ENSO REPORT
No. 37
September 21, 2009
East Asian Monsoon Activity Center
CAS，World Meteorological Organization
Beijing
Climate
Center
China Meteorological Administration
El Niño conditions lasting and an El Niño event to form
Recent monitoring results showed that El Niño conditions remain slowly enhanced
over the tropical Pacific, with warmer sea surface across the central and eastern
equatorial Pacific. While the present equatorial conditions should be paid more
attentions on the lasting warmth in the western Pacific, the suppressed convection over
the central and eastern Pacific and the slow drop of SOI. The El Niño conditions are
supposed to last in future 3 months and an El Niño event is expected to form in the
central and eastern equatorial Pacific.
1. Recent monitoring on ENSO evolution
Sea Surface Temperatures (SSTs): During summer 2009, equatorial SSTs were 0.5℃
above average across most of the Pacific Ocean, with positive anomalies at least above 1.0℃
in most of the central and eastern Pacific (Fig.1). In early autumn, the Pacific Ocean sea
surface is significantly warmer than normal across almost all equatorial regions.
Fig.1 Sea surface temperatures(upper) and the anomalies(lower) in summer 2009 (unit:℃)
ENSO indices: The ENSO indices have been above 0.5℃ since the beginning of June
2009, and keep increasing. In August, indices of NINO 1+2, NINO 3, NINO 4, NINO 3.4 and
NINO Z were 0.9℃, 1.0℃, 0.8℃, 0.8℃ and 0.9℃, respectively. Correspondingly, Southern
Oscillation Index (SOI) was -0.3 in August, with a drop of 0.4 from July (Fig.2).
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Fig.2 Evolution of Nino Z, Nino 3.4, Nino3 SSTA indices (unit:℃) and SOI
Subsurface Temperatures and Warm Pools: The anomalously warm subsurface water
has prevailed over most of the equatorial Pacific since June 2009. During August, Positive
equatorial subsurface temperature anomalies weakened in the eastern Pacific, while
strengthened in the central Pacific with the largest anomalies near 125m depth (Fig.3).Both
the area and intensity of the western Pacific and Indian Ocean warm pool were above normal
in the summer season.
Fig.3 Equatorial depth-longitude section of monthly mean subsurface temperature
anomalies (unit:℃) across the Pacific Ocean in August 2009
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Wind Field and Convections over the Tropics: During August, the monthly mean
outgoing longwave radiation (OLR) anomalies showed that the convection was near normal
in most of the equatorial Pacific. At lower troposphere (850hPa), westerly wind anomalies
covered large portions of the equatorial Pacific, except the regions near the Date Line (Fig.4).
At upper troposphere (200hPa), easterly wind anomalies covered most of the equatorial
Pacific. In early September, westerly wind anomalies at 850hPa evidently enhanced in
western equatorial Pacific and the excitated warm Kelvin wave shifted eastward which would
be in favor of increasing warmth in the central and the eastern Pacific.
Fig.4 Time-longitude section of 850hPa equatorial zonal wind anomalies (unit: m/s)
2. Diagnosis and outlook
1) Diagnosis
We compared the present El Niño conditions with the evolution of 14 El Niño events
from 1951. The results showed that usually under El Niño conditions, the warmth
concentrated in the central or eastern equatorial Pacific with cooler than normal temperatures
in the western Pacific. However, the present equatorial conditions differed from the previous
El Niño events about the lasting warmth in the western Pacific and the suppressed convection
over the central and eastern Pacific. Hence, the El Niño conditions were of particularity in
2009.
In addition，visa the analysis of onset and end time of El Niño events since 1951, we
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found that El Niño conditions would generally keep development in autumn and end till
winter after onset in summer.
2) Model predictions
Most statistical and dynamic climate models predict that equatorial SST anomalies will
keep above 0.5℃ in the central and eastern Pacific till winter 2009-2010.
3) Outlook
Based on current ENSO monitoring, diagnosis and prediction, El Niño conditions are
supposed to last in future 3 months and an El Niño event is expected to form in the central and
eastern equatorial Pacific. We will continue to monitor the further development of the El Niño
conditions and to update the information on ENSO monitoring, diagnosis and prediction in
time.
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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, Leng
Chief Editor: 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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