El Niño/Southern Oscillation (ENSO)

El Niño/Southern Oscillation (ENSO)

During September, the equatorial Pacific Ocean was in an El Niño state, but the atmosphere continued to display only weak El Niño conditions. Sea surface temperatures were above normal across most of the equatorial Pacific, and intensified slightly in September. NINO 3 & 4 anomalies were +1.2°C and +1.0°C in September, with the 3-month means for July– September at +1.2°C and +0.9°C. Subsurface oceanic heat content remains above average, but the positive anomaly is centred more towards the central Pacific rather than in the east.

The atmospheric circulation still is not well coupled with the ocean: in September the SOI reverted to positive values (+0.3 for the month), resulting in a near–zero three–month mean. Westerly wind anomalies continue west of the Dateline, as they have done since May, but have only sporadically penetrated to the east. The trade winds are generally slightly stronger than normal east of the Date Line. Convection was enhanced in September over the Equatorial Pacific, west of the Date Line, and somewhat suppressed over the Maritime Continent and Australia. Equatorial OLR anomalies remain weak east of the Date Line. The TRMM ENSO precipitation index is positive at +0.30, but well short of the +1 threshold for a moderate El Niño. An MJO event has developed during September, with enhanced convection north of Australia, but it appears to be slow-moving and is predicted to weaken in–situ during the first half of October.

The global climate model ensemble assessed by NIWA show all dynamical models (and all but one statistical model) indicating warm conditions through the end of the year. Most models show El Niño persisting to the end of the southern summer, with some strengthening between now and the end of the year, and a decline in the autumn. The NCEP ENSO discussion from 10 September suggests further strengthening of El Niño conditions and persistence through summer. The IRI summary indicates an 85–90% probability for El Niño persisting through to the end of southern summer. 

Mean sea surface temperatures (ºC) for September 2009

Sea surface temperature anomalies (ºC) for September 2009