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

Abstract

A variable-density, 1½-layer model is used to investigate the dynamics of the fresher-water plumes generated by river outflow. Solutions are found in a north-south channel, and the transport M_r and salinity S_r of the outflow are specified as boundary conditions along a 25 km-wide segment of the western boundary. In most cases, the river water discharges into a preexisting, oceanic mixed layer with thickness H ₁. When Mr 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 fresher water reverses direction, and this water, together with some oceanic water, flows southward on either side of the offshore density front between the fresher and salty waters. Two processes cause this upstream movement: geostrophic 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 Mr 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_r and S_r increase and as H ₁ decreases. The strength of the northward propagating plume is weakened as H ₁ decreases and Sr increases. When H ₁ = 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.