NIWA Outlook: December 2015 - February 2016

December 2015 – February 2016 Issued: 2 December 2015

NIWA Outlook: December 2015 - February 2016

Overview

Strong El Niño conditions continue in the Tropical Pacific. The latest monthly sea surface temperature anomalies exceed +2oC in the central and eastern Pacific. Sub-surface temperature anomalies in the eastern Pacific increased over the last month (November 2015) and exceed +7oC at 75-100m depth near 120oW. The SOI showed a – temporary – weakening in November (November value estimated on the 30th of November is -0.3), but overall the atmosphere remains well coupled with the ocean. In particular, strong westerly wind anomalies (weaker easterly trade-winds) dominate the western and central Pacific. Convection and rainfall are also much more intense than normal in the central and eastern Pacific, while overall large-scale convection was reduced over the Maritime Continent.

International guidance indicates that El Niño will continue (100% probability) over the next three months. Models have the event continuing through the following season, March-May 2016, and the majority predicts a rapid return to ENSO-neutral conditions in June-August 2016. By some measures, the current event is on par with the 1997/98 El Niño (the strongest since 1950).

For December 2015 - February 2016, above normal pressure is forecast to the north of New Zealand, while below normal pressure is expected to the south of the country. This circulation pattern is likely to be accompanied by anomalous westerly wind flows - a signature of El Niño conditions.

Outlook Summary

December 2015 - February 2016 temperatures are most likely (45% chance) to be near average for the north and east of the North Island as well as the east of the South Island. Temperatures are about equally likely to be near average (40% chance) or below average (40-45% chance) for all remaining regions of the country.

December 2015 - February 2016 rainfall is most likely (45%) to be below normal for the north and east of the North Island. Seasonal rainfall totals are about equally likely to be near normal (35-40% chance) or below normal (35-40% chance) for the west of the North Island as well as the north and east of the South Island. Rainfall is equally likely to be near normal (40% chance) or above normal (40% chance) for the west of the South Island.

December 2015 - February 2015 soil moisture levels are about equally likely to be normal (35-40% chance) or below normal (40-45% chance) in all regions of the North Island as well as the east of the South Island. Above normal soil moisture levels are most likely (45% chance) for the west of the South Island and below normal soil moisture levels are most likely (45% chance) for the north of the South Island.

December 2015 - February 2015 river flows are about equally likely to be normal (40-45% chance) or below normal (40-45% chance) in the north and west of the North Island. Seasonal river flows are most likely to be below normal (45-50% chance) in the east of the North Island and the north and east of the south Island, and most likely (45% chance) to be above normal in the west of the South Island.

Full summary with regional outlooks: SCO_Dec2015_Final.docx

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