El Niño/Southern Oscillation (ENSO)

The tropical Pacific is in an ENSO–neutral state. TheNINO3 and NINO4 indices for July changed little from June, and were +0.4°C and +0.1°C respectively (3–month means +0.4 and 0.1°C).

Sub–surface temperature anomalies are positive in the top 50–100 m across the Equatorial Pacific, but have weakened slightly. The positive SST anomaly "horseshoe" from the recent La Niña is still evident at about 40 degrees latitude in both hemispheres at 160W. The former equatorial 'cold tongue' has itself become a 'horseshoe' as the equatorial SSTs have become slightly warmer than normal.

In the atmosphere, the SOI rose slightly to +0.6 in July (estimated), with a 3–month mean close to +0.2. The TRMM ENSO index has weakened substantially to a value of –0.2 for the 30 days to 26 July. A region of enhanced convection around Papua New Guinea remains, but has contracted in the past month.

The ITCZ is south of its normal position (i.e., closer to the equator), but the SPCZ is still displaced to the southwest of its normal position. Convection remains suppressed in a small region near the Date Line, south of the Equator. A weak MJO pulse is moving into the western Pacific at the end of July. 

All of the models NIWA monitor are predicting ENSO–neutral conditions over the August–October period. Two dynamical models suggest La Niña redevelopment at the end of 2011, but the strong majority favour ENSO–neutral continuation. The NCEP ENSO discussion of 7 July indicates ENSO–neutral conditions to continue through austral spring 2011, with an uncertain status beyond that point.

The IRI summary of 21 July indicates neutral conditions as the most likely outcome through 2011, but both El Niño conditions or re-emergence of La Niña conditions cannot be ruled out. 

Surface temperature anomalies (ºC) for July 2011