Moored Mixing Measurements at the Equator

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0424133A fundamental problem in understanding ocean dynamics is our inadequate knowledge of how the ocean is mixed. In turn, this means that numerical models used to predict future climate scenarios cannot properly parameterize oceanic mixing, which is a sensitive component of their computations. This is especially true in equatorial regions, where currents are strong, near-surface waters are stratified, mixing is energetic and complex, and feedbacks to the atmosphere are critical. A series of short process experiments between 1984 and 1991 revealed the qualitative nature of small-scale interactions responsible for generating turbulence and thereby mixing at the equator. Most importantly, these revealed that significant variations in mixing occurred on time scales longer than could possibly be sampled in short ship-borne process experiments. Indeed, there are strong hints that variations in mixing rates-weak mixing prior to the 1992-93 El Nino and strong mixing to conclude the 1997-98 El Nino-contribute positive feedback toward modulating equatorial SST on El Nino/La Nina time scales. New ways to measure mixing on time scales longer than a ship-borne process experiment must be found.A self-contained device to measure temperature microstructure using glassbead thermistors will be designed, build and tested locally before being deployed on a deep-water mooring. The objective is to develop a simple, reliable and affordable device to be readily integrated into existing moorings. Specifically, a suite of 10 devices will be deployed on the TAO mooring at 0 140W for an initial period of 6 months. This deployment will be augmented by 4 days of turbulence profiling at the mooring location. The moored devices will provide the data to determine the turbulent diffusivity of heat (or buoyancy) and hence the turbulent heat flux at each sensor location. By deploying many devices vertically, the vertical divergence of the turbulent heat flux, or the heating rate of a parcel of water due to turbulent mixing will be computed. By deploying over a long time period, the influence of turbulent mixing on sea surface temperature changes associated with long time scale events will be assessed. As well, fundamental aspects of the narrowband internal wave field that are closely linked to turbulence at the equator will be investigated - thereby contributing to our understanding of the physical relationship between mixing and internal waves above the core of the Equatorial Undercurrent. Broader Impacts: The successful demonstration of moored mixing measurements will lead to an affordable and reliable community tool that can be used to obtain long records elsewhere in the ocean. This is especially important in determining the role of vertical mixing as a feedback process to longer time scale events such as El Nino and La Nina. This tool will contribute to the measurement suite available for operational observing systems. Development and scientific analysis will contribute to the training of one graduate student, several summer undergraduate students and College engineers.