NIWA meteorologist Ben Noll says the agency has been fielding many calls from people over the past 12 months wanting to understand what is behind the ongoing dry conditions that now extend along much of the east coast of both islands.
He says the reasons are a complex jigsaw of climate pieces, one that stretches well beyond NZ to the Indian Ocean.
“In late 2019, the Indian Ocean Dipole became highly positive, which meant ocean temperatures to the north and west of Australia became very cool,” Noll said.
The dipole is an irregular “see-saw” oscillation of sea surface temperatures in the Indian Ocean that brings cooler temperatures to the northern and western Pacific Ocean as it moves into positive values.
As a result, the atmospheric rivers of moisture that usually pass from the tropics and down over NZ dried up.
“The dipole shift basically turned off the tap,” he said.
Accompanying this was a positive Southern Annular Mode (SAM), meaning high pressure systems tended to sit higher up across more of NZ than usual, often blocking low pressure systems.
The final piece of the puzzle has been a complex interplay of El Nino and La Nina-like conditions over the past two summers, with events that have not behaved like historic El Nino/La Nina systems.
This has meant ocean temperature extremes usually experienced with those events in the eastern Pacific have moved west, or closer to NZ’s location.
“That apparently little change has an impact on our ocean and weather patterns, and a climate change handprint is all over the things we have observed,” he said.
To break the dry here would require a major climatic event to spark a shift.
“Right now, we are not seeing that spark of change, an object in motion likes to stay in motion,” he said.
That “spark” could be a major El Nino event as witnessed in 2015. This rippled through 2016 and 2017, ultimately delivering major precipitation for NZ.
“But we are now in no-man’s land with El Nino and La Nina. We are in a country almost entirely covered with below average precipitation,” he said.
In his WeatherWatch outlook for May, chief executive and forecaster Philip Duncan has highlighted the continuing dominance of high pressure systems well into autumn, bringing little prospect of rain for dry east coast regions almost the length of the country.
Predictions at WeatherWatch are for Northern Southland, Otago, Canterbury, Marlborough, Nelson, and all the North Island east coast to receive only .2-15mm of rainfall for the first half of May, and only in isolated patches.
High pressure systems are forecast to continue to keep a tight grip on the ability of any rainmaking events to penetrate across NZ, either from the north-east or west.
“This pattern has become well established over the past two years and for some regions they are entering their third dry year, dominated by some unusually large and intense high pressure systems,” Duncan said.
He says summertime had experienced high pressure systems reaching 1040hPa, intensities more common over colder winter months and accompanied by still, frosty weather.
“We are seeing a weather pattern that is similar to New South Wales for much of NZ’s east coast now, being consistently drier than average and warmer than average over the past two years,” he said.
Across the Tasman, New South Wales and Queensland farmers are scrambling to secure high-value livestock to meet grass growth boosted by regular rainfall events.
In the meantime, farmers in along much of the east coast in NZ are hesitant about their winter stock levels after low autumn growth, and breeding stock productivity is threatened by their second hard autumn.
MetService meteorologist Jake Cope confirmed the extent of the dry weather experienced along the east coast of both islands over the past two years.
Tauranga had averaged 400mm of rainfall a year above average in 2017 and 2018, only to now be over 1000mm behind in total for the past two-and-a-half years.
Similarly, Hawke’s Bay was 100mm a year above the average for 2017 and 2018, only to be to now be 550mm behind in total for the past two-and-a-half years.
MetService’s long-range forecast over the coming two weeks reinforces WeatherWatch’s prediction of continuing dry weather over coming weeks.
Cope says the current high pressure system is set to remain until about May 10, with some northerly showers possible.
He says part of the reason for the extended dry period over the past two years along the east coast has been a positive SAM index, with most high pressure systems sitting higher across the upper South Island and North Island.
“Coming into this summer all eyes were on La Nina where it’s more common to see more low pressure through the Tasman and north-easterly patterns with rainfall,” Cope said.
“But we have not seen much out of this La Nina. The lows have been a bit higher up the country, the boundary has pretty much been the Kaipara Harbour.”
While Bay of Plenty and Gisborne would benefit from northerlies, dry Wairarapa and Hawke’s Bay could benefit from southerly showers, something a more negative SAM index could deliver if lows from the Tasman cross at the same time.
