Seasonal climate outlook: February 2017 - April 2017

03 February 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 marginally below average. In the sub-surface ocean, the pockets of cooler than average temperatures that remained in December 2016 have all but dissipated and slightly warmer than normal ocean waters are now present in the western equatorial Pacific. The Southern Oscillation Index is close to zero for January 2017. Except for some aspects of rainfall and convection anomalies along the equatorial Pacific, the weak La Niña-like signals that were present in previous months have now vanished.

International guidance favours ENSO-neutral conditions with high probability (90% chance) over the next three month period (February - April 2017). Later during the year, the models indicate substantial chances for a return to El Niño conditions (37% in August – October 2017), but note that ENSO forecasts beyond the Southern Hemisphere autumn are known to be less reliable than at other times of the year.

With the likely persistence of ENSO-neutral conditions, and the anticipated absence of other large- scale climate drivers over the next three months, the atmospheric circulation around New Zealand is expected to favour only slightly more westerly to south-westerly wind flows than normal.

Outlook Summary

February – April 2017 temperatures are most likely (45-50% chance) to be near average for the north and west of the North Island and the east of the South Island. In the east of the North Island, February – April 2017 temperatures are about equally likely to be near average (45% chance) or above average (40% chance), and seasonal temperatures are about equally likely to be near average (40% chance) or below average (35% chance) in the north and west of the South Island.

February – April 2017 rainfall totals are most likely to be below normal (45% chance) in the north and east of the North Island, and equally likely to be near normal (40% chance) or below normal (40% chance) in the west of the North Island. Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance) for the west of the South Island. Near normal rainfall is most likely (40% chance) for the north and east of the South Island.

February – April 2017 soil moisture levels and river flows are most likely (55% chance) to be below normal for the north of the North Island and about equally likely to be normal (40% chance) or below normal (35-45% chance) for the east and west of the North Island. Near normal soil moisture levels and river flows are most likely (40-35% chance) for all regions of the South Island.

Regional predictions for the February – April 2017 season

Northland, Auckland, Waikato, Bay of Plenty

The table below shows the probabilities (or percent chances) for each of three categories: above average, near average, and below average. In the absence of any forecast guidance there would be an equal likelihood (33% chance) of the outcome being in any one of the three categories. Forecast information from local and global guidance models is used to indicate the deviation from equal chance expected for the coming three month period, with the following outcomes the most likely (but not certain) for this region:

Temperatures are most likely to be near average (50% chance).
Rainfall totals are most likely to be in the below normal range (45% chance).
Soil moisture levels and river flows are most likely to be below normal (55% chance).

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	35	20	15	15
Near average	50	35	30	30
Below average	15	45	55	55