The US-French Topex-Poseidon satellite recently detected two humps of warm water moving across the Pacific. The first wave began in January and arrived on the west coast of South America in February. The second, a mass of warm water larger in area than the entire United States, reached South America in early May. These so-called Kelvin waves indicate that a moderate to strong El Nino is developing and should be at full strength by the end of the year.
Normally, westward blowing trade winds push water toward the west Pacific. As a result, the sea level in the Philippines is typically 23 inches higher than off the coast of Panama. The surface water heats up as it flows west, making the ocean water situated off Australia the warmest on earth. An El Nino, named after the Christ Child by Peruvian fishermen over a century ago, is produced when the trade winds weaken, allowing this pool of warm water to slosh eastward across the Pacific. The resulting change in sea water temperatures has profound effects on world weather - causing droughts in Australia and Indonesia and devastating storms on the coast of South America and California.
An extremely strong and long-lived El Nino occurred in 1899, causing the Indian monsoon to fail, which resulted in one of Indias worst famines. The British government sent Sir Gilbert Walker to see if he could forecast future monsoons. Sir Gilbert sifted through world weather records and discovered that the surface air pressure between Darwin, Australia and Tahiti see-sawed back and forth, when one was above average, the other was below. When the pressure was higher in Tahiti than Darwin, the surface air flowed westward, and the Indian monsoons were normal. When the pressure was higher in Darwin, the trade winds died down or even reversed. It was during these periods of weak trade winds that the monsoon failed. Sir Gilbert coined the term Southern Oscillation to designate this pattern of east-west air pressure variations. But it was not until the 1960s that Jacob Bjerknes finally identified the connection between the southern oscillation and ocean temperatures - the ENSO (El Nino - Southern Oscillation) phenomenon. Since then the ENSO model has proven to be one of the more important twentieth-century advances in meteorology - along with the concepts of fronts, air masses, and the jet stream.
The most intense El Nino event of the century occurred during the winter of 1982 - 83. The surface water temperature was 7.5 degrees (Fahrenheit) above normal off the coast of Peru. It produced torrential rain in the Southwest US and Australias worst drought. Five hurricanes hit French Polynesia; there were floods in Louisiana and Florida, and a serious drought in Hawaii. In the early 1990s we experienced the longest-lived El Nino on record. Just as the already unprecedented 1991-93 El Nino seemed to be fading, it resurged, causing massive flooding in the Midwest during the summer of 1993 and heavy rains that flooded much of California during the winter of 1994- 95.
Despite all the technological advances in meteorology such as radar and satellites, the most useful analytic tool in forecasting El Nino episodes is still Walkers Southern Oscillation Index - a measure of the difference in sea level pressure between Tahiti and Darwin. El Ninos are associated with negative values. After eighteen months of positive values, the SOI has moved to strongly negative ones - minus 7 in March, minus 14 in April, and minus 18 in May. The alarms are going off. An El Nino of major proportions is on the horizon.
The ENSO model provides significant insights into what lies ahead for the winter of 1997-98 and the following year. If an El Nino develops, there is about a 75 percent probability that Australia will experience a significant drought during their summer growing season (our winter 1997-98). The anchovy catch off the coast of Peru will drop drastically. Wheat and soybean prices will likely soar. The vast expanse of warm water in the central Pacific will produce powerful hurricanes. Iniki (1992) and Ewa (1982) both occurred during El Ninos. (Although hurricane John missed the islands in 1994, an El Nino year, it was the most intense hurricane on record in the central Pacific with winds of over 200 mph.) But in the Atlantic, an El Nino discourages the formation of hurricanes. The displacement of the jet stream during El Nino episodes typically takes it over the tropical Atlantic where it shears off the tops of tropical storms before they can develop into full-scale hurricanes. If an El Nino occurred the previous winter, the probability of two or more hurricanes hitting the US mainland is only 20 percent, in contrast to the average 46.
Forecasting an El Ninos effects on Californias weather is tricky. California can be either wetter or drier than normal depending on how the El Nino affects the position of the jet stream. If it displaces the southern branch of the jet stream so it washes over California, unusually heavy precipitation results. But this does not occur with every El Nino. Eighty percent of the heaviest winter precipitation years are associated with El Ninos, but an El Nino triggers such precipitation only 60 percent of the time.
On the bright side, Hawaiis south shore surf will be magnificent during the summer of 1998.
Mac Williams
Cosmos Mariner
Destination Unknown
© November 11, 1997