Weekly ENSO report (PDF, 36 pages)La Niña is defined by cooler-then-usual temperatures of the equatorial Pacific (although it also brings many other effects to other parts of the world). The standard convention instructs you to measure the temperature anomaly in the ENSO 3.4 region, an equatorial rectangle near the middle of the Pacific (page 5/36). If the anomaly is -0.5 °C or even more negative, we deal with La Niña conditions (or regime).
But an official La Niña episode only occurs if the average temperature anomaly stays at or below -0.5 °C for five consecutive overlapping 3-month intervals. Because July-August-September 2007 was the first recent period of this kind when the anomaly was -0.5 °C (page 26/36), the more than sufficient -1.4 °C average anomaly in November-December-January 2007-08 guaranteed that we deal with a full-fledged La Niña episode.
The previous La Niña episode ended in March 2001. It is expected that La Niña will continue through Spring 2008 and many models indicate that it may last up to Summer 2008 (in Northern Hemisphere English). Even though the Pacific waters near Southern America seem to be warming up back to normal, a prominent NCEP CFS dynamical model (page 28/36) suggests that La Niña will probably last up to 2009.
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