Weathernotes Archive September 2004

Weathernotes for Thursday, September 16, 2004

According to Dr. Klaus Wolter in the September 3, 2004 update of the Multivariate ENSO Index (MEI) page, the latest increase of the MEI Index from .454 to .602 "virtually guarantees" weak to moderate El Niño conditions this Fall and Winter. At this point the MEI behavior resembles that recorded during 1979, which evolved to a moderate El Niño event during the Winter of 1979-80.

The Climate Prediction Center's ENSO Diagnostic Discussion, issued September 9, reports that the operational definition an El Niño was satisfied for the period June-August 2004, and that it "seems most likely that SST anomalies in the Niño 3.4 region will remain positive, at or above +0.5°C, through early 2005."

If mild to moderate El Niño conditions persist this Winter, what might be the impact on Southern California? Climate Diagnostics Center (CDC) data analysis tools show the expected positive correlation of seasonally averaged November to April Southern California precipitation to Niño 3.4 (East Central Tropical Pacific) SST. Generally, the larger the SST anomaly in the Niño 3.4 region, the more precipitation in Southern California.

However, there is growing evidence that other ocean-atmosphere interactions, such as the Pacific Decadal Oscillation (PDO), can lessen or amplify ENSO impacts. The PDO characterizes the state of Pacific SSTs north of the tropics. Results vary from region to region, but generally ENSO impacts are the most defined when ENSO and the PDO are in phase. The warm phase of the PDO is characterized by cooler than normal SSTs in the central Pacific, east of Japan; and warmer than normal SSTs off the west coast of North America, and in the tropical Pacific. The PDO index has been positive since August of 2002, with higher index values the Winter of 2002-2003, than in the Winter of 2003-2004. Current SST animations suggest the PDO could be moderately to strongly positive this Winter. CDC data analysis tools show a weak positive correlation of seasonally averaged November to April Southern California precipitation to the PDO index.

The long term Precipitation Outlooks, issued by the CPC September 16, 2004, start to show an El Niño like pattern with the Dec-Jan-Feb Precipitation Outlook. The CPC's discussion of these forecasts takes a wait and see attitude, and stresses that El Niño impacts will "depend critically on the evolution of SSTs in the eastern Pacific and the coupling between the atmosphere and SST anomalies."

Should the pattern continue to evolve like the El Niño of 1979-80, and behave in a similar manner, Southern California could experience a very wet Winter. That rainy season Los Angeles received over 26" [typo corrected] of rain. This weekend an unseasonably strong upper trough may result in some showers. We'll see what this Winter brings!

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