California Getting Wetter to the North, Drier to the South: Natural Variability or Climate Change?

Dan Killam,Joshua Willis,Pedro Ramirez,Steve Ladochy,Ann Bui,William Patzert

doi:10.3390/cli2030168

Abstract

Current climate change projections anticipate that global warming trends will lead to changes in the distribution and intensity of precipitation at a global level. However, few studies have corroborated these model-based results using historical precipitation records at a regional level, especially in our study region, California. In our analyses of 14 long-term precipitation records representing multiple climates throughout the state, we find northern and central regions increasing in precipitation while southern regions are drying. Winter precipitation is increasing in all regions, while other seasons show mixed results. Rain intensity has not changed since the 1920s. While Sacramento shows over 3 more days of rain per year, Los Angeles has almost 4 less days per year in the last century. Both the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) greatly influence the California precipitation record. The climate change signal in the precipitation records remains unclear as annual variability overwhelms the precipitation trends.

Highlights

Recent studies have shown that both precipitation and precipitable water have increased in many regions of North America as the atmosphere has warmed over the last century [1,2,3,4]
In California, any analysis of precipitation patterns must account for regional differences in climate
The northern and central parts show modest to slight increases in annual precipitation, while the southern regions show negligible change or decreases since the early 1900s and large decreases since the 1970s

Summary

Introduction

Recent studies have shown that both precipitation and precipitable water have increased in many regions of North America as the atmosphere has warmed over the last century [1,2,3,4]. In recent decades warmer climate has led to changes in the California hydrological cycle that influences state water supplies These include the decreasing spring snowpack [9], earlier snowmelt runoff [10] and trends towards more precipitation falling as rain rather than snow [11]. When comparing the 1948–1975 period to the later 1976–2004 years, Higgins et al [6] found that the large increase in total precipitation in the western United States could be explained by the warm phase of the Pacific Decadal Oscillation. More recently the PDO shifted from a warm to cool phase around 1998 and precipitation has decreased in the western United States This PDO-precipitation connection especially explained the increases in heaviest precipitation (heavy precipitation was defined as the top 10% of daily totals) [6]. Regions of the U.S The records allow us to test for the influence of climate change on California precipitation trends

Data and Methods

Results

Discussion and Conclusions