Global setting: May 2017
The tropical Pacific continued to exhibit an ENSO (El Niño – Southern Oscillation) neutral state (neither El Niño nor La Niña) during May 2017, although is now close to the threshold of a weak El Niño. Across the entire equatorial Pacific, sea surface temperatures (SSTs) are near or marginally above normal. Ocean subsurface temperatures in the eastern equatorial Pacific rose markedly between the end of April and end of May, but there is no immediate indication that these warmer waters will be realised at the ocean’s surface. The Southern Oscillation Index (SOI) is currently negative with a value of 0.1 for May 2017; i.e. suggestive of ENSO neutral conditions.
International guidance still suggests that a transition toward El Niño conditions over the next three month period (June – August 2017) is more likely than not, with a 57% chance, versus 42% chance for persistence of the current ENSO neutral state. Compared to last month, the forecast is less aggressive on El Niño development, now reaching a peak (60% chance) during the August-October 2017 period versus 69% last month.
Sea Surface Temperatures
SSTs surrounding New Zealand remain warmer than average all around the country, with anomalies exceeding +1.0oC along the west coast of the South Island and east of the North Island and close to +1.0oC along the north of the North Island. SSTs remain much warmer than average in the southern part of the Tasman Sea and off the coast of southeast Australia. Warmer than average SSTs may help to “lessen the sting” of any colder southerly outbreak, especially early in the June – August 2017 period.
The dynamical models’ forecasts indicate that warmer than average SSTs around New Zealand are likely to persist through June and either remain above average or near average through the winter season as a whole. Thus, for June – August 2017, coastal waters around New Zealand are forecast to be average or above average.