Monday, 10 January 2005: 1:45 PM
Predictability of Northern California precipitation during warm-phase El Nino events
Observed precipitation in Northern California has been greater than normal during most, but not all, recent El Nino events. For example, the most recent El Nino year (1997-1998) was the wettest in 50+ years in San Francisco; by contrast, the previous El Nino year (1991-1992) was drier than normal. Our goal is to understand the cause of these varying regional precipitation responses to El Nino. Using a high-resolution global model of the atmosphere, we performed ensembles of simulations of the 1991-1992 and 1997-1998 winters. All simulations used observed sea-surface temperatures (SSTs) as boundary conditions; different ensemble members used different atmospheric and land surface initial conditions. We find that 9 of 10 simulations of 1997-1998 are wetter than normal in Northern California (as observed), and 1 of 10 is very dry. By contrast, only half the simulations of 1991-1992 are wetter than normal. This suggests that, while there is strong chaotic variability, the varying precipitation responses to El Nino in Northern California are to a significant extent predictable; i.e. result from varying patterns and/or amplitudes of SST anomalies.