"Say what? We had El Niño this winter?". Yes, between November 2019 and March 2020, the primary metric used to identify El Niño events trickled along the minimum criteria to be classified as a "weak" event.
The Oceanic Niño Index (ONI) is the de-facto standard that NOAA uses for identifying El Niño (warm), and La Niña (cool), events in the tropical Pacific. It is the running 3-month mean Sea Surface Temperature (SST) anomaly for the Niño 3.4 region (i.e., 5°N-5°S, 120°-170°W). If events persist for 5 of these consecutive overlapping 3-month periods at or above +0.5° threshold it is classified as a "historical" warm (El Niño) event. See https://ggweather.com/enso/oni.htm.
This was the weakest El Niño seen in the data going back to 1950. For this past winter the ONI ranged between 0.5 and 0.6 for the period of October-November-December through February-March-April with an average of +0.52. The next closest "barely weak" El NIño was in the winter of 1958-59 when the average ONI for the same months was +0.57.
It is very important to note, that SST's also need to couple with the atmosphere for an El Niño to have a significant impacts. And furthermore, that impact can be increased or decreased by the "alphabet soup" of other climatic events like the PDO (Pacific Decadel Oscillation), PNA (Pacific-North American Oscialltion), AO (Arctic Oscillation) and MJO (Madden-Julian Oscillation).
This past winter year is also a good reminder that all El Niños are not the same and that they don't always mean a wet winter for California. That and other El Niño misconceptions are addressed at https://ggweather.com/enso/enso_myths.htm and https://ggweather.com/enso2016/us_elnino.html.
Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services