The El Niño/Southern Oscillation (ENSO) may have more far-reaching effects on the global climate than researchers used to think. Not only the Pacific Ocean, but the Indian Ocean experiences cyclical fluctuations in upper ocean temperature that have repercussions for the world's weather, according to a new analysis of surface and subsurface temperature, data confirming the phenomenon for the first time.
In fact, the two oceans may be linked in a related pattern that occurs over a 3- to 7-year period, scientists reported last week at AGU's Fall Meeting in San Francisco. To track ENSO in the Pacific, Indian, and Atlantic oceans, Yves M. Tourre, a research meteorologist at Columbia University's Lamont-Doherty Earth Observatory and Warren B. White, a research oceanographer at the Scripps Institution of Oceanography at the University of California, San Diego, analyzed a data set of surface and subsurface temperatures, which cover the upper quarter mile of ocean. They used more than 650,000 measurements that were collected over a recent decade by the Ship of Opportunity Program (SOOP) and compiled by the Integrated Global Ocean Services System (IGOSS). The data were scientifically quality controlled at Scripps.
What the scientists discovered was an El Niño pattern in the central Indian Ocean that is locked in phase to that of the Pacific Ocean. The scientists also observed related effects in the Atlantic that trail the El Niño in the Pacific and Indian oceans by about 12 to 18 months.
"For the first time we were able to look at the subsurface and put the story together," Tourre says. Michael McPhaden, a research oceanographer at the National Oceanic and Atmospheric Administration's (NOAA's) Pacific Marine Environmental Laboratory in Seattle, explains that for a long time meteorologists and oceanographers have wondered about the role that the Indian Ocean plays in the El Niño cycle, yet scientists have been unable to identify recurrent and robust patterns of sea surface variability in the Indian Ocean on El Niño timescales. But now this signal detection problem may be resolved, as Tourre and White offer one definition of these temperature paterns, McPhaden says. However, McPhaden says that "it's still speculative to say that the patterns have any effect on the overlying atmosphere and El Niño - that piece of research is yet to be done".
An El Niño begins in the Pacific when warm surface waters and heat in the upper layers of the ocean build up in the central equatorial Pacific and subsequently are transported eastward toward the coast of the Americas. When this movement, associated with Kelvin wave propagation, reaches the coastline, it is "reflected" and moves back across the Pacific in the form of a Rossby wave that continues to affect climate and ocean circulation. The net effect brings disruptive floods to some areas, while bringing droughts to others, ranging from eastern Australia to Brazil and Africa, and from North America to India. The 1982-1983 Pacific El Niño was the worst such event this century, but the most recent one occurred in 1993 following on the heels of the 1991-1992 El Niño warming.
Recently, some scientists predicted that another El Niño warming episode may be on its way, according to data presented at AGU's Fall Meeting from the Tropical Atmosphere-Ocean (TAO) Array of about 70 deep-ocean moorings, which span the equatorial Pacific. The data reveal that water temperatures and heat content of the upper layers, or 400 m, of the central Pacific have once again entered a warming period. In fact, the warm water, now at 2.5°C above normal near the dateline, has expanded into the Eastern Equatorial Pacific. In addition, the trade wind intensity over large areas of the central and western Pacific has decreased by as much as 4-5 m/s which is also characteristic of El Niño, researchers report. Scientists predict that these conditions will persist through May and June of 1995.
But, McPhaden says, "Nature may yet play tricks on us." Indeed, the seeming increase in frequency of El Niño warnings presents an area of study in itself. While some scientists have suggested that this period of extended El Niño conditions may be a result of anthropogenically produced global warming, it is equally plausible that "what we're seeing is an example of natural variability of the climate sytem," McPhaden says. In the new analysis, Tourre and White observed that migrations of warm water pools in the Pacific were mirrored in the Indian Ocean. In both oceans, the warm waters move eastward along the equator, with the warmest sea temperatures pooling, respectively, along the South American coast in the Pacific Ocean and in the central region of the Indian Ocean.
For example, when the warm water pool in the Pacific is
centered at a longitude of about 160°W, the warm water pool in
the Indian Ocean is beginning to be displaced eastward from about
50°E longitude of the tip of India (Figure 1). When the pool
arrives during the course of a 3- to 7-year cycle along the coast
of the Americas in the Pacific, the pool in the Indian Ocean is
off the coast of India. During this state, droughts are seen
throughout India, while flooding and its related economic and
health perturbations are occurring across Central and parts of
South America.
Fig. 1. During a 3- to 7-year cycle, a warm pool in the Indian Ocean moves eastward (1 to 3). Meanwhile, the same phenomenon occurs in the Pacific Ocean (4 to 6). The 1 and 5 events and the 2 and 6 events are in sync. In the Atlantic, a warm pool (7) develops 12-18 months later.
What's more, the scientists hypothesize that the ENSO cycles in the Indian and Pacific oceans may influence each other. Study and constant monitoring of El Niño events in the Indian Ocean may help predict rainfall patterns in the Austral-Asia region 6 12 months in advance in the way that scientists are now attempting to make better predictions of climate variations due to Pacific ENSO phenomenon. Indeed, learning more about the global aspects of ENSO may advance understanding of the Pacific El Niño.
Yet NOAA's McPhaden says at this point "the cause and effect relationships are somewhat ambiguous." In other words, it is not clear how to separate the effects because the Pacific and Indian Ocean El Niños are coincident.
Nevertheless, the dynamic processes within each ocean appear to be different, the scientists observe. For starters, the warm pool propagates more slowly along the equator in the Indian Ocean that the warm pool does in the Pacific Ocean. After the full-fledged El Niño in the eastern Pacific dissipates, the warm pool in the Indian Ocean continues eastward to Indonesia and southeastward into the Timor Sea north of Australia. In addition, the circulation of the pool in the Indian Ocean appears to be linked with the 3- to 7-year perturbation in monsoon winds that blow along the coastlines of East Africa, India, and Indonesia. During an Indian Ocean El Niño, the southwesterly summer monsoon is much weaker, and less moisture is being advected over India, Tourre and White report.
Meanwhile, Tourre and White also observed links between the Pacific El Niño and the Atlantic Ocean. But the warming in the Atlantic does not seem to be the result of eastward movement of warmer water pools. Instead, the scientists observed a passive ocean response caused by atmospheric forcing: shifts in global atmospheric sea level pressure and surface winds that appear to be related to ENSO in the Indo-Pacific region. While the El Niño is peaking in the Indian and Pacific oceans, southeasterly winds across the equatorial Atlantic are increasing and cooling Atlantic surface waters. About 12 to 18 months after the Indo-Pacific El Niño occurred, the easterly winds' relaxation is accompanied by warmer surface waters in the equatorial Atlantic. Understanding the changing Atlantic Ocean surface temperatures and shifting winds may help improve climate forecasts for these regions as well, Tourre says.
Yet it is clear McPhaden says, "A lot of work remains to link these patterns in the Indian Ocean to atmospheric effects and the El Niño cycle, in particular."
Return to Science and
Society 