El Nino forms, expected to strengthen, say NOAA forecasters

NOAA’s National Weather Service announced today that El Nino has developed in the tropical Pacific, and issued an El Nino Advisory. El Nino, the warm phase of the El Nino Southern Oscillation (ENSO), is predicted to intensify to a moderate or strong level this fall. Forecasters predict a 63% chance of sea surface temperatures exceeding 2.0°C in the Nino-monitored region of the Pacific. If this threshold is surpassed, NOAA considers the event a “very strong” El Nino. 

What is El Nino?

NOAA declares an El Nino (Spanish for “little boy”) has formed when temperatures in the equatorial Pacific are 0.5°C above average for several consecutive months. Additionally, NOAA monitors the atmosphere above this region of the Pacific, looking for a pattern called the Walker Circulation, a massive east-to-west air flow driven by the temperature and pressure differences between the warm western and cool eastern oceans. When the Walker Circulation breaks down and warmer water shifts east toward South America, El Nino is declared.

What does El Nino mean for U.S. weather?

El Nino tends to be strongest during the winter months, and its global impacts are typically most significant in the northern hemisphere winter. During a typical El Nino winter, the jet stream over the north Pacific Ocean tends to shift southward, bringing the storm track over the southern tier of the U.S. The southward shift in the storm track also leads to drier conditions over the Northern Rockies and Ohio and Tennessee valleys. For temperatures, El Nino often leads to a warmer than usual winter over the northern U.S. 

“Every El Nino is not the same; each one is unique with its own imprint on our weather,” said Ken Graham, director, NOAA’s National Weather Service (NWS). "Advanced monitoring and an improved understanding of El Nino patterns allow the NWS to better predict and better prepare the public and our core partners for what is to come.”

Usual impacts of El Nino can include:

• Stronger upper-level winds that tend to suppress storm and hurricane development in the Atlantic Basin, while weaker winds tend to enhance tropical development in the eastern and central Pacific basins.
• Stormier weather is more likely in the Southern U.S., with chances of both rain and snow increased during El Nino winters.
• High tide flooding could become a higher risk in parts of the U.S., especially on the West Coast. 
• Changes in the migration of fish and other oceanic organisms, with warm water species moving north while cold water species move farther north or into deeper waters. These behavioral changes impact growth, survival, and reproduction.
• Past El Nino episodes have also enhanced the formation of harmful algal blooms along the U.S West Coast. 

A new way of monitoring ENSO

In February, NOAA officially adopted the Relative Oceanic Nino Index (RONI) for monitoring sea surface temperatures and forecasting El Nino and La Nina events. Whereas the traditional Oceanic Nino Index (ONI) uses a static 30-year period of recorded ocean temperatures to calculate departures from average, RONI evolves from month to month, making this index more reliable for identifying El Nino and La Nina events.

NOAA’s ENSO team has been closely monitoring both the traditional ONI and RONI since 2021. After observing several years of ENSO phases and the corresponding atmospheric response, NOAA scientists concluded that RONI more closely correlated with the expected changes in the Walker Circulation, and therefore provided more useful information about El Nino and La Nina events to weather experts, emergency managers and the public.