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Dissertation - NSU Access Only
Ph.D. Oceanography/Marine Biology
Julian P. McCreary
David B. Enfield
Pijush K. Kundu
Russell L. Snyder
Gary S. Kleppel
Four ocean models are used to investigate the response of the coastal ocean to strong offshore winds: a linear 11/2-layer model, a linear 21/2-layer model, a nonlinear 11/2-layer model and a nonlinear 21/2-layer model. The nonlinear models include thermodynamics and entrainment, the latter allowing cool lower-layer water to move into the upper layer. The models are forced by wind stress fields similar in structure to the intense winter-time, mountain-pass jets (~20 dyne/cm2) that appear in the Gulfs of Tehuantepec and Papagayo and blow directly offshore for periods of 3 - 10 days. Analytic and numerical solutions are arranged in a hierarchy of increasing dynamical complexity in order to illustrate the important physical processes. They compare favorably with observations in several ways, including the large sea-level drop at the coast and the fast westward propagation speeds of anticyclones.
While the wind strengthens there is an ageostrophic current (not Ekman drift) that is directed offshore. This offshore drift forces coastal upwelling, thereby lowering the local sea level and sea surface temperature (SST). Although the drop in sea level at the coast can be large and rapid, none of this signal propagates poleward as an upwelling-favorable coastally trapped wave. While the wind weakens the ageostrophic current is directed onshore, and consequently the coastal ocean readjusts toward its initial state. Throughout the wind event, cyclonic and anticyclonic gyres spin up offshore on either side of the jet axis due to Ekman pumping. Entrainment cools SST offshore on and to the right (looking onshore) of the jet axis, and virtually eliminates the cyclonic gyre. The advection terms intensify the anticyclonic gyre and give it a more circular shape. After a wind event, the anticyclonic gyre propagates westward due to β. In all the 11/2-layer solutions, its propagation speed is less than observed values. In the 21/2-layer solutions, however, the Tehuantepec gyre moves westward and southward at a speed of 7.5 km/day and Papagayo gyre propagates westward at 12.8 km/day, both close to observed speeds.
The coastal sea-level drop is enhanced by several factors: horizontal mixing, advection of the upper-layer thickness field h, enhanced forcing, coastal geometry, and the existence of a second active layer in the 21/2-layer model. Horizontal mixing enhances the sea-level drop because the coastal boundary layer is actually narrower with mixing; consequently, the interface below the upper-layer must rise farther to balance the volume of water that is displaced offshore. Advection of h intensifies upwelling by carrying regions of shallow h offshore. The forcing τ/h is enhanced near the coast where h is thin. When the coastal geometry includes a bay, the wind has an alongshore component that intensifies upwelling at the apex of the bay. Finally, in analytic solutions to the 21/2-layer model the presence of two baroclinic modes increases the sea-level drop to some degree; in numerical solutions the maximum sea-level drop also increases because it is not as severely limited as it is in the 11/2-layer model. Of these factors the strengthened forcing τ/h has by far the largest effect on the magnitude of the drop, and when all of them are included the resulting maximum drop is -30.0 cm, close to observed values.
To investigate the processes that influence the propagation speeds of anticyclones, several test wind-forced calculations as well as additional numerical experiments with isolated eddies were carried out. Solutions to dynamically simpler versions of the 11/2-layer model show that the speed is increased both by β-induced self-advection and by larger h at the center of the gyre, both factors measuring the strength of the gyre circulation. Solutions to the 21/2-layer model indicate that the lower-layer flow field advects the gyres westward and southward, significantly increasing their propagation speed. Solutions with isolated eddies confirm that self-advection enhances the propagation speed of 11/2-layer anticyclones, and indicate that it is advection by a wind-forced background circulation that increases the speed of 21/2-layer anticyclones.
Hyong Sun Lee. 1990. The Coastal Ocean Response to Strong Offshore Winds in the Gulfs of Tehuantepec and Papagayo. Doctoral dissertation. Nova Southeastern University. Retrieved from NSUWorks, Oceanographic Center. (79)