El Niño/Southern Oscillation (ENSO)

El Niño/Southern Oscillation (ENSO)

During December, El Niño conditions in the equatorial Pacific were well established. Equatorial sea surface temperature anomalies continued to increase in December, associated with continued Equatorial westerly wind anomalies. In December, the NINO3 SST anomaly was +1.8°C and the NINO4 anomaly was +1.5°C (3–month OND means were +1.5°C for both regions). Upper-ocean equatorial heat content anomalies are still strongly positive east of the Date Line, with a large positive subsurface temperature anomaly of above +4°C east of 130°W.

In the atmosphere, the SOI for December was –1.0 (OND mean –1.1), associated with positive zonal wind anomalies (weakened Trade Winds) across the western and central Equatorial Pacific. Tropical OLR anomalies in December were consistent with El Niño conditions, showing strongly enhanced convection centred near the Date Line, and suppressed convection over the Indonesian region (but not northern Australia). The TRMM ENSO precipitation index was +0.61 for the 30 days to 4 January. An MJO event got under way in mid–December and had propagated over the eastern Indian Ocean by early January. The event was expected to continue eastwards, with enhanced convection over Indonesia and northern Australia. The MJO influence may account for some weakening in the SOI.

The global climate model ensemble assessed by NIWA show all dynamical models (and all but one statistical model) forecasting warm conditions (El Niño) through to the end of March 2010, but all models show a weakening towards neutral conditions in Austral autumn. The NCEP ENSO discussion from 10 December suggests the El Niño is expected to persist into the southern autumn. The IRI summary (17 November) indicates a 98% probability of continued El Niño conditions through February, and above 90% through April.

Mean sea surface temperatures (ºC) for December 2009

Sea surface temperature anomalies (ºC) for December 2009