Seasonal Climate Outlook: April - June 2015

Sea surface temperature anomalies in the tropical Pacific intensified significantly around the international Dateline during March 2015 and are currently showing a pattern consistent with weak El Niño conditions. Atmospheric patterns across the region were also generally consistent with weak El Niño anomalies.

International guidance indicates that the probability for conventional El Niño thresholds being crossed over the next three months (April – June 2015) is about 60%.

During April - June 2015, lower than normal mean sea level pressures are expected to the east and northeast of New Zealand with positive mean sea level pressure anomalies west of New Zealand.  This circulation pattern is likely to produce anomalous westerly quarter wind flows.

Coastal waters are forecast to be in the near average or above average temperature range for the April – June 2015 period.

Outlook Summary

April – June 2015 temperatures are most likely (50% chance) to be in the above average range in the east and north of the North Island. For all remaining regions of New Zealand temperatures are about equally likely (35-45%) to be in the near average or above average range. As autumn progresses, cold snaps and frosts can still be expected from time to time in some parts of the country.

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

April – June 2015 soil moisture levels and river flows are most likely (55-60% chance) to be below normal in the east of the South Island. Soil moisture levels and river flows are most likely (45% chance) to be in the near normal range in the west of the North Island and west of the South Island. In the north and east of the North Island, soil moisture levels are about equally likely (40-45% chance) to be in the near normal or below normal range, while river flows are most likely (45-50% chance) to be in the below normal range. Soil moisture levels and river flows are equally likely (40% chance) to be in the near normal or below normal range for the north of the South Island.

Regional predictions for the April to June 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 about 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 are about equally likely (40% chance) to be in the near normal or below normal range
• River flows are most likely (45% chance) to be in the below normal range.

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

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

Central North Island, Taranaki, Wanganui, Manawatu, Wellington

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 near average or 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 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	45	35	30	25
Near average	40	40	45	45
Below average	15	25	25	30

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 below normal range.
• Soil moisture levels are about equally likely (40-45% chance)  to be in the near normal or below normal range
• River flows are most likely (50% chance) to be in below normal range.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	20	15	15
Near average	40	40	40	35
Below average	10	40	45	50

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

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 near average or 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 (55-60% chance) to be in the below normal range.

The full probability breakdown is:

	Temperature	Rainfall	Soil moisture	River flows
Above average	45	25	10	15
Near average	40	35	30	30
Below average	15	40	60	55

 

Graphical representation of the regional probabilities

Background

Sea surface temperature anomalies in the tropical Pacific intensified significantly around the international Dateline during March 2015 and are currently showing a pattern consistent with weak ‘central Pacific’ or ‘Modoki’ El Niño conditions[1]. Atmospheric patterns were also consistent with weak El Niño conditions, with a negative Southern Oscillation Index value for March of -1.1 (preliminary value estimated the 31^st of March) and strong positive convective anomalies around the international Dateline, signalling that the atmosphere has weakly coupled with the Ocean.

Monthly SST anomalies are currently 0.55°C above normal in the NINO3.4 region (values updated to 29 March). The NINO3 monthly value is +0.26°C and the NINO4 (in the west-central Pacific) monthly value is +1.05°C.

Sub-surface ocean temperature anomalies in the Central Pacific (around and east of the International Dateline) at about 150m depth have increased significantly to reach up to +5°C. Positive anomalies (~+3°C) have also propagated further eastward in the upper Ocean (~50m) off the South American coast. Positive upper ocean heat content anomalies (upper 300m of the Ocean) have also intensified just east of the international Dateline and now reach above +2°C.

The latest Tropical Atmosphere Ocean (TAO) array observations show strong westerly wind anomalies along the equatorial Pacific west of the International Dateline. The NASA ENSO (El Niño Southern Oscillation) Precipitation Index (ESPI) for the 30 days to 30 March was +0.86 (i.e. on the El Niño side of Neutral).

International guidance indicates that the chances for conventional El Niño thresholds to be crossed over the April – June 2015 season are about 60%. This probability increases later on to reach about 70% during the winter (July – September 2015).

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.

Waters surrounding New Zealand are slightly warmer than average around the country (the preliminary value is +0.4^oC above normal) and are forecast to remain in the above normal range especially along the west coast of the country for the March – May 2015 period.

To find out more about normal conditions for this outlook period, refer to NIWA’s website.