Weathernotes Archive July 2009

Los Angeles Ends Water Year at 60% of Normal Rainfall. El Niño Here -- Will it Last? Los Angeles Temps Continue Below Normal.
Weathernotes for Thursday, July 9, 2009.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

Downtown Los Angeles (USC) ended the water year on June 30th having recorded 9.08 inches of rain, which is 6.06 inches below the 1971-2000 norm of 15.14 inches. Some additional water year rainfall totals:

According to the El Nino/Southern Oscillation (ENSO) Diagnostic Discussion issued July 9 by the Climate Prediction Center/NCEP/NWS, "During June 2009, conditions across the equatorial Pacific Ocean transitioned from ENSO-neutral to El Niño conditions." According to the report, El Nino conditions are expected to continue to develop, and last through the NH Winter.

The May-June Multivariate ENSO Index (MEI) value, reported July 6, has increased by 0.51 to +0.85. As was the case last month, the 3-month rise of the MEI (since February-March) is the 4th highest on record for this time of year, and was last exceeded by the strong El Niño of 1997. According to the MEI's originator, Dr. Klaus Wolter, "the combination of already border-line moderate El Niño conditions along with such a big rise in the MEI at this time of year has always been followed by continued El Niño conditions through the remainder of the calendar year, at least in the modern MEI record (since 1950)."

However, as the truncated 2006-07 El Niño event demonstrates, an El Niño is more than just warm Pacific equatorial SSTs. Through complex forcing and feedback mechanisms, the atmosphere and oceans have to cooperate on a global scale. As climate scientist Ed Berry cautions in his July 3rd Atmospheric Insights post, "Should total AAM departures become comparable to that observed during this past January and February, my concerns of an El-Nino 'false alarm' for the weather-climate dynamical system will be significantly raised."

Generally speaking, the momentum of the atmosphere increases when there is an El Niño, and decreases during a La Niña. Over the past 40 days the Global Wind Oscillation (GWO) has retreated from the higher AAM (El Niño) side of the GWO phase space to the lower AAM (La Niña) side. One concern is that two areas of tropical forcing (Indian Ocean and Pacific) may interact in such a way as to quash the El Niño engine. We'll see!

Closer to home, although temps in Southern California have recently been more seasonable, particularly in the Valleys, the last time the average daily temperature in Downtown Los Angeles was above normal was back on May 21 -- 48 days ago.

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