Global setting: April 2018

Weak La Niña conditions segued to ENSO-neutral in the tropical Pacific during April 2018.  The Southern Oscillation Index (SOI) over the last 30 days is positive at about +0.5.  Sea surface temperatures (SSTs) warmed over the past month, although they remain slightly colder than average in the eastern equatorial Pacific Ocean. 

The consensus from international models is for the tropical Pacific to persist in an ENSO-neutral state over the next three month period (79% chance over May – July 2018) and ENSO-neutral remains the most likely outcome over the late-winter and early spring seasons (July – September 2018).  However, the forecast models favour El Niño conditions emerging in the tropical Pacific over the austral spring (September – November 2018).

The atmospheric circulation around New Zealand is forecast to be characterised by lower than normal atmospheric pressure through to July.  This will result in mixed and changeable air flows across the country.  Warmer than average ocean waters that are present around the country are also expected to persist through the next three months, though forecasts suggest they will gradually weaken to the west of New Zealand.

Sea Surface Temperatures

The warmest coastal anomalies (for the past three months) are around the west of the South Island, where they remain at least +2.0°C above normal. According to the dynamical models’ forecasts, warmer than average SSTs are likely to persist for at least part of the next 3 months (May – July 2018).

Differences from average global sea surface temperatures for 1 April - 28 April 2018. Map courtesy of NOAA Climate Diagnostics. 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 0.5; February-April average 0.3.
Differences from average April surface temperatures in the seas around New Zealand