Seasonal Climate Outlook: May - July 2015

Overview

Warming of the sea surface across the equatorial Pacific Ocean continued in April 2015, building upon the warmer than normal waters observed in previous months. These patterns, in combination with weaker trade winds and increasing cloudiness near, and to the east of, the International Date Line are consistent with weak El Niño conditions. However, further coupling between the ocean and atmosphere is needed for El Niño to become established.

The international guidance monitored by NIWA indicates the probability of El Niño formally commencing over the next three months (May – July 2015) is close to 80% (up from 60% last month). Nonetheless, it is important to recognize that the skill of ENSO forecasts issued at this time of year during the northern hemisphere spring tends to be lower than at other times of the year (the so-called “spring predictability barrier”). NIWA’s National Climate Centre will maintain a close watch on the developing conditions.

During May - July 2015, higher than normal mean sea level pressures are expected over the New Zealand region, extending across the Tasman Sea. Lower than normal mean sea level pressures are expected well south of the country.  This circulation pattern is likely to produce more south-westerly quarter wind flows than normal.

Outlook Summary

May - July 2015 temperatures are most likely (35-50% chance) to be average or above average for all regions of New Zealand, although cold snaps and frosts can be expected in some parts of the country as autumn advances into winter. Coastal waters are forecast to be in the near average to above average temperature range.

May - July 2015 rainfall is equally likely (40% chance) to be normal or below normal for the east of the South Island, and most likely (40-50% chance) to be near normal for all remaining regions of the country.

May - July 2015 soil moisture levels and river flows are about equally likely (35-40% chance) to be in the near normal or above normal range in the west of the North Island and the west of the South Island. Soil moisture levels and river flows are most likely (45-50% chance) to be in the near normal range in the east of the North Island and the north of the South Island. In the north of the North Island, soil moisture levels and river flows are about equally likely (35-40% chance) to be in the near normal or below normal range. Finally, in the east of the South Island, soil moisture levels are about equally likely (40-45% chance) to be in the below normal or near normal range, and river flows are most likely (50% chance) to be in the below normal range.

Regional predictions for the May to July 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 (40-45% chance) to be in the above average or near average range.
  • Rainfall totals are about most likely (45% chance) to be in the near normal range.
  • Soil moisture levels and river flows are about equally likely (35-40% chance) to be in the near normal or below normal range.

Other outcomes cannot be excluded. The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

45

25

25

20

Near average

40

45

40

40

Below average

15

30

35

40

 

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

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

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

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

45

30

35

35

Near average

35

45

40

40

Below average

20

25

25

25

 

Gisborne, Hawke’s Bay, Wairarapa

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

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

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

45

20

20

20

Near average

35

50

50

45

Below average

20

30

30

35

 

Nelson, Marlborough, Buller

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

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

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

35

25

25

20

Near average

40

45

45

45

Below average

25

30

30

35

 

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

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

40

35

35

35

Near average

35

45

40

40

Below average

25

20

25

25

 

Coastal Canterbury, east Otago

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

  • Temperatures are about equally likely (40-45% chance) to be in the above average or near average range.
  • Rainfall totals are equally likely (40% chance) to be in the near normal or below normal range.
  • Soil moisture levels are about equally likely (40-45% chance) to be in the below normal or normal range.
  • River flows are most likely (50%) to be in the below normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

45

20

15

15

Near average

40

40

40

35

Below average

15

40

45

50

 

Graphical representation of the regional probabilities

Background

Sea surface temperature anomalies across the tropical Pacific increased during April 2015, with warm anomalies in excess of +1 °C extending from about 150°E to the South American coast. Much warmer than normal SSTs were also observed across much of the north-eastern Pacific Basin. Sub-surface ocean temperature anomalies in the eastern Pacific (around 140°W)  at about 150m depth have also increased from last month and range up to 5°C above normal. These warm anomalies have propagated eastwards over the past 3 to 4 months.

The warmer than normal sea surface temperatures in the equatorial Pacific, in combination with weaker trade winds and increasing cloudiness (also linked to more showers and thunderstorm activity than normal) near, and to the east of, the International Date Line are consistent with weak El Niño conditions. However, further coupling between the ocean and atmosphere will need to occur for El Niño to become established.

International guidance indicates the chances for conventional El Niño thresholds to be crossed during May – July 2015 are about 80% (up from 60% last month). Nonetheless, it is important to recognize that the skill of ENSO forecasts issued at this time of year during the northern hemisphere spring tends to be lower than at other times of the year (the so-called “spring predictability barrier”). NIWA’s National Climate Centre will maintain a close watch on the developing conditions.

For New Zealand, El Niño events are typically (but not always) associated with stronger and/or more frequent south-westerly quarter winds. Such a climate pattern typically leads to drier conditions in eastern and northern areas and more rain in western areas of the country.

The monthly sea surface temperature anomaly for New Zealand was +0.5°C in April. While the large region of warmer-than-normal water around New Zealand that was observed during March has contracted, local Argo float data still indicate that patches of warmer-than-normal waters extend to a depth (~1000m) north of New Zealand and south of Tasmania. Ocean models generally suggest that coastal waters will remain in the near normal to above normal range around the country for the May – July 2015 period.

To find out more about normal conditions for this outlook period, refer to NIWA’s website.

Download the PDF:

Seasonal Climate Update - May - June 2015 [599.69 KB]

For comment, please contact

Chris Brandolino, NIWA forecaster, NIWA National Climate Centre
Tel (09) 375 6335, Mobile (027) 886 0014

Darren Ngaru King, Scientist, NIWA National Climate Centre
Tel (09) 375 2053, Mobile (022) 122 8748