NWS: La Niña to Dissipate by May 2012 (Posted By Gary Cutrer)

Pressure departures: El Nino vs. La Nina.

Pressure departures: El Nino vs. La Nina.

End-of-2011 predictions called for the La Niña Pacific Ocean phenomenon to prolong extreme drought conditions in areas of North America through 2012.

But now, the National Weather Service’s Climate Prediction Center is predicting a transition to “ENSO-neutral conditions during March-May 2012.”

ENSO stands for El Niño/Southern Oscillation

From the NWS CPC FAQs Page:

ENSO-neutral refers to those periods when neither El Niño nor La Niña is present. These periods often coincide with the transition between El Niño and La Niña events. During ENSO-neutral periods the ocean temperatures, tropical rainfall patterns, and atmospheric winds over the equatorial Pacific Ocean are near the long-term average.

Of course, the prediction is based on computer models, which can be inaccurate. Does a weakening La Nina mean more rain for the Southwestern U.S? Not necessarily.  From the report:

A majority of models predict La Niña to weaken through the rest of the Northern Hemisphere winter 2011-12, and then to dissipate during the spring 2012.

. . . Over the U.S. during February – April 2012, there is an increased chance of above-average temperatures across the south-central and southeastern U.S., and below-average temperatures in the northwestern U.S.  Also, above-average precipitation is favored across most of the northern tier of states (except the north-central U.S.) and in the Ohio and Tennessee Valleys, and drier-than-average conditions are more likely across the southern tier of the U.S. (see 3-month seasonal outlook released on 19 January 2012).

El Nino / La Nina Influences on pressure differentials.

El Nino / La Nina Influences on pressure differentials.

The fluctuations in ocean temperatures during El Niño and La Niña are accompanied by even larger-scale fluctuations in air pressure known as the Southern Oscillation. The negative phase of the Southern Oscillation occurs during El Niño episodes, and refers to the situation when abnormally high air pressure covers Indonesia and the western tropical Pacific and abnormally low air pressure covers the eastern tropical Pacific. In contrast, the positive phase of the Southern Oscillation occurs during La Niña episodes, and refers to the situation when abnormally low air pressure covers Indonesia and the western tropical Pacific and abnormally high air pressure covers the eastern tropical Pacific. These opposite phases of the Southern Oscillation are shown above.

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