ENSO REPORT
No. 48
FEB 25, 2012
East Asian Monsoon Activity Center
CAS，World Meteorological Organization
Beijing
Climate
Center
China Meteorological Administration
1
La Niña weakened and will terminate in spring 2012
1. Current La Niña status
1) ENSO indices and Sea surface temperatures
In September 2011, negative SST anomalies below -0.5℃ appeared in most of the central
and eastern equatorial Pacific and NINO Z index decreased to -0.6℃, indicating that La Niña
conditions came into being (Fig. 1). Negative SST anomalies enhanced in most of the eastern
and central equatorial Pacific from October to November 2011, and NINO Z index was -0.7℃
and -0. 8℃, respectively. La Nina conditions peaked during the period from December 2011
to January 2012. During February 2012, La Niña conditions decayed, and negative SSTAs
weakened in the equatorial eastern Pacific and NINO Z index was -0.8℃ (Fig. 2 and Fig. 1).
From September to December, Southern Oscillation Index (SOI) correspondingly was
positive and reached its peak with 2.1 in December. In February 2012, SOI dropped rapidly
from 1.0 in January 2011 to 0.2 (Fig.1).
3
SOI
NINO Z
NINO 3.4
2
1
0
1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2
-1
2010
2011
2012
-2
Fig.1 Evolutions of Niño Z, Niño 3.4 SST index (unit: ℃) and SOI during Jan 2010 –Feb 2012 (The
value of February 2012 is obtained by the synthesis of 1-24 days)
2
Fig. 2 Sea surface temperature anomalies in February 2012 (unit: ℃)
（From 1 Feb. 2012 to 24 Feb. 2012）
2) Subsurface temperatures
Anomalously cold subsurface water controlled most of the equatorial central and eastern
Pacific and negative anomalies enhanced significantly, shifted eastward and uplifted obviously
from October to December 2011. Since February 2012, the intensity and extent of anomalous
cold subsurface water weakened and shrunk significantly (Fig. 3).
（a）
（b）
Fig. 3 Equatorial （5oS-5oN）depth-longitude section of pentadly mean ocean
temperature anomalies in February 2012 (a: 1st pentad; b:2nd pentad)
3）850hPa wind field
Since January 2012, the low-level equatorial easterly anomalies (850hPa) weakened
3
apparently over the equatorial central Pacific. Furthermore, the low-level equatorial westerly
anomalies strengthened across the equatorial eastern Pacific, which was favorable for
decaying of La Niña conditions (Fig. 4).
Fig. 4 Time-longitude section of equatorial (5S-5N) zonal wind anomalies
2. Outlook
Among 13 La Niña events since 1951, 6 La Niña events ended in winter, 4 in spring, and
only 3 in summer. Recent monitoring results showed that the La Niña conditions had peaked
during the period from December 2011 to January 2012 and subsurface and surface cold
waters warm up obviously in the tropical central and eastern Pacific in February, which means
that La Niña conditions begins weakening .
Based on model predictions and current monitoring of the ocean and atmosphere features,
the La Niña conditions is expected to transit to ENSO-neutral conditions during March to
April 2012.
4
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: Si Dong, Han Rongqing
Chief Editor: Wang Qiyi, Ke Zongjian
Technical assistant: Tang Jinyue
BCC’s ENSO monitoring website:
http://bcc.cma.gov.cn/Website/index.php?ChannelID=67&show_product=1
5