Moored Mixing Measurements at the Equator

To progress toward the goal of establishing extended observations of mixing and internal gravity waves, we have developed, tested and deployed a string of moored temperature microstructure recorders at the 0 140W TAO mooring site. The initial deployment yielded a highly-resolved, 4-month time series of temperature (T), the rate of dissipation of temperature gradient fluctuations () and estimates of the turbulence diffusion coefficient () and the turbulent kinetic energy dissipation rate () at 3 depths over the range 30-90 m. We have since attempted to maintain a presence at 0 140W and have deployed at 3-6 depths continually since September 2005. We expect these measurements will provide a meaningful beginning toward the objectives of dynamically linking the mixing to the larger spatial scales and longer time scales at this and other locations.

deployment tutorial movie (15MB)

 

One adverse consequence of cable motion forced by surface wave pumping is that the measured dT/dt can be dominated by vertical pumping of the sensor through a background vertical temperature gradient dT/dz, when vertical cable velocity, wc, is large. In the swell data band (near 0.1 Hz), the example 5 h spectrum of dT/dt corresponds identically with that of wcdT/dz at both 49 m and 84 m, where dT/dz is also large at this point in time. At 29 m, dT/dz is smaller, and surface wave contamination is not apparent over the band 0.1 < f < 1 Hz. Significantly, it is also evident that this band of contamination is distinct from the dissipation subrange used to compute (f > 1 Hz), and the internal gravity wave range f < 0.1 Hz.

The purpose of this measurement is to obtain long, uninterrupted time series of equatorial mixing. In examining the entire record of this and following deployments, there appear to be many different mixing states. This is the focus of ongoing scientific analysis. For the purpose of developing confidence in this new measurement, we consider a short, 4 day record that shows strong variability on a daily time scale. Here, the TAO buoy measurement of short wave radiation distinguishes between night and day. Local temperature maxima (~0.1C) at 29 m depth occur after sunset and are accompanied by high frequency fluctuations (these are narrowband and near N - not shown here). The daily cycle is echoed in , and . In each case, day-night differences are about 3-4 orders of magnitude.