El Niño/Southern Oscillation (ENSO)

The tropical Pacific remained in a neutral state (neither El Niño nor La Niña) in July 2013, despite the persistence of colder than normal sea surface temperatures (SSTs) in the eastern equatorial Pacific.

July NINO values estimates are 0°C for NINO 3.4 , – 0.3°C for NINO 3 and 0.2°C for NINO 4, not very different from June values.

The large area of cool SSTs that were present in June over the north Tasman Sea and the Coral Sea eased in July.

The subsurface ocean is currently slightly warmer than normal from about 100 m to the surface, while colder than normal below (~ 150 m). However, in the eastern Pacific, a shallow cool layer remains at the surface, and is responsible for the weak La Niña – like pattern present in the SST field.

The trade winds remained close to normal for this time of the year. The Intertropical Convergence Zone (ITCZ) was situated north of its climatological position in the eastern Pacific, while convection and rainfall within the ITCZ was mostly suppressed west of the Dateline.

The South Pacific Convergence Zone (SPCZ) was 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 July is –2.93 (weak La Niña) and the monthly SOI for July is +0.8.

The Madden – Julian Oscillation (MJO) was inactive during most of July and is forecast to remain so 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 August – October 2013 period, with 66 % chance, versus 22 % for La Niña and 12 % chance El Niño (source IRI / CPC).