Global setting: April 2016

El Niño conditions in the tropical Pacific weakened further during April 2016, with sea surface temperatures now typically only about +1oC warmer than normal.

Moreover, cooler than normal sub-surface waters have spread eastward from the western Pacific, and temperatures are more than 3oC below normal between 50 and 100m depth east of 160oW. These changes in sub-surface temperatures mean the tropical Pacific is poised to make a rapid transition into La Niña conditions.

International guidance indicates that neutral ENSO conditions are very likely (76% chance) over the next three month period (May – July 2016), as a whole. The likelihood of La Niña development increases into early spring, with a 52% chance over August – October 2016, and a further increase to 60% over November 2016 – January 2017. Forecast models indicate it is very unlikely (less than 10% chance) for a return of El Niño conditions during the rest of 2016. Because of the good chance of      La Niña forming later in 2016, NIWA is on a La Niña “Watch”.

For May – July 2016, above normal pressure is forecast to the north and northeast of New Zealand. This circulation pattern is likely to be accompanied by anomalous north-westerly wind flow.

Sea Surface Temperatures

Sea surface temperatures are forecast to be above normal over the next three months, especially to the west of New Zealand.

Differences from average global sea surface temperatures for 3 - 30 April 2016. Map courtesy of NOAA Climate Diagnostics Centre (http://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 -1.8; February to April average -1.3.
Differences from average April surface temperatures in the seas around New Zealand.