September situation:
September started out stormy, with low pressures bringing significant rain to western regions. Unusually cold southwesterlies prevailed across the middle of the month, before highs parked over southern New Zealand for the rest of September. This produced an extended dry and sunny period for the west and south of South Island, while easterly winds delivered significant rain to the east.
It was the fifth wettest September for Gisborne Airport (183mm, records began in 1905) and the wettest September for Napier Airport (171mm, records since 1950). Wairarapa, eastern Marlborough and north Canterbury also picked up useful easterly rain. Masterton and the Kaikoura Coast totalled about 130mm, Cheviot recorded 80mm, while Culverden clocked up around 30mm.
The ocean situation:
Since our last update, El Nino conditions have reached very strong levels in the tropical Pacific Ocean. It is the strongest El Nino seen since 1997-98. The 30-day Southern Oscillation Index (SOI) remains strongly negative at -1.8, while the NINO3 index sits at 2.6C. The atmosphere and the ocean are reinforcing one another, meaning that this El Nino is locked in and is likely to persist through into autumn 2016. The latest forecasts predict that this El Niño will strengthen further between now and Christmas, peaking around December, and should then ease during autumn.
Importantly, it is by no means certain that it will reach the ‘super El Nino’ levels seen in 1997-98 and 1982-83. Even if it does, there is no guarantee that we will see ‘super’ (extra strong) impacts here in New Zealand.
Seas around New Zealand continue colder than usual, particularly to the east.
Figure 2: Regions which show a large increase in the chances of a dry summer under El Nino conditions (highlighted in orange).
The Outlook
The highs that have recently been blocking over southern New Zealand are expected to shift to their more typical spring position (mid Tasman Sea and extending onto the North Island) during October. More westerly winds than usual are forecast across the country during the month.
Eastern regions should expect spells of above average temperatures due to foehn warming in the westerlies. Overall, average to above average temperatures are forecast for October in the east of both Islands. In comparison, temperatures are likely to be below average in western areas.
A drier than normal October is predicted across the entire North Island, due to a mix of ridging and westerlies (which don’t yield overly much rain). October rainfall for the northern and eastern South Island is forecast to be below normal, too. For the west and south of the South Island, October rainfall is likely to be in the near normal range.
The Bottom Line
Spring westerlies kick-start in October. Average to above average temperatures are expected for eastern areas, with below average temperatures elsewhere. A drier than usual October is likely for the entire North Island, and as well for the north and east of the South Island. Near normal October rainfall is predicted for the west and south of the South Island
