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NIWA Outlook: March – May 2017

NIWA Outlook: March – May 2017

Overview

The tropical Pacific is currently in an ENSO (El Niño – Southern Oscillation) neutral state (neither El Niño nor La Niña). Sea surface temperatures (SSTs) in the central Equatorial Pacific Ocean are near average. However, during February, warming occurred across parts of the central and eastern equatorial Pacific, with the easternmost part of the equatorial Pacific observing its warmest sea-surface temperature anomalies since December 2015. In the subsurface ocean, warmer than average temperatures (+2.0oC anomalies) are present in the western equatorial Pacific but remain near or slightly below average across the central and eastern equatorial Pacific. The Southern Oscillation Index (SOI) was weakly negative during February 2017. Rainfall has been well below normal across the western equatorial Pacific, but above normal in the eastern equatorial Pacific toward South America, a climatic trait more typical of El Niño conditions.

International guidance favours ENSO-neutral conditions with high probability (85% chance) over the next three month period (March – May 2017). Later during the year, models indicate significant chances for a return to El Niño conditions (over 50% in August – October 2017). As of 28 February, the Australian Bureau of Meteorology (BoM) ENSO tracker has been escalated to the “El Niño Watch” category. This means, according to the BoM, that the likelihood of El Nino forming in 2017 is approximately 50%.

However, over the next three months, with the likely persistence of ENSO-neutral conditions and intermittently active periods in the Madden-Julian Oscillation (pattern of increased or decreased rainfall and cloudiness along the equator that has a typical lifespan of 60 to 120 days) changeability in New Zealand’s weather patterns is likely to occur. The atmospheric circulation around New Zealand is expected to favour more westerly wind flows than normal.

Outlook Summary

March – May 2017 temperatures are about equally likely to be average (40-45% chance) or above average (35-40% chance) in the north and east of both the North and South Island. In the west of both Islands, March – May 2017 temperatures are most likely to be near average (45-50% chance). As autumn progresses, frosts may occur from time to time in cooler locations.

March – May 2017 rainfall totals are about equally likely to be near normal (35-40% chance) or below normal (35-40% chance) in all regions of the country except the west of the South Island where near normal rainfall is most likely (45% chance).

March – May 2017 soil moisture levels and river flows are about equally likely to be normal (35% chance) or below normal (35-40% chance) in the North Island. In the South Island, below normal soil moisture levels and river flows are most likely (50% chance) for the east and about equally likely (35% chance) to be normal or below normal in the north and west.

Full outlook: SCO_Mar2017_Final.docx

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