Tracking El Niño and La Niña

This month NOAA began using a new index to track El Niño and La Niña in the equatorial Pacific Ocean. These names describe distinct patterns of tropical Pacific rainfall and winds are a factor in middle and high latitude monthly to seasonal scale climate, especially from late autumn to early spring.
For over 20 years, NOAA used sea surface temperature departures from normal in a large area (about two million square miles) straddling the equator south and southeast of Hawai’i. Work done in the 1990s showed this measure was better correlated with rainfall and wind patterns in the tropical Pacific than an older definition, which was based on the average air pressure difference between Tahiti and Darwin, Australia.
However, in recent years it has become clear that, with global oceans warming rapidly, temperature departures from normal in this so called “bell-weather” region were often not as well correlated with the atmosphere in the tropical Pacific as in the past.
The new index still uses the difference from normal in the bell-weather area, but it’s now adjusted to take into account ocean temperatures across tropical areas around the world.
This week’s graphic shows the difference between the new and old indices, and it’s obvious that since about 2010 the new index is consistently lower, meaning more “La Niña-like.”
Globally, using the new index improves overall seasonal forecasts. But will this change help improve seasonal outlooks in western Alaska? While only time will answer that question, no matter how El Niño and La Niña are defined, it is still just one factor of many factors than can influence what actually transpires during the cold season. 
Bering Sea ocean temperatures and sea ice timing and thickness are major regional factors. Variations in Arctic winds 20 miles above the ground, unrelated to anything happening in the tropics, can sometimes have prolonged influence on our weather for weeks at time. We can think of the new NOAA El Niño/La Niña index as an upgrade to one tool in the climate outlook toolbox, but other tools will continue to be useful for constructing seasonal outlooks for western Alaska and the Bering Strait region.