Seasonal Climate Outlook: August - October 2014

31 July 2014

Overview

The equatorial Pacific Ocean continues to remain ENSO-neutral at the end of July 2014, with atmospheric and oceanic conditions failing to sufficiently couple to initiate an El Niño event. Other El Niño indicators have also weakened in the last month. Thus, the chances of an El Niño event developing over spring appear to be lessening.

Seasonal Climate Outlook: August - October 2014 PDF file.

International guidance still indicates that El Niño is the most likely outcome (about 70% chance) over the coming three seasons through to the end of summer 2015. However, it should be recognised that this guidance is based on model simulations from end-of-June conditions so does not take account of the rapid changes observed in July. The behaviour of the atmosphere over the next month or two will be critical to whether an El Niño event initiates or not.

During August–October 2014, mean sea level pressures are expected to be lower than normal to the northeast of New Zealand, with higher pressures than normal to the southeast of the country. This pressure pattern is expected to be accompanied by generally anomalous flow from the easterly quarter.

Sea surface temperatures for the coming three months are expected to be near average off the west coast of New Zealand and above average to the east.

Outlook Summary

August-October temperatures are most likely (50% chance) to be above average for the east of the North Island, and likely (40-45%) to be average or above average for all remaining regions of New Zealand. Cold snaps and frosts can still be expected in some parts of the country as winter advances into spring.

August-October rainfall is equally likely (40% chance) to be normal or above normal in the north and east of the North Island, and normal or below normal in the west of the North Island and in the north of the South Island. In remaining South Island regions, seasonal rainfall is most likely (45%) to be in the near-normal range.

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

Regional predictions for the August to October 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 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 (40-35% chance) to be in the near normal or above normal range.

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

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