Moored Buoys Used for El Niño Prediction

Since the early 17th century, mariners have noted the presence of an unusually cold current moving north towards the equator off the coast of Peru during most of the year, causing typically cool and dry conditions near the coast. Early Spanish chronicles also noted the appearance of heavy rains and floods in the coastal regions of Peru, which indicated an oceanographic environment quite different than that caused by the usual cold Peru current. Modern measurements have shown that the latter conditions occur when the cold waters off Peru are overrun by a warm, south flowing current in the winter months. This warm ocean current that typically appears near Christmas and lasts a few months was called "El Niño" by Spanish fishermen. The appearance of this current is regular although the degree of warming is variable. At irregular intervals (typically three to seven years), the warming is exceptional and persistent, raising the sea surface temperature by more than 3 degrees Celsius and lasting for six to 18 months. The term El Niño is now used mainly to describe these abnormally warm events.

El Niño events are accompanied by unusual and often catastrophic weather events around the globe. The large scale ocean/atmosphere interaction causing warming events across the Pacific is termed ENSO (El Niño Southern Oscillation). As an ENSO event begins, warm waters of the western tropical Pacific shift eastward due to a weakening of the easterly tradewinds. An example of this basin wide surface temperature warming can be seen during late 1994 in figure one.

Fig. 1 - Sea surface temperature and departures from climatology along the equator from 135E to 95W from July 1993 through July 1995, as measured from the TAO buoys.

Migration of warm water from west to east disrupts the large scale atmospheric circulation patterns, resulting in global changes on land and at sea. During ENSO events there are often drought conditions in northern Australia, Indonesia, and the Philippines and excessive rain in the island states of the central tropical Pacific and along the west coast of South America. These events affect non tropical regions as well. For example, the northeastern U.S. and Canada and the Pacific Northwest tend to be warmer than normal; and it is rainier than normal in the states bordering the Gulf of Mexico. In California, ENSO can be associated with either excessively wet or excessively dry winters. ENSO events also disrupt the marine ecology of the tropical Pacific and the Pacific coast regions of the Americas. The warm water is not as rich in nutrients as the usual cool water and does not support marine life. The amount and distribution of commercially valuable fish stocks is affected. Though it originates in the tropical Pacific, the consequences of ENSO are felt on a global scale.

The Tropical Atmosphere Ocean (TAO) array of buoys was developed in order to better describe, understand, and predict ENSO events. The array is part of a multinational research program sponsored by the governments of France, Japan, Taiwan, and the United States. The array consists of nearly 70 buoys, moored with steel cable, spanning the equatorial Pacific between the Galapagos Islands and New Guinea (see figures two and three).

Fig. 2 - Locations of the buoys in the TAO array.

Fig. 3 - Schematic drawing of a TAO Buoy. Each buoy measures and transmits surface and subsurface data. The moorings are securely anchored in place.

The buoys are separated by approximately 900 nautical miles in the east-west direction, and about 150 nautical miles in the north-south direction. Actual buoy positions are listed periodically in Notice to Mariners publication.

Each buoy measures surface wind, air temperature, humidity, sea surface temperature, and subsurface temperatures down to 500 meters. A few buoys also measure ocean currents, rainfall and solar radiation. Environmental data and buoy positions are transmitted via satellite and received daily at the TAO Project Office. Data are also transmitted in real-time over the Global Telecommunications System (GTS) to operational weather centers around the world where they are incorporated into numerical weather and climate forecasting models. The data can also be downloaded directly by scientists and other interested parties via Internet.

Fig. 4 - Photograph of the mooring after it has been deployed. The surface instruments are located on the tower of the buoy; care should be used when working near these moorings.

The TAO Array provides critical real-time data from the equatorial Pacific region where there are few islands or ships providing weather information. These data are used by several weather centers to produce charts of sea surface temperature and storm forecasts which are distributed to the maritime community via radiofax broadcasts. Wind and ocean current data from the buoys have also been used by nations in the western Pacific to help locate missing or overdue vessels.

The ability to anticipate future El Niño events and other disruptions in the world's climate is a benefit not only to maritime communities, but to all peoples of the world. By steering clear of these moorings, you can help us to learn more about these events and improve the prediction of them in the future.

For further information, please contact NDBC