Significant modulation of California precipitation by variability and projected changes in extratropical cyclone activity

Abstract:

Extratropical cyclones give rise to much of the precipitation over the U.S., especially in winter. Shifts in the storm tracks or changes in cyclone activity, such as those associated with ENSO or global climate change, can have significant impacts on the hydroclimate of many regions in the U.S. Our research provides a quantitative measure of the strong modulation of California precipitation by extratropical cyclone activity. Observed winter precipitation is shown to be highly correlated to a metric of extratropical cyclone activity over the Eastern Pacific just offshore of California, with a year-to-year correlation of 0.8. The recent lack of precipitation over the past 3 winters over California is coincident with three consecutive years of much below average extratropical cyclone activity. Both extratropical cyclone activity and California precipitation exhibit a small downward trend since 1979 which are consistent with each other, but both trends are not statistically significant.

We have examined the variability and projected changes in extratropical cyclone activity and California precipitation made by models participating in phase 5 of the Coupled Model Intercomparison Project. Our results indicate that most models can simulate the relationship between extratropical cyclone activity and precipitation quite well. Examination of projected change suggests 1) no strong evidence of a long-term downward trend in California-region storm track activity within the examined scenarios; and 2) that the inter-model spread in California precipitation projection can be largely explained by the inter-model spread in the projection of extratropical cyclone activity. The outstanding question is to assess and understand what gives rise to the spread in projection of cyclone activity.

Apart from long term climate projections, we have also examined whether short-term (months to seasons) prediction of the variability of extratropical cyclone activity is feasible. Preliminary results suggest that the current generation of climate prediction models (including the NOAA Climate Forecast System) has some skill in predicting extratropical cyclone variability out to at least several months in advance, suggesting the potential for predicting cyclone related precipitation over California across a similar time frame.