An Analysis of El Niņo,  La Niņa and California Rainfall
Jan Null, CCM
(always a work in progress, last updated October 2004)


Early indications are that winter 2004-2005 will see a marginal El Ni
ņo develop in the tropical Pacific But what does all this mean for rainfall across California?  If all the hype of previous El Niņo and La Niņa events is to be believed then it's time to start looking for high ground.  Or maybe just to look at what really has happened during past events. 

El Niņo is characterized by an increase in the sea surface temperatures (SST) in the tropical waters of the eastern and central Pacific Ocean.  These "warm" events occur on an irregular cycle, generally ranging from 2 to 7 years and last from as little as 6 month to as long as 4 years.  El Niņos also vary in strength and location and are measured by a variety of criteria including SST, the Southern Oscillation, low level wind flow and outgoing long-wave radiation.  Even though El Niņo forms and is only present in the tropical Pacific, it is indirectly felt in other parts of the world as it disrupts the "usual" position of the jet stream and consequently precipitation patterns.

The "cool" event counterpart of El Niņo is La Niņa.  These events also disrupt the normal weather patterns in other parts of the world, though their impact is generally not as significant as El Niņo in the United States.

Even within these occurrences there is considerable variation and not only the intensity but even the beginning and ending dates may vary from researcher to researcher.  For the purposes of this analysis the list developed at will be used as delineated in Table 1.  Since these events impact winter precipitation in the Northern Hemisphere the indicated season is for the beginning of the event (i.e., 1950 would indicate the winter of 1950-1951).

Table 1.  El Niņo and La Niņa Seasons
El Niņo - Weak to Moderate 57-58, 65-66, 77-78, 87-88, 92-93, 94-95, 02-03
El Niņo - Strong 72-73, 82-83, 91-92, 97-98
La Niņa - Weak to Moderate 50-51, 56-57, 64-65, 70-71, 71-72, 74-75, 98-99, 00-01
La Niņa - Strong 55-56, 73-74, 75-76, 88-89

Composite United States maps for the period of 1950-51 through 2002-03 were constructed for all the years in each of the event types listed in Table 1. as well as for all El Niņo and all La Niņa events.  These maps (Figures 1-6) show the precipitation anomaly based upon climate divisions (Fig 7). 
[click images to enlarge]

Fig. 1
All El Niņo

Fig. 2
Strong El Niņo

Fig. 3
Weak-Moderate El Niņo


Fig. 4
All La Niņa

Fig. 5
Strong La Niņa

Fig. 6
Weak-Moderate La Niņa


Fig 7
Climate Divisions

Seasonal rainfall totals were also calculated for each of the 7 California climate divisions, t, the California Department of Water Resources Northern Sierra 8 Station Index (an average of precipitation totals for Mt. Shasta City, Shasta Dam, Brush Creek, Mineral Ranger Station, Quincy, Sierraville Ranger Station, Blue Canyon and Pacific House), and for 6 key city locations (Eureka, San Francisco, Sacramento, Fresno, Los Angeles and San Diego.  These were then normalized and plotted as a percentage of the average rainfall for the period and color-coded by the type of event (Figs 8-21).


Fig 8
Climate Division 1

Fig 9
Climate Division 2

Fig 10
Climate Division 3

Fig 11
Climate Division 4

Fig 12
Climate Division 5

Fig 13
Climate Division 6

Fig 14
Climate Division 7

Fig 15
Eight Station Index

Fig 16
Fig 17
San Francisco
Fig 18
Fig 19
Fig 20
Los Angeles
Fig 21
San Diego


The most notable precipitation signal across California are from El Niņo events, which show an overall positive anomaly in all 7 climate divisions (Fig. 1).  The signal for La Nina is less distinct with no significant signal for the northern three climate divisions and a weak to moderate negative signal in the south (Fig. 4).

For the four strong El Niņos (Fig 2) the positive signal is even stronger with no significant signal for weak to moderate events (Fig 3).  Interestingly, the weak to moderate La Niņas show a more significant signal than the stronger events.  In these cases all but Northwest and Northeast California (i.e., climate divisions 1 and 3) show a negative anomaly, yet in the strong La Niņa cases the Northwest (Climate Division 1) is wet, Southern California (Divisions 6 and 7) are dry and Climate Divisions 2,3, 4 & 5 are neutral.

In addition to looking at the response of rainfall to various El Niņo and La Niņa events for the individual climate divisions and cities (Fig. 8-14 and Fig. 16-21) a key indicator is the 8 Station Index which reflects rainfall into the important watersheds of the Northern Sierra Nevada and Shasta Ranges (Fig. 15).  The signal for weak to moderate El Niņos is mixed with 4 of the 9 events being above normal and rest below and the weak La Niņas were equally mixed with 4 cases above normal and 4 below.

With the exception of the strongly positive rainfall anomaly in Southern California during strong El Niņos the presence of either El Niņo or La Niņa is not a guarantee of either a significantly wet or dry year in California.  It should also be noted that previous work (i.e.,
El Niņo and La Niņa...Their Relationship to California Flood Damage) found that there is NOT even a strong correlation between either El Niņo or La Niņa and flood damage in California.


For Additional El Niņo and La Niņa resources see:


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