Title

Coastal Circulations Driven by River Outflow

Author

Shuliang Zhang, Nova Southeastern UniversityFollow

Defense Date

5-1997

Document Type

Dissertation

Degree Name

Ph.D. Oceanography/Marine Biology

Department

Oceanographic Center

First Advisor

Julian P. McCreary

Second Advisor

Richard E. Dodge

Third Advisor

Barry A. Klinger

Fourth Advisor

Donald B. Olson

Fifth Advisor

Claes G. H. Rooth

Sixth Advisor

Russell L. Snyder

Abstract

Variable-density, 1^1/2- and 2^1/2- layer models are used to examine the behavior of plumes resulting from a fresher outflow of transport M_r and salinity S_r into a pre-existing oceanic layer of initial thickness H₁ and salinity S₁. It is found that the plumes exhibit a variety of features depending on conditions of the outflow, the situation of the ambient ocean, and external forcing. Perhaps the most interesting feature is that the plume can flow along the upstream (to the left of the river mouth, looking seaward in the northern hemisphere) coast by itself, and the research discussed here is focused on this topic.

To illustrate how density variations associated with river plumes drive circulations, several solutions of geostrophic adjustment to an initially-imposed, y-independent density front are investigated. In these solutions, a frontally-trapped alongfront geostrophic current with the fresher water to its right (facing in the current direction) is always generated in response to the initial pressure gradient across the density front. This density-driven geostrophic current is dynamically similar to that resulting from initial disturbances in layer thickness h (equivalently, potential vorticity q = f / h) in constant-density models, with low salinity (density) in the variable-density model being analogous to the low q in the constant-density model.

Solutions to the 1^1/2-layer model driven by river outflow are fundamentally different in low-R⁰ (Rossby number) and high-R₀ regimes. In the low-R₀ case, plumes advance along both upstream and downstream coasts. If M_r is less than a critical value M_cr (determined by ΔS = S_j - S_r and H₁), plumes are coastally-trapped and all the river water-first flows upstream, with some of it, together with some salty water, reversing direction near the plume nose to flow along the offshore front, this return flow passes the river mouth and continues to flow along the downstream (to the right of the river mouth) coast. When M_r > M_cr, the plumes expand offshore indefinitely, and some river water must flow downstream directly. The evolution of the river plume for the low-R₀ solutions can be understood in terms of two distinct flow patterns. One is a downstream coastal current ("coastal mode") directly forced by the river transport; it is dynamically similar to the response in a linear, constant-density, 1^1/2-layer model, and is responsible for the downstream motion. The other is an anticyclonic circulation ("gyre mode") due to geostrophic adjustment of the river plume; the coastal current of this circulation is responsible for the upstream motion. Analytical solutions illustrate that geostrophic adjustment along the offshore density front generates the return flow and that Kelvin waves originating from the plume nose cause the upstream flow. They also allow the plume width L and the upstream nose speed c of the nose to be determined as a function of model parameters.

For the high-R₀ solutions, river water flows directly offshore in a narrow jet. The angle in which the jet emerges from the river mouth is found to depend on several non-dimensional parameters. Inclusion of entrainment significantly inhibits the upstream plume propagation, and makes it difficult to distinguish low-R₀ and high-R₀solutions.

In solutions to the 2^1/2-layer model, the upper-layer circulation is not significantly different from that in their 1^1/2-layer counterparts. A pre-existing downstream coastal current significantly weakens upstream plume propagation; indeed, the upstream advance can be completely stopped if the background current is strong enough. Ekman flow and alongshore currents induced by upwelling-favorable winds push the plume offshore and upstream, whereas downwelling-favorable winds result in a coastally trapped plume that is advected downstream.

Comments

Research funded by ONR Grant N00014-90-J-1054 and by NSF Grants OCE 92-03916 and OCE-95-03379 awarded to Julian P. McCreary.

NSUWorks Citation

Shuliang Zhang. 1997. Coastal Circulations Driven by River Outflow. Doctoral dissertation. Nova Southeastern University. Retrieved from NSUWorks, Oceanographic Center. (45)
https://nsuworks.nova.edu/occ_stuetd/45.