Weather Up­date

Some­times, we see the at­mos­phere seem to re­peat it­self. Weather maps can have the same gen­eral ‘look’ for a cou­ple of weeks run­ning.

The Orchardist - - Contents - By Ge­orgina Grif­fiths, MetSer­vice Me­te­o­rol­o­gist

Re­peat pat­terns

By that I mean, some­times we see Lows for each week­end, fol­lowed by a pat­tern of High pres­sure mid-week. This is the some­what in­fa­mous “seven day cy­cle” that kiwi farm­ers (and sailors, and surfers, and sports coaches ev­ery­where….) like to com­ment on, usu­ally over a beer! Or per­haps we see a series of cold fronts moving over the Tas­man Sea, with each of them stand­ing up and de­vel­op­ing a wave (small low) in roughly the same place, one af­ter an­other. Re­peat pat­terns of­ten mean a run of poor weather in a par­tic­u­lar farm­ing re­gion, and an ex­tended spell of set­tled weather in an­other (usu­ally the district over the hill). This is be­cause New Zealand is a land of hills and moun­tains, with wind­ward rain and lee shel­ter­ing.

There are phys­i­cal mech­a­nisms that cause re­peat­ing weather maps – usu­ally the jet streams over the Tas­man Sea, which are zones of ex­tremely strong winds high up in the at­mos­phere. Fig­ure 1. A plot of the South­ern Ocean stormi­ness (also known as SAM) for 1 Jan­uary 2016 – 27 March 2017. Note the ex­tended pos­i­tive phase of the SAM for the first four months of 2016 (cir­cled in green), ver­sus the sus­tained neg­a­tive phase Novem­ber 2016 – Jan­uary 2017 (cir­cled in pur­ple). The weather maps in the first few months of 2016 were dom­i­nated by High pres­sure over and to the east of New Zealand. Much of the coun­try ran very dry over this pe­riod un­der the block­ing High, and it was very hot, due to fre­quent northerly winds. In com­par­i­son, the weather maps dur­ing sum­mer 2016/2017 were fre­quented by ac­tive fronts and stormy south­west­er­lies, which pro­duced wet and cold weather for most of New Zealand.

These jet streams cause cer­tain lo­ca­tions to favour de­vel­op­ing Low pres­sure, and other ar­eas to favour High pres­sure. An­other ma­jor driver of the weather map is the South­ern Ocean, with its pre­vail­ing storms blow­ing their way around the south­ern hemi­sphere. Some­times these storms get an os­cil­la­tion go­ing (rather like a swim­ming pool when the kids de­cide to start a whirlpool), with the storms tracks some­times ex­tend­ing fur­ther north, wash­ing up and over New Zealand, and some­times stay­ing away well to the south of the coun­try. Lastly, un­usual sea sur­face tem­per­a­tures, some­times in quite lo­calised re­gions, can af­fect weather sys­tem in­ten­sity, too.

The MetSer­vice long-range fore­cast­ers use un­der­stand­ing of these mech­a­nisms, and a whole lot of com­puter mod­el­ling, to fore­cast the next few weeks for our ru­ral cus­tomers. The most im­por­tant thing of all is to spot when the pat­tern change will oc­cur. One of the most use­ful in­di­ca­tors of all is the South­ern Ocean (see Fig­ure 1).

MARCH 2017

Dur­ing March 2017, the weather maps in the New Zealand re­gion ex­hib­ited a repet­i­tive pat­tern. Sev­eral slow-moving, com­plex (multi-cen­tred) Lows sat over the north Tas­man Sea, send­ing mul­ti­ple hu­mid, north­east­erly rain bands across the up­per North Is­land, and even­tu­ally, across the South Is­land, too. Ridg­ing, or High pres­sure ar­eas, tended to favour south­ern lat­i­tudes (in the vicin­ity of Ste­wart Is­land).

LOOK­ING AHEAD?

As we move into April, the pat­tern of the Highs favour­ing south­ern lat­i­tudes, and low pres­sure be­ing par­tial to north­ern New Zealand, looks likely to stay in place for the short termm. How­ever, at some point mid to late April, ex­pect a sharp pat­tern change to the more usual cir­cu­la­tion pat­tern of west­erly winds fir­ing up over the South Is­land.

You can catch our lat­est think­ing about com­ing weather pat­terns at www.metser­vice.com/ru­ral/monthly-out­look, in­clud­ing monthly fore­casts of re­gional rain­fall and tem­per­a­ture. If you sign up to the Monthly Out­look at www.metser­vice.com/emails, you will re­ceive FREE lon­grange in­tel and fore­cast maps. MetSer­vice Me­te­o­rol­o­gists are also happy to an­swer hor­ti­cul­tural ques­tions on Twit­ter and Face­book.You can find us at MetSer­vice New Zealand on Face­book and @metser­vice on Twit­ter.

Fig­ure 2. Mean sea level anal­y­sis for a) Left: mid­day 7 March 2017, b) Cen­tre: mid­day 10 March 2017, and c) Right: mid­day 26 March 2017. In all three sit­u­a­tions, the ridge or High pres­sure lies over the cen­tral South Tas­man Sea, and a com­plex Low lies west of the North Is­land.

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