Seasonal climate outlook November 2017 - January 2018

Seasonal climate outlook November 2017 - January 2018

1 November 2017

Overview

The tropical Pacific is still officially in a ENSO (El Niño Southern Oscillation) neutral state, but some indicators have leaned more towards La Niña conditions during the course of October 2017. After a brief period of warming early in the month, sea surface temperatures (SST) in the eastern equatorial Pacific Ocean have cooled significantly, especially off the South American coast. The NIWA Southern Oscillation Index has been positive since July 2017 and is currently at +0.9.

The international consensus is that the tropical Pacific Ocean will cool further over the next 3 months (November 2017 – January 2018), with La Niña conditions likely to be met over the same period (with 70% chance). However, the models indicate that if La Niña does develop, it is likely to remain in the weak category and be short-lived: a return to neutral conditions is most likely (58% chance) over the February-April 2018 period.

For November 2017 – January 2018 as a whole, the atmospheric circulation around New Zealand is forecast to be characterized by higher pressure than normal to the south-east of the country, and lower pressure than normal to the north. This pressure pattern is expected to be associated with north-easterly to easterly flow anomalies, a pattern which is consistent with regional conditions typically observed during La Niña events.

For the coming tropical cyclone season (November 2017 to April 2018), outlooks indicate that the risk for New Zealand is normal or above normal. On average, at least one ex-tropical cyclone passes within 550km of New Zealand each year. If an ex-tropical cyclone comes close to the country, it has equal probability of passing east or west of Auckland and the North Island. Significant rainfall, damaging winds and coastal damage can occur leading up to and during these events.

Outlook Summary

November 2017 – January 2018 temperatures are forecast to be above average for all regions of New Zealand (60% to 70% chance for above average temperatures). Coastal water temperatures around New Zealand are forecast to remain above average over the next three-month period, especially along the east coast of the South Island.

November 2017 – January 2018 rainfall totals are about equally likely to be normal (35% chance) or above normal (40% chance) for the north and east of the North Island, and most likely to be near normal (45% chance) in the west of the North Island and the north and east of the South Island. Rainfall totals for the next three months are about equally likely to be below normal (40% chance) or near normal (35% chance) in the west of the South Island.

November 2017 – January 2018 soil moisture levels and river flows are most likely to be below normal (45% chance) in the west and east of the South Island and most likely to be above normal (45% chance) in the east of the North Island. Soil moisture levels and river flows are equally likely to be near normal (35% chance) or above normal (35% chance) in the north of the North Island. In the west of the North Island and the north of the South Island, soil moisture levels and river flows are most likely to be in the near normal range (40% chance).

Full Seasonal Climate Outlook: Seasonal climate outlook November 2017 - January 2018 | NIWA

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