Date of Award

Spring 1995

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Committee Director

G. T. Csanady

Committee Member

C. E. Grosch

Committee Member

J. M. Klinck

Committee Member

J. C. Marshall

Abstract

Potential vorticity (PV) defined as: q = Δθ .(fk + Ω)

where θ is density anomaly, Ω (Δ x u) is relative vorticity, k is unit vertical vector and f the coriolis parameter, is used as a dynamical tracer to study the interior thermocline circulation. Using the generalized flux form of PV equation (Haynes and McIntyre, 1987), wind stress and buoyancy fluxes at surface outcrops of isopycnal surface are translated into PV fluxes. The PV flux condition so derived considers seasonal movement of the isopycnal outcrops and geostrophic turbulence.

A constant layer depth model, forced by the above flux condition, is used to study its influence on the interior circulation. The impermeability theorem of Haynes and McIntyre (1987), justifies treatment of a single isopycnic layer in isolation. Non-linear, quasi-geostrophic equations are used to study the dynamics on a rectangular basin model. The model is forced by PV at the northern boundary of the domain, which represents the location where the PV flux enters the thermocline interior.

PV input at the northern boundary allows the circulation to build up until opposite PV input at some other boundaries limits the process. The model simulation shows active northern, eastern and western boundary layers, and an interior circulation pattern with properties similar to those inferred by the homogenization theory of Rhines and Young (1982a). However, in the present study, the boundary layers control the key features of the circulation unlike in the classical models driven by Ekman pumping. The results show that the anticyclonic gyre forming in response to negative PV input on the northern boundary, has a strength depending on the intensity of the forcing, lateral diffusivity and the eastern boundary condition. In the case of an upwelled isopycnal (free slip eastern wall), the eastern boundary layer is stable and penetrates to a considerable distance south. On the contrary, for isopycnal intersecting the eastern boundary (no-slip wall), the eastern boundary layer separates at a short distance from the northeast corner, injecting massive amounts of positive vorticity into the basin. Cyclonic eddies are shed at a constant frequency near the eastern boundary, in the no-slip case, propagate towards the west and dissipate near the western boundary. Experiments with realistic subduction rates show that the PV transport due to the total pressure gradient along the isopycnal outcrop dwarfs the transport due to subduction. The results obtained mimic to a certain extent features of subtropical gyre circulation near eastern boundaries, notably in the Azores frontal area.

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DOI

10.25777/fkst-8n33

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