La Niña: The Kid's Sister
El Niño refers to the warm portion of ENSO, but it is an oscillation, and its temperature goes through natural reversals. The warm water surface will oscillate between the eastern and western portion of the Pacific. When the warm water shifts to the western Pacific, deeper, colder water reaches the surface in the eastern Pacific. This cold portion of ENSO is called La Niña—"the girl."
La Niña is the cold phase of the El Niño oscillation. The waters of the Pacific near the equator and off the west coast of South America are much colder than normal during the La Niña. La Niña is the cold water sibling of El Niño.
An earlier figure showed how El Niño formed and weakened. La Niña occurs as soon as the western Pacific high-pressure system diminishes. That reduction in atmospheric pressure is hastened by the colder water that surfaces off the coast of Asia during El Niño. The high-pressure system gets El Niño rolling, but once it forms, the cold waters that surface in the west lead to its demise. Then as the high weakens, the normal east-to-west flow in the atmosphere resumes. Warm surface water is forced westward and upwelling causes colder water to return to the eastern portion of the ocean. That is La Niña. A strong El Niño is frequently followed by a strong La Niña. It's nature's way of balancing the score, but there are no guarantees. Numerous factors contribute to global weather patterns. During the spring of 1998, the cold water of La Niña surfaced in a matter of days, and the weather patterns responded almost immediately.
Fire and Ice
Although record warm temperatures occurred across the northern states during the first four months of 1998, La Niña turned the pattern around very quickly. The polar air masses began pushing southward. Snow fell in northern New England during June, and storms occurred with great frequency across the Northeast. Rainfall amounts for June were three times above the normal. Through July, most of the Northeast failed to experience a single heat wave, which is defined as a spell of 90 degree or higher temperatures for three or more days. Through June and most of July, the only encounter Albany, New York, had with 90-degree heat occurred at the end of March when El Niño still raged. It is no coincidence that a cool jet stream dips into the northern United States as soon as the tropical jet stream weakens. The resistance to the cold air breaks down as El Niño melts away. The winter of 1995-1996 was the snowiest on record for many northern states. That occurred during a La Niña period. During the 1998 La Niña, record warmth did occur in early December from the Great Lakes to New England, but in California the cold was severe. A disastrous citrus freeze caused $600 million of damage. In late December, the arctic blast moved into the Midwest with record snowfalls.
Since 1950, El Niños have been around 31 percent of the time and La Niñas 24 percent of the time. That means that for 55 percent of the time, there's an El Niño or a La Niña stirring.
The fading of El Niño 1998 wasn't exactly followed by universal cooling. The weather did cool, but globally, June 1998 was still the warmest June on record—thanks to the blistering heat that was trapped across the southern portion of the United States and Mexico. Overall, thanks to El Niño of 1998, globally the year became the warmest on record, and although the winter began on the cold side, the 1998-1999 season turned out to be mild from The Great Lakes to New England.
During 1998, hundreds of thousands of wooded acres burned in Florida. Scores of homes and buildings were destroyed. Crop losses reached more than $100 million. In Texas, temperatures reached 100 degrees or higher on a daily basis, with no rain in sight. Texas is famous for heat, but the unrelenting string of 100 degrees was of record proportions. During La Niña, there is a trend toward warmer weather than normal in the southern states, but other global factors might have contributed to the extreme nature of the 1998 heat wave. Some of those factors include changes in the global circulation patterns and even some astronomical changes, such as possible changes on the sun.
Generally El Niño appears off the coast of South America every few years. The extreme 1982-1983 El Niño claimed more than 2,000 lives and resulted in losses of $13 billion. Several additional El Niño patterns developed during the next 15 years, but these were much weaker. After the string of weak to moderate ENSO occurrences, a record El Niño developed in 1997. Overall, El Niño patterns will continue to appear, but only occasionally should they have the amplitude of the types that surfaced in 1982 and 1997.
Weather relationships are more difficult to determine during weak or moderate El Niño years. The pattern is far less clear-cut when water temperatures are two or three degrees above normal, rather than when those temperatures are five to ten degrees above. Meteorologists confidently predicted the impact of the 1997-1998 event months in advance. It was hard to miss that one. But the basic lack of understanding of all the interactions between the atmosphere and oceans will make future predictions far less reliable when those less-than-dramatic ENSO patterns come along. During late 1999, conditions became close to neutral in the eastern Pacific. The pattern remained weak through 2001. During those weak oscillations, old-fashioned winter weather returned to the northern United States.
Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.