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El Niño/Southern Oscillation (ENSO)

The La Niña event in the tropical Pacific is continuing to ease, and relaxation of anomalies previously observed are more evident in the ocean than in the atmosphere.

The La Niña event in the tropical Pacific is continuing to ease, and relaxation of anomalies previously observed are more evident in the ocean than in the atmosphere. Tropical Pacific SST anomalies continue to weaken: NINO3, NINO4, and NINO3.4 were –0.2°C, –0.6°C, and –0.7°C respectively for April, compared to around –1.4°C for all regions at the start of 2011. Sub–surface temperature anomalies are positive across the Equatorial Pacific, apart from a shallow near–surface region in the central Pacific. Upper-ocean heat content (top 300m) in the tropical Pacific is now positive right across the Equatorial Pacific. The SST pattern of an Equatorial cold tongue and an extra–tropical positive SST “horseshoe” remain, but are weakening. In the atmosphere, the SOI rose to +2.9 (second highest on record for April in the NIWA SOI time series), with the three–month mean for FMA 2011 at +2.4, even though tropical zonal wind anomalies are near zero or slight positive across much of the Pacific. The TRMM ENSO index eased to –1.1 for the 30 days to 26 April. Convection remains enhanced over the Maritime Continent, the Philippines, and northern Australia, and is suppressed farther east. The ITCZ and SPCZ both remain displaced poleward of their normal positions. The MJO is currently very weak, and models are equivocal about any future development.

All but one of the models NIWA monitors is predicting ENSO–neutral conditions over the coming May–July period. Almost all models indicate neutral conditions through October, and a few suggest El Niño conditions developing by the end of 2011. Only one model suggests a return to cool La Niña conditions. The NCEP ENSO discussion of 7 April described the continued weakening of the La Niña event and states that ENSO–neutral conditions are expected by June. The IRI summary of 21 April describes the current La Niña event as weak and expects neutral conditions by mid–May. The IRI suggests roughly even chances of La Niña or neutral conditions through June 2011.

 

  Surface temperature anomalies (ºC) for April 2011