Archive of Spotlight Feature Articles

The Tropical Atmosphere Ocean (TAO) Array Expands into Indian Ocean

By Sidney Thurston, NOAA Office of Climate Observation

The Indian Ocean is unique among the three tropical ocean basins in that it is blocked at 25°N by the Asian land mass. Seasonal heating over this land mass sets the stage for dramatic monsoon wind reversals and intense summer rains over the Indian subcontinent and adjoining areas of Southeast Asia. Recurrence of these summer monsoon rains is critical to agricultural production that provides life-sustaining support for hundreds of millions of people in the region. The reversing monsoon winds also generate a unique system of currents unlike those observed in the tropical Atlantic and Pacific, which are under the influence of steadier trade winds. Blocked by Asia, these currents cannot export heat to the northern subtropics. The resulting oceanic thermal structure produces feedbacks to the overlying atmosphere that affect not only the monsoon circulation and rainfall patterns, but also weather and climate in remote parts of the globe through atmospheric teleconnections.

Despite the importance of the Indian Ocean in the regional and global climate system, it is the most poorly observed and least well understood of the three tropical oceans. To help remedy this situation, Dr. Mike McPhaden of NOAA's Pacific Marine Environmental Laboratory (PMEL), together with a group of international climate scientists, has developed a plan for systematic, sustained, and comprehensive in situ observations in the Indian Ocean to complement both present and planned space-based satellite measurements. Some elements of the in situ observing system are already in place. However, what has been missing until recently is an agreed upon strategy for integrated measurements across all platforms and, in the case of moored buoy measurements, an array design that meets present day requirements for climate research and forecasting.

McPhaden and colleagues have formed a new Indian Ocean Panel to provide the framework for developing observing system implementation plans and strategies. The Panel, chaired by Dr. Gary Meyers of the Commonwealth Scientific Industrial and Research Organization of Australia (CSIRO), is jointly sponsored by the Climate Variability and Predictability (CLIVAR) program of the World Climate Research Program (WCRP) and the International Oceanographic Commission's (IOC) Global Ocean Observing System (GOOS). The first meeting of the Panel, held at the Indian Institute of Tropical Meteorology in Pune, India during 18-20 February 2004, lead to a draft report describing the scientific rationale for and potential societal benefits of a sustained Indian Ocean observing system. The report also provides design criteria and implementation guidelines for each of the observing system components, including Argo floats, drifting buoys, tide gauge stations, ship-of-opportunity expendable bathythermograph lines, and a moored buoy array. The plan for the moored buoy array represents, in essence, an expansion into the Indian Ocean of the highly successful Tropical Atmosphere Ocean (TAO) array, which was originally developed in the Pacific for improved understanding and prediction of El Niño and the Southern Oscillation (ENSO).

In a case of scientific serendipity, McPhaden learned while in Pune that the National Institute of Oceanography (NIO) was planning a cruise later in the year to deploy some of its own current meter moorings in the equatorial Indian Ocean from the Ocean Research Vessel (OVR) Sagar Kanya. With the encouragement of scientists on the Panel, McPhaden and V.S.N. Murty of NIO wrote a proposal to the Indian Department of Ocean Development, which operates the Sagar Kanya, for a few additional days of ship time to deploy PMEL ATLAS moorings and an Acoustic Doppler Current Profiler (ADCP) mooring. This proposal was accepted and, with funding provided by NOAA's Office of Climate Observation (OCO), four ATLAS moorings and one ADCP mooring were deployed on a cruise that took place from 9 October to 17 November 2004. The excitement generated by these deployments was such that, even before the cruise had ended, NOAA Administrator VADM Lautenbacher was showcasing real-time data from the moorings in his keynote address to the Oceans '04 International Conference in Kobe, Japan in early November. Success of the cruise provided VADM Lautenbacher with the opportunity to highlight a very recent example of international cooperation in the development of Global Earth Observing System of Systems (GEOSS) of which the Indian Ocean observing system is a component.

McPhaden has spent that last two decades developing similar moored buoy observing systems in the tropical Pacific and Atlantic Oceans. In the Pacific, the TAO array (renamed TAO/TRITON in 2000 to recognize the significant contribution of TRITON moorings in the western Pacific by the Japan Agency for Marine-Earth Science and Technology) focuses on ENSO variability and predictability. The Pilot Research Moored Array in the Tropical Atlantic (PIRATA), a trilateral program supported by NOAA in collaboration with science agencies in Brazil and France, addresses ocean-atmosphere interactions that affect rainfall variability over South America and Africa. PIRATA also provides data on oceanic and atmospheric conditions in the main development region (10°-20°N) of Atlantic hurricanes.

McPhaden notes that U.S. participation in the development of a sustained observing system in the Indian Ocean has lagged that in the other two basins for a variety of logistical, scientific, and historical reasons. However, the importance of the Indian Ocean for understanding and predicting seasonal and longer-term climate variability has been underscored by several recent discoveries. Among these are the Indian Ocean Dipole (also referred to as Indian Ocean Zonal Mode), which is an ENSO-like fluctuation involving coupled ocean-atmospheric interactions in the Indian Ocean region. Also, new observations indicate that interactions between the ocean surface mixed layer and the Madden-Julian Oscillation (MJO)—a 30-60 day fluctuation in atmospheric winds, pressure, and rainfall that originates over the Indian Ocean—are highly energetic. Understanding these interactions is important because the MJO has significant impacts on Asian monsoon rainfalls, west coast U.S. weather, tropical Atlantic hurricane formation, and the evolution of the ENSO. Similarly, decadal warming trends in Indian Ocean sea surface temperatures have recently been shown to affect the North Atlantic Oscillation, Sahel rainfall, and other aspects of global climate.

The five PMEL moorings deployed on the ORV Sagar Kanya are just the first step for NOAA in the development of an Indian Ocean moored buoy array. The President's FY 2006 budget contains new funds for additional moorings as part of NOAA's ongoing commitment to the effort. Resource commitments from several interested nations over the next few years will lead to the establishment of this and other key elements of the Indian Ocean observing system, which will significantly advance our understanding and ability to predict the monsoons and related climate phenomena.

For information on the Office of Climate Observation program, go to http://www.oco.noaa.gov/. For more details on this program, contact:

Sidney Thurston
NOAA Office of Climate Observation
Sidney.Thurston@noaa.gov
301-472-2089 ext. 112

For information on the Pacific Marine Environmental Laboratory, go to http://www.pmel.noaa.gov/. For more details on this program, contact:

Michael McPhaden
NOAA/Pacific Marine Environmental Laboratory
Michael.J.Mcphaden@noaa.gov
206-526-6783

[4/4/05]

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