Seasonal Climate Outlook: December 2014 -February 2015

Overview

In November 2014, the equatorial Pacific Ocean warmed significantly, reaching El Niño levels towards the end of the month. Some – but importantly not all – atmospheric indicators also show patterns consistent with the development of a weak El Niño.

International guidance indicates that the chance for El Niño developing over the December 2014 – February 2015 period is about 75%. This figure has increased compared to forecasts issued last month.

During December 2014 – February 2015, lower pressures than normal are forecast over the New-Zealand region. This pressure pattern is expected to produce a weak southwesterly flow anomaly with perturbed conditions from time to time.

Sea surface temperatures for the coming three months are expected to be near average around the coasts of New Zealand.

New Zealand has a slightly elevated chance of having an ex-tropical system coming within 550km of the country during the 2014 - 2015 Tropical Cyclone season. The tropical cyclone outlook indicates this risk will be highest between February and April.

Outlook Summary

December 2014 – February 2015 temperatures are most likely (45-50% chance) to be average in all regions but for the east of the North Island, where seasonal temperatures are about equally likely (40-35% chance) to be near average or above average.

December 2014 – February 2015 rainfall totals are most likely (40-50% chance) to be in the near normal range for all regions.

Soil moisture and river flow are about equally likely (40-45% chance) to be in the near normal or below normal range in the north and east of the North Island and the north of the South Island. Soil moisture levels and river flows are most likely (45-50% chance) to be in the near normal range in all remaining regions of the country.

Regional predictions for the December to February 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 near average.
• Rainfall totals are most likely (45% chance) to be in the normal range.
• Soil moisture levels and river flows are about equally likely (40-45% 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	35	25	20	15
Near average	45	45	40	40
Below average	20	30	40	45

 

 

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

The full probability breakdown is:

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

 

Gisborne, Hawke’s Bay, Wairarapa

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

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

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	35	20	15	15
Near average	40	45	45	45
Below average	25	35	40	40

 

Nelson, Marlborough, Buller

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

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

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	30	30	20	15
Near average	45	40	40	45
Below average	25	30	40	40

 

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 (50% chance) to be in the near average range.
• Rainfall totals, soil moisture levels and river flows are all 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	30	30	25	25
Near average	50	45	50	50
Below average	20	25	25	25

 

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 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	30	25	20	20
Near average	50	50	45	45
Below average	20	25	35	35

Background

Sea-Surface-Temperatures in the equatorial Pacific Ocean steadily increased through November 2014, and have now crossed the conventional El Niño threshold of 0.7 ^oC above normal.  It remains to be seen whether SST’s will persist at these levels. The subsurface ocean is also currently warmer than normal (+ 4 ^oC) at about 100 to 150 m depth in the eastern Pacific around 120^oW.

The preliminary NIWA Southern Oscillation Index (SOI) for November 2014 is –0.7. This brings the  3-months September-October-November value to -0.8. Strongly negative SOI values (less than -1) are typically associated with El Niño.

Despite oceanic anomalies showing signs of a weak El Niño,  the atmosphere has not yet fully shifted to El Niño conditions. In particular, patterns of convection and rainfall are lower than expected in the central Pacific.

The international guidance places the chances of El Niño developing over the December 2014 to February 2015 period at about 75%, a sharp increase from outlooks issued last month.

Note that for New Zealand, El Niño events are typically (but not always) associated with stronger and/or more frequent westerly winds. Such a climate pattern typically leads to drier conditions in eastern areas and more rain in western areas of the country. The rainfall outlook - as synthesized from various dynamical and statistical models - indicates that near normal rainfall is the most likely outlook for all regions of New Zealand, although a significant chance  for below normal rainfall (35%) is present for the east of the North Island.

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. Ocean models forecasts indicate that SSTs are likely to be close to normal around the country over the next three months.

To find out more about normal conditions for this outlook period, refer to NIWA’s website, where daily updates on climate maps are available.

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.