California temperature and precipitation trends: climate variability or global warming

In this study, we examine recent climatic records from over 300 stations in California to determine which factors influence the temperature and precipitation trends over the last several decades. While temperatures for the state show a similar increase over the last century to those of the United States as a whole, the precipitation record is more complex. The large state displays several climate regions, each with their own unique seasonal patterns of temperatures and precipitation.

Throughout California, nearly all regions show a warming trend over several decades. However, cyclic variations within the trend corresponding mainly to Pacific Ocean patterns also occur. In particular, we find that the Pacific Decadal Oscillation (PDO), which represents large-scale sea surface temperatures (SST) patterns in the North Pacific, accounts for over 25% of the annual temperature variability. The strongest correlations occur along the entire west coast. However, all regions of California show a positive correlation between annual temperatures and the PDO. Both local SSTs and the PDO can be used to forecast California temperatures two to three seasons in advance, especially summer temperatures. Since the start of the 20th Century, California temperatures have risen fastest since the 1970s. During this period, spring and summer temperatures have increased fastest, with winters cooling somewhat. Similar to other regions of the U.S., the diurnal temperature range has decreased as minimum temperatures have risen faster than maximum temperatures.

Precipitation records show regional differences in trends and variability over several decades. Most of the northern and central California regions exhibit precipitation increases since 1925, while most of the southern regions display either little or no change or even some decreases throughout the same period. Seasonally, in all regions increases are greatest in winter, especially since the 1970s. Annual precipitation correlates significantly with ENSO measurements (Southern Oscillation Index, SOI, and Multivariate ENSO Index), with the SOI explaining over 30% of the variation in southern California. SOI values are useful predictors of precipitation, up to six months in advance for the southern California coastal region. SOI is less useful in explaining variability toward the northern regions of the state. During El Niño events, the southwestern U.S. receives above normal precipitation while the northwestern region is usually below normal. The opposite U.S. regional precipitation pattern is associated with La Niña periods. The precipitation trend is not useful in explaining variability, with an r2 of less than 2%. Statewide, precipitation intensity also shows some increase from the first half of the 1925-2009 record to the second half, although the differences are not significant. ENSO influences on precipitation are stronger when in phase with the PDO.

The results show that Pacific Ocean patterns, such as the PDO and ENSO, explain annual variability of California temperature and precipitation better than any trend line and are useful in predicting west coast climate.