the inertial radius, L i , and the local Rossby radius, L b , respectively. When r b y c , the bulge is held tightly to the wall, and a relatively large fraction of the inflow discharge is forced into the coastal current. For plumes with y c approaching r b , the bulge is further from the wall, and the coastal current flux is reduced. Once y c /r b > 0.7, the bulge separates from the wall causing flow into the coastal current to cease and the bulge to become unstable. In this state, the bulge periodically detaches from and re-attaches to the wall, resulting in pulsing transport in the coastal current. Scaling of the bulge growth based on h g , L i and L b predicts that it will increase as Ro 1/4 , where Ro is the inflow Rossby number. The bulge growth, inferred from direct measurements of the coastal current transport, is proportional to Ro 0.32 and agrees with the predicted dependence within the experimental error.

[1] River Influences on Shelf Ecosystems (RISE) is the first comprehensive interdisciplinary study of the rates and dynamics governing the mixing of river and coastal waters in an eastern boundary current system, as well as the effects of the resultant plume on phytoplankton standing stocks, growth and grazing rates, and community structure. The RISE Special Volume presents results deduced from four field studies and two different numerical model applications, including an ecosystem model, on the buoyant plume originating from the Columbia River. This introductory paper provides background information on variability during RISE field efforts as well as a synthesis of results, with particular attention to the questions and hypotheses that motivated this research. RISE studies have shown that the maximum mixing of Columbia River and ocean water occurs primarily near plume liftoff inside the estuary and in the near field of the plume. Most plume nitrate originates from upwelled shelf water, and plume phytoplankton species are typically the same as those found in the adjacent coastal ocean. River-supplied nitrate can help maintain the ecosystem during periods of delayed upwelling. The plume inhibits iron limitation, but nitrate limitation is observed in aging plumes. The plume also has significant effects on rates of primary productivity and growth (higher in new plume water) and microzooplankton grazing (lower in the plume near field and north of the river mouth); macrozooplankton concentration (enhanced at plume fronts); offshelf chlorophyll export; as well as the development of a chlorophyll ''shadow zone'' off northern Oregon.

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