ENSO REPORT
No. 38
October 27, 2009
East Asian Monsoon Activity Center
CAS，World Meteorological Organization
Beijing
Climate
Center
China Meteorological Administration
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El Niño will persist into winter 2009/2010
1. Recent monitoring on ENSO evolution
a) Distribution of SSTA
Since onset of El Niño in June 2009, the SSTs were more than 0.5℃ above normal in
most of the central and eastern equatorial Pacific, with SSTA indices exceeding El Niño
thresholds of 0.5℃ continuously (Fig.1). Currently, the SSTs in October(24 days) 2009 were
more than 0.5℃ above normal in the central-eastern equatorial Pacific, with maximum above
1.5℃(Fig.2). Compared with last month, positive SSTA in October increased in the central
equatorial Pacific and decreased in the eastern equatorial Pacific.
2.5
2
1.5
1
0.5
0
-0.5
-1
SOI
Nino 3.4
Nino Z
-1.5
-2
-2.5
Jun
2008
Aug
2008
Oct
2008
Dec
2008
Feb
2009
Apr
2009
Jun
2009
Aug
2009
Oct
2009
Fig.1 Evolution of Nino Z, Nino 3.4 SSTA indices (unit: ℃) and SOI
(SSTA and SOI indices in Oct. 2009 are calculated for the first 24 days )
Fig.2 Sea surface temperature anomalies in October (24days) 2009 (unit: ℃)
b) Subsurface Temperatures
Since June 2009，anomalously warm subsurface water has covered most of the central and
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eastern equatorial Pacific, and shifted eastward and upward continuously in the eastern
equatorial Pacific, which is favorable for maintaining warmer than normal condition in the
following months (Fig.3). During October, the anomalously warm subsurface water still
steadily covered the central equatorial Pacific indicating developing status of El Niño.
Meanwhile, the anomalously warm subsurface water weakened around the coasts of South
America as compared with past months (Figure omitted).
Fig. 3 Equatorial depth-longitude section of monthly mean ocean temperature
anomalies (unit: ℃) during June (left) and September (right) 2009
c) Southern Oscillation
The evolution of SOI index has exhibited a significantly fluctuating feature after El Niño
onset indicating an unstable response of Trade winds variation to warm SSTs in equatorial
Pacific (Fig.1). However, during October (24days) the SOI was -1.2 reflecting typical El Niño
feature.
d) Tropical Pacific Convections and 850hPa Zonal wind
Since June 2009，the convection has enhanced over the western to central equatorial
Pacific, and suppressed over the eastern equatorial Pacific(Fig.4). Correspondingly, at low
level (850hPa), westerly wind anomalies have prevailed over most portions of the central and
eastern equatorial Pacific indicating weaker than normal Trade winds. The most recent
monitoring showed that since the middle of September, the westerly wind anomalies
strengthened significantly over the western equatorial Pacific and extended eastward
significantly, which would be favorable for further warming in the eastern Pacific Ocean for
the coming months (Fig.5).
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Fig. 4 Time-longitude section of pentad outgoing longwave radiation (OLR)
anomalies averaged between 5ºS-5ºN (Units: W/m2)
Fig. 5 Time-longitude section of pentad 850hPa zonal wind anomalies
averaged between 5ºS-5ºN (Units: m/s)
2. Diagnosis and outlook
a) Diagnosis
Diagnostic analysis on historical El Niño events showed that the intensity of El Niño events
would easily be weak to moderate when onset in summer, and Trade winds variation
represented by SOI index usually exhibited a strong fluctuation after onset, which was very
similar to the evolutional features of current El Niño. Furthermore, the weak-moderate El Niño
events usually got peak phase in later autumn and early winter period, terminated in later winter,
then turned into near neutral conditions in the subsequent 3 months, with area-averaged SST
anomalies in the central and eastern equatorial Pacific slightly above normal. Considering
aforementioned characteristics of historical El Niño events and recent El Niño status, it seems
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that there is a great probability for persistence of a weak-moderate strength El Niño into winter
2009/2010, and maintain neutral conditions during spring 2010.
b) Model predictions
Predictions of most statistical and dynamic models indicated a moderate strength El Niño
during winter 2009/2010, and ENSO-neutral condition in the central-eastern equatorial Pacific
during spring 2010.
c) Outlook
‘The national consultation meeting on ENSO monitoring and outlook’ was hosted by BCC
(Beijing Climate Center) on 21, October 2009. Based on discussions about recent oceanic and
atmospheric conditions and characteristics of ENSO cycle, and referred to the aforementioned
ENSO models’ prediction results, the experts from Institute of Atmospheric Physics, National
Marine Environmental Forecasting Center, Peking University and Chinese Academy of
Meteorological Science and BCC reached a consensus that the weak-moderate El Niño
condition beginning from Jun 2009 will persist into winter 2009/2010. Then, it will turn into
near neutral conditions, with area-averaged SST anomalies in the central and eastern
equatorial Pacific slightly above normal during spring 2010.
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BCC operational definitions for El Niño and La Niña Event (condition)
El Niño (La Niña) event: which is characterized by a positive(negative) sea-surface
temperature departure from normal (for the 1971-2000 base period) in NINO Z (NINO
1+2+3+4) greater (less) than or equal to 0.5℃ (-0.5℃) for at least 6 consecutive months
(allowing below (above) 0.5℃(-0.5℃) for only one month) .
BCC considers El Niño (La Niña) conditions to occur when the monthly NINO Z index
greater (less) than or equal to 0.5℃ (-0.5℃) along with consistent atmospheric features. And,
these anomalies must also be forecasted to persist for at least 3 consecutive months.
*References
1. On Indices and Indicator of ENSO Episodes, 2000, Acta Metrological Sinica, 58(1):
102-109
2. Redefining ENSO Episode on Changed Reference, 2005, Journal of Tropical
Meteorology，2005, 21(1): 72-78
Distribution of the NINO regions for ENSO monitoring
Editor: Sun, Chenghu Chief Editor: Ren, Fumin Zhou, Bing
Technical assistant: Liu, Yunyun
BCC’s ENSO monitoring website:
http://bcc.cma.gov.cn/en/product.php?PID=67&WCHID=21&ChannelID=67
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