Seasonal Climate Outlook: July - September 2014

Overview

While above normal sea surface temperatures across the Tropical Pacific Ocean have crossed El Niño thresholds in June 2014, most atmospheric indicators (e.g. sea level pressure, convection, trade winds) have remained at neutral levels, indicating that El Niño conditions have not yet become fully established.

The latest climate model guidance indicates continued warming of the central and eastern tropical Pacific through July-September 2014, with peak strength expected during October-December 2014. However, the coupling of the ocean and atmosphere across the equatorial Pacific will be critical to the declaration of an El Niño event in the months ahead.

During July–September 2014, mean sea level pressures are expected to be lower than normal over northern areas of New Zealand, with higher pressures than normal to the south-east of the country. This pressure pattern is expected to be accompanied by generally anomalous flow from the easterly quarter.

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.

Outlook Summary

July-September temperatures are most likely (50% chance) to be above average for all regions across the North Island, and about equally likely (40-45%) to be average or above average for all regions across the South Island. Meanwhile, 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.

July– September rainfall is about equally likely (40-35% chance) to be in the normal or below normal range for the west 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 (40-50% chance) to be in the near normal range. 

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

July– September river flows are about equally likely (35-40 % chance) to be normal or above normal in east of the North Island, and about equally likely (35-40 % chance) to be normal or below normal in west of the North Island. For all remaining regions, river flows are most likely (40-45%) to be in the near normal range.

Regional predictions for the July to September 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 (50% 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 are about equally likely (40-35% chance) to be in the near normal or above normal range.
• River flows are most likely (40% chance) to be in the near normal range.

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

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	35	35	30
Near average	40	45	40	40
Below average	10	20	25	30

•  View the historic rainfall and temperature 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 (50% chance) to be in the above average range.
• Rainfall totals are about equally likely (40-35% chance) to be in the normal or below normal range.
• Soil moisture levels are about equally likely (40-35% chance) to be in the near normal or below normal range.
• River flows are about equally likely (40-35% chance) to be in the near normal or below normal range.

The full probability breakdown is:

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

•  View the historic rainfall and temperature for this region 

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

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	35	35	35
Near average	40	45	40	40
Below average	10	20	25	25

•  View the historic rainfall and temperature for this region 

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 about equally likely (40-35% chance) to be in the normal or below 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 (40% chance) to be in the near normal range.

The full probability breakdown is:

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

•  View the historic rainfall and temperature for this region 

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

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	30	30	30
Near average	40	40	45	40
Below average	20	30	25	30

•  View the historic rainfall and temperature for this region 

Coastal Canterbury, east Otago

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 (50% 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:

	Temperature	Rainfall	Soil moisture	River flows
Above average	40	25	40	35
Near average	40	50	40	45
Below average	20	25	20	20

•  View the historic rainfall and temperature for this region

Background

Above normal sea surface temperatures across the equatorial Pacific Ocean continued in June 2014 (particularly in the far eastern equatorial Pacific where surface anomalies are more than 4 °C above average) and these anomalies are consistent with El Niño conditions. However, most atmospheric indicators remained at neutral levels at the end of June, and consequently the necessary thresholds to declare an El Niño event have not been crossed.

The NIWA Southern Oscillation Index (SOI) for June was -0.1 (estimated at 30-June), which brings the April-May-June value to +0.1. This 3-month value is typically associated with ENSO-neutral conditions. Further, the NASA ENSO Precipitation Index (ESPI) for the 30 days to 28-June was -0.2 (down from ~ +0.2 last month). These values also indicate ENSO neutral conditions, and along with other atmospheric indicators, such as the direction and strength of the trade winds, collectively signal that the atmosphere has yet to respond to the warmer oceanic waters observed in the central and eastern Pacific.

The latest guidance from international climate models indicates continued warming of the central and eastern tropical Pacific Ocean through July-September 2014, with six out of ten dynamical models and one of the four statistical models monitored by NIWA indicating El Niño conditions. However, the peak strength in sea surface temperatures across the Tropical Pacific is actually expected during October-December 2014, with 7 out of 10 dynamical models and 3 out of 4 statistical models indicating El Niño index values. Atmospheric patterns linked with El Niño are expected to strengthen during this time. The coupling of the ocean and atmosphere will be critical to the declaration of any El Niño event in the months ahead.

Note that for New Zealand, El Niño typically reaches its peak during our summer, where it is related to stronger and/or more frequent westerly winds over the New Zealand region. Such a climate pattern typically leads to drier conditions in eastern areas and more rain in western areas of the country.

Meanwhile, waters surrounding New Zealand remain generally warmer than average, particularly to the east of the country and in the south-western Tasman Sea where they have intensified since last month. The estimated monthly sea surface temperature (SST) anomaly for New Zealand was +1.0°C in June (rising from +0.7^oC in May). SSTs have now been warmer than normal around New Zealand since January 2013.

For comment, please contact

Chris Brandolino, NIWA forecaster, NIWA National Climate Centre

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

Darren King, Senior Scientist, NIWA National Climate Centre

Tel (09) 375 2086.

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
   We therefore concluded that above average temperatures were very likely.
4. 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.
5. 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).
6. 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. www.niwa.co.nz/our-science/climate/publications/all/cu
7. 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.
8. 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.

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Background

The equatorial Pacific Ocean remains ENSO-neutral at the end of August 2014, and despite the Southern Oscillation Index being currently negative, a fully coupled (ocean and atmosphere) event has yet to initiate. Sea-surface temperatures (SSTs) remain well under El Niño thresholds in the Nino3.4 region. The subsurface ocean is currently warmer than normal (+ 2^oC) at about 100 to 150 m. depth in the central Pacific around 160^oW.

The international guidance places the chances of El Niño developing over the September to November 2014 period at about 55%, a reduction from outlooks issued in the previous months. Later during the summer (December 2014 – February 2015), the chances for El Niño rise to about 70%. There is general agreement that this event – if it does indeed occur - would fall in the weak to moderate category.

The NIWA Southern Oscillation Index (SOI) for August is –1.1. This brings the  3-month June-July-August value to -0.5. Strongly negative SOI values (less than -1) are typically associated with  El Niño. In contrast, the NASA ENSO Precipitation Index (ESPI) for the 30 days to the 1^st of September was -0.82 (on the La Niña side of neutral).

Note that for New Zealand, El Niño events typically reach their peak during summer, when they are related to stronger and/or more frequent westerly winds over the New Zealand region. Such a climate pattern typically leads to drier conditions in eastern areas and more rain in western areas of the country.

Meanwhile, waters surrounding New Zealand remain slightly warmer than average around the South Island, and close to normal or slightly cooler than normal around the North Island. The  preliminary  monthly sea surface temperature anomaly for New Zealand was +0.3°C in August, a significant decrease compared to the previous months. Ocean models forecasts indicate that SSTs are likely to be close to normal around the country over the next three months.

For comment, please contact:

Chris Brandolino, NIWA forecaster, NIWA National Climate Centre

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

Dr Brett Mullan, Principal Scientist, NIWA National Climate Centre

Tel (04) 386 0508, Mobile (027) 294 1169.

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 We therefore concluded that above average temperatures were very likely.
4. 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.
5. 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).
   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