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Fast Response of the Meridional Overturning Circulation to Agulhas Leakage Perturbations

Erik van Sebille
Master Thesis, 2005, Utrecht University, 80 pages


In this thesis, the adiabatic response time of the Meridional Overturning Circulation to the introduction of an Agulhas Ring in the South Atlantic Ocean and the influence of the Mid-Atlantic Ridge on this response is investigated. To this end, a two-layer Primitive Equation model on a sphere was constructed. The ring is a circle-symmetric Gaussian mass of excess water, in cyclogeostrophic balance. To gain some insight in the strength of the response, the Meridional Overturning Circulation is modelled as two regions, one in the northernmost and one in the southernmost part of the basin, where water is exchanged between the layers. The magnitude of the exchange is governed by two different parameterizations. In the first it is related to the difference in zonally averaged pressure difference at the sea surface, between two predestined regions. In the second it is related to the difference between the meridional mass flux in the upper and the lower layer over 40N.
The Agulhas rings are simulated realistically. The ring signal travels the ocean via consecutively long Rossby waves and western coastal Kelvin waves on the southern hemisphere, equatorial Kelvin waves and eastern coastal Kelvin waves to finally radiate long Rossby basin modes in the northern hemisphere. The modelling of the overturning circulation has been less successful. Although parameter tuning gives consistent results, the model contains some unwanted features that are discussed in detail.
The impact of one Agulhas ring on the Meridional Overturning Circulation is consistent in the two parameterizations. In the first 3 years, the overturning slows down to accelerate beyond the unperturbed situation after that. The introduction of the Mid-Atlantic Ridge in an attempt to short-cut the wave propagation had little influence on the response time. The magnitude of the response is different for the two parameterizations. The pressure gradient driven parameterization yields a 0.05 Sv response, which is independent of the presence or absence of a Mid-Atlantic Ridge. The flux-driven one gives a 0.4 Sv response in a configuration with a ridge, and a 0.15 Sv response when no ridge is present.