Summer heat and the continuing influence of the La Niña weather pattern have continued to linger well past autumn’s halfway point, with farmers and growers in mixed minds on what it will mean heading into winter.
Weatherwatch director and forecaster Philip Duncan said New Zealand continues to experience sea surface temperatures well above normal for the time of year, while waters nearer the equator, where La Niña conditions are more apparent, are returning to normal.
Bay of Plenty water temperatures, for example, continue to hit 20degC, among the highest recorded for this time of year and well above the May average of 17.7deg C.
Niwa climate scientists have forecast NZ could be on track to experience a rare trifecta year for La Niña’s persistence, with a 60% likelihood predicted the conditions would continue from now to July and 50:50 from there to the end of the year. If so, that would make it three in a row for La Niña’s influence.
But Duncan said it was important not to get too far ahead in looking at La Niña implications, particularly when current patterns were indicating NZ was being subject to some peculiarities in shorter-term weather patterns.
“We have this marine heatwave here which is quite localised to NZ waters, and then we have these massive high pressure systems really influencing both water temperatures and our rainmaking potential, some of these systems are the size of Australia,” Duncan said.
Australia’s meteorologists said the La Niña system continues to fade, possibly reaching a neutral state by winter.
The pattern of large, dominating blocking high pressure systems has been a recurrent one for the past three years, with nor ’easterly La Niña-type rainmakers ‘bouncing’ off the edge of the dominant high pressure systems and missing NZ entirely.
“Fortunately, this year we have managed to get some of those systems slip through to bring some welcome rain, but some areas like Waikato and Southland are having an Indian type summer, with the risk of a ‘green’ drought,” he said.
Duncan likened La Niña to a dial of degrees rather than a switch, and NZ’s experience with it has only touched the “moderate” point on that dial this year.
“And we are only on the edge of the tropics where most of its impact will be felt,” he said.
He said in contrast La Niña has proven to be a ‘switch’ effect in Australia, where rainfall levels down the continent’s east coast have been consistently higher and more reliable than NZ’s for the past two years.
The Australia Bureau of Meteorology’s landscape water balance index paints a 100% root zone soil moisture level for almost the entire east coast from Cape York down and past Melbourne.
Niwa’s soil moisture deficit indicator highlights soils in much of Waikato, Whanganui and Otago all experiencing significant moisture deficits.
Duncan said the lack of snow in the usual catchments so close to the start of winter was also a concern.
Looking through to winter he anticipated the lower South Island may still continue drier than average.
In the meantime, the upper North Island was entering its fifth consecutive farming year with below normal rainfall. Longer-term weather modelling done by WeatherWatch and IBM has NZ continuing to trend drier through May.
To the end of April, Waikato had only had 50% of its average year to date rainfall.
