El Niño/Southern Oscillation (ENSO)

The tropical Pacific remained in a neutral state (neither El Niño nor La Niña) in Augut 2013, although cooler than normal Sea Surface Temperatures (SSTs) persist in the far eastern Pacific.

Accordingly both the NINO 3. 4 and NINO 3 indices are slightly negative, with August values estimates of – 0.2°C for NINO 3.4 and – 0.5°C for NINO 3. NINO 4 remains positive at 0.2°C.

SSTs are still warmer than normal over a large area in the central and western Pacific, extending from the Solomon Islands to the southeast, encompassing Fiji and Samoa.

The subsurface ocean is currently slightly warmer than normal from about 100 m to the surface, while colder than normal below (~ 150 m).

The trade winds remained close to normal in August. The Intertropical Convergence Zone (ITCZ) was situated north of its climatological position in the eastern Pacific, while convection and rainfall within the ITCZ was - as last month - much lower than normal west of the Dateline.

The South Pacific Convergence Zone (SPCZ) was not well defined in the western Pacific and positioned slighly southwest of its climatological position.

The latest value for the TRMM ENSO index for the 30 days to 31 August is –1.50 (on the La Niña side of Neutral).

The SOI is now slightly negative (– 0.2 for August) after having been positive for 3 consecutive months.

The Madden – Julian Oscillation (MJO) was inactive over the Pacific over most of last month. Reduced convective activity associated with the MJO is forecast in the western Pacific over the coming two weeks.

The ensemble of dynamical and statistical climate forecast models that NIWA monitors indicates that neutral ENSO conditions are likely to persist over the September – November 2013 period, with 66 % chance, versus 21 % for La Niña and 13 % chance El Niño (source IRI / CPC).