Background
El Niño 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 Niños 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 Niño 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 Niño is La Niña.  These events also disrupt the normal weather patterns in other parts of the world, though their impact is generally not as significant as El Niño 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 http://ggweather.com/enso/years.htm 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).

Data
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 Niño and all La Niña events.  These maps (Figures 1-6) show the precipitation anomaly based upon climate divisions (Fig 7).  [click images to enlarge]

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 Eureka	Fig 17 San Francisco	Fig 18 Sacramento	Fig 19 Fresno
Fig 20 Los Angeles	Fig 21 San Diego

Analysis
The most notable precipitation signal across California are from El Niño 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 Niños (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 Niñas 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 Niña 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 Niño and La Niña 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 Niños is mixed with 4 of the 9 events being above normal and rest below and the weak La Niñas were equally mixed with 4 cases above normal and 4 below.

Conclusion
With the exception of the strongly positive rainfall anomaly in Southern California during strong El Niños the presence of either El Niño or La Niña 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 Niño and La Niña...Their Relationship to California Flood Damage) found that there is NOT even a strong correlation between either El Niño or La Niña and flood damage in California.