El Niño/Southern Oscillation (ENSO)

The tropical Pacific remains in an ENSO–neutral state,with most signs of the preceding La Niña event having dissipated. However, recent developments suggest a possible resurgence of La Niña conditions, at least temporarily.

The NINO3.4 index for August was –0.4°C (3 month mean –0.1°C). Sub–surface temperature anomalies show a strengthened cold anomaly centred near 140°W, with a negative heat content anomaly (0–300m) redeveloping in the central Pacific during August. These  changes in the ocean may be related to the July increase in the SOI to above +1, and associated temporary strengthening of the Trade winds near the Date Line.

The Trades have strengthened again in August, west of 170°W, while the SOI has weakened to +0.1 (from +1.1 in July). SST anomalies are mostly positive in Southern Hemisphere mid–latitudes, with a remnant cool "horseshoe" still evident in the tropics and subtropics, east of the Date Line.

The TRMM ENSO index was 0.0 for the 30 days to 27 August. OLR anomalies are patchy, though there is a region of suppressed convection centred near the Date Line, south of the Equator, and a suggestion that the SPCZ remains displaced to the southwest.

An MJO pulse is developing over the Indian ocean but is predicted to weaken as it propagates eastwards to the western Pacific.

All of the models NIWA monitor are predicting ENSO–neutral conditions during spring. Several dynamical models suggest cool conditions the next few months, but only one indicates La Niña redeveloping over summer. The NCEP ENSO discussion of 4 August indicates ENSO–neutral conditions through spring, but neutral or La Niña  conditions equally likely for summer. The IRI summary of 18 August states neutral conditions as the most likely outcome through the remainder of 2011.