Pests and path­ways – mon­i­tor­ing biose­cu­rity risks to New Zealand’s fresh veg­etable and arable in­dus­tries

NZ Grower - - Contents - By Ge­orgina Grif­fiths, MetSer­vice Me­te­o­rol­o­gist

At this time of year, the South­ern Ocean be­comes the key driver of our tem­per­a­tures. When the south­ern storms fre­quently wash up and across New Zealand, ex­tended pe­ri­ods of un­set­tled south­west­er­lies pro­duce cooler than av­er­age tem­per­a­tures across the coun­try.

The re­verse sit­u­a­tion is char­ac­terised by mostly set­tled weather, with South­ern Ocean lows and fronts dis­placed well to the south of the coun­try.

The South­ern Ocean stormi­ness is so in­flu­en­tial on what sort of weather maps we see in the New Zealand re­gion, that it is mon­i­tored by MetSer­vice me­te­o­rol­o­gists. We use an in­dex called the South­ern An­nu­lar Mode (also known as SAM). When the SAM in­dex (Fig­ure 1) re­mains neg­a­tive over a few weeks, this usu­ally spells trou­ble – ex­pect cold, stormy, and wet­ter weather to pre­vail.

If the SAM main­tains in its pos­i­tive phase for sev­eral weeks, ex­pect rel­a­tively ‘quiet’ weather. Highs of­ten sit to the east and south of the coun­try. Northerly flows are com­mon dur­ing pos­i­tive SAM phases, as are above av­er­age tem­per­a­tures. For those re­gions ex­posed to northerly rain, an ex­tended spell of pos­i­tive SAM weather maps may not mean drier con­di­tions (for ex­am­ple, in Nel­son or the Bay of Plenty). Win­try tem­per­a­tures

The ex­pec­ta­tion for a neg­a­tive SAM (stormy South­ern Ocean) dur­ing the sec­ond half of May con­trib­uted to the MetSer­vice pre­dic­tion of win­try tem­per­a­tures, and a much colder than usual fort­night. This was a brave call in the face of the per­sis­tent warmth that had been ob­served for much of the first half of May!

You can see our fore­cast weekly tem­per­a­ture de­vi­a­tions, is­sued midMay (Fig­ure 2). Much of the South Is­land was pre­dicted to run un­usu­ally cold dur­ing the week 21 – 27 May. Weekly tem­per­a­tures were fore­cast at be­tween 2 and 3 de­grees be­low the late May av­er­age. (Note that this is a very large weekly de­vi­a­tion!) For the week 28 May – 3 June, a colder than usual week was fore­cast across most of the coun­try.

Th­ese fore­cast maps use en­sem­ble data. This is just a fancy way of say­ing we use a group of weather mod­els

(51 in fact), and run them as a pack out through time. The av­er­age out­come of the en­sem­ble group of­ten val­i­dates well, even when run out for three or four weeks. This is es­pe­cially true for tem­per­a­ture fore­casts, and for very ‘strong’ cli­mate sig­nals (e.g. strong Highs or deep Lows).

While you can’t find ‘within-week’ in­for­ma­tion (such as what might hap­pen on a par­tic­u­lar day) us­ing weekly en­sem­ble data, you can see the big pic­ture. In this case, the en­sem­bles are strongly sig­nalling an un­usu­ally cold fort­night for late May/early June.

You can find out more at https://blog.metser­vice.com/Ensem­bleFore­cast­ing.

Look­ing ahead

Go­ing for­ward through win­ter, the on­go­ing state of the SAM will be key to our tem­per­a­tures, and weather maps. In ad­di­tion, some of the global cli­mate mod­els sig­nal an El Nino in the wings for spring. How­ever, this is not yet cer­tain, as it depends on the ocean and the at­mos­phere work­ing in tan­dem. You can catch our lat­est think­ing at www.metser­vice.com/ ru­ral/monthly-out­look, or sign up to free long-range in­for­ma­tion and the weekly en­sem­ble fore­cast maps at www.metser­vice.com/emails.

▶ Fig­ure 1: A plot of the SAM for 1 Jan­uary 2017 – 28 May 2018* The sus­tained pos­i­tive phase ear­lier in 2018 has been re­placed by in­ter­mit­tent neg­a­tive for­ays dur­ing au­tumn. *Fore­cast SAM data were used in the pe­riod 15-28 May.

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