Publication Date

2014-12-16

Availability

Open access

Embargo Period

2014-12-16

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2014-10-21

First Committee Member

Igor Kamekovich

Second Committee Member

Pavel Berloff

Third Committee Member

William Johns

Fourth Committee Member

Tamay Ozgokmen

Abstract

The discovery of multiple nearly zonal oceanic jets enriches the classical view of mid-latitude ocean circulation, which was thought to be dominated by turbulent eddies and large-scale oceanic gyres. These jets emerge as a result of baroclinic instability and are maintained by baroclinic eddies. Potential importance of interactions of these ocean currents with major topographic features, such as the mid-Atlantic ridge, calls for the studies of the effects of bottom topography on the dynamics of eddies and the eddy-driven jets. The importance of bottom topography in the linear baroclinic instability of zonal flows on the beta-plane is examined by using analytical calculations and a quasi-geostrophic (QG) eddy-resolving numerical model. The particular focus is on the effects of a zonal topographic slope, which are compared with the effects of a meridional slope. A zonal slope always destabilizes background zonal flows that are otherwise stable in the absence of topography, regardless of the slope magnitude, whereas the meridional slopes stabilize/destabilize zonal flows only through changing the lower-level background potential vorticity (PV) gradient beyond a known critical value. Growth rates, phase speeds and vertical structure of the growing solutions strongly depend on the slope magnitude. In the numerical simulations configured with an isolated meridional ridge, unstable modes develop on both sides of the ridge and propagate eastward of the ridge, in agreement with our analytical results. In the second part of this study, we describe a novel mechanism for the generation of oceanic alternating jets by topographic ridges. The dynamics of these jets is examined using a baroclinic QG model configured with an isolated meridional ridge. Zonal topographic slopes of the ridge lead to the formation of a system of currents, consist- ing of mesoscale eddies, meridional currents over the ridge, and multiple zonal jets in the far field. Dynamical analysis shows that transient eddies are vital in sustaining the deep meridional currents, which in turn play a key role in the upper-layer PV balance. The zonal jets owe their existence to the eddy forcing over the ridge but are maintained by the Reynolds and form stresses in the far field. Locally nonzonal mean currents are shown to produce zonal jets in the far field, but the presence of local vorticity source appears to be the truly fundamental cause of jet existence. We conclude that local nonzonality of the mean PV contours, due to either a topographic ridge or a nonzonal background flow, can generate multiple zonal jets through a remote mechanism. Lastly, we explore the relationship between eddies and striations in a baroclinic double gyre ocean. Results show that alternating vertically coherent striations exist in the eastern part of the double gyre in short time mean field. Both the baroclinic QG model simulation and the analysis of altimetry sea level anomalies suggest that striations are intrinsically associated with eddy trains. Eddy vorticity forcing is closely associated with the striations and tends to drive the striations to drift meridionally. The concept of eddies being intrinsically related to striations helps to come up with a better parameterization scheme to represent the effects of eddies/striations in comprehensive models.

Keywords

Mesoscale eddies; jets; baroclinic instability; topography

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