NIWA Outlook: October – December 2018

NIWA Outlook: October – December 2018

Overview

For the sixth consecutive month, ENSO-neutral conditions persisted across the tropical Pacific. In September 2018 however, the Southern Oscillation Index (SOI) continued to trend down and is now in the El Niño range with a value of –1. On the other hand, sea surface temperatures (SSTs) in the central and eastern equatorial Pacific remain in the neutral range: anomalies for the conventional NINO3.4 index remain just under the +0.3oC mark.

The consensus from international models is for the tropical Pacific to transition toward El Niño over the next three-month period (68% chance over October – December 2018). The probability for El Niño conditions being established remains high until autumn 2019, with a 71% chance for El Niño conditions over the April – June 2019 period. As was the case last month, most models forecast a rather weak event, characterised (at least initially) by maximum SST anomalies located in the central – rather than eastern – equatorial Pacific Ocean.

In other words, the upcoming El Niño event, should it come to fruition, is not expected to be of a similar intensity or type to what was experienced during 2015-16, 1997-98, or 1982-83.

The October – December 2018 atmospheric circulation around New Zealand is forecast to be characterized by higher pressure than normal to the west and southwest of New Zealand. This circulation pattern is expected to be associated with anticyclonic conditions extending over the country, interspersed by episodes of southerly and southwesterly winds. This circulation pattern may be associated with cold nights and mornings, as well as the potential for late-season frosts and fog, particularly during the month of October.

Furthermore, soil moisture is well below normal for the time of year across parts of the upper South Island and the expectation is for normal or below normal rainfall for the coming three-month period.

Outlook Summary

October – December 2018 temperatures are forecast to be average (45% chance) for all regions of New Zealand except the north of the South Island, where temperatures are forecast to be below average (40% chance) or near average (35% chance).

October – December 2018 rainfall totals are forecast to below normal (35 – 40% chance) or near normal (35 – 40% chance) for all regions of New Zealand except the east of the North Island, where near normal rainfall is most likely (45% chance).

October – December 2018 soil moisture levels and river flows are forecast to be below normal (40 –45% chance) or near normal (40% chance) for all regions of New Zealand.

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