El Niño/Southern Oscillation (ENSO)

El Niño/Southern Oscillation (ENSO)

Sea surface temperature anomalies (ºC) for July 2009 (Click for enlargement)

Mean sea surface temperatures (ºC) for July 2009. (Click for enlargement)

During July, conditions in the equatorial Tropical Pacific indicated that the ocean moved into an El Niño state, but the atmosphere has yet to respond. Sea surface temperature anomalies in the equatorial Pacific are generally positive and are broadly consistent with El Niño conditions. NINO 3 & 4 anomalies for July were +1.4°C and +0.8°C, respectively (3-month means +1.0°C and +0.6°C). Subsurface conditions are also consistent with El Niño, with a substantial positive heat content anomaly in the eastern Equatorial Pacific. However, positive temperature and heat anomalies extend right across the Pacific Basin, though cooling in the far west occurred from mid–July.

The atmospheric circulation in the region does not yet appear to be coupled with the ocean. The July SOI reverted to positive values (+0.2 for the month) with a three-month mean also near zero. OLR anomalies show a region of enhanced convection about and (mostly) west of the Date Line, and the TRMM ENSO precipitation index is only weakly positive (around +0.1). Significant positive zonal wind anomalies have been evident over the western Pacific throughout July, but have not yet penetrated east of the Date Line. The MJO has been weak since late June, but an event may develop in August

The global climate model ensemble assessed by NIWA show almost all dynamical models with warm (El Niño) conditions through the end of the year, while all statistical models indicate neutral conditions through October. The July NCEP ENSO discussion suggests weak-moderate El Niño conditions through the rest of 2009. The IRI summary estimates at least an 80% probability of El Niño through the rest of the year. The Bureau of Meteorology indicates that the ocean is in an El Niño state and that the atmosphere is now moving in that direction.