Sunday, June 27, 2010

[californiadisasters] Goodbye, El Niño; hello, La Niña

Goodbye, El Niño; hello, La Niña

Published: Sunday, Jun. 27, 2010

With summer under way, a new rain season beginning Thursday and the Mid-State Fair just around the corner, I thought it would be interesting to look at the long-range forecast.

The moderate El Niño condition (warmer than normal ocean water in the equatorial and eastern Pacific), which shifted the position of the jet stream further south, thereby directing more Pacific storms toward the Central Coast last winter, has ended.

Over the last few months, ocean water temperatures have decreased. In fact, the National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Springs, Md., has issued a La Niña watch (colder than normal ocean water in the equatorial and eastern Pacific) for late summer into the fall and winter as ocean water temperatures are expected to continue to diminish.

Normally the seawater temperature at the Diablo Canyon nuclear power plant averages 52.1 degrees during the month of May. This May the average was 51 degrees.

This shift can have profound ramifications all around the world.

There is a good chance that the Central Coast will experience normal temperatures this summer. The average maximum temperature at the Paso Robles airport during July is 94 degrees, while the average maximum temperature in Atascadero in July is 86 degrees, according to the Air Pollution Control District's monitoring station there. San Luis Obispo is cooler, with an average July maximum temperature of 78 degrees at Cal Poly.

Unfortunately, this condition, combined with another large-scale ocean water temperature cycle, can also produce lower-than-average winter rainfall.

The other large-scale ocean water temperature cycle is the Pacific Decadal Oscillation (PDO), which can increase or decrease the La Niña effect. The PDO is found primarily in the North Pacific. The phases of the PDO are called warm phases or cool phases.

Unlike El Niño and La Niña, the PDO stays in one phase for a lot longer — between 10 and 40 years. The El Niño and La Niña phase usually last for about a year or so. It appears that we are still in the cool phase of the PDO, which means it should increase the La Niña effect and produce a greater chance of below-normal rainfall. These are long-range forecasts and should be taken with a grain of salt. Only time will tell the story.

A La Niña condition can also help produce an active hurricane season as it diminishes the hurricane-killing wind shear in the upper atmosphere that often shuts down any tropical cyclones that try to form in the Western Atlantic and Eastern Pacific oceans.

This condition, combined with a region of unusually warm water in the eastern Atlantic Ocean that is currently reported, could produce a vigorous hurricane season. This is not good news for those desperately trying to contain the largest oil spill in United States history in the Gulf of Mexico.

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