Synopsis: Warm-episode conditions are expected to continue
into early 2005.
Positive
sea surface temperature (SST) anomalies greater than +0.5°C persisted in the central and
western equatorial Pacific (Niņo 4 and Niņo 3.4 regions), and expanded
eastward into the eastern equatorial Pacific (Niņo 3 region) during
September 2004 (Fig. 1). By the end of the month,
positive SST anomalies greater than +0.5°C (~1°F) were found
between 155°E and 110°W, with anomalies greater than +1°C extending from
160°E eastward to 120°W (Fig. 2). The
increase and eastward expansion of the area of anomalous warmth in the
central equatorial Pacific during July-September indicate the early stages
of a warm (El Niņo) episode. Through
the end of September conditions were not yet indicative of a basin-wide El
Niņo, particularly due to the presence of below-average SSTs in the far
eastern equatorial Pacific between 95°W and the South American coast.
Through
most of 2004 MJO activity has resulted in week-to-week and month-to-month
variability in many atmospheric and oceanic indices.
In the past few months the warmth in the central equatorial Pacific
has supported eastward shifts of enhanced convection associated with the
convectively active phase of the Madden-Julian Oscillation (MJO) across the
western equatorial Pacific. This
activity has been associated with periods of weaker-than-average easterlies
that initiated eastward-propagating oceanic Kelvin waves, which contributed
to a deeper-than-average oceanic thermocline and an increase in surface and
subsurface temperature anomalies in the central and eastern equatorial
Pacific (Fig. 3).
The
NOAA operational definition for El Niņo
[Oceanic Niņo Index (ONI), a three-month running mean of the Niņo 3.4 index,
greater than or equal to +0.5°C]
was satisfied for the period June-August 2004, with an ONI value of
+0.7°C. Based on the recent evolution of oceanic and atmospheric conditions
and on a majority of the statistical and coupled model forecasts, it seems
most likely that SST anomalies in the Niņo 3.4 region will remain positive, at or above +0.5°C,
through early 2005.
Expected
global impacts include drier-than-average conditions over Indonesia(through early
2005), northern and northeastern Australia (November
2004-February 2005), and southeastern Africa (November 2004-March 2005). If the warming in
the tropical Pacific should strengthen and spread eastward to the South
American coast, then wetter-than-average conditions can be expected in
coastal sections of Ecuador and northern Peru during the first few months of
2005, and drier-than-average conditions can be expected to develop in the
eastern Amazon late this year and spread to Northeast Brazil during February
through April 2005.
El
Niņo wintertime impacts over the United States vary considerably
depending on the character (distribution and intensity) of the warming in
the tropical Pacific. Composite impacts for selected ranges of the ONI for
El Niņo episodes since 1950 (Fig. 4) show that the areal extent of
warmer-than-average (wetter-than-average) conditions increases across the
northern (southern) United States, as the strength of El Niņo increases.
The current warming in the tropical Pacific is expected to continue through
the upcoming winter, with models indicating an ONI in the range of +0.5ēC
to +1.4ēC. Thus, the winter outlook for the United States (Fig.
5) is a blend of the composite impacts for comparable
historical El Niņo episodes and the effects of long-term trends.
Warmer-than-average conditions are expected in the West and in the northern
Plains, while cooler and wetter-than-average conditions are expected for
portions of the South and Southeast.
This discussion is a consolidated effort of NOAA and its
funded institutions. Weekly updates for SST, 850-hPa wind, OLR and features of the equatorial subsurface
thermal structure are available on the Climate Prediction Center web page at
http://www.cpc.ncep.noaa.gov (Weekly Update). Forecasts for the evolution of
El Niņo/La Niņa are updated monthly in the Forecast Forum
section of CPC's Climate Diagnostics Bulletin. The
next ENSO Diagnostics Discussion is scheduled for 10 November 2004. To receive
an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send
your e-mail address to: ncep.list.enso-update@noaa.gov
or to Vernon.Kousky@noaa.gov
www.nws.noaa.gov 
