James Stephens

J. C. Stephens, 1998, PhD thesis, University of Reading, UK, 165pp.

Abstract

Two complementary aspects of the thermohaline circulation are considered. In Part I, we examine the linear stability of rotationally-influenced gravity currents with zero potential vorticity, flowing over a topographic slope. Several classes of unstable normal mode are found, and the theory of resonant instabilities provides a simple explanation of the origin of these instabilities and allows one to identify the physical mechanisms primarily responsible for them. Two new types of instability that rely on the presence of topography are found. The first, which is relatively weak, arises as a result of a resonance between gravity waves on the jet and topographic Rossby waves. The second is more powerful, occurs at relatively long wavelengths, and is brought about by a resonance between a vortical wave on the jet and topographic Rossby waves.

In Part II, we develop a 2.5 layer planetary-geostrophic model of the North Atlantic, driven by climatological winds and eastern-boundary ventilation, to investigate the effect of the wind-field in shaping the Mediterranean salinity tongue. The upper-layer depth from our model shows a clear similarity to observations, both in terms of the location and intensity of the subtropical gyre and also the position of the outcropping line in the northern basin. Potential vorticity in layer two reproduces the sweep of potential-vorticity contours south-westwards from the eastern boundary, and provides the pathways along which Mediterranean Water spreads into the model interior. We solve for the steady salinity field in the second layer, including sources of Upper Labrador Sea Water and Antarctic Intermediate Water on the isopycnal surface. The shape and spreading latitude of the model salinity tongues bear a striking resemblance to observations. We also include a simple parameterisation of meddies, which we show act to push the salinity tongue southward.

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