October brought less than 10mm of rain to the already dry Canterbury and Marlborough, making it one of the driest Octobers on record for these regions. Most of the North Island also observed lower than normal rainfall as well as Nelson. But the southwest of the South Island had a wet month.
Temperatures were warm for inland and eastern parts of the South Island and close to average for the western coastline of both Islands. Elsewhere temperatures for October were mild.
The Ocean
Over the last few months, the 2015 event has ranked as the strongest El Nino seen since 1997-98. The 30-day Southern Oscillation Index (SOI) remains strongly negative around -2.0, while the latest weekly NINO3.4 index was steady at 2.5C.
There has been discussion as to whether this is the strongest event since 1950. Metservice says this is only true when examining a single index (the NINO4 index, which represents sea temperatures along the equator, in the western Pacific region 160E to 150W), and only if comparing September 2015 values to September values of other events. It is more important to understand that the peak seen in this El Nino so far has not yet reached the peak observed in 1997-98.
Looking ahead, computer models remain very consistent. They peak this event around December, and then weaken it through the first quarter of 2016. It may well be that there is a return to neutral conditions by the end of autumn next year.
In the Australasian region, there is also another climate player in the mix – the Indian Ocean Dipole. This reinforces the El Nino recipe of ‘drier and hotter’ for eastern Australia in the lead up until Christmas. And for New Zealand, it can lead to more highs over the North Island and northwesterly winds over the South Island a little later on, during the summer. This is something MetService will continue to monitor, as this may exacerbate summer dryness for eastern areas.
Seas around New Zealand continue to be cooler than usual, particularly to the east of the country.
The Outlook
Expect a shifty November, with large swings in both temperature and rainfall across the month. The month starts relatively warm and wet under northwesterly conditions, with frequent Lows around New Zealand. However, southwesterlies return in the second week of the month, and likely prevail over the country for the remainder of November.
Temperatures rapidly swing from above average at the start of November, to below average in the second week of the month. The second half of November looks unusually cold for the time of year.
With regards to rainfall, all regions run the best chance of significant rain in the gauge in the first week of November. For the remainder of the month, rainfall then looks normal to below normal across the country.
Bottom line: November starts unusually warm, but ends unseasonably cold, for all regions. Low pressures and a good chance of some useful rain are forecast in the first week of the month, followed by a prevailing (somewhat drier) southwest regime. Monthly rainfall totals are predicted to be normal to below normal in the east of the South Island, above normal between New Plymouth and Wellington, and near normal elsewhere.
