ENSO REPORT
No. 37
September21, 2009
El Niño conditions lasting and an El Niño event to form
Recent monitoring results showed that El Niñoconditions remain slowlyenhancedover the tropical Pacific, with warmer sea surface across the central and eastern equatorial Pacific. While the present equatorial conditionsshould be paid more attentions on the lasting warmthin 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 were0.5℃aboveaverage across most of the Pacific Ocean, with positiveanomalies 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 beenabove 0.5℃since the beginning of June 2009, andkeep increasing.InAugust, indices of NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z were0.9℃,1.0℃,0.8℃,0.8℃and0.9℃, respectively. Correspondingly, Southern Oscillation Index (SOI) was -0.3 in August, with a drop of 0.4 from July (Fig.2).
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 prevailedover most of theequatorial Pacific since June 2009. During August, Positive equatorial subsurface temperature anomalies weakenedin the easternPacific, while strengthened in thecentral Pacific with the largest anomalies near 125m depth(Fig.3).Both the area and intensity of the western Pacific and Indian Oceanwarm pool were above normal in the summer season.
Fig.3 Equatorial depth-longitude section of monthly mean subsurface temperature
anomalies(unit:℃)across thePacific Ocean in August 2009
Wind Field and Convections over the Tropics: During August,the monthly mean outgoing longwave radiation (OLR) anomalies showed that the convection was near normalin mostof theequatorial 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 anomaliescovered most of theequatorial Pacific. In early September, westerly wind anomalies at 850hPa evidently enhancedin western equatorial Pacific and the excitatedwarm Kelvin wave shifted eastward which would be in favor of increasing warmth in thecentral and the eastern Pacific.
Fig.4Time-longitudesection of 850hPa equatorial zonal windanomalies(unit: m/s)
2. Diagnosis and outlook
1) Diagnosis
We compared the presentEl Niño conditions with the evolution of 14 El Niño eventsfrom 1951. The results showed thatusually 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 Niñoevents about the lasting warmthin 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 found that El Niñoconditions would generally keep development in autumn and end till winter after onset in summer.
2) Model predictions
Moststatistical and dynamic climate models predict that equatorial SSTanomalies will keep above 0.5℃in the central and eastern Pacific till winter2009-2010.
3) Outlook
Based on currentENSOmonitoring, diagnosis and prediction,El Niño conditions are supposed to lastin future 3 months and an El Niño event is expected to form in the central and eastern equatorial Pacific. We willcontinue to monitor the further development of the El Niño conditions and to update the informationon 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 indexgreater(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:
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