El Niño/Southern Oscillation (ENSO)

A strong La Niña event is in place in the tropical Pacific, and it intensified during December.

A strong La Niña event is in place in the tropical Pacific, and it intensified during December. There is strongly enhanced convection over the Maritime Continent and northern Australia, and convection is strongly suppressed near the equator in the western and central Pacific. The ITCZ and SPCZ both remain displaced poleward of their normal position. The TRMM ENSO index was –1.2 for December (values of –1.0 or less are considered typical of La Niña conditions). The easterly trade winds were stronger than normal west of 150°W, and the SOI rose to +2.8 for December (+1.6 in November and +2.1 in the three–month mean for OND). 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 extra–tropics of both hemispheres. NINO3 and NINO4 SST anomalies were around –1.5°C and –1.3°C respectively, on average for December (OND averages –1.4°C and –1.2°C, respectively). A strong negative subsurface heat content anomaly continues to migrate eastwards along the Equator and continues to weaken slowly. A positive SST anomaly is gradually spreading eastwards in the western Pacific. There is a weak region of MJO–related convection over the Australian/western Pacific region which is expected to propagate eastwards across the Pacific during the rest of January, and may strengthen.

Almost all the global climate models NIWA monitor predict the tropical Pacific to be in a La Niña state over the coming three months, with most easing towards neutral conditions during autumn (MAM) and into winter. The NCEP ENSO discussion of 6 January states that La Niña conditions are near their peak and the event is likely to persist well into southern autumn 2011 at a lesser intensity. The IRI summary of 16 December indicates a 98% probability for moderate–strong La Niña conditions continuing through February 2011, 94% through March 2011, and at least 50% until April–June 2011.

Surface temperature anomalies (ºC) for December 2010