Matthießen, Jan-Dirk (2017) Modelling the Equatorial Deep Jets. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 93 pp.

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Abstract

The Equatorial Deep Jets are a series of stacked zonal jets below the Equatorial Undercurrent with a vertical scale of several hundred metres and a time scale of many years (4.5 years in the Atlantic Ocean). They are an ubiquitous feature of the equatorial ocean system, but to this day realistic domain ocean models fail to reproduce their basic features. This thesis begins by describing a set of idealised ocean model simulations that shed some light on why ocean models have such difficulties with the deep jets. The idealised model solutions are able to reproduce the basin mode like variability that can be found in observations. A sufficient vertical resolution is seen to be necessary for the evolution of deep jets in the model, and in contrast to previous studies the introduction of a realistic coastline does not eliminate the deep jet variability. But another feature, the prominent upward energy propagation of the deep jets found in observations, cannot be reproduced in a simulation with realistic bottom topography, albeit even though in this simulation the deep jets have the vertical scale and corresponding time scale of theoretical basin modes. In the next part of the thesis we explore a surface influence of the equatorial deep jets. The North Equatorial Countercurrent (NECC) exhibits a modulation in its meandering with the same time scale as the deep jet period. This was shown by comparing satellite data of the NECC with moored observations of the deep jets. The surface influence was then confirmed in two idealised model simulations with different basin widths, where the shorter basin width leads to a shorter basin mode period. We also diagnosed the vertical energy flux associated with the deep jets, the first time this has been done in a model, to give a reasoning for the deep jets’ influence on the NECC. Following that is a chapter about the emergence of deep jets in the idealised model simulation. The spin-up of these deep jets can be explained by the linear superposition of equatorial basin modes, where the time at which the model starts to exhibit downward phase propagation corresponds to the beat period of two superposed basin modes. The basin modes are excited at the start of the model run as Rossby and Kelvin waves at the eastern and western boundaries of the model and appear to be amplified by the non-linearity in the solution.

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