We should have been in for a cracking summer. Seasonal forecasts late last year, taking their cue from a La Niña phase in the Pacific, tantalised with visions of beaches, basking and barbecues.
But in mid-December, Nelson wallowed under one-in-50-year floodwaters. Rain and drizzle, along with gale northerlies, spoilt many a North Island holiday, and disenchanted vacationers began to suspect the forecasters had got it wrong. But did they?
Only a little, says NIWA Principal Scientist, James Renwick. "The overall weather patterns were not too far from what we were expecting, but the local pattern was a bit different."
NIWA's seasonal climate outlooks draw on several global models that predict the climate a season ahead from current atmospheric and oceanic conditions. They take account of large-scale patterns that strongly affect New Zealand's climate, such as the El Niño Southern Oscillation (ENSO), which affects water temperatures in the Pacific, and the Southern Annular Mode, which affects the position of westerlies over the Southern Ocean.
About half of season-to-season climate variations can't be predicted, he says. "The deluge over Nelson and Golden Bay before Christmas is some of the unpredictability we're not going to capture." Each La Niña or El Niño plays out slightly differently over New Zealand. "A little like the commute to work, perhaps – we know roughly how the drive will go, but traffic conditions vary each time.
"Typically, in La Niña conditions, we get highs sitting east of New Zealand, bringing warmer temperatures. That happened this summer, and all December's climate models were pointing to a little warmer and drier than normal," says Renwick. But the highs drifted further east and south this time, allowing lows from the Tasman to deliver wetter, windier and cooler weather than expected.
Middle latitude climates such as New Zealand's, he says, can be highly variable, but NIWA's forecasting success rate matches that of most other mid-latitude agencies, such as those in the United States and western Europe.