Claus, Martin (2016) Shallow Water Models of the Atlantic Equatorial Deep Jets. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 111 pp.

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Abstract

In this thesis, the dynamics of the Equatorial Deep Jets (EDJs) are investigated with a special focus on the Atlantic basin. The EDJs are vertically stacked, geostrophically balanced, alternating zonal jets along the equator that are present in all equatorial oceans below the pycnocline. In the Atlantic basin, they have a vertical scale of 300-700 m, a time scale of about 4.5 years and represent the dominant mode of interannual zonal flow variability along the equator in the intermediate depth range that is associated with upward energy propagation. The tool of choice is a linear shallow water model (SWM), given the good agreement of the EDJs with linear equatorial dynamics, and for the reason that the EDJs are presently not or not realistically represented in state-of-the-art ocean models. A striking aspect of the EDJs is their meridional width, which is about 50% broader than expected from inviscid linear dynamics based on the vertical scale. Two possible effects that can lead to a broadened meridional scale shall be investigated: Dissipation of momentum in the absence of sufficient diapycnal diffusion and the presence of a barotropic mean flow. The former mechanism is investigated by conducting a sensitivity experiment with the linear SWM with respect to lateral eddy viscosity and the results suggests that a lateral eddy viscosity of 300 m2 s−1 is sufficient to account for the observed broadening. To study the influence of the barotropic mean flow on the EDJs, the SWM is linearised about a mean state derived from observations. The results indicate a shielding of the equatorial wave guide by the North and South Equatorial Intermediate Currents but little influence of the mean flow at the equator itself, if sufficient lateral mixing is applied. The zonal coherence of the EDJs and their long time scale can lead to a significant transport of dissolved oxygen along the equator, and hence might contribute to the observed equatorial tongue of dissolved oxygen spreading out from the western boundary. A simple advection- diffusion model is driven with the velocity field of the EDJs, simulated with the SWM, that are superimposed on the mean flow at selected depth levels to investigate the effect of the EDJs on the mean oxygen distribution and its variability. The model is able to correctly simulate the phase difference of zonal velocity and oxygen anomaly and hence produces an eastward net oxygen flux. The simulated EDJ induced oxygen variability depends strongly on the mean zonal gradient of oxygen along the equator, which is mainly set by the mean flow, and can explain a substantial part of the observed oxygen variability at the equator. Based on dense, long-reaching, near-full-depth observations of zonal velocity at 23°W on the equator, a linear representation of the EDJs in the Atlantic basin is reconstructed by using a multi-mode SWM. As a by-product, the vertical structure of the zonal forcing is inferred that is required to drive the EDJs. From the resulting vertical profile of mean power input into the EDJs, it follows that the EDJs are maintained over a considerable depth range and that the apparent absence of downward energy propagation is due to the strong dissipation
and missing support of the jets below the peak power input at 1300 m.

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