Two op­pos­ing pat­terns pre­dicted for sum­mer

It might pay to standby for SAM.

NZ Grower - - Biosecurity -

Lately, there’s been a lot of talk about El Nino and a lot of air time around a hot Novem­ber. In­stead, as MetSer­vice ex­pected, Novem­ber tem­per­a­tures swung sharply dur­ing the month.

MetSer­vice high­lighted that Novem­ber tem­per­a­tures were likely to be ex­tremely volatile, bounc­ing from un­usu­ally hot to ex­tremely cold. Have a look at the weekly tem­per­a­ture anom­aly charts that were is­sued to our hor­ti­cul­tural clients on Novem­ber 5 as seen in Fig­ure 1.

These plots show the de­vi­a­tion from the nor­mal tem­per­a­ture for the time of year. The strength of the colder-than-usual sig­nal in week three is ab­nor­mally large for three weeks out, and we noted this in our com­men­tary. This was the week of the un­sea­sonal snow down south.

And while these weekly tem­per­a­ture fore­casts aren’t per­fect – and don’t show day-to-day de­tail - they do il­lus­trate the power of en­sem­ble-based fore­cast­ing, which you can find out more about at https://blog.metser­ Ensem­bleFore­cast­ing].

Look­ing ahead to sum­mer

With re­spect to sum­mer weather from De­cem­ber to Fe­bru­ary, two op­pos­ing weather driv­ers may well be in play.

MetSer­vice mon­i­tors both the trop­ics, the cen­tre of ac­tion for El Nino/La Nina, and the South­ern Ocean. This is crit­i­cal, given that New Zealand lies half­way be­tween the trop­ics and the pole. Both driv­ers can con­trib­ute strongly to our weather pat­terns.

All the en­sem­ble-based guid­ance that MetSer­vice mon­i­tors is cur­rently in­di­cat­ing a rel­a­tively quiet South­ern Ocean dur­ing sum­mer. This is known as a pos­i­tive South­ern An­nu­lar Mode (SAM). This pat­tern favours norther­lies across NZ, per­sis­tent highs near the Chatham Is­lands, and an el­e­vated chance of a wet­ter-than-nor­mal sum­mer in north­east­ern re­gions of the coun­try,as seen in Fig­ure 2.

If El Nino forms this sum­mer, and this is not yet a cer­tainty at the time of writ­ing, then it looks likely that we see two op­pos­ing weather pat­terns. Farm­ers and grow­ers should not ex­pect to see a typ­i­cal El Nino sum­mer, as seen in Fig­ure 3, given that the South­ern Ocean is fore­cast to act in the op­pos­ing di­rec­tion dur­ing the sum­mer pe­riod.

Bot­tom line - make in­formed de­ci­sions

Given the ex­pected bat­tle for the NZ weather map this sum­mer it will pay to keep up to date with the longer-range fore­casts and make in­formed de­ci­sions. You can sign up for free to get the Monthly Out­look emails straight to your in­box at www.metser­ These emails in­clude some en­sem­ble-based fore­cast maps. But more im­por­tantly, they also con­tain the lat­est fore­caster com­men­tary. Even if the en­sem­bles weren’t so handy, this alone can be worth sign­ing up for.

▴ Fig­ure 2: Typ­i­cal sum­mer rain­fall anom­aly, dur­ing mod­er­ate to strong pos­i­tive phases of the South­ern An­nu­lar Mode (SAM).

▴ Fig­ure 3: Re­gions with an el­e­vated chance of be­low nor­mal sum­mer rain­fall dur­ing mod­er­ate to strong El Nino events (shaded orange).

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