El Niño/Southern Oscillation (ENSO)

The tropical Pacific is in an ENSO–neutral state, but bordering on La Niña conditions.

The Equatorial Pacific ocean circulation continues to trend towards La Niña, and there is evidence of atmospheric coupling. The NINO3 and NINO4 anomalies were around –0.2 and +0.2°C for June (AMJ means both around +0.5°C). The SST anomaly pattern across the tropical and subtropical Pacific is quite La Niña-like, with near-Equatorial negative anomalies east of the Date Line, and a warm “horseshoe” in the west, extending into the extra-tropics of both hemispheres. At the sub-surface, a strong negative heat content anomaly has developed east of the Dateline, with temperature anomalies as low as –4°C.

The SOI weakened to near zero on average in June (down from +1.0 in May, and around +0.9 for AMJ). OLR anomalies for June show strongly suppressed convection near the Solomons, extending to east of the Date Line. Consistent with this, the TRMM ENSO index has dropped further to –1.2 in the 30-day mean to 27 June (values of –1.0 or less are typical of La Niña conditions). The trade winds have been consistently stronger than normal west of the Date Line over the past month. The MJO has been weak recently, but an event is now developing with enhanced convection over the Indian Ocean. It is forecast to move only slowly eastwards through early July.

Most models monitored by NIWA predict the tropical Pacific to be in a La Niña state over the coming 6 months. The NCEP ENSO discussion of 3 June states that conditions are favourable for transition to La Niña conditions during JJA. The IRI summary of 17 June indicates a ~58% probability for developing La Niña conditions from June though August (and 41% probability for maintaining neutral conditions), increasing to ~62% probability of a La Niña from the August to October season through the remainder of 2010.

 Surface temperature anomalies (ºC) for June 2010