How do we define an El Nino? The Climate Prediction Center uses the ONI (Oceanic Nino Index) to define El Nino and La Nina events. Their definition for El Nino/La Nina using the ONI is as follows:
An El Nino event is defined when the three month running mean of (ERSST.v4) sea surface temperature anomalies in the Niño 3.4 region climb above the threshold of 0.5o for five consecutive overlapping three month periods.In other words, we take the average ocean temperature anomaly in the 3.4 region over a 3 month period, say September through January. Take the next five month overlapping periods--October through December and November through January--(the first two for example) and find these averages. That is your five consecutive, 3 month running mean. If this period's average is at or above +0.5 degrees, we classify this as an El Nino. The ERSST.v4 dataset goes back to 1850. Using this methodology, the top 10 strongest E Ninos are easy to find. Hat tip to Eric Webb for compiling the ERSSTv4 data.
The top ten El Ninos are as follows:
1877-78, 1888-89, 1982-83, 1997-98, 1896-97, 1902-03, 1905-06, 1940-41, 1957-58, 1972-73
What do the sea surface temperatures for each of these El Ninos look like? I created an animation of each El Nino's six, three month overlapping period starting with the September-November period ending with the February-April period. Each El Nino evolves differently. The position of the equatorial warm water, position and strength of the warm/cool pools in the PDO region are different with some, similar to others. Here is each sea surface temperature anomaly animation from the list above.
|1877-78 El Nino
|1888-89 El Nino
How does the atmosphere respond in each of these cases at the 300mB level? More on this later.
What were the winter temperatures like in each of these strong (both eastern and central) El Ninos?
One glaring similarity is that each of these El Nino events (1888-89, 1940-41 and 1957-58) average temperatures across the eastern US stayed close or slightly above average in December and in January. Then in February, the bottom falls out and temperatures drop significantly below average across a large area of the east. If you look at the Pacific sea surface temperature configuration of these winters (eyeballing the ENSO region, Eastern Pacific/west coast/Gulf of Alaska regions and Pacific Meridional Mode area), they closely resemble this year!