Current Climate - September 2012
Spring westerlies arrived during the first few days of September, and prevailed during the first half of the month. These stormy westerly quarter winds produced very wet conditions on the West Coast of the South Island, but in contrast, a rather dry month in eastern areas of both islands. From mid-month, a pattern change saw more anticyclones than usual lie over New Zealand and to the east of the country. This combination of patterns resulted in more northwest winds than normal over New Zealand for the month of September, overall.
The frequent west to northwest winds of September produced a very wet September for western regions of the South Island, as well as for Nelson, much of Southland, and Central Otago, with totals exceeding one and a half times September normal rainfall in these regions. Above normal rainfall was also observed in eastern parts of Northland. In contrast, it was a rather dry September in eastern areas of both Islands, illustrating the lee effect of westerly winds. In particular, rainfall was less than half of September normal for parts of Gisborne, Hawkes Bay and Canterbury. Below normal rainfall was also experienced between Wanganui and Wellington. Across the remainder of the country, rainfall was generally near normal.
At the end of September, soils were slightly drier than is usual for the time of year for Taranaki, Waikato, Central Plateau, and parts of the Bay of Plenty, Gisborne, Hawkes Bay, and coastal Wairarapa. Soil moisture levels in Dunedin and across south Canterbury remain above normal for the time of year. Elsewhere, soil moisture levels were generally close to normal.
It was a mild September over the lower South Island, as well as for north Canterbury, Coromandel, the Hauraki Plains, and the Rodney District, with temperatures between 0.5°C and 1.2°C above the September average. Elsewhere, temperatures were generally near average. The nation-wide average temperature in September 2012 was 10.8°C (0.4°C above the 1971-2000 September average), using NIWA’s seven-station temperature series which begins in 1909. Notably, an unusually cold southerly air stream for the time of year affected New Zealand between 11 and 13 September, with record or near-record low September temperatures observed at many locations. In stark contrast, the last two days of September were extremely warm.
It was an extremely sunny September for eastern areas of both Islands (with sunshine totals typically exceeding 125 percent of September normal), reflecting the high frequency of westerly winds during the month. In contrast, it was a rather dull September for Westland. Elsewhere, sunshine totals were generally close to normal.