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IRI Climate Information Digest
May 1998


El Nino/Southern Oscillation (ENSO): Cold Episode Looms

The strong 1997/98 ENSO warm episode lost virtually all of its strength during the month of May. Equatorial Pacific Sea surface temperatures dropped dramatically becoming somewhat cooler than normal in some parts of the basin. This was accompanied by corresponding changes in the atmosphere as the Tahiti-Darwin Southern Oscillation Index rose to slightly above normal (See Digest Figures 1 and 2). Statistical and numerical indicators the likelihood of colder than normal equatorial Pacific waters (La Nina - Definition URL) perhaps as early as July or August but some models hold back until later in the calendar year.

El Nino/Southern Oscillation (ENSO): Warm Episode Impacts Linger

Despite the rapid demise of one of this centuries strongest El Nino/Southern Oscillation Episode some parts of the Western Hemisphere continue to experience climate anomalies which had their roots in the warm episode. Notably, much of Mexico and Central America continue to suffer from drier than normal conditions with the area of extreme dryness spreading into southern part of the United States. Meanwhile excessive rainfall and flooding continued to threaten southeastern regions of Brazil, Northern Argentina, and much of Uruguay.

Asian Monsoon shows slow evolution:

The 1997/98 ENSO warm episode was characterized by an extreme distortion of average Asian Monsoon circulation and rainfall patterns. While the Indian sub-continent enjoyed an average 1997 summer monsoon rainfall despite the strong ENSO warm episode the monsoon evolution thus far during 1998 has been somewhat sluggish. Both the land surface temperatures and the Indian Ocean temperature remains considerably warmer than average for this time of year providing conflicting influences on the further evolution of the monsoon circulation patterns in that part of southern Asia. The situation is further clouded by the rapid demise of the ENSO warm episode and possible rapid development of (La Nina) cold episode conditions.