NIWA Outlook:September-November 2014

NIWA Outlook: September-November 2014

Issued: 2 Sep. 2014

Overview

The equatorial Pacific Ocean remains ENSO-neutral at the end of August 2014. Despite the Southern Oscillation Index being currently negative, a fully coupled (ocean and atmosphere) event has yet to initiate.

International guidance indicates that the chance for El Niño developing over the September –November 2014 period is about 55%. This figure has decreased compared to forecasts issued in previous months, and all signs indicate that this event would – if it does indeed occur – fall in the weak to moderate category.

During September–November 2014, mean sea level pressures are expected to be lower than normal to the north as well as over most of the country. This pressure pattern is expected to be accompanied by generally anomalous easterly flow and perturbed conditions.

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

Outlook Summary

September-November temperatures are likely (40-45% chance) to be average or above average in all North Island regions, and also in the north of the South Island. Seasonal temperatures are most likely (45% chance) to be near average for the east and the west of the South Island. Cold snaps and frosts can still be expected in some parts of the country as we advance into spring.

September-November rainfall totals are about equally likely (40–45%) to be in the normal or above normal range in all North Island regions, and most likely (45% chance) to be normal in the north of the South Island. Seasonal rainfall is about equally likely (40–35%) to be in the near normal or above normal range in the east of the South Island, but about equally likely (40–35% chance) to be near normal or below normal in the west of the South Island.

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

Regional predictions for the September to November 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 equally likely (40-45% chance) to be average or above average. • Rainfall totals are about equally likely (40-45% chance) to be in the normal or above normal range.

• Soil moisture levels and river flows are about equally likely (35-40% chance) to be in the nearnormal or above normal range.

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 average or above average.

• Rainfall totals, soil moisture levels and river flows are all equally likely (40% chance) to be in the normal or above normal range.

Gisborne, Hawke’s Bay, Wairarapa

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

• Temperatures are about equally likely (40-45% chance) to be average or above average.

• Rainfall totals are equally likely (40% chance) to be in the normal or above normal range.

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

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 aboveaverage range.

• Rainfall totals, soil moisture levels and river flow are all most likely (40-45% chance) to be in the normal range.

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

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

Coastal Canterbury, east Otago

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 about equally likely (40-35% 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 (45% chance) to be in the below normal range.

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 (+C) at about 100 to 150 2om. depth in the central Pacific around 160o

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 1st 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.

ENDS

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