Seasonal Climate Outlook: August - October 2016

1 August 2016

Overview

ENSO (El Niño – Southern Oscillation) neutral conditions are currently present in the tropical Pacific: Sea surface temperatures (SSTs) along the eastern equatorial Pacific are near or slightly below normal, and the atmospheric conditions over the tropical Pacific are generally consistent with an ENSO-neutral state. As a whole the tropical ocean-atmosphere system still shows a leaning towards La Niña, but with a slight weakening of the signals that were observed last month (June 2016).

The Southern Oscillation Index (SOI) is currently slightly positive but well within the neutral range. Slightly enhanced trade-winds are present in the western and central Pacific west of about 150^oW. The subsurface ocean remains cooler than normal in the central and eastern Pacific, however these anomalies have weakened compared to last month.

International guidance still favours development of La Niña conditions (50% chance) over the next three month period (August – October 2016), but note that this probability is lower than this time last month. The likelihood of La Niña conditions becoming established in the Pacific increases slightly later on, reaching 55% in November – January 2016/2017. In summary, both the current state and recent evolution of the ocean-atmosphere system in the Pacific, as well as the models’ forecasts, suggest that this La Niña event, if it develops, will be characterized by a relatively late onset, short duration and weak amplitude.

For August - October 2016, there is no clear guidance or indication on seasonal airflow anomaly. However, weak anomalously low pressures are forecast around New Zealand and are likely to be accompanied with unsettled conditions. Anomalously high ocean temperatures around the country mean warmer and more humid air masses are likely to affect New Zealand, especially the North Island. Consequently, there remains an elevated risk for significant rainfall events and severe storms.

Seasonal Climate Outlook Video

In the video below NIWA Forecaster Chris Brandolino describes what New Zealanders can expect, in broad terms, in the way of rainfall and temperature over the August-October 2016 period:

Summary

August – October 2016 temperatures are very likely (65-70% chance) to be above average in all regions of the country. Nevertheless, frosts and cold snaps will occur from time to time in cooler locations. Sea surface temperatures are forecast to remain above normal over the next three months, especially to the west of New Zealand.

August – October 2016 rainfall totals are about equally likely to be in the near normal range (40% chance) or above normal (35-40% chance) range in the north and west of the North Island and the west of the South Island. Seasonal rainfall is most likely (40% chance) to be in the near normal range in the east of the North Island and the north of the South Island. In the east of the South Island, rainfall for the August – October 2016 period is about equally likely to be near normal (40% chance) or below normal (35% chance).

August - October 2016 soil moisture levels and river flows are most likely (45% chance) to be below normal in the east of the South Island. In the north of the North Island, soil moisture levels and river flows are about equally likely to be in the near normal range (35-40% chance) or above normal range (35% chance). In the west of the North Island, soil moisture levels are about equally likely to be near normal (40% chance) or above normal (35% chance), and river flows are most likely to be in the near normal range (40% chance). Seasonal soil moisture levels and river flows are about equally likely to be near normal (35-40% chance) or below normal (35% chance) in the east of the North Island and the north of the South Island. In the west of the South Island, soil moisture levels are most likely to be near normal (40% chance) while river flows are about equally likely to be near normal (40% chance) or above normal (35% chance).

Regional predictions for the August – October 2016 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 very likely to be above average (70% chance).
Rainfall totals are equally likely (40% chance) to be above normal or near normal.
Soil moisture levels are equally likely (35% chance) to be in the near normal or above normal range.
River flows are about equally likely to be near normal (40% chance) or above normal (35% chance).