Dynamics of the Equatorial Boundary Layer: A comparison of equatorial wave theory and observations
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The equatorial region (+/- 2 degrees in latitude) differs from the mid-latitude regions in that there is the presence of strong zonal jets, known as equatorial deep jets. The overarching goal of this thesis is to search for eastward propagating waves that may either be explained by or produce these strong zonal currents. Equatorial zonal jets are traditionally thought to be driven by east to west surface winds, which create a pressure gradient between the eastern and western boundaries resulting in these underlying currents. As equatorial jets are stacked, with a westward, very shallow surface current and eastward undercurrent at about a 100m depth, it is ambiguous as to whether these zonal mean flows lead to instabilities in the form of waves or vice versa, whether the waves actually play a role in driving these currents. The first part of the thesis reviews the theoretical models used to describe equatorial wave theory: the linear approach assuming the beta plane approximation versus the non-traditional approach, which includes the full Coriolis term. Phase speeds for different types of waves are obtained from the dispersion relation using the two approaches. The second part analyzes satellite sea surface height anomaly data to obtain phase speeds of propagating equatorial ocean waves and uses a spectral analysis to obtain information on spectral peaks. A 2D Fourier transform is applied to create a filter where only eastward/westward propagating signals are isolated. The focus is on eastward propagation as the Equatorial Under Current may steer waves eastward. Alternatively eastward moving waves may be rectified and drive this eastward propagating jet. By gaining insight into the properties and types of eastward moving waves, the mechanisms driving the eastward zonal jets may be more tangible, as most of the observed equatorial waves appear to propagate zonally. The Fourier filter is followed by a Radon transform to precisely determine observational phase speeds, which are then compared to theoretical phase speeds to identify different wave types. Pacific ocean wave speeds were higher on average for all frequency bands in comparison to the Atlantic and Indian oceans. When the annual cycle was removed, the presence of a mode 1 classical Rossby wave, lacking meridional nodal lines, was found in the Indian ocean. Equatorial Kelvin waves were found in the Pacific ocean at the tri-annual and 70 day cycle. A gravity and westward propagating mixed Rossby-gravity wave were found only at the 25 - 40 day period band in the Atlantic. The presence of eastward propagating Rossby waves was not confirmed from the observational data sets, however examples of classical waves were obtained.