ENSO REPORT
No. 35
March 30, 2009
East Asian Monsoon Activity Center
CAS，World Meteorological Organization
Beijing
Climate
Center
China Meteorological Administration
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La Nina condition will terminate in April 2009
1. Recent oceanic and atmospheric conditions
(a) Sea Surface Temperatures (SSTs)
Since December 2008, taking the Pacific basin as a whole, the equatorial Pacific has
exhibited a typical La Nina condition (Nino Z≤-0.5℃), with the negative SST anomalies
(SSTa) persisting in the eastern equatorial Pacific and the positive SSTa occupying the western
equatorial Pacific. However, the negative SSTa have significantly weakened in the
central-eastern equatorial Pacific since Feb 2009, accompanied by fluctuation of the negative
SSTa in the eastern equatorial Pacific (Fig.1). The monitoring of Nino Z and Nino 3.4 indices
in March show that the indices are both -0.6℃(28days) and SOI index is 0.2(28days) (Fig.2).
Fig. 1 Time-longitude section of pentad SSTa averaged between 5ºS-5ºN（unit: ℃）
2.5
SOI
Nino 3.4
Nino z
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
2008年1月
2008年4月
2008年7月
2008年10月
2009年1月
Fig.2 Nino Z, Nino 3.4 SSTa indices (unit: ℃) and SOI during Jan 2008 - Mar 2009
(Nino and SOI indices in Mar 2009 are calculated for the first 28 days )
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(b) Subsurface Temperatures
Since early 2009，anomalously cold subsurface water has weakened in the eastern
equatorial Pacific, and the cold center has shifted eastward and upward (Fig.3). At present, the
cold center migrates into the east of 110ºW. Meanwhile, the anomalously warm subsurface
water in the western equatorial Pacific has strengthened gradually and extended eastward
(Figure omitted).
Fig. 3 Equatorial depth-longitude section of ocean temperature
anomalies (unit: ℃) during January (left) and February (right) 2009
(c) 850hPa Zonal wind
Since Jan 2009, low level (850hPa) easterly wind anomalies have prevailed over the
central-eastern equatorial Pacific, while westerly wind anomalies have persisted over the
western equatorial Pacific. Recently, the intensity of easterly wind anomalies weaken in the
central-eastern equatorial Pacific and the westerly wind anomalies extend eastward slightly
(Fig.4).
Fig. 4 Time-longitude section of pentad 850hPa zonal wind anomalies
averaged between 5ºS-5ºN (Units: m/s)
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(d) Convections over the Tropics
Since early 2009，the convection has enhanced over the western equatorial Pacific, and
suppressed over the central equatorial Pacific. However, current monitoring indicates that the
convection center over the western equatorial Pacific shifts eastward slightly (Fig.5).
Fig. 5 Time-longitude section of pentad outgoing longwave radiation (OLR)
anomalies averaged between 5ºS-5ºN (Units: W/m2)
The aforementioned monitoring on recent tropical atmospheric and oceanic
conditions indicates that La Nina conditions persist. However, we believe that the La
Nina conditions will soon weaken into the neutral conditions when considering recent
atmospheric and oceanic status including the eastward propagation of anomalously
warm subsurface water in the western equatorial Pacific, the weakening of anomalously
cold subsurface water in the central and eastern equatorial Pacific, and the persistence
and extending of westerly anomalies in the west equatorial Pacific.
2. Diagnosis and outlook
(a) Diagnosis
To reveal the possible tendency of oceanic condition in the spring and summer 2009, six
cases which are similar to 2008/09 La Nina-like conditions during 1951-2007 periods have
been selected to analyze. The results show that they generally exhibit a significantly
weakening tendency during the subsequent spring and summer period after reaching their
mature phase of La Nina-like in prior winter. Thus, during summer most of cases will weaken
into neutral conditions, with area-averaged SST anomalies in the eastern equatorial pacific
slightly above normal (Fig.6). Considering aforementioned historical characteristics of ENSO
cycle and recent status, it seems that there is a great probability for termination of La Nina
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condition in April 2009, which has begun from Dec 2008.
2.5
1967/68
1971/72
1975/76
1984/85
1996/97
1999/00
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
8月(+0)
6月(+0)
4月(+0)
2月(+0)
12月(-1)
10月(-1)
8月(-1)
6月(-1)
Fig. 6 The evolution of Nino Z index for six historically analogous
cases with 2008/09 La Nina –like condition
(b) Model predictions
Predictions of most statistical and dynamic models indicated the negative SST anomalies
in the eastern equatorial Pacific will weaken gradually during March-August 2009, it will
turned into the neutral condition, with area-averaged SST anomalies in the eastern equatorial
pacific slightly below normal during spring 2009, and with area-averaged SST anomalies
slightly above normal during summer 2009.
‘The national springtime consultation meeting on ENSO monitoring and outlook’ was
hosted by BCC (Beijing Climate Center) on 24, March 2009. Based on discussions about
current oceanic and atmospheric conditions and characteristics of ENSO cycle, and
referred to the ENSO models’ prediction results, the experts from Institute of
Atmospheric Physics, National Marine Environmental Forecasting Center, Peking
University and Chinese Academy of Meteorological Science reached a consensus that La
Nina condition beginning from Dec 2008 will terminate in April 2009. And, the negative
SST anomalies in the eastern equatorial Pacific will turn into the neutral condition, with
area-averaged SST anomalies slightly below normal during spring 2009, and with
area-averaged SST anomalies slightly above normal during summer 2009.
However, considering frequent occurrence of rapidly adjustment in the tropical
Pacific during spring season, we will closely monitor the ENSO status and issue our
updating prediction on it.
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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 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: Zhu, Yanfeng Technical assistant: Wang, Yanjiao
BCC’s ENSO monitoring website:
http://bcc.cma.gov.cn/en/product.php?PID=67&WCHID=21&ChannelID=67
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