Climatology of El Niño Events

and California Precipitation
by Jan Null, CCM
Updated May 2014


 Summary:   With the periodic reoccurrence of El Niño there are efforts to try and assess what the impact will be on California, especially the state's water supply.  This project examines the precipitation for warm ENSO events across California.

Overview:  El Niño events have are defined from the Oceanic Niño Index (ONI) into Weak, Moderate and Strong events.  All events classified and then also parsed as just weak or moderate or strong.  Then the seasonal (July 1 through June 30) percent of normal precipitation for each category, plus a collective category of "all" warm and cool events, was collated for a number of cities and geographic areas.  One of these areas is the Northern Sierra Nevada Eight Station index (8SI) which is the average of the precipitation totals for Mt. Shasta City, Shasta Dam, Brush Creek, Mineral Ranger Station, Quincy, Sierraville Ranger Station, Blue Canyon and Pacific House.  Additionally the seven NCDC climate division (see map) averages were compared to the ENSO events.  These comparisons was done in terms of both average seasonal precipitation and also the number of occurrences within percent of normal categories.

Observations:  The best "signal" associated with warm ENSO (El Niño) events is, as would be expected, with the "Strong" events, especially in Southern California.  For six seasons categorized as strong, all regions averaged above normal precipitation with the highest percentages of normal (PON) in Los Angeles, Climate Divisions 6 and 7.  During weak and moderate warm events there is no such signal with the number of years of above and below normal being fairly evenly split statewide.

Important Caveat:  The data presented here is a climatology of past events and is made up of a broad range of events from a small sample that are averaged together.  Consequently, like all averages, this type of data should not be used as a forecast! 


El Niño

Table 1.  All El Niño Events - Average Percent of Normal Precipitation (22 events)

ENSO Type San Francisco Eureka Sacramento Fresno Los Angeles 8SI Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7
All El Niño 110% 103% 106% 116% 127% 104% 105% 104% 98% 112% 110% 126% 128%
Weak El Niño 88% 93% 87% 94% 104% 92% 94% 86% 84% 90% 91% 103% 110%
Moderate El Niño 106% 105% 102% 121% 127% 105% 104% 107% 104% 111% 112% 128% 129%
Strong El Niño 145% 113% 137% 139% 172% 128% 132% 134% 119% 156% 131% 167% 158%

Table 2.  All El Niño - Years by Percent of Normal Categories (22 events)
% of Normal San Francisco Eureka Sacramento Fresno Los Angeles 8SI Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7
<80% 7 3 9 6 6 8 7 9 7 8 7 5 5
80-100% 4 8 3 5 3 2 1 2 6 3 5 4 3
100-120% 3 3 1 2 2 5 7 4 3 2 1 3 4
120-140% 1 6 4 2 0 4 3 3 3 0 3 2 1
>140% 7 2 5 7 11 3 4 4 3 9 6 8 9

Table 3.  Weak El Niño - Years by Percent of Normal Categories (8 events)
% of Normal San Francisco Eureka Sacramento Fresno Los Angeles 8SI Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7
<80% 4 1 4 3 4 3 3 4 4 4 3 4 4
80-100% 2 4 2 3 2 1 0 1 1 2 3 2 1
100-120% 0 2 0 0 0 3 4 2 3 0 0 0 1
120-140% 1 1 1 0 0 1 1 1 0 0 1 0 0
>140% 1 0 1 2 2 0 0 0 0 2 1 2 2

Table 4.  Moderate El Niño - Years by Percent of Normal Categories (8 events)
% of Normal San Francisco Eureka Sacramento Fresno Los Angeles 8SI Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7
<80% 2 2 3 1 2 3 3 3 2 2 2 1 1
80-100% 1 1 1 2 0 0 0 1 3 1 2 1 2
100-120% 3 1 1 2 2 2 2 1 0 2 1 2 1
120-140% 0 4 3 1 0 2 2 2 1 0 1 1 0
>140% 2 0 0 2 4 1 1 1 2 3 2 3 4

Table 5.  Strong El Niño - Years by Percent of Normal Categories (6 events)
% of Normal San Francisco Eureka Sacramento Fresno Los Angeles 8SI Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7
<80% 1 0 2 2 0 2 1 2 1 2 2 0 0
80-100% 1 3 0 0 1 1 1 0 2 0 0 1 0
100-120% 0 0 0 0 0 0 1 1 0 0 0 1 2
120-140% 0 1 0 1 0 1 0 0 2 0 1 1 1
>140% 4 2 4 3 5 2 3 3 1 4 3 3 3

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