El Niño/Southern Oscillation (ENSO)

The tropical Pacific remained in a neutral state (neither El Niño nor La Niña) in May 2013.

Since April, slightly cooler than normal sea surface temperatures (SSTs) have developed in the eastern equatorial Pacific. The region of warmer than normal SSTs that extended from Tasmania to New Zealand in previous months has weakened this month. May NINO values estimates are + 0°C for NINO 3.4 , – 0.3°C for NINO 3 and 0.1°C for NINO 4.

Higher than normal subsurface temperatures still exist along the Equator at about 150m depth west of the Dateline, while lower than normal temperatures are present east of the Dateline at similar depths. The thin warm surface layer that existed in the eastern Pacific in April has vanished, allowing the surface expression of the deeper cool anomalies.

Equatorial oceanic integrated heat content (0 – 300 m) is currently slightly warmer than normal in the West and cooler in the East. The trade winds remained slighly stronger than normal in the western Pacific.

The Intertropical Convergence Zone (ITCZ) was north of normal in the eastern Pacific. The South Pacific Convergence Zone (SPCZ) was positioned south of normal in May in the western Pacific.

The latest value for the TRMM ENSO index for the 30 days to 3 June is – 1.68 (on La Niña side of neutral) and the monthly SOI for May is 0.6.

The Madden – Julian Oscillation (MJO) was mostly inactive over the region in May and is forecast to remain weak 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 June – August 2013 period, with 67 % chance, versus 26 % for La Niña and 7 % chance El Niño (percentages are from the latest IRI/CPC statement).