Subtle processes make forecasts hazy
It is an ironic twist of the meteorological world that the biggest systems that don't bring severe weather actually determine our day-to-day weather.
The tracks of the rain, snow and wind-bearing low-pressure features that whizz across the country causing chaos are generally ordained by the position of the large, slower-moving high-pressure systems, which contain little in the way of cloud or rain.
It was the dominance of those anticyclones that parked themselves over the country and refused to move on that caused this summer's crippling drought.
The highs fended off all possible sources of rain.
In fact, their influence was so strong that not only did they "block" horizontally across New Zealand and the Tasman Sea but they extended vertically all the way to the top of the weather-containing layer of the atmosphere, about 12,000 metres up.
MetService spokesman Dan Corbett said the blocking highs at all levels were the architects of the drought.
"It was their size and prevalence. The upper ridge sat in place for much of February, and any other weather systems didn't have a chance and were squeezed to death by it. The surface anticyclonic ridge was massive, it was huge, and the upper ridge was in place for weeks and weeks supporting that. If the upper levels don't support it, it will be a bag of bones and fizzle out.
"The other interesting point is we didn't get the easterlies and the subtropical lows that creep down from the north. A few came down from the southern Cooks or the tropics but they tended to develop east of New Zealand and because of that they missed us."
The location of the "block" was crucial. In this case it had been on top of the country, but a blocking high a bit further east over New Year 2011-12 brought a persistent mild northerly flow and flooding in Nelson.
"Blocking happens as a thing of nature. It can be temperature differences that cause it, can be sea temperature differences, subtle differences that can affect how a low deepens or a high moves."
The block was now breaking down as the nights lengthened and the days shortened, altering the temperature balance.
"We're past the equinox.
"What was this big jewelled necklace of anticyclones that was the ridge axis, strung across through southern Australia, New Zealand and the Pacific, is now starting to shift northwards as it always does at this time of year.
"That opens us up to the Roaring Forties."
Victoria University school of geography, environment and earth sciences Associate Professor James Renwick said it was not yet clear how much influence climate change would have on the frequency and severity of future drought.
"We're not sure exactly yet how it will play out. The devil is in the detail, as it often is."
People should not expect that with climate change what had been experienced this summer would automatically be replayed again next year or suddenly become the norm.
But more extreme weather would become a way of life.
"It's pretty clear that climate change means heavier rains when it does rain, but the flipside is longer intervals between the rains.
"The chance of what we've had this summer goes up, and the background level of dryness will probably go up as well - the land is warming up faster than the oceans, so moisture is tending to get sucked out faster.
"It's easier to get to severe soil-moisture deficit conditions if you are starting from a lower base."
The "wild card" in the climate change-drought debate was the future strength and position of the westerly wind belt, measured by the Southern Annular Mode (Sam) index, he said.
Sam had been positive in the past couple of months with westerly winds further south than average, allowing the slow-moving subtropical high pressure systems to also move south on to New Zealand. There had been a trend in the past 40 years towards that pattern and that "looked to continue into the future", Renwick said.
However, the recovery of the ozone hole over Antarctica could act to counter that.
Warmer temperatures in the atmosphere over the Antarctic under that scenario would push back the westerlies further north.
National Institute of Water and Atmospheric Research principal climate scientist Dr Brett Mullan said predicting the drought last spring would have been "exceptionally" difficult. The general outlook for summer had been for "average" conditions - with neither an El Nino or La Nina phenomenon - meaning there would be a mixture of weather. The eight global and three local computer models used as the basis for the institute's seasonal outlooks did not . . . allow them to pick the extended dry spell.
"In October we had been looking at fairly normal conditions, but by November and December we were going for normal or below normal [rainfall] in the North Island.
"So we were seeing some signal there but when the models don't agree, you can't just say there is going to be a drought. I don't know that we could have foreseen the drought any better."
He believed it was the worst drought for many places since 1946.
"Of course it is not the worst everywhere in the country for 60-odd years, but is so particularly over the North Island. There are different factors for drought.
"In the east it's the strong westerly winds, which are very prominent in El Nino seasons, but the other factor is the high pressure belt across the country, which has been the dominant factor with this one," Mullan said.
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