In order to understand California's precipitation patterns and the influence of El Niño better, Bor-Ting Jong from the Lamont-Doherty Earth Observatory at Columbia University and her colleagues used sea surface temperature and weather data from as far back as 1901.

Analysing the data month by month and region by region, they teased out some patterns in the behaviour of the weather. In particular, they showed that El Niños that remain strong into late winter are more likely to bring heavy precipitation to southern California (in eight out of ten cases this put southern California in the wettest tercile).

It seems that the El Niño-related warmer sea surface temperatures in the eastern equatorial Pacific in late winter cause deep convection patterns to shift eastward. "The anomalous atmospheric circulation moves the Pacific storm track eastward and southward, steering more storms toward southwestern North America, including central and southern California," explained Jong, whose findings are published in Environmental Research Letters (ERL).

However, the most recent El Niño has seemingly bucked the trend. "It didn’t really fit our research based on historical data, which claims that a moderate-to-strong El Niño in late winter is very likely to cause northern California not to be dry and southern California to be anomalously wet," said Jong. Instead this El Niño brought a little extra precipitation to northern California in the form of heavy snow in March 2016, and around 80% of average rainfall to southern California.

In this case the timing of the weakening of El Niño may have been crucial. Jong and her colleagues’ analysis suggests that the ideal conditions to bring rain to California are created by an El Niño that remains strong into late winter. But the most recent El Niño faded during late winter. "We are now investigating if the El Niño weakened in late winter faster than expected, and this was the reason why California did not receive all the drought alleviation expected," said Jong.

Evidently the interplay between El Niño and precipitation in California is complex, but understanding the relationship better will be of great value for farmers and water managers across the state.

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