That July 15 forecast from the International Research Institute for Climate Prediction isn't what you'd hear from your local weatherman. But, as the animation above shows, it was right on. El Ni¿a ("the girl"), the twin and opposite of more well-known El Ni¿o ("the boy") is now in full swing.
Over the past months, sea surface temperatures in the South Pacific have dropped steadily to below normal levels (blue). This cooling of the central and eastern tropical Pacific may already have influenced this fall's string of powerful Atlantic hurricanes, from Bonnie to Mitch. This winter, La Ni¿a's impact on the weather could help trigger dramatic temperature swings across the central and eastern United States and increase the likelihood of drought across the Southeast and Southwest.
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In reality, these thermal Gemini are part and parcel of the same phenomenon, which climatologists refer to as ENSO, for El Ni¿o /Southern Oscillation. ENSO is a steady periodic warming and cooling cycle, with "normal" periods of various duration in between. The warm peaks--El Ni¿o--were first named by South American fishermen because the warm, oxygen--and nutrient--poor water caused fish catches to plunge around Christmas time (El Ni¿o is also a reference to the Christ child) in certain years. During these periods, the water temperature, normally in the 60s to 70s Fahrenheit, may exceed 80 degrees F as the "warm pool" located in the central and western Pacific expands to cover the tropics.
La Ni¿a was less noticed by the coastal fishermen because it marked a period of abundance--the upwelling of cold, nutrient rich water along the coast resulted in good catches. According to the National Center for Environmental Prediction, this century's previous La Ni¿a's began in 1903, 1906, 1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, and 1988. During the most recent La Ni¿a, in the winter of 1988-89, sea surface temperatures were about 7 degrees F below normal. La Ni¿a's don't always follow El Ni¿o's; since 1975, La Ni¿a's have been roughly half as frequent as El Ni¿o's.
Both ends of the ENSO spectrum became the subject of intense study in the 1980s when scientists noticed that their impact went far beyond the South American fisheries but had, in fact, a powerful global effect on weather. In addition, it also appears to have a significant influence on the prevalence of many diseases, such as malaria.
Just this past summer, the world's first international summit on the impacts of La Ni¿a was organized by the National Center for Atmospheric Research in Boulder, Colo. and the United Nations University, based in Tokyo. The researchers identified what is known and still unknown about La Ni¿a and what is needed in order to issue forecasts and prepare for La Ni¿a's impacts. Their findings are included in an extensive executive summary.
The scientists at the meeting agreed that better ocean monitoring is needed for more accurate predictions of El Ni¿o and La Ni¿a. They suggested that the current network of 70 buoys across the Pacific should be expanded to higher latitudes and to the Indian and Atlantic oceans because of their influence on the climate signals that emanate from El Ni¿o and La Ni¿a. Early and significant warnings of the onset of La Ni¿a picked up by an earth-sensing satellite called TOPEX/Poseidon occurred in areas not covered by the ocean buoys.
TOPEX/Poseidon can measure very small changes in the ocean's surface from orbit. Where the water is less warm and less dense, the surface bulges upward; in areas of cold, dense seawater it decreases. Indeed, recent data suggest that the changes in sea level induced by ENSO are global and that the 1997-98 El Ni¿o event may have been a major contributor in the average sea level rising about eight-tenths of an inch before it returned to normal levels. "This is the first time we have been able to identify that El Ni¿o may cause a change in average global sea level," said R. Steven Nerem, a TOPEX/Poseidon science team member at the Center for Space Research at the University of Texas at Austin.
But so far, predictions of this latest La Ni¿a seem to be fairly close to the mark. Meteorologists are reporting that heavier than average rainfall has been observed in the far western Pacific, including much of Indonesia and Australia, with relatively dry conditions in portions of southeastern South America and the southeastern United States. Meanwhile, the traditional end of the hurricane season saw two Atlantic hurricanes, Lisa and Mitch, bringing the seasonal total to thirteen named storms. Hurricane Mitch was marked by its duration, strength and persistent destructive rains over Central America. In addition, forecasters say that average global temperatures have continued above the 1961 to 1990 average, virtually ensuring that 1998 will be a record warm year.
So, for the next year or so, we'll all have to learn to get along with El Ni¿o's twin sister.