The tropical Pacific is now exhibiting moderate to strong La Niña conditions, as the current event continues to strengthen.
There is enhanced convection over Indonesia and much of northern Australia. Convection remains suppressed along the Equator in the western and central Pacific. Easterly trade winds are stronger than normal west of about 160°W, and the ITCZ and SPCZ are both poleward of their normal positions. The SOI has been near or above +2.0 for the past three months (+2.5 for September, +2.1 for JAS)). The TRMM ENSO index is –1.4 for September, which is slightly weaker than the August value of –1.9 (values of -1.0 or less are considered typical of La Niña conditions). Sea surface temperatures show a cold tongue extending along the Equator from 160°E eastwards, with positive SST anomalies in the far western Pacific and in the extra–tropics of both hemispheres. The NINO3 and NINO4 anomalies have both intensified from August, and are now around –1.2°C for September (wth a JAS average of –0.8°C). Sub–surface ocean temperatures exhibit a strong negative heat content anomaly of about –5°C to the east of the Date Line, centred near 140°W and 100 m depth. This anomaly is more intense and 10° further east compared to last month. The MJO is currently very weak and projections are unreliable.
All of the dynamical and statistical models NIWA monitor predict the tropical Pacific to be in a La Niña state over the coming 3–6 months. A weakening is predicted in autumn 2011 for models that project beyond three months. However, JAMSTEC is forecasting La Niña to be a protracted event that will persist through all of 2011 to early 2012. The NCEP ENSO discussion of 9 September states that La Niña is expected to persist at least through southern summer 2010/11. The IRI summary of 16 September upgrades the event to moderatestrong, and indicates at least a 95% probability for La Niña conditions continuing through to December-February 2010/2011, and at least 50% until March–May 2011.
Surface temperature anomalies (ºC) for September 2010