It has been noted in several studies (Mo and Higgins, 1997 J Clim) that tropical influences have a profound impact on California precipitation. Specifically wet events are noted by tropical intraseasonal oscillations (Madden Julian Oscillation) near 120E. During ENSO neutral years there is a higher frequency of the MJO. During this past cool season in California a well-defined intraseasonal oscillation occurred with a 30 to 60 day cycle. Two major precipitation episodes occurred from two of the three negative phases in central and northern California. Fig. 1 shows an example of the stream gage for the Russian River in coastal central California when the MJO was identified at 120E. The MJO was identified using the MJO index posted daily to the NCEP’s Climate Prediction Centers web site:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_mjo_index/mjo_index.html

In terms of impacts on forecast operations, this maybe one of the only tools this author is aware of that provides any type of reliable indication of potential flooding well ahead of any numerical models indicating such a wet pattern. It is well known that current operational models have little sensitivity to the MJO. It turns out that for this past winter season, these two MJO periods accounted for most of the winter precipitation in the arid southwestern US.

In California, most reservoirs have space set aside for floodwaters. It is sometimes beneficial to encroach into this flood space during dry periods. Given that the MJO may provide lead times of as much as two weeks, sufficient time is available for reservoir operators to make controlled releases that provide hydroelectric power and still bring the reservoir below the encroachment levels required. The key to making such critical decisions is the reliability of the MJO forecast. A third strong negative phase of the MJO occurred in March, 2004 with no impact on California. Thus it is necessary to understand the relationship between the MJO and the large-scale circulations that may or may not support the breakthrough of the subtropical jet stream into California.