Its official! NOAA has announced that an El Niño event is underway in the Pacific Ocean. But what, you may wonder, is an El Niño and how will it impact weather in the US?
El Niño is the warm phase of the larger El Niño-Southern Oscillation, known as ENSO. Developing every 3 to 7 years, it is a naturally occurring oceanic-atmospheric coupled phenomenon that influences weather around the globe.
To detect its presence, scientists monitor the Oceanic Niño Index (ONI). This index is based on sea surface temperature anomalies in a rectangular region of the tropical Pacific between 5°N and 5°S latitude and from 120°W to 170°W longitude. When sea surface temperature anomalies in this region exceed +0.5°C, a weak El Nino is present. When they reach +1°C, it is considered a moderate El Niño, and when they go above +1.5°C it is classified as a strong El Niño event.
But warming ocean temperatures are only half the story. There also needs to be a corresponding change in the atmosphere for an El Niño event to be declared. Specifically, there needs to be a weakening of the east-to-west flow of the Trade Winds and a change in tropical rainfall patterns.
Influencing the position of the polar and sub-tropical jet streams, El Niño is most noticeable during autumn and winter. In the US, its impacts typically include wetter than average conditions in California and most of the southern states while drier than average conditions settle across the Ohio Valley, Great Lakes, and northern Rockies. In terms of temperature, the southwest and southern plains tend to be cooler than average while the northern tier of the country is generally warmer than average.
Since every El Niño is different, it is important to remember that none of the regional impacts listed above are guaranteed. We will have to wait and see how this newest El Niño event plays outs.