Total rainfall across New Zealand this month has varied widely, such as 483 percent of normal in Central Otago, and zero percent in the lower North Island. Photo / Michael Craig
January may fall as one of the oddest months in the weather book, with a picture of feast-or-famine rainfall saturating the south – and the northern tip again in severe drought.
Total rainfall across New Zealand this month varied as much as 483 percent from normal, in Central Otago, to zero percent in the lower North Island.
“Obviously it’s unusual to see this spatial pattern, which looks so random,” said astrologer Niwa Ben Noll.
The regional contrast can be seen clearly on the Niwa rainfall map for most of the month to date, showing patches of green, or rainfall much higher than average, and orange, indicating much drier places.
Among the wetest places are northern, central and southern Otago – receiving 335, 483 and 202 percent of normal rain for January – along with southern Northland, northern Waikato and the Bay of Plenty, which have received about 150 percent of their usual total.
It is a very different picture for the lower North Island and the upper South Island, with totals of zero and 25 percent, respectively.
Total numbers were also significantly less around the East Cape on the North Island (62 percent), Hawke’s Bay (58 percent), northern Canterbury (51 percent) and Christchurch (33 percent).
“This is really very high rainfall,” he said.
“Another interesting observation is that we saw another major flood event this month, making it the third in as many months.”
Noll said the three major floods that hit Napier in November, Plimmerton in December and central Otago this month could be attributed to the presence of eccentric climate drivers.
The La Nina climate system with moderate strength has a clear influence on our summer weather – but this year, it is behaving very out of line with traditional patterns.
Under classic La Nina conditions, northern and eastern New Zealand will be wetter now, given its tradition of bringing storms and rain from the northeast to those places, and drought in the south and southwest.
In contrast, the northern region is currently abnormally dry – the tip of the North Island is now classified as severe meteorological drought – while the fire hazard in the south and southwest ranges from low to moderate.
Added to this unusual image is a “destructive disturbance” to La Nina’s classic taste of a separate natural phenomenon.
It is the Madden-Julian oscillation (MJO) – a system that forms the greatest element of intra-seasonal variability in the tropical atmosphere.
“So we’ve seen until January, La Nina and other coercive patterns like the MJO are basically pedaling in the opposite direction,” he said.
“It makes it difficult to say which will dominate our weather patterns – and what remains is mixed conditions across the country.
“Trying to summarize that picture can be very difficult, so, put simply, be nice to your local weather forecaster.”
Noll said there was potential for summer to take another interesting turn, with a possible spike in tropical cyclone activity in the southwest Pacific in late January and early February.
“And while there are increasing opportunities for activity, where it is happening over the wider region is the million dollar question,” he said.
“The whole picture here is we need to be alert, and maybe even alert, for such activity to start here after a slight lull in mid-summer.”
Niwa’s forecast for January to March estimates temperatures are likely to be warmer than normal in all regions.
They will be punctuated with brief but very unstable weather periods, with rainfall likely almost above normal everywhere except in the western part of the South Island.
More swelling is also taking place, with spells of high humidity expected over time – especially in the north.