In brief: Dry run

Over the three summer months, most of New Zealand sweltered under record or near-record sunshine, while much of the North Island, Canterbury and Westland got less than half their normal seasonal rainfall.

NIWA Principal Scientist Brett Mullan told media that the long dry spell came courtesy of huge, “blocking” high pressure systems: “They sat over the country and steered the rain-bearing weather systems around them.”

Normally, he said, blocking highs tended to sit over eastern New Zealand, but high pressure systems this summer had been especially broad, stretching across the country and out over parts of the Tasman Sea.

So it was that on 15 March the Ministry for Primary Industries (MPI) declared the entire North Island as experiencing “a medium-scale adverse event due to drought”. Earlier that week, authorities declared a fire ban over practically the entire North Island – the first such sweeping measure in the country’s history. On 22 March, MPI extended the drought declaration to the Buller and Grey districts.

NIWA has confirmed that for parts of southern Northland, Auckland, Waikato, Bay of Plenty, Hawke’s Bay and the west of the South Island, the 2012–13 drought was the worst in 40 years, more severe even than the 2007– 08 Waikato drought. In Hawke’s Bay, it was second only to the devastating 1997–98 drought. In Marlborough and North Canterbury, it was less severe than 1997–98 and 2000–01. However, this drought was the most pervasive yet: only the dry of 1972–73 (Wairarapa, Tasman, Otago and Southland) comes close to matching its sweeping geographical extent.

In some locations, the Potential Evapotranspiration Deficit (PED) index – a measure of combined soil moisture loss to transpiration by plants and evaporation – confirms the driest conditions in 70 or so years. A little rain in mid-March did nothing to alleviate soil moisture deficits up to 50mm below the normal late-March average.

As autumn unfolded, those high pressure systems were not expected to monopolise the skies over, and to the south of, New Zealand as much, and indeed they did not. However, March remained dry, and the PED index climbed even higher, taking the 2012-13 drought well clear of 1972-73 drought. The 2012–13 drought has challenged both records and profits. (Alan Blacklock) In brief New Zealand droughts typically break in autumn, although sometimes this is delayed until early winter. In 2013, the rains came in April – well above normal rainfall occurred in Waikato, Bay of Plenty, Manawatu and the upper South Island. By the end of April, soils had been recharged with water throughout much of New Zealand. By late May, some dry regions still remained, particularly Hawkes Bay and Wairarapa.

The 2013 drought cannot be categorically linked to climate change, but NIWA’s climate modelling indicates that droughts are likely to become more frequent, and more severe, in eastern and northern parts of New Zealand in coming decades. For example, a drought in eastern or northern New Zealand with a one-in-20 year return period based on data from 1971 to 2000 may occur twice, or even four times, as often by the end of this century.

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Principal Scientist - Climate