July to be 30th month of heat streak
New Zealand is about to mark its 30th straight month of above-average temperatures — something a meteorologist has partly put down to climate change’s “tail wind”.
Niwa also expected this month to be among the warmest Julys the country has experienced, in a year that’s so far also been a near record-topper in the temperature stakes.
The 30-month run, in which each month had finished above respective mean temperatures for the 1981-2010 period, included some of the most dramatic climate events ever observed in New Zealand.
Among them: our hottest summer (2017-18), our second hottest year (2018), our hottest month (January 2018) and two marine heatwaves — one which would likely be considered freakish amid 2050 conditions.
The combined effect could be seen in the Southern Alps’ snow-starved glaciers, which one scientist recently described as “sad and dirty” after another major melt.
Niwa meteorologist Ben Noll cited a mix of climate factors for the long, dry run. “It’s been like a novel made up of a whole lot of novellas and short stories.”
Much of it had to do with what had unfolded out in the equatorial Pacific Ocean, which brewed natural systems that accounted for around a quarter of what Noll called New Zealand’s “climate driver pie”.
Over the period, there hadn’t been a traditional El Nino event, which traditionally formed in the eastern equatorial Pacific.
This usually brought prevailing colder southerly winds in winter, and stronger and more frequent southwesterlies in spring and autumn, bringing a combination of summer and winter-like effects.
Instead, the period had been coloured by a variety of La Nina systems stemming from the central and western Pacific, and bringing warmer conditions on the whole.
“What’s happened is the warm water in the Pacific has been shifted further west,” Noll said.
“And in order to get cooler temperatures in New Zealand, we need that warm water sitting in the eastern Pacific.
“That’s been absent for the better part of three years now, and stands as one of the leading drivers of this long-term warmth.”
Another part of the picture could be put down to the Southern Annular
Mode, or SAM, which was effectively a ring of climate variability that encircled the South Pole and extended out to the latitudes of New Zealand.
In its positive phase, the SAM was associated with relatively light winds and more settled weather over New Zealand latitudes, together with enhanced westerly winds over the southern oceans.
Yet, even when colder winds from the south did manage to blow up here, they were still modified by sea surface temperatures around New Zealand that had long been on the warmer side of average.
At the start of this month, our seas were sitting at more than 0.7C above average — for the fourth consecutive month.
They’d climbed much higher further back in the 30-month period — reaching a massive 6C above average in the Tasman Sea at the height of the marine heatwave of 2017-18, and then as high as 4C during last summer’s marine heatwave.
Noll expected this month, which had been tracking at 1.5C above average, to finish up as one of the hottest Julys on the books.
“We are definitely looking at a top five finish, if not top three.”
The warmest July ever recorded, at 1.8C above average, came in 1998 — just as that year’s devastating El Nino was dissipating.