Seasonal Climate Outlook: March - May 2014

During March–May 2014, mean sea level pressures are expected to be lower than normal to the north of the country, with higher pressures than normal to the southeast of New Zealand.

This circulation pattern is likely to be associated with anomalous north-easterly flows over the North Island and anomalous weak easterly flows over the South Island.

Sea surface temperatures are expected to be near average around New Zealand, for the coming three months as a whole.

The equatorial Pacific Ocean continued in a neutral ENSO-state (neither El Niño nor La Niña) in February 2014. International guidance indicates that ENSO-neutral conditions are the most likely outcome for autumn, but in the following winter season about half the international models develop El Niño or borderline El Niño conditions.  

News: 'El Nino Watch' called by U.S. climate experts

U.S. climate experts have raised their El Nino alert system from 'Not Active' to 'El Nino Watch'. There is no change to NIWA's March-May climate outlook, and the probability of an El Nino later this year (winter) currently remains at about 50%. However, this situation could change quickly, so NIWA is keeping a close watch on the state of the ocean-atmosphere system in the tropical Pacific.  

Find out more about El Nino Watch →

Outlook summary

March–May temperatures are most likely (45-50% chance) to be near average for all regions of the country. As autumn progresses, cold snaps and frosts can be expected from time to time in some parts of the country. Sea surface temperatures are expected to be near average for the coming three months around New Zealand.

March–May rainfall is likely (40-45% chance) to be near normal or below normal for the north and west of the North Island, and for the north, west and south of the South Island. For the eastern regions of both Islands, three-month rainfall totals are most likely (50% chance) to be in the near normal range.

Soil moisture levels and river flows are most likely (45-55% chance) to be below normal in the north and west of the North Island and in the west and south of the South Island. They are also about equally likely (40-45% chance) to be near normal or below normal in the north and east of the South Island, but most likely (45% chance) to be near normal in the east of the North Island.  

Regional predictions for the March to May 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 (45% chance) to be in the near average range.
• Rainfall totals are equally likely (40% chance) to be in the near normal or below normal range.
• Soil moisture levels and river flows are most likely (55%) to be in the below normal range.

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

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

The full probability breakdown is:

Click here to 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 (45% chance) to be in the near average range.
• Rainfall totals, soil moisture levels and river flows are most likely (45-50% chance) to be in the near normal range. 

The full probability breakdown is:

Click here to 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 most likely (45% chance) to be in the near average range.
• Rainfall totals, soil moisture levels and river flows are all equally likely (40% chance) to be in their near normal or below normal ranges. 

The full probability breakdown is:

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

The full probability breakdown is:

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

The full probability breakdown is:

Click here to view the historic rainfall and temperature for this region

Background  

The equatorial Pacific Ocean continued in a neutral ENSO-state (neither El Niño nor La Niña) in February 2014. Colder than normal sea surface temperatures appeared in the eastern equatorial Pacific, but at 150m sub-surface depths an extensive region of warmer than normal water developed during the month. The NIWA Southern Oscillation Index for February is -0.1 (estimated at 26-February). This brings the 3-month December-February value to +0.4. International guidance indicates that the tropical Pacific Ocean is very likely (>80% chance) to remain neutral over the next three months (March-May). However, in the following season (winter) about half the international models develop El Niño or borderline El Niño conditions, with the other half remaining near neutral. A similar situation persists into the spring of 2014, with approximately 50:50 chances of neutral or El Niño conditions.

The monthly sea surface temperature (SST) anomaly for New Zealand was approximately +0.1°C in February 2014. While this is the 14th consecutive month that SSTs have been warmer than normal around New Zealand, there has been rapid cooling of surrounding waters (from +1.0°C in December 2013), particularly to the northwest of the North Island. Sea surface temperatures are expected to be near average for the coming three months around New Zealand.

The tropical cyclone season in the southwest Pacific still has two months to run (March-April). Historically, Ex-Tropical Cyclones (ETCs) approach within 550km of New Zealand for about 9 out of 10 years, with the most active period being February-April. For ENSO-neutral years (the current situation), any ETCs approaching the North Island are twice as likely to pass to the east of Auckland as to the west of the city.

At the end of February 2014, soil moisture levels continued lower than normal across much of New Zealand, with the main exceptions being the West Coast of the South Island and eastern Northland. The entire west of the North Island from Auckland down to Wellington received less than 40% of the normal February rainfall. Soil moisture deficits are not as extensive as those a year ago in the 2013 drought, but may be as severe as 2013 in isolated regions. Soil moisture levels are lowest, relative to normal at this time of year, in the Waikato, Waitomo and Taupo districts.

These current dry conditions developed during a period when there was not a clear consensus between the various global model rainfall forecasts. However, the latest forecast guidance from February is pointing more consistently towards drier than normal conditions than seen in previous months.  

For comment, please contact  

Dr Brett Mullan, Principal Scientist, NIWA National Climate Centre
Tel (04) 386 0508, Mobile (027) 294 1169

Mr Darren King, Environmental Scientist, NIWA National Climate Centre
Tel (09) 375 2086, Mobile (021) 399 637  

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. See our New Zealand Climate Update.
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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