Golden Gate Weather Services

El Niño Category Changes  - Upon Further Review

Just like a sporting event where they go to instant replay to possibly overturn a decision, the same is true with the classification of El Niño's and La Niña's.  This has happened with the dataset used to calculate the Oceanic Niño Index (ONI) and consequently the "category" of nearly a dozen El Niño and La Niña events has changed.  This was the result of the Extended Reconstructed Sea Surface Temperature (ERSST) dataset being upgraded to ERSSTv4 from ERSSTv3.  (See details and previous ONI dataset) 

The ONI is a retrospective tool and is used to classify past events, and sometimes the most recent month or two values may shift slightly.  It is also not the best indicator in real-time events because it is a 3-month average and may not represent recent dynamic changes very well.  For a year to be classified historically as a warm (El Niño) or cool (La El Niña) event there must be at least 5 consecutive over-lapping 3-month periods above/below the +0.5 degrees C criteria.

This updated ONI data and list of events is annotated with an asterisk to show the ones that changed a category.  In total 23% of the 3-month periods (from 1950 to present) are cooler by 0.1 deg C, 4% cooler by 0.2 deg C, 1% cooler by 0.3 deg C, 20% warmer by 0.1 deg C, 5% warmer by 0.2 deg C and 47% remaining unchanged. This resulted in 9 events decreasing a category while 2 bumped up a category.

Of most recent interest is that last winter (2014-15) is no longer historically counted as an El Niño year because JFM was adjusted to 0.4 deg C, breaking the string of 5 consecutive periods at or above the 0,5 deg C criteria for a weak El Niño.

Summary of event changes:
2014-15  - no longer El Niño
2008-09  - no longer La Niña
2005-06  - no longer La Niña
1999-00 - from strong to moderate El Niño
1994-95 - from moderate to weak El Niño
1987-88 - from strong to moderate El Niño
1979-80  - increased to weak El Niño 
1968-69 - from moderate to weak El Niño
1967-68 - increased to weak La Niña
1956-57 - no longer La Niña
1951-52 - from moderate to weak El Niño 

There are now a total of 23 El Niño events since 1950; with 12 being "weak", 6 being "moderate", 3 "strong" and 2 "very strong".  Of the cool La Niña events, there is a total of 20; with 11 in the "weak" category, 5 "moderate" and 4 "strong".

These changes necessitated a reworking of several analyses of El Niño and precipitation.  These include El Niño / La Niña Winter Historic Precipitation & Temperatures  and Climatology of El Niño Events and California Precipitation.

Jan Null, CCM
Golden Gate Weather Services