Climatology of El Niño Events

and California Precipitation
by Jan Null, CCM
Updated July 21, 2015


 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, Strong and Very Strong events.   Then the seasonal (July 1 through June 30) percent of normal precipitation for each category, plus a collective category of "all" warm events, was collated for a number of cities and geographic areas. These include three major watersheds, the Northern Sierra Nevada Eight Station Index (8SI, map), the Southern Sierra Nevada Five Station Index (5SI, map) and the Tulare Basin Six Station Index (6SI, map). Additionally the seven NCDC climate division (see map) and the entire state of California (CA) 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 "Very Strong" events all at least 140% of normal except for Climate Divison 3 which was between 120% and 140%.  In the "Strong"category Southern California showed a significant positive signature with the three seasons categorized as strong, all  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 (23 events)
ENSO Type Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7 CA 8SI 5SI 6SI SF EKA SAC FAT LAX
All El Niño 108% 106% 105% 114% 111% 127% 124% 111% 103% 105% 111% 102% 100% 103% 110% 126%
Weak El Niño 107% 107% 111% 110% 112% 126% 122% 111% 105% 109% 112% 95% 99% 99% 109% 126%
Moderate El Niño 87% 81% 78% 86% 83% 99% 109% 87% 79% 77% 83% 78% 90% 79% 83% 91%
Strong El Niño 115% 110% 105% 131% 116% 133% 120% 116% 102% 100% 112% 123% 96% 116% 113% 140%
Very Strong El Niño 162% 167% 150% 198% 182% 212% 190% 174% 162% 174% 191% 181% 146% 183% 190% 209%

Table 2.  All El Niño - Years by Percent of Normal Categories (23 events)
% of Normal Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7 CA 8SI 5SI 6SI SF EKA SAC FAT LAX
<80% 7 8 8 7 7 7 5 5 8 9 8 8 3 9 8 6
80-100% 2 2 4 4 5 1 4 5 2 4 3 3 9 3 4 3
100-120% 5 5 3 2 1 4 4 4 6 1 2 5 6 2 2 2
120-140% 5 4 5 2 3 2 1 3 4 4 3 2 3 5 3 1
>140% 4 4 3 8 7 9 9 6 3 5 7 5 2 4 6 11

Table 3.  Weak El Niño - Years by Percent of Normal Categories (12 events)
% of Normal Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7 CA 8SI 5SI 6SI SF EKA SAC FAT LAX
<80% 3 3 4 3 3 5 5 3 3 4 3 4 1 4 5 4
80-100% 0 1 1 3 3 1 1 2 1 2 3 2 5 2 1 2
100-120% 4 3 2 0 0 0 0 1 4 0 0 3 5 1 1 0
120-140% 4 4 3 2 2 0 0 3 3 3 2 2 1 4 2 0
>140% 1 1 2 4 4 6 6 3 1 3 4 1 0 1 3 6

Table 4.  Moderate El Niño - Years by Percent of Normal Categories (6 events)
% of Normal Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7 CA 8SI 5SI 6SI SF EKA SAC FAT LAX
<80% 4 4 3 3 3 2 0 2 4 4 4 3 2 4 2 2
80-100% 0 1 3 1 2 0 3 2 0 1 0 1 2 1 3 1
100-120% 1 1 0 2 1 3 2 2 2 1 2 2 1 1 1 2
120-140% 1 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0
>140% 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1

Table 5.  Strong El Niño - Years by Percent of Normal Categories (3 events)
% of Normal Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7 CA 8SI 5SI 6SI SF EKA SAC FAT LAX
<80% 0 1 1 1 1 0 0 0 1 1 1 1 0 1 1 0
80-100% 2 0 0 0 0 0 0 1 1 1 0 0 2 0 0 0
100-120% 0 1 1 0 0 1 2 1 0 0 0 0 0 0 0 0
120-140% 0 0 1 0 1 1 1 0 1 1 1 0 1 1 1 1
>140% 1 1 0 2 1 1 0 1 0 0 1 2 0 1 1 2

Table 6.  Very Strong El Niño - Years by Percent of Normal Categories (2 events)
% of Normal Div. 1 Div. 2 Div. 3 Div. 4 Div. 5 Div. 6 Div. 7 CA 8SI 5SI 6SI SF EKA SAC FAT LAX
<80% 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
80-100% 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
100-120% 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
120-140% 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0
>140% 2 2 1 2 2 2 2 2 2 2 2 2 2 2 2 2

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