Q&A: Where did the wind come from?

We asked NIWA's weather gurus the questions on everyone's lips this spring: what's with the wind and droughts?

Was there a lot of wind over spring? 

Yes, there was a lot of wind in October and above average in September. Note that spring officially includes November, which hadn't ended at time of writing.

Octobers are generally windier in New Zealand than other months, but this October was particularly windy. The airflow was dominated by anomalous westerly pressure gradients. Several vigorous and damaging storms affected much of the country.

The main centres all experienced more days than usual where the wind was over 60km/h. In Auckland, 23 per cent of the days in the month experienced winds over 60km/h (usually 18 per cent). In Wellington, 65 per cent of the days in the month experienced winds over 60km/h (usually 54 per cent). In Christchurch, it was 26 per cent (usually 18 per cent).

Across central New Zealand, October had eight days where the winds averaged over 60km/h. Typically, the region experiences just over five days where the winds average over 60km/h. Over the past 50 years there have only been six Octobers that had more windy days.

September was similar – six days of wind over 60km/h – but not as prominent in the records as October. The highest wind gust was 165km/h, recorded at Cape Turnagain on 11 September.

But it was the wind events of October that may stay longest in people's minds. On 8 and 9 October, wind in Devonport flattened a number of trees, lifted roofs and knocked out electricity. Strong southerly winds in Wellington resulted in the cancellation of flights and the harbour ferry.

The worst day of the month was 14 October. The highest wind gust of the month was recorded this day: 167km/h, at Mount Kaukau (Wellington). Damaging winds were experienced throughout New Zealand. In Wellington, at least 40 flights were cancelled, whilst power was cut to hundreds of homes and uprooted trees were reported. Further north, trees falling on power lines caused power outages in Bulls, Kimbolton and parts of Dannevirke. In Canterbury, strong winds brought down trees, with trees falling on power lines causing a loss of power for more than 2000 homes in areas including Geraldine and Temuka. In Marlborough, downed trees were reported in Blenheim and caused traffic delays on SH6 near Long Valley Road. Downed trees on power lines and lifting roofs were reported throughout the Nelson region.

The wind moved northward. On 15 October, strong winds in Auckland cancelled flights, brought down power lines in South Auckland and caused minor damage to numerous buildings across the city.

The winds returned from 24 to 26 October. In Wellington, they forced the diversion and cancellation of flights and the harbour ferry. In Canterbury strong winds knocked trees into power lines, leaving a number of communities temporarily without electricity.

Was it a warm spring?

Yes, especially for those living in the North Island. Although it was wetter than usual for many parts of New Zealand, it was also an unusually warm month for eastern parts of the North and South Islands. It was very dry in the north and east of the North Island.

The nationwide average temperature in October was 12.9°C (0.8°C above the 1971–2000 October average from NIWA's seven-station temperature series, which begins in 1909). Well above average temperatures (more than 1.2°C above the October average) were recorded in parts of Hawke's Bay and north Canterbury near Kaikoura.

Near average temperatures (within 0.5°C of the October average) were recorded in western and southern parts of Southland  and Otago, the Canterbury High Country, Westland, northwest Tasman, northern Taranaki, and northern parts of Waikato,  Auckland and Northland. Above average temperatures (0.5–1.2°C above the October average) occurred throughout most  remaining areas of New Zealand. 

It was also a month of sunshine. It was a very sunny month natural resources for Northland, Gisborne, Hawke's Bay and  mid-Canterbury (well above normal sunshine totals; more than 125 per cent of the October normal). Above normal sunshine (110–125 per cent of normal October sunshine) occurred throughout central parts of the North Island, Bay of Plenty and northern Auckland.

Below normal sunshine (75–90 per cent of normal October sunshine) was recorded for the northwest and southwest of the South Island. 

Are we in for another drought this summer?

At the moment it doesn't look like it. The prediction is for weather conditions to be near normal. But note that last year's drought conditions really didn't start appearing until November.

Last year's drought was caused by a slow moving high pressure system that blocked other weather systems from approaching the country. That was somewhat due to the equatorial Pacific Ocean being in a neutral state (neither El Niño nor La Niña).

That neutrality continues this year, but this time the seasurface temperatures are slightly warmer than normal west of the Dateline. This is likely to persist for the next three months (November–January) and into autumn 2014.

So over the coming three months, lower pressures than normal are forecast in the Tasman Sea and across the North Island, and higher pressures than normal are expected to the south of the country. This circulation pattern is expected to produce disturbed northerly quarter flows over the north of the country, and slightly enhanced easterly flows over the South Island.

Temperatures over the November–January period are likely (40 per cent chance) to be near average or above average in all regions of New Zealand, except for the west of the North Island and east of the South Island, where near average temperatures are the most likely outcome (45 per cent chance). The chance of below normal temperatures across all regions is about 20 per cent.

Rainfall totals over the November–January period as a whole are most likely (45–50 per cent chance) to be near normal in all regions except for the north and east of the North Island, where rainfall is equally likely (40 per cent chance) to be near normal or above normal.

Soil moisture levels and river flows are most likely (45 per cent chance) to be below normal in the north of the North Island, and most likely (50 per cent chance) to be above normal in the east of the North Island, and likely to be near normal for all  remaining regions. 

October winds. [NIWA]