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A Northwest Tropical Atlantic Station for Flux Measurement (NTAS)

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July 1, 2003 to June 30, 2004

PI: Albert J. Plueddemann
Woods Hole Oceanographic Institution
Woods Hole, MA 02543-1541

Program Manager: Michael Johnson, NOAA OCO

Related NOAA Strategic Plan Goals: Goal 2 (80%) – Understand Climate Variability and Change to Enhance Society’s Ability to Plan and Respond and Goal 3 (20%) – Serve Society’s Needs for Weather and Water Information.

Background: The Northwest Tropical Atlantic Station (NTAS) project for air-sea flux measurement was conceived in order to investigate surface forcing and oceanographic
response in a region of the tropical Atlantic with strong SST anomalies and the likelihood of significant air-sea interaction on seasonal to decadal time scales. The strategy is to maintain a meteorological measurement station at approximately 15 N, 51 W through successive (annual) turn-arounds of a surface mooring. Redundant meteorological systems measure the variables necessary to compute air-sea fluxes of heat, moisture and momentum using bulk aerodynamic formulas.

Objectives: NTAS has two primary science objectives. First, to determine the air-sea fluxes of heat, moisture and momentum in the northwest tropical Atlantic using highquality, in-situ meteorological measurements from a moored buoy. Second, to compare the in-situ fluxes to those available from operational models and satellites, identify the flux components with the largest discrepancies, and investigate the reasons for the discrepancies. An ancillary objective is to compute the local (one-dimensional) oceanic budgets of heat and momentum and determine the degree to which these budgets are locally balanced.

Accomplishments: Three Air-Sea Interaction Meteorology (ASIMET) systems were calibrated, assembled and tested. Two systems, comprised of the best performing sensors, were mounted on a three-meter discus buoy in preparation for deployment. The previous year’s mooring (NTAS-3, deployed 15 February 2003) was replaced by the new mooring (NTAS-4) on 21 February 2004 during a mooring turn-around cruise on the NOAA ship Ron Brown. The period between deployment and recovery was dedicated to an intercomparison of the two systems, with the shipboard system as an independent benchmark. The NTAS-3 sensors are undergoing post-calibration and a cruise report is in preparation. Post-processed, hourly meteorological data from the NTAS-1 and 2 deployments, and uncorrected hourly data from NTAS-3 are available on-line from the Upper Ocean Processes (UOP) group web site (http://uop.whoi.edu/ntas). Complete, but uncorrected, data from the NTAS-3 deployment and 20 weeks of data from NTAS-4 are also available.

The first three deployment years have been very successful, with 100% data return of meteorological variables needed for bulk flux estimation. Figure 1 shows the annual cycle at the NTAS site as depicted by selected meteorological variables averaged over 1 week on a 13 month time base. Spring (MAM) is characterized by SST increasing from its annual minimum and very low levels of precipitation. Summer (JJA) is characterized by steady northeast winds at 6-8 m/s and continuing increases in SST. Episodic precipitation begins in late summer. Fall (SON) is characterized by reduced solar radiation, SST decreasing from its annual maximum, persistent precipitation, and variable winds. By mid winter (DJF), solar radiation begins to increase, precipitation decreases, and winds become steadier. Monthly averages show a clear tendency for strong precipitation to be associated with SST above 27°C, a characteristic of the Inter-Tropical Convergence Zone (ITCZ) system.

Initial comparisons of the NTAS 1 and 2 fluxes with gridded products (Fig. 2) indicate a variety of issues for further investigation. European Centre for Medium Range Weather Forecasts (ECMWF) fluxes were from the output of the operational forecast model. National Centers for Environmental Prediction (NCEP) fluxes included both the first (NCEP-1) and second (NCEP-2) reanalysis. The Southampton Oceanography Centre (SOC) climatology is based on ship reports from 1980-1993. It is clear that the two-year mean net heat flux is significantly underestimated by the three models (mean differences are 4-5 times larger than the expected error of about 10 W/m2 from the buoy data). For ECMWF and NCEP-1 this is due to overestimation of latent heat losses and underestimation of shortwave gains. NCEP-2 shows a dramatic improvement in shortwave flux relative to NCEP-1, but still has a large net heat flux error due to substantial overestimation of latent heat losses. As a result, the amplitude of the annual cycle and the timing of positive to negative heat flux transitions are poorly reproduced by the models. In addition, all three models indicate a negative two-year mean net heat flux, whereas the observed value is +40 W/m2. Interestingly, the climatological net heat flux is a better match to the observations
than any of the models.

References:
Goldsmith, R.A. and A.J. Plueddemann, 2002. Moored buoy site evaluations. In: Marine Geography, J. Breman, Ed., ESRI Press, pp 73-77.

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
Turnaround Cruise Report. WHOI Tech. Rept. WHOI-2001-07, 55 pp.

Plueddemann, A.J., 2002. In-situ meteorology from the Northwest Tropical Atlantic Station, Proc. U.S. CLIVAR Atlantic Conf., Wash. DC, pp 9-13.

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 Turnaround Cruise Report. WHOI Tech. Rept. WHOI-2002-07, 68 pp.

Plueddemann, A.J., 2003. In-situ meteorology and air sea fluxes in the Northwest Tropical Atlantic, NOAA Climate Observation Program Workshop, Silver Spring, MD,
13-15 May 2003, (poster).

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
Turnaround Cruise Report. WHOI Tech. Rept. WHOI-2003-04, 69 pp.

Plueddemann, A., 2004. Multi-year, in-situ surface fluxes in the northwest tropical Atlantic, CLIVAR International Science Conference, Baltimore, MD, 21-25 June 2004,
(poster).

Plueddemann, A. and R. Weller., 2004. Meteorology and air sea fluxes from Ocean Reference Stations, NOAA Climate Observation Program Workshop, Silver Spring, MD, 13-15 April 2004, (poster).

Weller, R., L. Yu, A. Plueddemann, D. Hosom, and S. Sathiyamoorthy, 2003. Synthesis of basin scale air-sea flux fields, CLIMAR-II: Second JCOMM Workshop on Advances in Marine Climatology, Brussels, Belgium, 17-20 Nov 2003, (poster).

Weller, R., A. Plueddemann, D. Hosom, R. Payne, J. Smith and F. Bahr, 2003. The quality of surface meteorology from buoys and volunteer observing ships, CLIMAR-II:
Second JCOMM Workshop on Advances in Marine Climatology, Brussels, Belgium, 17-20 Nov 2003, (poster).

Web site: http://uop.whoi.edu/ntas

Last updated: August 19, 2008
 


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