No.27
2006.10.25

The positive SSTa had emerged in the equatorial mid-eastern Pacific since spring 2006 and was gradually rising, and the SSTa of all Nino areas exceeded +0.5ºC in
August 2006, with Nino Z and Nino 3.4 being 0.6ºC and 0.5ºC respectively, which indicated a coming of El Niño conditions (Fig.1). The recent observations (Fig. 2) showed that the SSTa in the equatorial Pacific east of 160ºE exceeded +0.5ºC and Nino Z index reached +1.0ºC while the SSTa in the equatorial western Pacific showed a declining characteristic. The sea surface temperatures (SST) in the mid-eastern equatorial Pacific are still enhancing and in El Niño conditions so far.
Fig. 1 Time-longitude section of monthly mean SSTa ( ℃ ) along the equatorial Pacific

Fig. 2 SSTa for the period of 1-21 October 2006
The strong positive subsurface sea temperature anomalies were sustained during September but the center apparently shifted eastward to the equatorial eastern Pacific Ocean with central values over 4ºC. Meanwhile the subsurface sea temperatures in equatorial western Pacific tended to becoming cold conditions (Fig. 3).
Most of the equatorial Pacific was covered by the westerly anomalies at 850hPa in September 2006 while it was dominated by the weaker easterly anomalies at 200hPa (Fig. 4).
The Southern Oscillation index (SOI) has successively been less than zero for 5 months since May
2006, which corresponded to the positive values for Nino Z and Nino 3.4 indices (Fig.5).
The anomalous outgoing long-wave radiation (OLR) showed that convections had been suppressed over the western part of the equatorial western Pacific but enhanced in the eastern part of it since July
2006.
Fig. 3 Equatorial depth-longitude section of monthly mean ocean temperature anomalies ( ℃ ) in

September 2006
Fig. 4 Monthly mean zonal wind anomalies (Units: m/s) 2006 .09 (left) 850hPa (right) 200hPa
1.5
1
SOI
Nino 3.4
Nino Z
0.5
0
1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 2 3 4 5 6 7 8 9
-0.5
-1
2005 2006
-1.5
-2
-2.5
-3
Fig. 5 SSTa indices （℃） Nino Z (Nino1+2+3+4; red) and Nino 3.4 (blue), and SOI time series for the period of January 2005-September 2006
Monthly surface and subsurface sea temperature anomalies in the equatorial mid-eastern Pacific were increasing recently. In addition, both lower and higher troposphere wind fields, sea level pressures, and convective activities showed the characteristics of warm phase which was suitable for the El Niño conditions. But t he monthly PDO index was negative value for August-September 2006 and the PDO index for the recently several years seems to fall into the lower value stage for a decadal time scale, which seems not suitable for development of a strong
El Niño. As Fig. 6 showed, during 1982-September 2006, there were two weak warm events (1994/1995 and 2002/2003) and a moderate warm

event (1991/1992) when PDO index was less than zero, and the Nino Z and PDO indices were out of phase with each other during the early stages of all these events and such situations remained no more than 4-5 months; whereas all warm events (1982/1983, 1986/1988, and 1997/1998) that occurred during the positive PDO index period exceeded the moderate intensity, and the stages of PDO and Nino indices in phase remained more than 10 months.
A workshop on ENSO Outlook was hosted by BCC on October 20,
2006. Experts from NMEFC(National Marine Environment Forecast
Center)/SOA(State Ocean Administration), IAP (Institute of Atmospheric
Physics)/CAS(Chinese Academy of Sciences), Beijing University, CAMS
(Chinese Academy of Meteorological Sciences) and BCC (Beijing
Climate Center) discussed the current oceanic and atmospheric conditions and made outlook for El Niño prediction based on diagnosing recent tropical oceanic and atmospheric conditions, historical analog analysis, and most dynamic and statistical model’s predictions: T he present El
Niño conditions will continue for 3-6 months in future and will become a weak-moderate El Niño event.
Fig. 6 Monthly indices evolution of Nino Z and PDO for the period of January
1982-September 2006.
El Niño: A phenomenon occurs in the equatorial mid-eastern Pacific Ocean, which is characterized by a positive sea-surface temperature departure from normal

(for the 1971-2000 base period) in Nino Z (Nino 1+2+3+4) greater than or equal to
0.5℃ for at least 6 consecutive months (allowing below 0.5℃ for only one month) .
La Niña: A phenomenon occurs in the equatorial mid-eastern Pacific Ocean, which is characterized by a negative sea-surface temperature departure from normal
(for the 1971-2000 base period) in Nino Z (Nino 1+2+3+4) smaller than or equal to
-0.5℃ for at least 6 consecutive months (allowing above 0.5℃ for only one month) .
*References
1. On Indices and Indicator of ENSO Episodes, 2000, Acta Meteorogica 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