University of California Water Resources Center

The water supply in California is subject to large variations on a variety of timescales ranging from intraseasonal to decadal. Interannual variations were the focus of the research undertaken in this project. The primary source of water in California is precipitation associated with winter storms originating over the North Pacific Ocean. Thus, the variability in the water supply is ultimately linked to variations in the precipitation. It is known that a significant amount of the interannual variability in precipitation is related to variations in sea surface temperatures (SSTs) in the tropical eastern Pacific Ocean - El Niño and La Niña events. However, the response in California precipitation varies from event to event, in part because the SST anomalies do not evolve in the same way during each event. In addition, extreme events such as the flood of January 1997 can occur even when tropical Pacific SST anomalies are weak, suggesting that mechanisms other than El Niño/La Niña forcing can produce seasonal to interannual variations in California precipitation. The natural variability of the Pacific storm track is one such possible mechanism. These storm track variations can modulate the frequency, strength, and location of landfall of winter storms.