August 2013 was characterised by mean sea level pressure (MSLP) which was much lower than normal across all of New Zealand, with strongly negative anomalies to the south of Australia. In contrast, MSLP anomalies were well above normal to the east of New Zealand. These regional pressure patterns resulted in more frequent northerly and north-easterly flows across the country, resulting in high mean temperatures for the time of year across much of the country.
New Zealand Climate Update 171 – September 2013
What happened in August, how our climate outlook for the previous three months turned out, global and local sea temperatures, and our outlook for September to November.
The equatorial Pacific Ocean as a whole remains in a neutral state (neither El Niño nor La Niña), although colder than normal (La Niña-like) sea temperatures persist in a shallow surface layer in the eastern equatorial Pacific. International guidance indicates that ENSO-neutral is the most likely outcome for the next three months (September–November). For the New Zealand region, higher pressures than normal are forecast south of the country, while lower pressures than normal are expected from Queensland eastwards. This circulation pattern is expected to produce more northerlies or north-westerlies affecting the North Island, and a weaker than normal westerly flow over the South Island.
Rainfall for the September–November period as a whole is likely to be normal or below normal in the east of the North Island and west of the South Island, near normal in the east of the South Island, but normal or above normal for other regions.
Temperatures over September to November period as a whole are likely to be above average in the east of the North Island and the north of the South Island, and likely to be average or above average in other regions. However, frosts and snow conditions may occur in some areas from time to time in early spring.
Soil moisture levels are likely to be normal or below normal in the east of the North Island, near normal in the north and west of the South Island, and normal or above normal in other regions. River flows are likely to be normal or below normal in the east of the North Island, normal or above normal in the north of the North Island, and near normal in all other regions.
For winter (June–August) 2013 higher than normal pressures were forecast for the south and southeast of the country, with lower than normal pressures are expected to the west and north of New Zealand. This circulation pattern was expected to be associated with more northerly and north-easterly airflow than usual. Weak MSLP's dominated over New Zealand during winter with much higher than normal to the east and south-east of the country, and lower than normal to the north, west and south of New Zealand.