Regions which are far from coasts, and especially in the mountainous western section of North America, including large portions of Idaho, are more subject to rapid and repeated short-term climate variations. Unfortunately, many of these variations are, by nature, not predictable. Rather, they contribute to the climate noise, often making any small, persistent anomalies which do occur indistinguishable from the climate noise.
The sea surface temperature near the equator in the Pacific Ocean changes little from day-to-day, week-to-week, or even month-to-month. This means that sea-surface temperature anomalies, i.e., the sea-surface temperature which is warmer or colder than the 30-year average for the location and date, lasts for months, seasons or even years. Sea-surface temperature in the tropics is closely related to the occurrence of large thunderstorms throughout the tropics. These storms transfer the warmth from the ocean surface into the atmosphere overhead.
At an average of every three to five years, ocean surface temperatures in the equatorial central and eastern Pacific become abnormally warm. The abnormal warming in the eastern Pacific often begins in mid to late December and peaks during the northern spring.
During El Niño episodes, abnormally low pressure is observed in the eastern tropical Pacific and abnormally high pressure is found over Indonesia and northern Australia. The normal pattern of tropical cloudiness and precipitation is also disrupted. Rainfall that normally falls over Indonesia shifts eastward over the abnormally warm ocean waters of the central equatorial Pacific.
The effect of El Niño upon the global atmospheric circulation together with the location of the United States just to the northeast, make the U.S. particularly vulnerable to increased storminess over its southern sections during El Niño. Another effect of El Niño is reduced storminess over the northern U.S., along with unusual warmth and dry conditions.