Warmer than normal water in the east-central equatorial Pacific Ocean is called El Niño, and cooler than normal water is called La Niña. We often see year-to-year variations in sea surface temperature, rainfall, air pressure and atmospheric circulation in the equatorial Pacific Ocean. ENSO, or El Niño Southern Oscillation, describes the year-to-year variations between La Niña and El Niño. ENSO-neutral means water temps are "normal" based on long-term averages for the equatorial Pacific Ocean.
The atmosphere likes to remain in balance. That said, the oscillation back and forth between La Niña and El Niño occurs roughly every 2-7 years. Changes in both global ocean temperatures, pressure patterns and trade winds keep the oscillation going. It's like the atmosphere is in a tug-of-war with the ocean to maintain balance.
Why is ENSO Important?
Under ENSO, long-term weather patterns are altered from what's considered "normal." This has impacts on winter snowfall patterns across the United States and Canada. As the oceans interact with the atmosphere, the entire system operates in a feedback loop whereby warmer water produces local atmospheric pressure changes, putting a domino effect into motion. Local atmospheric pressure differences drive a circulation across the equatorial Pacific Ocean, and that drives changes in winds at different altitudes. So, the difference between a La Niña and El Niño can be significant and felt globally.
The pattern can stay ENSO neutral but will normally start tilting in one direction or another, either toward La Niña or El Niño. The challenge is to figure out which phase will dominate the winter season across the United States. To do that, we use an array of computer models. Historical trends can be important, too, and there are other considerations outside of ENSO that must be examined when putting together a winter forecast.
