Prototype ATLAS moorings providing measurements of air temperature, SST and subsurface temperature to 500 m were first deployed in 1984 at 2°N, 108°W and 2°S, 110°W. All data were transmitted to shore in real-time via Service Argos, utilizing NOAA's polar orbiting weather satellites for data relay. These initial deployments were followed in 1985 by the installation of regional scale meridional arrays that spanned the equator along 110°W and along 165°E, the latter in collaboration with the Institut Français de Récherche Scientifique pour le Développement en Coopération (ORSTOM) in Noumea, New Caledonia. Realizing the importance of the surface wind field in driving the tropical ocean circulation lead to the addition of real-time wind measurements to ATLAS moorings in 1986. Beginning in 1989, relative humidity sensors were added for studies of atmospheric boundary layer dynamics and air-sea exchange processes. The early technical successes of the ATLAS mooring program and the recognized value of the data for short-term climate studies led to multi-national plans for a basin-scale expansion of the array during the second half of TOGA (1990-1994). This expansion was feasible because the relatively low cost of the ATLAS mooring allowed for its deployment in large numbers.
TOGA-TAO as it is now constituted also includes a small number of Profile Telemetry of Upper Ocean Currents (PROTEUS) and conventional current-meter moorings along the equator. PROTEUS and ATLAS moorings are similar in design and instrumentation. PROTEUS in addition measures and telemeters current profiles in the upper 250 m from a downward-looking acoustic Doppler current meter mounted in the surface buoy (McPhaden et al., 1991b).
Design criteria for TOGA-TAO are based on general
circulation model simulations
of wind-forced oceanic variability and on empirical studies of
space/time correlation
scales. These
studies indicate that basin-scale wind measurements within
~7° of the equator are
required
to simulate accurately the seasonal to interannual evolution of
SST variability in the
cold-tongue
region of the equatorial Pacific and that the ocean responds most
sensitively to zonal
wind rather than meridional wind forcing on these time scales
(Harrison, 1989). Zonal
wind field variations
are minimally coherent over 2-3° latitude and 10-15°
longitude (Harrison and
Luther, 1990), and
approximately one sample per day is required to achieve an
accuracy of 0.5-1.0 m s
for monthly
mean zonal wind speeds at a particular location (Halpern, 1988;
Mangum et al.,
1992).
The space/time scales of upper ocean thermal structure are depth
dependent and
nonstationary in time. However, the most stringent thermal field
sampling
requirements (for thermocline temperature during non-ENSO
periods) are comparable to
those for zonal winds (e.g., Meyers et al., 1991; Hayes
and McPhaden, 1992).
Zonal current variations are coherent over 20-
30° longitude on monthly time scales along the equator
(McPhaden and Taft, 1988),
where direct
velocity measurements are required because of the limited utility
of the geostrophic
approximation.
Enhancements to the TAO array at present include additional moorings west of the date line as part of the TOGA Coupled Ocean Atmosphere Response Experiment (COARE; Webster and Lukas, 1992) to provide finer than 10° zonal resolution of surface winds, upper ocean temperatures, and currents along the equator over a 2-y period beginning in early 1992 (Fig. 1). Also, sensors have been added to several moorings in the western Pacific to measure salinity, rainfall, and incoming shortwave radiation for specialized research purposes. Similarly, bio-optical sensors have been added to the 0°, 140°W PROTEUS mooring for the Equatorial Pacific Joint Global Ocean Flux Study (Murray et al., 1993).
TAO data are made available to the research community directly from PMEL via Internet anonymous file transfer protocol (ftp) procedures, and via a dial-up phone line data base. In addition, PMEL distributes TAO Workstation Display software, which allows remote users to display interactively real-time TAO data and animations on Unix workstations using a point-and-click windows interface (Soreide and McPhaden, 1993). A subset of the real-time TAO data stream is retransmitted on the Global Telecommunications System (GTS) by Service Argos, so that the meteorological measurements are available for assimilation into atmospheric numerical weather prediction models at, for example, the European Center for Medium Range Weather Forecasting (ECMWF), the Fleet Numerical Oceanography Center (FNOC), and the U.S. National Meteorological Center (NMC). Real-time TAO SST measurements are included in weekly blended analyses of in situ and satellite data at NMC (Reynolds, 1991), subsurface thermal data are assimilated directly into the NMC operational ocean model (e.g., Leetmaa and Ji, 1989), and wind data are incorporated into the Florida State University (FSU) monthly ship wind analyses (Legler, 1991). Data from the TAO Array have also been used to validate satellite-derived estimates of SST (e.g., Liu, 1988), wind speed (e.g., Atlas et al., 1991; Bates, 1991), sea level (e.g., Cheney et al., 1989; Picaut et al., 1992), surface geostrophic currents (Picaut et al., 1990), rainfall (McPhaden et al., 1993) and most recently estimates of surface wind velocity from the ERS-1 scatterometer (Halpern et al., 1993).
A skeletal version of the TAO Array was used to describe the evolution of the 1986-87 ENSO event (McPhaden et al., 1990; McPhaden and Hayes, 1990), and the mechanisms responsible for SST variability along the equator (Hayes et al., 1991a; McPhaden and Picaut, 1990). The array in late 1987 consisted of 15 moorings, primarily concentrated along 110°W, 140°W, and 165°E. In the following section, we present a preliminary description of the 1991-93 ENSO from a much more extensive array of buoy measurements across the Pacific basin.
Fig. 1: The TOGA-TAO Array in August 1993 and in its final configuration December 1994. ATLAS moorings (diamond), and current-meter moorings (square).
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