Outlook Summary

A weak, central Pacific El Niño continued during June as sea surface temperatures (SSTs) remained more than 0.7˚C above average (i.e. the El Niño threshold) for the fourth consecutive month.

While El Niño is expected to continue over the coming three-month period (60% chance), it may gradually weaken and become less influential on the global climate system.

For the July to September period, air pressure is forecast to be higher than normal to the northwest of New Zealand and lower than normal to the south. This is expected to be associated with more westerly quarter winds to start the season and mixed flow patterns later. There may be a trend toward more unsettled weather during the coming season, particularly in September.

Temperatures are forecast to be near average or above average for all regions of New Zealand, owing to warmer than average coastal and regional sea surface temperatures.

Warm spells are possible on occasions during July and particularly August when air flows extend from Australia from time to time.

Rainfall is forecast to be near normal or below normal in the north of the North Island and north and east of the South Island. Near normal rainfall is forecast in all other regions of New Zealand.

As the season progresses, high pressure systems may become less frequent in the New Zealand region, contributing to more regular rainfall events.

Video of NIWA Seasonal Climate Outlook: July-September 2019

July – September 2019 temperatures are about equally likely to be near average (40-45% chance) or above average (40-45% chance) for all regions of New Zealand. Despite a low chance for a season with below average temperatures, cold snaps and frosts remain likely to occur.

July – September 2019 rainfall is about equally likely to be near normal (40% chance) or below normal (35% chance) in the north of the North Island and north and east of the South Island. Rainfall is forecast to be near normal (45% chance) in all remaining regions of New Zealand.

July – September 2019 soil moisture levels and river flows are most likely to be below normal (40% chance) in the north of the North Island. For the north of the South Island, soil moisture levels and river flows are about equally likely to be near normal (35-40% chance) or below normal (35-40% chance). Near normal soil moisture levels and river flows are forecast for the remaining regions of New Zealand (40-45% chance).

Regional predictions for the July – September 2019 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 about equally likely to be near average (40% chance) or above average (35% chance).

Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance).

Soil moisture levels and river flows are most likely to be below normal (40% chance).

The full probability breakdown is:

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

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 near average (40% chance) or above average (35% chance).

Rainfall totals, soil moisture levels, and river flows are most likely to be in the near normal range (40-45% chance).

The full probability breakdown is:

Temperature Rainfall Soil moisture River flows Above average 35 30 30 25 Near average 40 45 40 45 Below average 25 25 30 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 near average (40% chance) or above average (35% chance).

Rainfall totals, soil moisture levels, and river flows are most likely to be in the near normal range (40-45% chance).

The full probability breakdown is:

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

Tasman, Nelson, Marlborough, Buller

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

Temperatures are about equally likely to be above average (40% chance) or near average (35% chance).

Rainfall totals, soil moisture levels, and river flows are about equally likely to be near normal (35-40% chance) or below normal (35-40% chance).

The full probability breakdown is:

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

West Coast, Alps and foothills, inland Otago, Southland

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

Temperatures are about equally likely to be above average (40% chance) or near average (35% chance).

Rainfall totals, soil moisture levels, and river flows are most likely to be in the near normal range (40-45% chance).

The full probability breakdown is:

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

Coastal Canterbury, east Otago

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

Temperatures are about equally likely to be above average (40% chance) or near average (35% chance).

Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance).

Soil moisture levels and river flows are most likely to be near normal (40% chance).

The full probability breakdown is:

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

Graphical representation of the regional probabilities

Background

During June 2019, sea surface temperature anomalies remained above El Niño thresholds (i.e. +0.70˚C) in the central Pacific for the fourth consecutive month. The NINO3.4 Index (in the central Pacific) anomaly for June (to the 23rd) was +0.75˚C. The NINO 1+2 Index (in the far eastern Pacific) was near normal.

Upper-oceanic heat content anomalies once again increased in the central Pacific (170W-150W) but continued to decrease in the eastern (110W-100W) and western (160E to the Dateline) Pacific. The distribution of ocean heat content across the Pacific reflects ongoing El Niño-Modoki conditions.

The Southern Oscillation Index (SOI) was on the El Niño side of neutral for the second consecutive month (preliminary June value -0.7).

Trade winds were slightly stronger than normal in the western and central Pacific during June. A strong westerly wind event is expected across the central Pacific during late June and early July. Later in July, enhanced trade winds are again expected to develop. Therefore, sea surface temperatures may increase during the first half of July followed by a decrease later in the month.

Model guidance suggests that El Niño conditions will most likely (60% chance) continue over the next three months; however, a gradual weakening of sea surface temperature anomalies is possible in the equatorial Pacific. For the October – December 2019 period, the probability for El Niño continues at 60% before increasing to 65% during January – March 2020.

The Madden-Julian Oscillation (MJO) was active in the Indian Ocean and western Pacific during June. In early July, the MJO is forecast to move into phases 1 and 2 (the Indian Ocean). Phase 1 is associated with wetter than normal conditions in the west of New Zealand and slightly above average temperatures nationwide.

New Zealand’s coastal water temperatures for June 2019 remain above average for all areas except the east of the North Island. Sea surface temperatures over the Tasman Sea and far Southwest Pacific remain, in general, above average for the time of year with anomalies of around +1.0˚ to 1.5˚C. Warmer than average seas may help to modify cold air masses that move toward New Zealand from late winter into early spring and contribute to spells of above average temperatures.

New Zealand Coastal Sea Surface Temperatures during June 2019

North NI West NI East NI North SI West SI East SI +0.34˚C +0.48˚C -0.05˚C +0.50˚C +0.42˚C +1.00˚C



Warmer than average sea surface temperatures are forecast to persist during the coming three-month period.

The Southern Annular Mode (SAM) was positive for much of June after a brief negative spell very early in the month. Indications are for the index to continue positive, associated with higher pressure than normal in the New Zealand region, early in the month followed by a downturn during mid-July.

Download

Seasonal Climate Outlook July - September 2019 [PDF 3.56MB]

Contacts

Chris Brandolino, Principal Scientist – Forecasting, NIWA National Climate Centre

Tel (09) 375 6335, Mobile (027) 886 0014

Ben Noll, Meteorologist/Forecaster, NIWA National Climate Centre

Tel (09) 375 6334, Mobile (027) 405 3052

Notes to reporters and editors