An investigation of oceanic gap-traversing boundary currents and shelf-slope dynamics through analytic and experimental models
Date
2020
Authors
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Publisher
University of Delaware
Abstract
The problem of oceanic gap-traversing western boundary currents is considered. Examples of such situations are the Gulf Stream forming the Loop Current in the Gulf of Mexico, and the Kuroshio Current traversing the Luzon Strait of the South China Sea. These systems are dominant drivers in the regions they occupy and they affect ecosystems, weather, climate, and humanity. The currents must traverse the gap, either by leaping across it or by penetrating into it. Transitions between these two general states are difficult to predict and the driving mechanisms are not well understood. ☐ Motivated by these problems, recent works have studied gap-traversing systems with simplified models. Notably, these models exhibit hysteresis: transitions between penetrating and leaping states do not necessarily occur for the same flow rates of the incoming boundary current, depending on if the flow rate is increasing or decreasing. Thus, multiple states may exist for a given parameter configuration. This hysteresis may be a large factor in the unpredictability of loop current systems in the ocean, if it is indeed present in the environmental systems. ☐ This dissertation will build upon recent studies through simplified numerical and laboratory experiments. Relevant oceanographic effects and leading theories on the dynamics of the gap-traversing systems in the ocean will be adapted to the models. For the Kuroshio intrusion, there is often significant net transport directed westward or eastward through the Luzon Strait. The effect of a positive or negative mean throughflow through the gap in a simplified model will be studied in barotropic experiments. For the Loop Current, the dominant theory behind eddy detachment is baroclinic instability (i.e. two-layer effects). Thus, baroclinic effects will be studied in a model and in particular, lower layer forcing will be examined. According to dynamical systems theory of hysteretic systems, there should exist an unstable third state between the penetrating and leaping solution branches of the system. This unstable state will be calculated from numerical model results for the first time and used in an effort to reveal neutral modes active when the system nears transition. ☐ As a preface to the experiments, a better understanding of fundamental details of stratified flow is sought. First, an idealized case of lower layer flow at a slope is considered with the derivation of an analytic solution. This provides a framework that explains how Ekman pumping may control interface evolution in intensified lower layer flow. Second, as an aside from the analytic solution, the results of a cross-shelf transport study at the shelf-slope region are presented. A model is devised to characterize transport across a jet due to interactions with a topographic wave and tidal forcing. Such a mechanism is much different in nature than the episodic events that are thought to be main drivers of cross-shelf exchange (e.g. eddy impingement). This study also provides an example of the utility of analytic solutions. Lastly, returning to fundamental studies on layered systems, a viscous 1.5-layer western boundary current (WBC) is investigated. A problem for the structure function of this WBC is derived and solved numerically, which demonstrates the effect of the layer interface. This allows for explanation of the north-south asymmetry of WBCs observed in layered laboratory experiments, which previously could not be explained with standard quasigeostropic theory.
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Keywords
Boundary, Current, Dynamics, Fluid, Geophysical, Western