Collaborative Research: Equatorial Internal Gravity Wave Shear, Strain, Instabilities and Mixing--A Moored Process Study

University Of Washington, Seattle WA

Investigators

Abstract

Understanding the physics of instabilities that lead to the unique strong turbulent flux below the equatorial cold tongue is an imperative step toward developing a physics-based turbulence parameterization for improving the model prediction of the El Nino-Southern Oscillation (ENSO) phenomenon. The primary goal of this experiment is to make long-term observations of detailed evolutions of internal waves, instabilities, and mixing at time scales from 1-min to months and at vertical scales from 0.5 m to O(100) m below the equatorial cold tongue. The tongue of cold water straddling the equator in the eastern Pacific is one of the major sea surface temperature (SST) features influencing tropical air-sea interactions. Vertical turbulent fluxes through the base of the surface mixed layer are major factors in the heat budget that determines SST. Strong turbulence during La Nina cools the mixed layer, strengthening the cold tongue. Weak turbulence during El Nino allows the mixed layer to warm, weakening or erasing the cold tongue. Several periods of intensive microstructure profiling discovered the role of small-scale mixing in modulating the cold tongue, but attempts to parameterize the turbulent fluxes failed because the physics leading to instabilities and turbulence has not been captured by these measurements. Previous observations indicate that internal waves play the key role by providing the finescale shear that trigger instabilities. High spatial and temporal resolutions of measurements, sufficiently capturing instabilities below the cold tongue, are needed, but have not been made. In the present experiment, shear and strain will be measured at 1-minute intervals over vertical scales of O (1-100) m and turbulent scalar diffusion rates immediately beneath the surface mixed layer in the cold tongue. Co-investigators Moum and Nash at OSU have recently developed a moored microstructure package that can reliably measure turbulent scalar diffusion rates in the equatorial undercurrent. Closely vertically spaced instruments measuring velocity, temperature, salinity and mixing rates will be mounted on a surface mooring. The mooring will be 5 miles from the long-term TAO mooring at 0 140 W. The experiment will be coordinated with another funded observational program. In spring 2008, 10 pods, evenly spaced between 30 and 90-m depth, will be deployed on the long-term TAO mooring at 0 140 W. The deployment will be Immediately with18 days of shipboard microstructure profiler measurements. Intellectual Merit. Internal waves produce much of the diapycnal mixing in the ocean and must be understood and quantified before accurate mixing parameterizations can be developed. In the eastern equatorial Pacific, the stratified high-shear zone between the undercurrent core and the base of the mixed layer provides a unique environment for internal waves. Understanding it well enough to accurately parameterize the diapycnal mixing is a major intellectual challenge whose solution will advance the general understanding of internal waves, instabilities, and turbulence, as well as provide necessary support for climate studies. Broader Impact This work is expected to lead to more accurate parameterizations of equatorial mixing in general circulation models (GCM) and thus to more realistic climate models. SST, air-sea interaction, and ENSO in the cold tongue are strongly controlled by the turbulence entrainment flux. To improve the large-scale model prediction of ENSO, we need to understand the physics of instabilities and turbulence and improve the turbulence parameterization in GCMs. The proposed project will provide support for the education of a graduate student. The observations and analysis results will be published on a publicly accessible website.