Seasonal Climate Outlook: June - August 2014

Overview

During June–August 2014, mean sea level pressures are expected to be lower than normal to the northeast of New Zealand, and weak lower than normal pressures are expected over the country. This pressure pattern is expected to be accompanied by generally anomalous easterly flow and perturbed conditions.

Sea surface temperatures for the coming three months are expected to be near average off the west coast of New Zealand and above average to the east.

In May 2014, oceanic conditions in the equatorial Pacific were borderline between neutral and weak El Niño. Above normal sea surface temperatures warmed further in the eastern Pacific and persisted around the International dateline. International guidance indicates that El Niño is now the most likely outcome for June-August 2014. However to date the atmosphere has not fully responded to the oceanic anomalies; for example, the Southern Oscillation Index is close to zero.  It is still too early to estimate the strength of this event.

Outlook Summary

June–August temperatures are most likely (45-50% chance) to be above average in the North Island and about equally likely (40-45%) to be above average or average in the South Island. As we progress into winter, cold snaps and frosts are to be expected in many parts of the country. Sea surface temperatures for the coming three months are expected to be near average off the west coast of New Zealand and above average to the east.

June–August rainfall totals are equally likely (40% chance) to be near normal or above normal for the east of the North Island and the north of the South Island. For the remaining regions of New Zealand, three-month rainfall totals are most likely (45-50% chance) to be in the near normal range. 

Soil moisture levels and river flows are most likely (45% chance) to be below normal in the north of the North Island. They are most likely (45% chance) to be near normal in the west and east of the North Island. Soil moisture levels and river flows are about equally likely (35-40%) to be near normal or above normal in the north of the South Island. Soil moisture levels are about equally likely (35-40 % chance) to be near normal or below normal in the west of the South Island and equally likely (40% chance) to be normal or above normal in the east of the South Island. River flows are likely to be near normal (45 % chance) in these two latter regions.

Regional predictions for the June to August 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:

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

The full probability breakdown is:

Gisborne, Hawke’s Bay, Wairarapa

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

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

The full probability breakdown is:

Nelson, Marlborough, Buller

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 average or above average range.
• Rainfall totals are equally likely (40% chance) to be in the near normal or above 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:

West Coast, Alps and foothills, inland Otago, Southland

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

• Temperatures are equally likely (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 are about equally likely (35-40% chance) to be in the near normal or below normal range.
• River flows are most likely (45% chance) to be in the near normal range.

The full probability breakdown is:

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 most likely (45% chance) to be in the near normal range. 
• Soil moisture levels are equally likely (40% chance) to be in the near normal or above normal range.
• River flows are most likely (45% chance) to be in the near normal range.

The full probability breakdown is:

Background

In May 2014, oceanic conditions in the equatorial Pacific were borderline between neutral and weak El Niño. Above normal sea surface temperatures warmed further in the eastern Pacific and persisted around the International dateline. All the sea surface temperatures indices used to monitor El Niño have now reached values close to or at the conventional El Niño thresholds, but are required to persist at these levels for at least three months in order to officially declare El Niño status.

 The NIWA Southern Oscillation Index (SOI) for May is +0.2 (estimated at 28-May). This brings the 3-month March-May value to -0.2. This – along with other indicators – signals that the atmosphere has yet to respond to the oceanic anomalies. During El Niño episodes, the SOI tends to persist at values near -1.0 or more negative.

International guidance now indicates that the tropical Pacific Ocean is likely (~60% chance) to transition to El Niño conditions over the next three months (June-August). The chances of an El Niño being active over spring and summer are higher still (>70%). If the system develops as expected, El Niño typically reaches its peak during our summer, where it is related to stronger or more frequent westerly winds over the New Zealand region, typically leading to dry conditions in east coast areas and more rain in the west. Despite similarities in the oceanic anomalies with those recorded before the 1997/1998 El Niño, it is still too early to comment on the potential strength of the event.

The monthly sea surface temperature (SST) anomaly for New Zealand was approximately +0.9°C in May 2014 (up from about +0.7°C in April). SSTs have beenwarmer than normal around the country since January 2013, having reached a peak (over 1^oC warmer than normal) in December 2013. Sea surface temperatures for the coming three months are expected to be near average off the west coast of New Zealand and above average to the east.

For comment, please contact

Chris Brandolino, NIWA forecaster, NIWA National Climate Centre
Tel (04) 375 6335, Mobile (027) 886 0014

Dr. Nicolas Fauchereau, Climate Scientist, NIWA National Climate Centre
Tel (09) 375 2053.

Notes to reporters and editors

1. NIWA’s outlooks indicate the likelihood of climate conditions being at, above, or below average for the season as a whole. They are not ‘weather forecasts’. It is not possible to forecast precise weather conditions three months ahead of time.
2. The outlooks are the result of the expert judgment of NIWA’s climate scientists. They take into account observations of atmospheric and ocean conditions and output from global and local climate models. The presence of El Niño or La Niña conditions and the sea surface temperatures around New Zealand can be a useful indicator of likely overall climate conditions for a season.
3. The outlooks state the probability for above average conditions, near average conditions, and below average conditions for rainfall, temperature, soil moisture, and river flows. For example, for winter (June–July–August) 2007, for all the North Island, we assigned the following probabilities for temperature:
   ·  Above average: 60 per cent
   ·  Near average: 30 per cent
   ·  Below average: 10 per cent
4. We therefore concluded that above average temperatures were very likely.
5. This three-way probability means that a random choice would be correct only 33 per cent (or one-third) of the time. It would be like randomly throwing a dart at a board divided into three equal parts, or throwing a dice with three numbers on it. An analogy with coin tossing (a two-way probability) is not correct.
6. A 50 per cent ‘hit rate’ is substantially better than guesswork, and comparable with the skill level of the best overseas climate outlooks. See, for example, analysis of global outlooks issued by the International Research Institute for Climate and Society based in the US published in the Bulletin of the American Meteorological Society (Goddard, L., A. G. Barnston, and S. J. Mason, 2003: Evaluation of the IRI’s “net assessment” seasonal climate forecasts 1997–2001. Bull. Amer. Meteor. Soc., 84, 1761–1781).
7. 1. Each month, NIWA publishes an analysis of how well its outlooks perform. This is available online and is sent to about 3500 recipients of NIWA’s newsletters, including many farmers. See: www.niwa.co.nz/our-science/climate/publications/all/cu
   2. All outlooks are for the three months as a whole. There will inevitably be wet and dry days, and hot and cold days, within a season. The exact range in temperature and rainfall within each of the three categories varies with location and season. However, as a guide, the “near average” or middle category for the temperature predictions includes deviations up to ±0.5°C for the long-term mean, whereas for rainfall the “near normal” category lies between approximately 80 per cent and 115 per cent of the long-term mean.
   3. The seasonal climate outlooks are an output of a scientific research programme, supplemented by NIWA’s Capability Funding. NIWA does not have a government contract to produce these outlooks.

Visit our media centre at: www.niwa.co.nz/news-publications/media-centre