NIWA Outlook: October - December 2016

NIWA Outlook: October - December 2016

Overview

ENSO (El Niño – Southern Oscillation) neutral conditions are still present in the tropical Pacific Ocean as a whole, although some indicators are currently in the weak La Niña category. Sea surface temperatures (SSTs) in the central equatorial Pacific Ocean are near or slightly below average, and slightly above average in the western Pacific and off the South American coast. The atmospheric conditions over the tropical Pacific are generally consistent with an ENSO-neutral state, but show a leaning towards La Niña, as was already the case last month.

The Southern Oscillation Index (SOI) is currently positive (+1.3, value for September 2016 estimated on the 30th of September) and thus technically in the La Niña category. The trade winds are slightly stronger than normal in the western Pacific (west of ~ 140oW). Subsurface ocean temperature anomalies are relatively weak and cooler than normal waters are mostly found in the central (rather than the eastern) Pacific. In summary, the ocean – atmosphere system in the tropical Pacific shows a mix of ENSO-neutral and weak La Niña signals.

International guidance still slightly favours La Niña conditions (54% chance) over the next three months (October - December 2016) however neutral conditions are forecast to become slightly more likely than La Niña by January – March 2017 (50% chance for neutral) and the probability for La Niña drops sharply later on, with only 24% chance in April – June 2017. In summary, La Niña conditions are only slightly more likely than not over the next 3 months, and become less likely as we progress into the first half of 2017.

Despite the current borderline La Niña conditions and the only modest probability for La Niña to develop by the end of the year, the circulation pattern expected for October - December 2016 for the New Zealand region is broadly consistent with the typical La Niña signature: higher pressure than normal is forecast to the south and southeast of the country, while lower pressures than normal are forecast to the north of the New Zealand. These pressure anomalies are likely to be accompanied with easterly quarter flow anomalies.

Outlook Summary

October-December 2016 temperatures are most likely (50-55% chance) to be above average in the north of the South Island, east of the North Island and east of the South Island and very likely (60% chance) to be above average in the north and west of the North Island and the west of the South Island. Sea surface temperatures around New Zealand are forecast to remain near or above normal over the next three months.

October-December 2016 rainfall totals are about equally likely to be in the near normal range (40% chance) or above normal range (40-45% chance) in the North Island. Seasonal rainfall totals are most likely (45% chance) to be in the near normal range in the north of the South Island. In the east and west of the South Island, rainfall for October-December 2016 is about equally likely to be near normal (40% chance) or below normal (35-40% chance).

October-December 2016 soil moisture levels and river flows are most likely to be above normal (45% chance) in the east and west of the North Island and equally likely (40% chance) to be near normal or above normal in the north of the North Island. Seasonal soil moisture levels and river flows in the north of the South Island are most likely (40% chance) to be in the near normal range. Below normal soil moisture levels and river flows are most likely (45% chance) for the west of the South Island. In the east of the South Island, soil moisture levels and river flows are about equally likely to be below normal (40% chance) or near normal (35% chance).

Full outlook: SCO_October_2016.pdf

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