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El Niño ends: La Niña may begin
NIWA Science   
Tuesday, 03 April 2007

The El Niño event of the latter half of 2006 has now ended. Pacific basin-wide conditions are now neutral. Despite being at the time of year when it is most difficult to predict future developments, there are sufficient indications to suggest that a transition to La Niña has recently become a substantial possibility. This transition is generally considered by experts to have some, though weak, chance of developing in the next 2–3 months. At this time, re-development of El Niño is considered an unlikely outcome over the next few months.

Rapid dissipation of El Niño conditions occurred during January and February of 2007. Sea-surface temperatures in December were up to 2 degrees Celsius warmer than normal in the central and eastern Equatorial Pacific, and were still generally around 1 degree Celsius warmer than normal in early January. By the end of February, sea surface temperatures in the region were colder than normal and surface winds were near normal. Sub-surface equatorial Pacific Ocean was cooler-than-normal in the east and warmer-than-normal in the west. These conditions are typical of the end of an El Niño event.

Prior to the dissipation of El Niño, climate patterns over several months displayed many characteristics usually associated with El Niño events, including drier than normal conditions across many parts of Australia, Indonesia and Fiji and unusually heavy rains and flooding across parts of eastern Africa and extended dry spells across many southwestern parts of southern Africa.

This is now the time of year when predictions of developments across the basin-wide tropical Pacific are least accurate. Dynamical and statistical forecast models show a range of possible outcomes over the next 3–6 months. Most models have been indicating cooling after the November-December peak of El Niño conditions to near-average before or by mid-2007. However, the observed rate of cooling has been more rapid than most models predicted. Currently, several, but not all, models indicate the likelihood of an emerging La Niña over the next several months. Furthermore, experts have noted the presence of a substantial pool of cooler than normal water just beneath the surface of the central and eastern Equatorial Pacific. This water is expected to reinforce, over the next few weeks, the already cooler than normal waters at the surface. The system at this time of the year is finely balanced and can be quite easily deflected from an apparent track, but the pre-requisite conditions appear to be in place for the development of a La Niña event. The next 2–3 months will be crucial for determining whether neutral conditions continue, or a La Niña event does indeed transpire.

It is important to consider that El Niño and La Niña are among a number of factors that lead to information about the regional climate patterns to be expected over the next several months. More complete information can be found in detailed seasonal climate outlooks, as produced by National Meteorological and Hydrological Services.

In summary:

  • An El Niño event prevailed during the second half of 2006, exerting substantial influence on climate patterns during that period.
  • A rapid dissipation of the El Niño event occurred in the early part of this year, especially during February 2007, and leading to neutral conditions in the Equatorial Pacific. Indeed, sea-surface temperatures had already become cooler than normal in the central and eastern Equatorial Pacific by the end of February.
  • Forecasts made at this time of year notoriously lack skill, and the period March-May is often referred to as the "spring barrier" in the predictability of El Niño and La Niña. Nonetheless, there are indications that cooler than normal waters may prevail over the next several weeks in the central and eastern Equatorial Pacific such that a La Niña event becomes established. If such an event does become established, then given the timing in the year, it would be likely that the event would persist for much of the remainder of the year.
  • A further update of the situation will be made in the next 2–3 months, given the uncertainty that currently exists on the expected conditions in the tropical Pacific.

The situation in the tropical Pacific will therefore continue to be carefully monitored. More detailed interpretations of regional climate fluctuations will be generated routinely by the climate forecasting community over the coming months and will be made available through National Meteorological and Hydrological Services.

Editor's Note: Original news release can be found here.
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