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Upper Ocean Instrumentation for the Northwest Tropical Atlantic Station

OCCI Project Funded: 2001

Proposed Research

This proposal would support the purchase of instrumentation that will allow the local oceanic budgets of heat and momentum to be determined at a critical location in the tropical Atlantic. With support from the National Oceanic and Atmospheric Administration (NOAA), a surface mooring site called the Northwest Tropical Atlantic Station (NTAS) has been established at 15°N, 51°W. This site will be maintained for a period of at least four years, perhaps for as long as ten years. The site is of interest due to strong sea surface temperature variability on seasonal to decadal time scales, and the expectation of significant air-sea interaction. The existing NOAA project provides support for a mooring with high-quality meteorological sensor systems on the surface buoy, but not for subsurface measurements. The temperature and velocity sensors proposed here, placed along the upper 100 m of the mooring line, would provide substantial added value to the NTAS surface flux estimates in the form of local budget estimates.

Final Report

Background
An air-sea interaction mooring site, denoted the Northwest Tropical Atlantic Station (NTAS), was established in 2001at 150 N, 510 W with support from the National Oceanic and Atmospheric Administration (NOAA). The intent of the NOAA project is to provide high-quality, surface flux estimates at this site for up to 10 years through successive 1 year mooring deployments. The site is of interest due to strong sea surface temperature variability on seasonal to decadal time scales, and the expectation of significant air-sea interaction. A fundamental issue is to determine regional air-sea interaction processes and feedbacks, and a necessary first step is to determine the degree of local balance in the heat, moisture and momentum budgets. NOAA provided support for moorings with high-quality meteorological sensor systems at the surface, but not for subsurface measurements. The Ocean and Climate Change Institute (OCCI) grant funded the purchase of subsurface temperature and velocity sensors for the moorings. This provided substantial added value to the NTAS mooring data because allows the local oceanic budgets of heat and momentum to be determined.

Approach
Funds from this grant were used to purchase 20 Seabird SBE-39 temperature sensors and a RDI Acoustic Doppler Current Profiler (ADCP). A second ADCP was obtained from our inventory. The SBE-39 temperature sensors provide high-accuracy (0.005 oC) measurements with 5-10 m resolution in the upper 80 m (the expected seasonal range of the mixed layer). Inexpensive, low-accuracy (0.2 oC) “Tidbit” temperature sensors are used in the upper thermocline (90-120 m) where temperature gradients are larger. The ADCPs provide velocity profiles with 8 m vertical resolution and 1 cm/s precision within the upper 100 m. One ADCP and 10 temperature sensors were deployed on the NTAS-2 mooring (Plueddemann et al., 2002). These data are now being analyzed. The second ADCP and the remaining 10 temperature sensors were deployed on the NTAS-3 mooring (Plueddemann et al., 2003; Fig. 1), and are awaiting recovery in 2004. The NTAS-2 instrumentation will be refurbished for NTAS-4, the NTAS-3 instrumentation will be refurbished for NTAS-5, etc. In this way all NTAS deployments after the first year will benefit from high-quality subsurface temperature and velocity data.

Results
The ADCP and 9 of the 10 SBE-39 temperature sensors on the NTAS-2 mooring returned complete records. Daily average temperature in the upper 130 m is shown in Fig. 2 for the 11.5 month NTAS-2 deployment period (4 Mar 2002 - 16 Feb 2003). Seasonal heating and cooling is concentrated in the upper 60 m. A difference of about 2 oC is found between the warmest temperatures in September (yearday 244-273) and the coolest in February (yearday 397-424).The relatively weak vertical stratification (~0.5 oC) in the upper 60 m is also notable relative to the stronger gradient (2-3 oC) between 80 and 120 m.

High-accuracy temperature measurements in the upper 80 m are necessary to reduce errors in the heat budget calculations and to distinguish the surface mixed layer from deeper, weakly stratified water. This can be seen in Fig. 2 where the vertical gradient of temperature is plotted for the first 8 months of the record. The mixed layer (white line), defined by a temperature difference from the surface of 0.01 oC (Plueddemann and Weller, 1999; Thomson and Fine, 2003), is typically less than 20 m deep and is often well separated from the top of the main thermocline (black line). These two levels would be indistinguishable using the low-accuracy temperature sensors. Wind-driven shear flow, typically concentrated within the "transition layer" between the mixed layer and the upper thermocline (Plueddemann and Weller, 1999), will be detected by the ADCP.

In further analysis, the oceanic heat content will be computed by vertical integration from either a fixed reference level or an isotherm below the mixed layer. Comparing the rate of change of heat content to the air-sea heat flux will determine the degree of local balance. The momentum budget will be assessed by isolating the wind-driven flow using the combination of a vertical reference level, coherent averaging (e.g., in crosswind/downwind coordinates), and comparison of the integrated flow to the Ekman transport predicted from the wind stress. The objective is to determine the degree to which the oceanic budgets of heat and momentum are locally balanced at the mooring site.


References

Thomson, R.E. and I.V. Fine, 2003. Estimating mixed layer depth from oceanic profile data, J. Atmos. Ocean. Technol., 20(2), 319-329.

Plueddemann, A.J. and R.A. Weller, 1999. Structure and evolution of the oceanic surface boundary layer during the Surface Waves Processes Program, J. Mar. Sys., 21, 85-102.

Plueddemann, A.J., N.R. Galbraith, W.M. Ostrom, G.H. Tupper, R.E. Handy and J.M. Dunn, 2001. The Northwest Tropical Atlantic Station (NTAS): NTAS-1 mooring deployment cruise report. Woods Hole Oceanog. Inst. Tech. Rept. WHOI-2001-07, 55 pp.

Plueddemann, A.J., W.M. Ostrom, N.R. Galbraith, P.R. Bouchard, G.H. Tupper, J.M. Dunn and M.A. Walsh, 2002. The Northwest Tropical Atlantic Station (NTAS): NTAS-2 mooring deployment cruise report. Woods Hole Oceanog. Inst. Tech. Rept. WHOI-2002-07, 68 pp.

Plueddemann, A.J., W.M. Ostrom, N.R. Galbraith, J.C. Smith, J.R. Ryder, J.J. Holley, and M.A. Walsh, 2003. The Northwest Tropical Atlantic Station (NTAS): NTAS-3 mooring deployment cruise report. Woods Hole Oceanog. Inst. Tech. Rept. WHOI-2003-03, 69 pp.

Originally published: January 1, 2001