AUV Turbulence Measurements in the LOCO Field Experiments 

The Objective of this program is to quantify the fine and microstructure (turbulence) fields during the LOCO DRI field observations and relate them to mixing and dispersion. Particular attention will be given to the role of turbulence in the formation, maintenance and dissolution of so-called “thin layers.” Experimental focus will be on the intensive heavily-instrumented, process-oriented part of the LOCO field experiments, which are expected to be of order 1 to 10 kms in horizontal spatial extent. We will employ the new SMAST turbulence AUV which is a second generation turbulence vehicle (REMUS/HYDROID) based on the pioneering work of Levine and Lueck (1999). The vehicle is equipped with a full suite of turbulence and fine structure sensors and is proposed to also have optical backscattering and chlorophyll sensors.

A recently developed technique utilizes the various turbulence and fine structure AUV-borne data to directly estimate the eddy and scalar turbulent fluxes as well as the dissipation rate, ε, and variance of scalar gradient fluctuations, χ. Estimation of these quantities along with estimation of the fine scale shear, temperature, salinity, and density gradient, allows calculation of all of the terms of the steady state, homogenous Turbulent Kinetic Energy (TKE) and Scalar Variance Budgets (Goodman and Levine, 2003). By doing this the usage of various assumptions to calculate mixing efficiency and the turbulent eddy and scalar diffusivities can be examined. This is particularly important in regimes characterized by weak turbulence and strong stratification (the conditions typically present for thin layers), since in those regimes it may not be possible to use classical turbulence theory along with various “standard” parameterizations about mixing to estimate the turbulent diffusivities.The AUV turbulence and finestructure observations coupled with this theoretical framework will be used to quantify the turbulence level, and the spatial structure and the relationship of turbulence to finescale shear and density gradients both within and around Thin Layers. This approach will allow one to directly address the issue of the role of shear straining and dispersion in the development, maintenance, and decay of biological and physical thin layers. Obtaining a very dense set of horizontal measurements will also allow estimate of turbulent advection terms. The analysis and modeling of the field data as well as experimental planning would be carried out in very close coordination with other LOCO investigators and their observational techniques, i.e. moored, profiling, and ship deployed physical, biological, acoustical, and optical sensors and systems.

LOCO Experiments 

A 5-year, $9 million project termed LOCO, for “Layered Organization in the Coastal Ocean,” funded by ONR (Office of Naval Research) was conducted in Monterey Bay, CA in August, 2005 and July, 2006 to study on and attempt to quantify the mechanism of thin layer formation and evolution.

Experimental Sites: Monterey Bay (MB) located along the central California coast between 36.55° and 37.00°N (Fig 1a), is the largest bay along the west coast of United States. It is a semi-enclosed, with an open connection to the ocean. It has a very limited of fresh water input. Along with its deepness and broadness, Monterey Bay is not an estuary as it is not significantly diluted by the fresh water influx except temporally during brief periods of large river discharge. The Bay is symmetrical in shape and divided approximately equally into a northern and southern sector by the Monterey Submarine Canyon (MSC). The MSC, is the major topographic feature in MB, and has a very important role in transporting nutrients from the deep ocean to the shallower costal regions by the internal tide (Shea and Broenkow, 1982). Although MSC represents a major depression of the bay, a major fraction of the bay is very shallow. Approximately 80% of the Bay, 440 km2, is shallower than 100m and only 5% is deeper than 400m (Breaker and Broenkow, 1994). There are two bights in MB, one to the south near Monterey and a second to the north between Santa Cruz and Aptos. The T-REMUS AUV LOCO surveys were conducted in the northern bight during 15th August to 4th September 2005 and July 13th to 26th, 2006. Bathymetry within the experimental site (Fig1.1b) is relatively uniform, from 16m to 23m across shelf. Fig 1b shows the detailed tracks of a small Autonomous Underwater Vehicle (AUV) and the fixed LOCO observatory stations during these two experiments.

Figure 1. a) Map of the complex topography within and offshore of Monterey Bay. b) Detailed map showing the tracks of the T-REMUS AUV during LOCO. The blue lines indicates the engineering runs of August 15th, 2005; the green rectangle was used for two daytime runs and three nighttime runs; the red square indicates location of the experiment at mid-night of August 26th, 2005; and the magenta square the experiment at the morning of September 04th, 2005. The black lines indicate the engineering run on July 13th, 2006; the salmon-colored lines indicate the location of the LOCO 2006, four, eight-hour runs. The black contour lines represent the isobath in 5m intervals. The red diamonds with characters (E, W, S, N and C) are the five fixed LOCO observatory stations deployed by Donaghay and Holiday etc during LOCO 2005. The red squares with characters (K0 to K4) are the K-lines stations deployed by Donaghay and Holiday etc during LOCO 2006.
Preliminary Result: The preliminary analyses from the two LOCO experiments show that we have collected a very complete data set. The high resolution CTD and the microstructure measurement package (RMMS) provide unique concurrent horizontal and vertical characterizations of temperature, salinity, density and direct estimation of turbulent dissipation rate. By combing that data with finescale measurements of density and shear, estimates can also be made of the Froude and Buoyancy Reynolds numbers and thus a characterization of the intensity of the turbulent field form an activity diagram (Ivey and Imberger, 1991). The upward and downward looking ADCP provides the profiles of the vector current field. In addition also measured are optical backscattering at two wavelengths (470 and 700nm) and chlorophyll fluorescence and acoustic backscattering at 1200 kHz from the ADCP. The BB2F sensors yield horizontal and vertical characterization of fine scale optical parameters useful for estimating phytoplankton biomass. In the absence of other factors, the profile of chlorophyll a production in the sea could be expected to be related directly to productivity (e.g. Wolf and woods, 1988). Furthermore, the ratio of the two backscattering channel to the chlorophyll fluorescence could be used to identify the living phytoplankton from the mineral and dense detrital particles. Fig 2 shows the contour maps of Chlorophyll a measured by T-REMUS during night experiment on July 17th, 2006.

Fig 2. Contour maps of Chlorophyll a obtained by T-REMUS during LOCO 2006.

Fig 3 is a three dimensional view of the same chlorophyll a data collected on July 17th, 2006 data. From this plot, it clearly shows two separate thin layers were measured at the upper water column during that night. And there are also some patchy structures of chlorophyll a in the deeper layer. Combined with the concurrently measured fine- and micro- scale physical and optical data, a statistics analysis will be conducted for the physical and biological characteristics both in and around the thin layers. The mechanisms of the formation, evolution and breakdown of thin layers will also be studies using these LOCO data.

Fig 3, one example of 3D view of Chlorophyll a thin layer measured by T-REMUS.