Coastal circulations driven by river outflow in a variable‐density 1½‐layer model

Abstract

A variable-density, 11/2-1ayer model is used to investigate the dynamics of the freshet-water plumes generated by river outflow. Solutions are found in a north-south channel, and the transport M• and salinity S• of the outflow are specified as boundary conditions along a 25 kin-wide segment of the western boundary. In most cases, the river water discharges into a preexisting, oceanic mixed layer with thickness H1. When M• is sufficiently low, plumes remain coastally trapped. Immediately after the outflow is switched on, a coastal Kelvin wave is excited at the river mouth that establishes a southward current of oceanic water along the right-hand coast. In contrast, all the river water first bends to the left as it exits the river mouth, and the resulting plume advances northward along the left-hand coast. At the plume nose, some of the freshet water reverses direction, and this water, together with some oceanic water, flows southward on either side of the offshore density front between the freshet and salty waters. Two processes cause this upstream movement: geostrophie adjustment generates the southward frontal current, and Kelvin-wave propagation from the nose thins the layer within the plume thereby establishing the northward, geostrophic, coastal jet. When M• is sufficiently high, coastally trapped plumes no longer exist. If the Rossby number of the outflow is also large enough, the river water flows directly offshore and only a portion of it recirculates to form a northward propagating coastal plume. The angle'at which the outflow leaves the river mouth becomes more southward as M• and S• increase and as H1 decreases. The strength of the northward propagating plume is weakened as H1 decreases and S• increases. When H1 = 0 so that there is no ambient oceanic layer, there is no northward plume at all. Likewise, the plume is weakened when the model includes entrainment, a process that acts to prevent the layer thickness from thinning appreciably. Introduction River water typically bends to the right (facing sea-ward) in the northern hemisphere as it flows into the ocean, forming a shallow plume along the right-hand coast. (Hereafter, we refer to the region •o the right of the river mouth as being "downstream" and the lefthand region as being "upstream.") Well-documented examples of this type of circulation are the outflows from the Delaware Bay [Miinchow and Garvine, 1993] and the CheSapeake Bay [Boicourt, 1981]. In contrast, salinity distributions near the Changjiang (Yangtze) and Ganges River mouths indicate that some river water flows directly offshore during the summer when the Copyright 1997 by the American Geophysical Union. Paper number 97JC00985. 0148-0227/97/97JC-00985509.00 discharge is high [BeardsIcy et al., 1983; Wang, 1988; Murty et al., 1992; $hetye et al., 1993]. There are also indications that river water can bend to the left to flow along the upstream coast. A dramatic leftward flow of the Mississippi River Outflow occurred after the major flood in 1993, with part of the plume flowing eastward ...

Structure and eastward extension of the Changjiang River plume in the East China Sea

“…For now, however, we may explain the detachment of the patches as follows. During heavy flooding, which occurs several times every summer, a good portion of the discharge may accumulate in the river mouth to form a patch similar to an anticyclonic eddy of fresh water shown by McCreary et al [1997] and Bang and Lie [1999]. The patch can be separated from the mouth by a sustained upwelling favorable wind [Fong and Geyer, 2001].…”

“…The observation of the northward alongshore current is noteworthy because it is different from the usual expectation that in most estuaries in the Northern Hemisphere, the discharge turns clockwise as it flows out of the river mouth. According to McCreary et al [1997], the northward current may be associated with two processes of geostrophic adjustment and Kelvin-wave propagation. We also believe that it is attributable to a northward coastal current generated by upwelling favorable winds, since a southward coastal current dominates the coastal current system in the winter monsoon.…”

“…It is difficult to deduce a plausible dynamics directly from our limited CTD and drifter data. However, according to a numerical study by McCreary et al [1997], some river water can bend to the left to flow along the left-hand coast, together with a return flow along the offshore density front formed between the fresher and salty waters. They explained the leftward bending as caused by two processes associated with the geostrophic adjustment and Kevin-wave propagation.…”

On the dynamics of internal waves in a nonlinear, weakly nonhydrostatic three‐layer ocean

“…models lies in their dynamical simplicity, which allows for a straightforward diagnosis of the processes retained. At the same time they are dynamically sophisticated enough to simulate realistically different oceanographic phenomena [McCreary et al, 1997]. Using this approximation, several studies have been carried out that demonstrate that considering a two-layer system suffices for the description of the main characteristics of internal solitary waves observed in the ocean, as the improvement brought by considering a more complex stratification is often only marginal.…”

The Rhine Outflow Plume Studied by the Analysis of Synthetic Aperture Radar Data and Numerical Simulations

“…For these reasons, more efficient ''process modeling'' strategies have been developed in the last three decades, which are still considered among the most valid tools for the description of oceanic flow features including frontal processes. In this context Garvine (1974aGarvine ( , 1987Garvine ( , 1996 developed different steady-state layered models while, for example, O'Donnell and Garvine (1983), O'Donnell (1990), andMcCreary et al (1997) developed time-dependent layered models, both enabling the assessment of the potential implications of plume fronts on the larger-scale dynamics. The advantage of using these layered models lies in their dynamical simplicity, which allows processes at work in them to be readily diagnosed.…”