This project is directed at understanding the formation of small vertical scale features in the equatorial thermocline, their interaction with other scales, and the overall impact on the larger scale dynamics of equatorial flows. The investigation is undertaken in the context of the wind forced basin scale flow. The stimulus comes from observations of such small vertical scale features in the thermocline of the equatorial Pacific and findings emerging from recent theoretical and numerical work. In addition to the familiar barotropic/baroclinic instabilities of the zonal mean flow, equatorial currents are also subject to instabilities that favor much smaller vertical scales of the order of tens of meters but with relatively large meridional and zonal extents. A manifestation of small vertical scale features in the equatorial thermocline is the presence of the interleaving of water masses. Previous work suggests that such features have a significant impact on the basin scale dynamics. We have devised a series of numerical experiments to elucidate the properties of these small vertical scale structures and their interaction with other scales. Making use of the considerable computational resource of the Japanese Earth Simulator we are able to put this study in the context of the basin wide dynamics.
Intellectual Merit: The extraordinary richness in spatial scales coupled with the high temporal variability of the equatorial ocean raises a number of intriguing and important questions concerning the scale selection process of features, meridional versus zonal coherence, and the potential interaction between scales. The small scale features themselves can induce considerable mixing of tracers, momemtum and potential vorticity, affecting the development of other modes and the large scale state. Our investigation covers aspects of equatorial dynamics that have only recently started to be studied. We will capitalize on new findings on the stability of equatorial flows. The outcome will be an improved understanding of equatorial dynamics and the impact of multi scale interactions in the system.
Broader Impacts: The robustness of climate predictions depends on the effectiveness of atmosphere and ocean models to capture the essential physics of the problem. The ocean state affects the seasonal and interannual variability of the coupled system. The ocean state is itself very sensitive to the form and level of lateral and vertical mixing. Here we plan to explicitly model the mixing induced by small vertical-scale structures in the equatorial thermocline and their overall impact on the basin-scale dynamics. The characteristics of the small vertical scale features are themselves dependent on the broader scale flow on intraseasonal through to interannual timescales. We have the intriguing possibility of multiple scale interactions on a broad range of space and time scales. The present study will go some way to assessing what aspects of these interactions need to be taken into account in climate models. A graduate student will be employed on the project. The student will receive training in fluid dynamics, numerical modeling and analysis of observations.
