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La Niña on the way?

By: Dr. Jeff Masters, 2:53 PM GMT on January 23, 2009

A La Niña event may be developing in the Eastern Pacific Ocean, as equatorial sea surface temperatures cooled to 0.73°C below average in December, and have cooled further to -1.1°C below average this week. A La Niña event is defined as occurring when the 3-month running mean temperature anomaly in the equatorial Eastern Pacific cools below -0.5°C for five consecutive months. The 3-month running mean as of January 1 was -0.4°C, so we are still officially experiencing neutral conditions. Approximately 50% of the El Niño forecast models predict that a full-fledged La Niña event will develop between January and May of 2009. Our last La Niña event ended just eight months ago, in May 2008. It is uncommon to have separate La Niña events develop two years in a row. This has only occurred twice since 1950, so I give it a 50/50 chance that a full-fledged La Niña event will develop in 2009. One argument against a La Niña event developing is the current burst of west-to east winds that has developed in the Western Pacific. These westerly winds have developed as a result of a series of cold air outbreaks associated with troughs of low pressure, plus a periodic flare-up of thunderstorm activity called the Madden-Julian Oscillation. This burst of westerly winds has triggered formation of a 200 meter-deep, long-period, eastward-propagating ocean wave known as an equatorial Kelvin Wave. This wave has been marching eastwards over the past few weeks, pushing warmer Western Pacific water into the Eastern Pacific. As the Kelvin wave continues to propagate eastwards towards South America over the next few weeks, it should keep La Niña conditions from amplifying through the rest of January.

Despite the unusually late start to La Niña conditions this winter, the cooler water developing in the Eastern Pacific is capable of having a significant impact on regional weather patterns. We can expect during January-March 2009 above-average precipitation over Indonesia and below-average precipitation over the central and eastern equatorial Pacific. For the contiguous United States, potential impacts include above-average precipitation in the Ohio and Tennessee Valleys and below-average precipitation across the South, particularly in the southwestern and southeastern states. Other potential impacts include below-average temperatures in the Pacific Northwest and above-average temperatures across much of the southern United States.

Figure 1. Latest three-month forecast of precipitation issued by NOAA's Climate Prediction Center.

Volunteers needed for disaster relief fund-raising
The portlight.org disaster relief charity is in the process of wrapping up its Hurricane Ike relief efforts, and is looking ahead to the future. According the new wunderground featured blog, Portlight Disaster Relief, "Our goals are to expand our network of supporters, continue to create a sense of ownership and community and create a financial reserve. Achieving these goals is critical to us being able to serve future hurricane victims in a strategic, pro-active and efficient manner." To this end, Portlight is sponsoring a fund-raising effort this March and April in 40 cities--a Spring Relief Walk. Volunteers in twenty cities have already committed to the effort, and more volunteers are needed! Check out the Portlight Disaster Relief blog for more information.

My next post will be Monday.

Jeff Masters

Climate Change

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.