Seasonal Climate Outlook

NIWA Outlook: July – September 2017

Overview

The tropical Pacific remained in an ENSO (El Niño – Southern Oscillation) neutral state (neither El Niño nor La Niña) during June 2017, but oceanic and atmospheric anomalies were mixed, with some indicators leaning towards El Niño and others towards La Niña. Across the equatorial Pacific Ocean, sea surface temperatures (SSTs) are currently slightly above normal. The large-scale atmospheric circulation continues to generally reflect ENSO-neutral conditions, but is showing conflicting signals: the Southern Oscillation Index (SOI) is currently negative (i.e. leaning towards El Niño), with a value of -1.0 for June 2017, but large-scale rainfall and convection anomalies are showing La Niña-like patterns, with more intense than normal convection and rainfall over large parts of the Maritime Continent.

International guidance – in contrast to previous months – now suggests that the tropical Pacific Ocean is more likely to remain in an ENSO neutral state (60% chance) than to transition towards El Niño (35% chance) over the next three month period (July – September 2017). The probability for El Niño increases only slightly later during the year, with a maximum of 43% over the October-December 2017 period. Chances for La Niña development remain low (less than 15%).

For July – September 2017 as a whole, the atmospheric circulation around New Zealand is forecast to be characterised by slightly higher than normal pressures southeast of the Chatham Islands, leading to weak easterly-quarter flow anomalies over the country.

Outlook Summary

July – September 2017 temperatures are about equally likely to be above average (45% chance) or near average (40% chance) for the west and east of the North Island. In all remaining regions of New Zealand, above average temperatures for July – September 2017 as a whole are very likely (60-70% chance). Nevertheless, frosts and cold snaps will occur during the winter and early spring seasons. Coastal waters around New Zealand are forecast to remain above average and may warm further over the next three month period.

July – September 2017 rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance) in the east of the North Island. For all remaining regions of New Zealand, rainfall totals are most likely to be near normal (40-50% chance) over the next three month period. In contrast to long periods of settled conditions during June, the first half of July may have more active, rainier intervals across New Zealand.

July – September 2017 soil moisture levels and river flows are about equally likely to be near normal (35-40% chance) or below normal (35% chance) in the north of the North Island and the north and east of the South Island. In the west of the South Island, soil moisture levels and river flows are both most likely to be in the below normal range (50% chance). Soil moisture levels and river flows are equally likely to be near normal (35% chance) or above normal (35% chance) in the east of the North Island. In the west of the North Island, soil moisture levels and river flows are most likely to be in the near normal range (45% chance).

 

Regional predictions for the July – September 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 very likely to be above average (60% chance).
  • Rainfall totals are most likely to be in the near normal range (50% chance).
  • Soil moisture levels are equally likely to be near normal (35% chance) or below normal (35% chance), and river flows are about equally to be near normal (40% chance) or below normal (35% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average                60 25 30 25
Near average 30 50 35 40
Below average 10 25 35 35

 

Central North Island, Taranaki, Whanganui, Manawatu, Wellington

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are about equally likely to be above average (45% chance) or near average (40% chance).
  • Rainfall totals, soil moisture levels and river flows are all most likely to be in the near normal range (45-50% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 45 25 30 25
Near average 40 50 45 45
Below average 15 25 25 30

 

Gisborne, Hawke’s Bay, Wairarapa

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are about equally likely to be above average (45% chance) or near average (40% chance).
  • Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance) .
  • Soil moisture levels and river flows are equally likely to be near normal (35% chance) or above normal (35% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 45 35 35 35
Near average 40 40 35 35
Below average 15 25 30 30

 

Tasman, Nelson, Marlborough, Buller

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are very likely to be above average (70% chance).
  • Rainfall totals are most likely to be near normal (40% chance).
  • Soil moisture levels and river flows are about equally likely to be near normal (40% chance) or below normal (35% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 70 30 25 25
Near average 20 40 40 40
Below average 10 30 35 35

 

West Coast, Alps and foothills, inland Otago, Southland

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are very likely to be above average (70% chance).
  • Rainfall totals are most likely to be in the near normal range (45% chance).
  • Soil moisture levels and river flows are most likely to be in the below normal range (50% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 70 20 20 20
Near average 20 45 30 30
Below average 10 35 50 50

 

Coastal Canterbury, east Otago

Probabilities are assigned in three categories: above average, near average, and below average.

  • Temperatures are very likely to be above average (70% chance).
  • Rainfall totals are most likely to be in the near normal range (40% chance).
  • Soil moisture levels and river flows are about equally likely to be in the near normal (40% chance) or below normal (35% chance) range.

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows
Above average 70 30 25 25
Near average 20 40 40 40
Below average 10 30 35 35

Graphical representation of the regional probabilities

Background

ENSO (El Niño – Southern Oscillation) neutral conditions persisted across the tropical Pacific overall during June 2017, but oceanic and atmospheric anomalies were mixed, with some indicators leaning towards El Niño, and others towards La Niña.

Across the equatorial Pacific Ocean, sea surface temperatures (SSTs) are slightly above normal, with an anomaly of +0.5oC in the NINO3.4 region (in the central Pacific) for the month of June (on the El Niño side of neutral). Overall, SSTs in the whole tropical Pacific Ocean (between about 30oN and 30oS) are currently close to or slightly above normal.

Positive subsurface temperature anomalies along the equator in the first 100 meters or so of the Ocean have remained mostly stable or weakened slightly during June 2017. In the central Pacific (between about 170oW and 140oW), these warmer than normal waters overlay a cooler than normal layer, observed at a depth of about 150m, and these negative anomalies have intensified during June 2017, in places now exceeding about -3oC.

The preliminary [value estimated on the 30th of June] Southern Oscillation Index (SOI) for the month of June 2017 is negative at -1.0, indicating borderline El Niño conditions. Note, however, that the SOI has shown significant intra-seasonal variability over the past few months; it is thus far from certain that negative SOI values will persist.

Zonal wind anomalies along the equator were close to zero during June and generally large-scale circulation patterns are consistent with an ENSO-neutral state. However, rainfall and convection anomalies in the tropical Pacific are still more typical of La Niña conditions; in particular, more intense than normal convection and rainfall was observed over large parts of the Maritime Continent. Accordingly, the ENSO Precipitation Index (ESPI) is significantly negative with a value of -2.2 (indicating La Niña state).

In contrast to the ENSO outlooks issued in previous months, international guidance now indicates that the tropical Pacific Ocean is more likely to remain in an overall ENSO-neutral state (60% chance) than to transition towards El Niño (35% chance) over the next three months (July – September 2017). The probability for El Niño increases only slightly later this year, with a maximum of 43% over the October-December 2017 period. Chances for La Niña remain low (less than 15%) through to the end of the year.

Coastal waters remain generally warmer than average all around the country, with anomalies exceeding +0.5oC along the west coast of the South Island. However, in the last two weeks, SSTs have trended slightly cooler near the North Island. The anomaly in the “NZ box” (160°E-170°W, 30-45°S) is currently reaching about +0.5oC. Ocean waters are still much warmer than average in the southern part of the Tasman Sea, off the east coast of Tasmania and southeast Australia, with anomalies exceeding +3oC in place.

The dynamical models’ forecasts indicate that warmer than average SSTs around New Zealand are likely to persist or intensify over the July – September 2017 period.

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