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El Niño/Southern Oscillation (ENSO)

Conditions in the tropical Pacific eased during May to lie in the neutral range by the end of the month.

Conditions in the tropical Pacific eased during May to lie in the neutral range by the end of the month. Remnants of the La Niña event that has been in place since mid–2010 are evident, but the main indicators are now near normal. The NINO3.4 SST anomaly was –0.4°C for May, with NINO3 and NINO4 at –0.1°C and –0.6°C respectively (3–month means –0.6, –0.2, –0.6°C).

Subsurface temperature anomalies are positive across the Equatorial Pacific, apart from a shallow near-surface region in the central Pacific, and upper-ocean heat content (top 300m) is also positive right across the Equatorial Pacific. The positive SST anomaly "horseshoe" in the extra-tropics is still evident, but the equatorial cold tongue has all but disappeared. In the atmosphere, the SOI fell to +0.2, after sitting near +2 or above for 10 months (3–month mean for MAM 2011 was +1.8). The TRMM ENSO index remains relatively strong at around –1 for the 30 days to 29 May.

Convection remains enhanced over the Philippines and parts of the Maritime Continent, and in the SPCZ (still displaced to the southwest of its normal position). Convection is suppressed in only a small region near the Date Line, south of the Equator. There are signs of an MJO event developing, but its amplitude is weak at present. All but one of the models NIWA monitors is predicting ENSO–neutral conditions over the June – August period.

Most models indicate neutral conditions for SON, but a few suggest weak El Niño conditions developing in spring. The NCEP ENSO discussion of 5 May indicates neutral conditions by May – June. The IRI summary of 19 May describes the present situation as ENSO-neutral and expects neutral conditions as the most likely outcome for the rest of 2011 (63% chance in MJJ).