TAO Array Completed

The Tropical Atmosphere Ocean (TAO) Array is Completed

Michael J. McPhaden

NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Bulletin of the American Meteorological Society, Vol. 76, No. 5, May 1995

The Tropical Atmosphere Ocean (TAO) Array of moored buoys in the Pacific Ocean was completed on 17 December 1994 with deployment of the 69th and final mooring at 8N,156E from the Taiwanese research vessel Ocean Researcher I. The TAO Array measures oceanographic and surface meteorological variables critical for improved detection, understanding and prediction of seasonal-to-interannual climate variations originating in the tropics, most notably those related to the El Nino/Southern Oscillation (ENSO). The array spans one third the circumference of the globe, from 95W near the Galapagos Islands to 137E off the coast of New Guinea. Moorings are deployed every 2-3 degrees of latitude between 8N and 8S along lines that are separated by 10-15 degrees of longitude. Data are reported in real time via Service Argos, which utilizes National Oceanic and Atmospheric Administration (NOAA) polar orbiting weather satellites for data relay. TAO data are then retransmitted on the Global Telecommunication System for immediate distribution to oceanographic and meteorological centers around the world. The TAO Array was developed under auspices of the recently completed 10-year (1985-1994) international Tropical Ocean Global Atmosphere (TOGA) program. A major objective of TOGA was to develop an ocean observing system to support studies of large scale ocean-atmosphere interactions on seasonal-to-interannual time scales. The need for such an observing system was dramatized in the early planning stages for TOGA by the occurrence of the 1982-83 ENSO, the strongest of the century, which was neither predicted nor even detected until nearly at its peak. TAO is now one of the cornerstones of this observing system, which also includes drifting buoy arrays, a volunteer observing ship expendable bathythermograph network, island and coastal tide gauges, an island wind profiler network, and remotely sensed measurements from both operational and research satellites.

ENSO events occur periodically at intervals of roughly 2-7 years and typically last for about 12-18 months. During ENSO, the Pacific trade winds weaken, causing eastward migration of the western Pacific warm pool into the central Pacific, and reduced equatorial upwelling in the eastern Pacific. Anomalously warm sea surface temperatures appear from the coast of South America to west of the international date line, which leads to further weakening of the trade winds. The ocean-atmosphere system therefore becomes locked in a positive feedback loop that is ultimately broken when planetary scale oceanic wave processes excited by the collapse of the trade winds readjust the upper ocean thermal structure and induce surface cooling.

In the atmosphere, the pattern of deep convection and precipitation shifts eastward with the warmest sea surface temperatures during ENSO, leading to drought conditions in northern Australia, Indonesia, and the Philippines; and excessive rains in the island states of the central tropical Pacific and along the west coast of South America. Anomalous latent heating of the atmosphere associated with this eastward shift in convection alters the general circulation of the atmosphere, extending the influence of ENSO not only to other tropical regions (drought conditions in northeast Brazil, southern Africa, and a weakened Indian monsoon), but also to higher latitudes. For example, during ENSO events, most of Canada and the northwestern U.S. tend to experience mild winters, and the states bordering the Gulf of Mexico tend to be cooler and wetter than normal. California has experienced a disproportionate share of its episodes of heavy rainfall during ENSO winters such as 1982-83, 1991-92, and 1994-95, but other ENSO winters, such as 1976-77, have been among the driest on record. ENSO also disrupts the marine ecology of both the tropical Pacific and the Pacific coast regions of the Americas, affecting the mortality and distribution of commercially valuable fish stocks and other marine organisms. Thus, though originating in the tropical Pacific, ENSO has socio-economic consequences that are felt worldwide.

The history of moored measurements for climate studies in the equatorial Pacific dates back to the late 1970's when David Halpern of NOAA's Pacific Marine Environmental Laboratory (PMEL) was the first to successfully deploy and maintain surface moorings in the strongly sheared flows of the equatorial current system. Based in part on the success of this early pioneering effort, the late Stan Hayes of PMEL conceived of a pan-Pacific moored array that would telemeter in real time the essential variables needed to improve the detection, understanding and prediction of ENSO. Hayes named this array the TOGA Tropical Atmosphere Ocean (TAO) Array. Key to the successful implementation of TAO was the development of the low cost Autonomous Temperature Line Acquisition System ATLAS mooring which measures and telemeters in real-time surface winds, sea surface temperature and subsurface temperatures to depths of 500 m. Ocean currents are involved in the zonal redistribution of upper ocean heat content on seasonal-to-interannual time scales, so a small number of moorings along the equator measure and telemeter upper ocean velocity data. All moorings measure surface air temperature and relative humidity; additional sensors, such as rain gauges, salinometers, short wave radiometers, and bio-optical sensors, have been added for specialized studies.

One measure of TAO's success is the growing number of publications (almost 200 in the past three years) that make use of the mooring data. The topics covered in these publications are diverse, ranging from detection of new phenomena (such as 60-90 day oceanic equatorial Kelvin waves), to description of recent ENSO events, tests of ENSO theories, studies of ocean current dynamics, definition of and diagnostic studies of the seasonal cycle, the upper ocean heat balance, studies of atmospheric boundary layer dynamics, verification of ocean and atmosphere models used for climate research, data assimilation into climate models, satellite validation (of sea-level, sea surface temperature, surface wind speed and velocity, and rainfall), etc. Moreover, the TAO Array provides a broad geographical perspective and long time history to aid in the interpretation of measurements from shorter duration, regional scale observational studies. Examples of recent process-oriented field programs embedded in the TAO Array include the Tropical Instability Wave Experiment to study the life cycle and energy sources for tropical instability waves in the eastern Pacific (1990-91), the TOGA Coupled Ocean-Atmosphere Response (COARE) program to study ocean-atmosphere interactions in the western Pacific warm pool (1992-94), the Joint Global Ocean Flux Studies (JGOFS) Equatorial Pacific Experiment to study biogeochemical cycling in the upper ocean (1992), and the Central Equatorial Pacific Experiment to study cloud-radiative feedbacks (1993).

In addition, TAO data have improved the quality of several important operational climate analysis and prediction products. TAO data available on the Global Telecommunications System are incorporated into surface wind analyses at weather prediction centers around the world, such as at NOAA's National Centers for Environmental Prediction (NCEP) in Washington, DC; the Fleet Numerical Oceanography Center in Monterey, California; the European Center for Medium Range Weather Forecasting in the United Kingdom; Meteo-France in Toulouse, etc. Also, operational short-term climate predictions at NCEP, based on state-of-the-art coupled ocean-atmosphere models, are dependent on assimilation of TAO subsurface temperature data for accurate ocean initialization. Finally, TAO sea surface temperature data are incorporated into the operational satellite/in situ blended analyses produced at NCEP, and TAO winds are a dominant data source for the tropical Pacific wind analysis performed at Florida State University.

ATLAS moorings and most current meter moorings are assembled at PMEL; all sensors are calibrated prior to deployment and after recovery. ATLAS moorings have a 1-year design lifetime and current meter moorings have 6-month to 1-year design lifetimes, so servicing the array is an ongoing effort. Accumulated over the 10 years of the TOGA program between 1985 and 1994, more than 400 TAO moorings have been deployed on 83 research cruises involving 17 ships from 6 different countries, requiring a total of 5.7 years of ship time. At present, nearly one year of dedicated ship time per calendar year is required to maintain the fully implemented array of 69 moorings. Data return from the array is greater than 80%, with a significant percentage of the loss due to vandalism by fishing fleets in the far eastern and western Pacific.

Scientific utilization of the moored measurements has been encouraged by the development at PMEL of sophisticated data management and dissemination capabilities. These include the TAO Workstation Software, which is a user friendly, Unix-based interactive data display, analysis and distribution system; the creation of an Internet anonymous-ftp data base at PMEL; and distribution of TAO data and analyses through the World Wide Web, utilizing interfaces such as MOSAIC (http://www.pmel.noaa.gov/tao/realtime.html). Yearly submissions of TAO data are made to the National Oceanographic Data Center (NODC) in Washington, DC; the National Climate Data Center (NCDC) in Asheville, North Carolina; and the TOGA Subsurface Data Center in Brest, France. TAO data are also available on the TOGA compact disk produced by the Jet Propulsion Laboratory in Pasadena, California.

The TAO Array was finally completed just as the TOGA program which sponsored it came to an end. However, new national and international research programs have been formulated to build on the successes of TOGA, namely the U.S. Global Ocean Atmosphere Land System (GOALS) program, and the World Climate Research Program's study of Climate Variability and Predictability (CLIVAR). These programs, scheduled for the 15-year period 1995-2010, will continue and expand upon seasonal-to-interannual climate research initiated during TOGA. Maintenance of the TAO Array, as well as other components of the TOGA observing system, is key to the success of these new climate research programs.

Long-term support of critical climate measurements has motivated planning for the Global Climate Observing System (GCOS) as well as the climate module of the Global Ocean Observing System (GOOS). Based on the success of TOGA, the observational requirements of CLIVAR/GOALS, and the potential for improving the skill of seasonal-to interannual climate forecasts, the TAO Array has been identified as a centerpiece of the initial U.S. contribution to the climate module of GOOS and to the ocean component of GCOS. Moreover, possible expansions of the TAO Array to other tropical ocean basins and/or to higher latitudes in the Pacific can be envisioned in response to evolving scientific objectives in CLIVAR/GOALS. To meet the observational challenges of the future, a new generation of low cost mooring is being developed with increase design lifetime and measurement capabilities. In addition, array design assessments are underway to optimize present and future sampling strategies for climate studies.

The TAO Project is a multi-national effort involving the participation of the United States, Japan (JAMSTEC) Taiwan, and France (ORSTOM). Funding in the United States has been provided primarily by NOAA's Office of Global Programs and by the Office of Oceanic and Atmospheric Research. The TAO Project Office, under the direction of Michael J. McPhaden, is located at NOAA's Pacific Marine Environmental Laboratory in Seattle, Washington.