El Niño/Southern Oscillation (ENSO)

During November, El Niño conditions in the equatorial Pacific strengthened during October and November. Equatorial sea surface temperature anomalies increased across much of the Pacific. For the last two months, NINO4 has been more positive than NINO3. In November, NINO3 and NINO4 anomalies were +1.5°C and +1.7°C (3–month SON means +1.3°C and +1.4°C). The upper-ocean equatorial heat content increased further in November, and there was a very large and positive subsurface temperature anomaly of +5°C near 125°W and 100m depth.

In the atmosphere, the SOI weakened slightly from its October value of –1.6, with a November average of –0.8 (and SON mean of –0.7). The strong westerly wind anomalies east of the Dateline that existed during October weakened and reversed in the first half of November, but westerlies returned towards the end of the month. Convection over the Tropical Pacific was quite unusual during November, with all the most intense convection well away from the Equator, and mostly centralised in the Southern Hemisphere. In what is perhaps an indication of this unusual convection pattern, the TRMM ENSO precipitation index crossed into the La Niña range at –0.49. There was a big movement of the MJO during November, but its phase was such as to promote equatorial convection in the Indian Ocean and suppress it in the Pacific west of the Dateline.

The global climate model ensemble assessed by NIWA show all dynamical models (and all but one statistical model) with warm conditions through to the end of February 2010. The dynamical models continue the warm conditions through autumn 2010 but with a general easing to neutral conditions by winter. The NCEP ENSO discussion from 5 November suggests the El Niño is expected to continue to strengthen, and persist at least through southern summer. The IRI summary (19 November) indicates a 96% probability for El Niño conditions for the coming summer season.

Mean sea surface temperatures (ºC) for November 2009

Sea surface temperature anomalies (ºC) for November 2009