The USAPI have entered a critical phase of El Niño when very dry conditions typically occur. During past
El Niño events, dry conditions have plagued many of the USAPI from Micronesia to Hawaii during the winter
and spring months that follow the onset of an event. Historically, the most serious dry spells have occurred
following a strong El Niño. El Niño conditions began in mid-2009, strengthened throughout the fall and early
winter, and by December had become "strong". The potential for an extended period of dryness in many areas
of the USAPI prompted this Special Edition ENSO Bulletin. The regular edition of the Pacific ENSO Update
will follow in mid-February.
According to the U.S. Climate Prediction Center (CPC), El Niño strengthened during December 2009, with
oceanic and atmospheric anomalies indicating a strong El Niño. At this time, the 3-month Niño-3.4 SST average
is anticipated to exceed +1.5C during the winter (e.g. November-December-January and December-
January-February). Most models indicate that SST anomalies in the Niño-3.4 region will begin to decrease in
early 2010, and El Niño will persist through April-May-June 2010. Regardless of its precise peak strength,
the event is expected to exert a significant influence on global weather and climate in the coming months.
Widespread dry conditions are beginning to materialize in the USAPI and at many locations throughout the
state of Hawaii. However, in the USAPI a strong shearline passed through the Mariana Islands during the
week of January 18th, yielding cool temperatures, high surf, and 2-3 inches of rainfall. In addition, a tropical
disturbance delivered welcome rains to eastern Micronesia, and through January 22nd the monthly rainfall at
Kwajalein was above normal. American Samoa received abundant rainfall during December 2009. Overall,
predictions call for even drier conditions over the next 3 to 5 months for both Hawaii and Micronesia.
ENSO-related dryness occurs across Micronesia and into Hawaii during the winter and spring that follow
El Niño. Widespread and prolonged dry conditions have occurred during the course of very strong El Niño
events, such as those of 1983 and 1998. Although the current El Niño does not match the intensity of these
past events, CPC has declared it a "strong El Niño". There is a high probability that many island groups will
experience some very dry months during the first half of 2010, but we do not expect the severity of dryness
experienced in 1983 or 1998. A brief summary is given below for conditions expected at each island group.