What an odd year

The weather so far in 2017 has been un­usu­ally volatile – and very wet for most of the coun­try (see Fig­ures 1 and 2).

NZ Grower - - Metservice Update -

MetSer­vice fore­cast­ers reg­u­larly an­a­lyse weather map char­ac­ter­is­tics at the weekly to monthly time scale, to iden­tify why cer­tain re­gions of the coun­try have run wet­ter or warmer, cooler or drier, than is nor­mal. Fil­ter­ing out the de­tail, also known as the ‘weather noise,’ means we zoom out from in­di­vid­ual fronts, lows and highs, and in­stead look at the big pic­ture to ex­plain what is go­ing on. We also use this tech­nique to fore­cast what is com­ing next, at the in­tra-sea­sonal time scale (for the com­ing few weeks to months).

When we look at the ‘av­er­age’ weather map so far in 2017 (not shown), we see the year-so-far has been char­ac­terised by low pres­sures over the coun­try, over­all. This will come as no sur­prise to the growing com­mu­nity. Jan­uary 2017 was par­tic­u­larly stormy across the en­tire coun­try (ex­clud­ing North­land), with wet, cold and very windy weather ex­pe­ri­enced in most re­gions. How­ever, the door to the trop­ics soon opened once we moved into March and April. Sub-trop­i­cal, northerly airstreams pro­duced in­ter­mit­tent but dam­ag­ing down­pours over the north and east of both Is­lands dur­ing au­tumn, with a late Cy­clone

Sea­son to boot. In con­trast, High pres­sure favoured the south­ern South Is­land dur­ing au­tumn. Re­cently, we’ve seen the at­mos­phere re­vert to a more nor­mal pro­gres­sion of westerly fronts and mo­bile Highs across the coun­try.

THE SOUTH­ERN OCEAN

The South­ern Ocean is of­ten the lead in­flu­ence on New Zealand weather maps, on time scales weeks to months. The other main driv­ers of our weather pat­terns are the Tas­man Sea, and the trop­ics. So far in 2017, we’ve seen the South­ern Ocean flip-flop with rea­son­able speed be­tween its northerly and southerly phases (Fig­ure 1) – in stark con­trast to 2016, when the at­mos­phere be­came ‘stuck’ for ex­tended pe­ri­ods in one gear, or the other.

LOOK­ING AHEAD

The num­ber one in­flu­ence on early win­ter weather pat­terns for New Zealand will be whether the South­ern Ocean con­tin­ues to flip-flop reg­u­larly be­tween phases. If it does, farm­ers can ex­pect the full range of weather maps that New Zealand can of­fer over win­ter. Ex­pect change­able, and mo­bile, weather sys­tems in the New Zealand re­gion, with the Tas­man Sea and South­ern Ocean both con­tribut­ing at times. On the farm, ex­pect both east­erly winds and the more usual westerly regimes, rather than an ex­tended pe­riod of any one thing. Ba­si­cally, things look likely to be a real mixed-bag to start win­ter. >

Look­ing fur­ther ahead, the trop­i­cal Pa­cific re­mains in the neu­tral El Nino-South­ern Os­cil­la­tion (ENSO) range. De­spite equa­to­rial sea sur­face tem­per­a­tures warm­ing since the start of 2017, they re­main be­low El Nino thresh­olds. The most re­cent model pre­dic­tions have pulled back from de­vel­op­ing a strong El Nino event. Around half of the in­ter­na­tional cli­mate mod­els de­velop an El Nino event in the sec­ond half of the year. From a New Zealand per­spec­tive, even if an El Nino were to de­velop to­wards spring, it won’t be a ma­jor player for us over the win­ter pe­riod. How­ever, it will prob­a­bly pay to re­mem­ber one thing: the usual New Zealand re­sponse to mod­er­ate or strong El Nino events is a cold spring.

You can catch our lat­est think­ing about fu­ture New Zealand weather pat­terns at http://www.metser­vice.com/ru­ral/ monthly-out­look, in­clud­ing monthly fore­casts of re­gional rain­fall and tem­per­a­ture. MetSer­vice Me­te­o­rol­o­gists are also happy to an­swer farm­ing ques­tions on Twit­ter and Face­book. You can find us at MetSer­vice New Zealand on Face­book and @metser­vice on Twit­ter.

▶ Fig­ure 1: Rain­fall ac­cu­mu­la­tion plots for Kerik­eri (top map) and Te Puke (bot­tom map). The av­er­age an­nual rain­fall ac­cu­mu­la­tion is shown in black, the to­tal for this year shown in red (data 1 Jan­uary – 20 May 2017).

▶ Fig­ure 2: Rain­fall ac­cu­mu­la­tion plots for Blen­heim (bot­tom map) and Christchurch (top map). The av­er­age an­nual rain­fall ac­cu­mu­la­tion is shown in black, the to­tal for this year shown in red (data 1 Jan­uary – 20 May 2017).

Fig­ure 3: A plot of the South­ern Ocean stormi­ness (also known as SAM) for 1 Jan­uary 2016 – 20 May 2017. Note the ex­tended pos­i­tive phase of the SAM for the first four months of 2016, ver­sus the sus­tained neg­a­tive phase Novem­ber 2016 – Jan­uary 2017. The weather maps in the first few months of 2016 were dom­i­nated by High pres­sure over and to the east of New Zealand (cir­cled in green). 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 ab­so­lute con­trast, the weather maps in late 2016/early 2017 (cir­cled in pur­ple) were fre­quented by ac­tive fronts and stormy south­west­er­lies, which pro­duced wet and cold weather for most of New Zealand. No­tice that the South­ern Ocean has been reg­u­larly switch­ing phases since Fe­bru­ary 2017.

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