Block­ing pat­tern brought on drought

Matamata Chronicle - - Rural Delivery -

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.’’

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