Blocking pattern brought on drought
It is an ironic twist of the meteorological world that the biggest systems that do not bring severe weather determine our day-to-day weather.
The tracks of the rain, snow and windbearing low-pressure features that whiz across the country causing chaos are generally determined by the position of 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 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,’’ he said.
‘‘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 2011-2012 brought a persistent mild northerly flow and flooding in Nelson.
Victoria University school of geography, environment and earth sciences Associate Professor James Renwick said it was not yet clear just how much influence climate change would have on the frequency and severity of droughts.
‘‘We’re not sure exactly yet how it will play out. The devil is in the detail, as it often is.’’
He said 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 flip side is longer intervals between the rains,’’ he said.
‘‘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.’’
The ‘‘wild card’’ in the climate changedrought 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 slowmoving 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 the summer had been for ‘‘average’’ conditions, meaning there would be a mixture of all sorts of weather.
The eight global and three local computer models used as the basis for the institute’s seasonal outlooks did not have strong enough guidance from weather patterns at the time to 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,’’ he said.
‘‘So we were seeing some signal there, but when the models don’t agree, you can’t say just 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,’’ he said. ‘‘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 highpressure belt across the country, which has been the dominant factor with this one.’’