From WikiClimateImpacts
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S. California
El Niño/La Niña and Southern California Rainfall PatternDuring El Niño when the sea surface temperature (SST) in the eastern-equatorial Pacific Ocean is anomalously high, there is a tendency for Southern California, as well as other parts of the southwestern U.S. and northern Mexico, to receive above-normal rainfall. During La Niña when the SST in the eastern-equatorial Pacific is below normal, the association tends to be the opposite. These tendencies are related to the effects of El Niño and La Niña on the storm track over the North Pacific Ocean that carries moisture in the atmosphere from Asia towards North America.
How does SST affect rainfall through the storm track?The climatological Pacific storm track—the region of preferred propagation of storm systems—heads eastward from Japan and, when it reaches the eastern Pacific, tends to move northeastward. Consistent with the precipitation variations, during El Niño winters, the storm track tends to maintain an eastward route from the central Pacific and head into southwestern North America. During La Niña winters, the northeastward deflection over the eastern Pacific is exaggerated. These changes in the storm track are fundamental to creating seasonal precipitation anomalies and long-term droughts. Other Factors?Because of the small amount of total annual rainfall in Southern California and the relatively large amount of rainfall associated with some individual storms, sometimes a single storm can tip the balance between above- and below-average annual rainfall in Southern California. In addition to El Niño and La Niña, the Arctic Oscillation also affects the latitude of the storm track and, thus, has an important effect on Southern California rainfall. During the positive phase of the Arctic Oscillation, the storm track is driven northward, resulting in drier conditions in California. In general, the opposite precipitation pattern is exhibited during the negative phase. see the current state of the Artic Oscillation index ENSO DiversityIn the past two decades, there has been a tendency to have more central-Pacific El Niño (events that have maximum warming in the central-equatorial Pacific) than the classical eastern-Pacific El Niño (events that have maximum warming in the eastern-equatorial Pacific). The two flavors of El Nino are associated with different precipitation pattern in the west coast of the US.
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