By Drew Lerner
KANSAS CITY (B)--La Nina conditions that returned for a brief period last autumn weakened during December and have generally disappeared in the past couple of months. A rather neutral equatorial Pacific ocean temperature pattern has evolved, and the odds continue against La Nina returning anytime soon. Official forecasts from several sea surface forecasting groups around the world are calling for neutral ocean temperatures through the spring and early summer. El Nino may evolve later this year, however.
All signs suggest La Nina conditions have ended and will not return anytime soon. The recent warming of ocean temperatures off the South American Coast is beginning to suggest recent forecasts for a developing El Nino in 2001 may prove correct.
El Nino events often begin with a pool of warmer-than-usual sea surface temperatures off the South American coast during the early spring months. El Nino events--as they evolve--usually allow warmer-than-usual water off the South America coast to expand west to the International Dateline while continuing to warm. Water temperatures in the western equatorial Pacific Ocean usually turn cooler as an El Nino event evolves.
If an El Nino event is going to evolve in 2001, additional warming should take place over the eastern equatorial Pacific Ocean the next few weeks.
El Ninos usually send the Southern Oscillation Index into negative territory. The index measures air pressure differences between Darwin, Australia, and the island of Tahiti. The SOI had recently been holding rather steady, but a decline in the index is now under way with today's reading of +8.51 (.85 in U.S. terms).
The index will have to become solidly negative as sea surface temperatures rise above average to confirm the developing El Nino event. SOI values of -10 (-.10 in U.S. terms) or smaller usually suggest an El Nino event has developed with values of -20 or smaller suggesting a strong El Nino. However, changes in atmospheric conditions associated with El Nino will usually begin long before the SOI says El Nino conditions have developed.
A weakening of the easterly trade winds in the Pacific Ocean also would be associated with El Nino events.
Official U.S. forecasts for sea surface temperatures suggest neutral conditions through spring and into early summer 2001. Warmer-than-usual temperatures are expected to show up during the second half of summer through the final quarter of this month, according to some of the U.S. National Weather Service forecast models.
However, some experts at the University of Hawaii have reminded the world that the 1997 El Nino event began with significant warming of ocean temperatures off the South American coast in the late winter and early spring of that year. The forecasters cautioned about writing off 2001 for an El Nino event until after the spring season has determined the true direction of sea surface temperature anomalies currently present off the South American coast.
The last El Nino event to affect the world began in the spring of 1997 and continued into 1998. The event began in March and April as warmer water began to rapidly accumulate off the South American coast similar to what is occurring now. The 1997 event became the most significant of the 20th century with sea surface temperatures rising well above average by mid-summer. The intense event was predicted to have a huge negative impact on world weather, but as it turned out the impact was not nearly as great as many had anticipated.
El Nino events are known for producing dryness in eastern Australia, Indonesia and South Africa. Wet weather often occurs in parts of South America, and North America usually has warm winters, although a wet, cool, pattern often occurs in the southern United States.