Seasonal Climate Outlook: February - April 2015

Overview

Sea surface temperatures across the equatorial Pacific Ocean are borderline between neutral and weak El Niño conditions.  However - as was the case over the past few months – the atmospheric circulation in the Pacifc is still inconsistent with El Niño.

International guidance indicates that the probability of El Niño conditions developing  over the next three months (February – April 2015) is about 60%.

During February – April 2015, higher pressures than normal are forecast over and to the southeast of the country, and slightly lower than normal pressures are likely to the north of New Zealand. This atmospheric pressure pattern is expected to be associated with weak easterly flow anomalies.

Sea Surface Temperatures around New Zealand are forecast to be in the above normal range all around the country.

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 during the present forecast period (February – April).

Outlook Summary

February – April 2015 temperatures are most likely (45-50% chance) to be in the above normal range for  all regions of New Zealand.

February – April 2015 rainfall is most likely (45-50% chance) to be in the near-normal range in the north of the North Island and the north and west of the South Island. Rainfall totals for the season as a whole are about equally likely (35-40% chance) to be near-normal or below normal in the east of the South Island. In the east and west of the North Island, the coming season’s rainfall is about equally likely (35-40% chance) to be near or above normal.  

February – April 2015 soil moisture levels are most likely (45% chance) to be below normal in the east of the South Island and about equally likely (35-40%) to be near-normal or below normal in all regions of the North Island and the west of the South Island. Soil moisture levels are most likely (40% chance) to be near normal in the north of the South Island. River flows are most likely (50% chance) to be below normal in the east of the South Island and about equally likely (35-40% chance) to be near or below normal in the north and west of the North Island and the west of the South Island. Near normal river flows are likely (40% chance) in the east of the North Island and the north of the South Island.

Regional predictions for the February to April 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 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

50

35

25

25

Near average

30

45

40

35

Below average

20

20

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

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

50

40

30

25

Near average

30

40

35

35

Below average

20

20

35

40

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

50

35

25

30

Near average

30

40

40

40

Below average

20

25

35

30

Nelson, Marlborough, Buller

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 and 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

50

30

30

30

Near average

30

45

40

40

Below average

20

25

30

30

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

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

45

20

20

20

Near average

35

50

40

40

Below average

20

30

40

40

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 above average range.
  • Rainfall totals are about equally likely (35-40% chance) to be in the near-normal or below normal range.
  • Soil moisture levels and river flows are most likely (45-50% chance) to be in the below normal range.

The full probability breakdown is:

 

Temperature

Rainfall

Soil moisture

River flows

Above average

50

25

20

15

Near average

30

40

35

35

Below average

20

35

45

50

Background

Sea-Surface-Temperatures in the Pacific Ocean remain warmer than normal all along the Equator, but are warmer in the west than the east (i.e. the anomaly gradient is opposite to what is typical during El Niño), and anomalies have eased off quite significantly compared to last month.

Sub-surface ocean temperature anomalies over +2°C persist at about 150m depth just east of the international Dateline and some weak positive anomalies (~+1°C) are present in the upper Ocean (~50m) off the South American coast. Ocean heat content anomalies (upper 300m of the Ocean) are currently slightly negative east of ~ 130°W and positive in the western part of the basin.

As in previous months, the atmospheric indicators of ENSO are inconsistent with each other and with a typical El Niño situation. The Southern Oscillation Index (SOI) value for January 2015 (preliminary estimate on the 30th of January) is -0.9, i.e. in the warm-neutral range, but rainfall and convection anomalies are again at odds with typical El Niño patterns, and indicate enhanced convective activity and precipitation over the western Pacific Ocean.

The international guidance suggests the chances of El Niño developing over the February to April 2015 period is about 60%, this is down compared to probabilities that were indicated in previous months. It is also worth noting that the current forecast period (February – April) is reaching into what is usually the decay phase of a typical El Niño event: an El Niño starting to develop during this season would be very unusual.

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 and northern areas and more rain in western areas of the country. The regional atmospheric circulation and rainfall outlook for February to April 2015 – as synthesized from various dynamical and statistical models – is not typical of El Niño conditions.

Waters surrounding New Zealand are warmer than average all around the country and are forecast to remain so over the February to April 2015 period.

To find out more about normal conditions for this outlook period, refer to this web page, where daily updates on climate maps are available. As of 30 January, severely to extremely drier than normal soils exist for most of the South Island along and east of the Divide, including central and eastern Southland.  For the North Island, severely to extremely drier than normal soils continue for the vast majority of the island.  Exceptions include south-coastal Gisborne, parts of Hawke’s Bay, isolated pockets of the Bay of Plenty and Waikato regions as well as along the western slopes of the central and northern Tararuas. Even where normal to above-normal rainfall conditions are predicted over the next three months, these dry conditions are likely to persist because of the high evaporation rate over the summer months; note also that temperatures are forecast to be above normal in average over the forecast period for all regions of New Zealand.

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.

Graphical representation of the regional probabilities. [NIWA]