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WEATHERWISE THE NEUTRAL WINTER OF VARIABILITY BY DANIEL CORBETT, METEOROLOGIST T his past winter season has had a good mix of weather types ranging from a brief polar blast in early June, to blocked southwesters, anticyclones and stuck Tasman lows. The winter may not have felt as cold for some people as the previous winter and perhaps appeared a bit back to front, with a polar blast and snow at the start of the season, then a milder weather later on. The good mix of weather systems is very typical of the passing phase in the ENSO cycle of Neutral. In other words, moving away from La Nina of the previous summer but not quite reaching the other extreme of El Nino. In the neutral phase there are other factors that can influence the makeup of the weather for the season. These include such things as sea temperature anomalies, SAM (Southern Annular Mode) and winter blocking. The other interesting thing about the winter of 2012 was the source region for the air that moved across New Zealand at various times during the winter. The source region is important because it influences how cold or even how warm the air feels in the various weather systems. In the first part of winter the source region was the southern ocean. During the second half of the winter it was more so from the milder Tasman Sea. This year the coldest part of the winter occurred in the first part of the season. The Southern Ocean opened up for business and began to feed polar chilled air towards New Zealand. Some of this air collided with a moist Tasman Sea low which helped to bring significant snow to parts of the South Island on June 6. Christchurch city picked up 15cm of snow and some of the higher elevations received 50-100cm by the time the storm system had departed. In mid-June the southern ocean was still breathing its cold icy breath, and continued to be the main source region for air across New Zealand. It was colder than normal then, with daytime highs barely above eight degrees celsius in some parts of central and Southern New Zealand. A blocked (stuck) weather pattern kept a bone chilling southerly flow in place with wintry showers peppering southern and eastern coasts for over a week in mid-June. July 29, 2012 – interesting weather patterns The blocked weather and stubborn lows. In late July the upper wind flow changed to a more blocking regime, causing many of the approaching lows from the Tasman Sea to get stuck and linger. This led to excessive rainfall and flooding in places, particularly along northern and eastern areas of New Zealand. And the low that would not go away in late July A stubborn low spent nearly a week spinning off the west coast of the North Island in the last days of July 2012. The weather system began as an active low that moved in from the Tasman Sea. The rain spread across the north of the North Island during Sunday July 30, and by the Monday morning over 100mm had already fallen in the ranges of Coromandel with 115mm at Kopu. Several days later the low was still in place providing wet weather to much of New Zealand. Finally, the blocking eased in mid-August and a more typical winter westerly flow returned with a procession of rain bearing fronts interspersed with anticyclones, from the milder Tasman Sea. What is to come for Late Spring/Summer? Westerly flows will be a major feature over the next several weeks, bringing spells of rain with active troughs. As we head further into the Spring season don't be surprised to see some variability in our weather patterns especially once the weak El Nino has settled into the driver's seat for the ride towards summer. A weak signal El Nino could also mean that other factors may play some part in our weather patterns for late spring/summer. June 6, 2012 – nasty stuff, batten down the hatches 56 Professional Skipper November/December 2012 Settled periods, with clear sunny days and frosty or foggy mornings associated with passing anticyclones, are expected. Fronts and troughs rolling in from the Tasman Sea may be followed by episodes of cool southwest winds across the whole country lasting several days. Occasionally a low pressure centre may move onto the country from the north, preceded by an easterly flow with some heavy rain for north-eastern areas. Keep an eye out for a blocking pattern over the next several weeks where the weather patterns get stuck and anomalies can sometimes occur as a result.