A Later Onset of the Rainy Season in California

Study key findings & significance

• The start of California's annual rainy season has been pushed back 27 days since the 1960s from November to December, prolonging the state's increasingly destructive wildfire season by nearly a month.
• This delay occurs due to a number of conditions that controls precipitation: the summer circulation pattern has been extending throughout November across the North Pacific, and the wintertime strengthening of the Aleutian Low is delayed.
• The findings provide observational evidence for the projected rainfall change over California and inform ongoing discussion about the drying/wetting tendencies of the rainy season in California.

Author quotes

What we've shown is that it will not happen in the future, it's happening already. The onset of the rainy season has been progressively delayed since the 1960s, and as a result the precipitation season has become shorter and sharper in California.

Jelena Luković, lead author and climate scientist at the University of Belgrade in Serbia

Abstract

Californian hydroclimate is strongly seasonal and prone to severe water shortages. Recent changes in climate trends have induced shifts in seasonality, thus exacerbating droughts, wildfires, and adverse water shortage effects on the environment and economy. Previous studies have examined the timing of the seasonal cycle shifts mainly as temperature driven earlier onset of the spring season. In this paper, we address quantitative changes in the onset, amounts, and termination of the precipitation season over the past 6 decades, as well as the large‐scale atmospheric circulation underpinning the seasonal cycle changes. We discover that the onset of the rainy season has been progressively delayed since the 1960s, and as a result the precipitation season has become shorter and sharper in California. The progressively later onset of the rainy season is shown to be related to the summer circulation pattern extending into autumn across the North Pacific, in particular, a delay in the strengthening of the Aleutian Low and later southward displacement of the North Pacific westerlies.