Research - Category I: Hydrology, Climatology & Hydraulics

Understanding and Predicting Seasonal-to-Interannual Fluctuations in California Precipitation Using an Atmospheric General Circulation Model
(Funded 1999-2000)

Principal Investigators:
John D. Farrara
Department of Atmospheric Sciences
UC Los Angeles
(310) 825-9205
jfarrara@ucla.edu

Jin-Yi Yu
Department of Atmospheric Sciences
UC Los Angeles
(310) 206-3743
yu@atmos.ucla.edu

Executive Summary:
The water supply in California is subject to large variations on a variety of timescales ranging from intraseasonal to decadal. This variability has been clearly evident during recent years with extremes that include both the abundant supplies during 1983, 1986, 1993, 1997, 1998 and the multi-year drought of 1987-1992. 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, there is no unique El Niño (or La Niña) signal in California precipitation - the spatial distribution and intensity of precipitation anomalies varies rather substantially from one event to another. In part, this is because tropical Pacific SST anomalies evolve differently from one event to another. Furthermore, recent research has shown that California precipitation can be influenced by SST variations in other tropical ocean basins even during winters in which there are strong SST anomalies in the tropical Pacific. Moreover, 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 large interannual variations in California precipitation. The natural variability of the Pacific storm track is one such possible mechanism. In the light of all this, projections of winter precipitation in California for water resource and emergency planning made using statistical methods which rely heavily on tropical Pacific SST anomalies have clear limitations.

In order to overcome these limitations, the approach proposed here is to use a numerical model of the global atmosphere to better understand the physical mechanisms underlying, and to predict, the intraseasonal to interannual variations in climate that influence precipitation in the western United States. Specifically, the project proposes to: 1) make a thorough assessment of the relative importance of natural variations in the Pacific storm track and variations associated with SST anomalies, and 2) perform and analyze simulations designed to increase the knowledge of the mechanisms by which SST anomalies influence California precipitation. As the understanding and modeling capability evolve it is proposed to develop and use a numerical model-based system for experimental prediction of winter precipitation in California.

The proposed research will result in a better understanding of the physical mechanisms responsible for the observed intraseasonal and interannual variations in California precipitation. In addition, a numerical model-based prediction system with an improved capability to predict these variations in extended range weather forecasts will be produced.