Global setting: April 2017

The tropical Pacific overall remained in an ENSO (El Niño – Southern Oscillation) neutral state (neither El Niño nor La Niña) during April 2017.

The strong ‘coastal El Niño’ which developed along the coast of South America (southern Ecuador and northern Peru) during February and March has now weakened. In the central equatorial Pacific Ocean, sea surface temperatures (SSTs) are close to or slightly above normal. Atmospheric patterns generally reflect ENSO-neutral conditions, but convective and rainfall anomalies in the central and western Pacific are still leaning towards weak La Niña-like conditions. Ocean subsurface waters are slightly warmer than normal at depth in the western Pacific and close to the surface in the eastern Pacific, and are generally consistent with an ENSO-neutral state. The Southern Oscillation Index (SOI) is currently negative with a value of -0.4 for April 2017; i.e. on the El Niño side of neutral.

International guidance indicates that a transition towards El Niño conditions over the next three months period (May – July 2017) is more likely than not, with 56% chance, versus 44% chance for persistence of the current ENSO-neutral state. The models indicate increasing chance for El Niño becoming established later during the winter, with nearly 70% chance for the August to October period. Note however that ENSO forecasts made just before the start of the winter season have lower accuracy than at other times of the year, and the current spread between the models’ forecasts is significant.

Sea Surface Temperatures

Ocean waters surrounding New Zealand are currently warmer than average all around the country, with anomalies exceeding +1oC along the west coast of the South Island and close to +1oC along the north and east of the North Island. SSTs remain warmer than average in the southern part of the Tasman Sea and along southeast Australia.  The dynamical models’ forecasts for SSTs indicate that this pattern is likely to persist over the next three months period, and waters in the south Tasman Sea are expected to warm further. However, several outbreaks of southerly winds during the start of May might lead to a short-term reduction in sea surface temperature anomalies around New Zealand. For May – July 2017 as a whole, coastal waters around New Zealand are thus forecast to be average or above average.

Differences from average global sea surface temperatures for 9 April - 6 May 2017. Map courtesy of NOAA Climate Diagnostics Centre (https://www.esrl.noaa.gov/psd/map/images/sst/sst.anom.month.gif)
Monthly values of the Southern Oscillation Index (SOI), a measure of changes in atmospheric pressures across the Pacific, and the 3-month mean (black line). SOI mean values: April SOI -0.4; February-April average 0.0.
Differences from average April surface temperatures in the seas around New Zealand