El Niño/Southern Oscillation (ENSO)

Strong La Niña conditions exist in the tropical Pacific, with continued intensity from last month.

Strong La Niña conditions exist in the tropical Pacific, with continued intensity from last month. The 3–month mean SOI for NDJ 2010/11 was +2.0, remaining at virtually the same level since JAS 2010. NINO3 and NINO4 SST anomalies were around –1.3°C and –1.4°C respectively in January, holding close to values observed in December. The monthly TRMM ENSO index was -0.95 to 24 January, again showing minimal change from a value of –1.2 in December. Strong convection over the Maritime Continent and northern Australia occurred last month, with suppressed convection near the Equator in the western and central Pacific. The ITCZ and SPCZ are displaced poleward of their normal positions, and the easterly trade winds remain stronger than normal west of 150°W. A very prominent cold tongue is visible in the SST anomaly field, centred on the Equator and extending from 150°E to the South American coast. SST anomalies are positive in the far western Pacific and in the extratropics of both hemispheres. The eastward migration of a strong negative subsurface heat content anomaly centred near 120°W has slowed, and the positive anomaly seen in the western Pacific near 160°E has moved little since December. Last month, a region of MJO–related enhanced convection over the Australian/western Pacific region propagated eastward and strengthened. For the next two weeks, this eastward propagation is expected to continue, but then weaken rapidly as the reduced convection ‘pole’ transits across northern Australia.

Almost all the models NIWA monitor predict continuation of La Niña over the coming three months, but with progressive weakening in intensity during this period. From mid-autumn, the global models suggest either weakening to ENSO–neutral conditions or La Niña persistence into austral winter a weaker intensity. The NCEP ENSO discussion of 6 January states that La Niña has neared a peak, and is likely to persist into southern autumn at lower intensity. The IRI summary of 20 January indicates a 88% probability for La Niña through February–April 2011, 67% through March–May 2011, and 46% through April–June 2011.

Surface temperature anomalies (ºC) for January 2011