2018 set to wet ____________________

Fol­low­ing on from a record hot sum­mer, the first half of 2018 has de­liv­ered one of the wettest Jan­uary to June pe­ri­ods in re­cent history. Some of the key grow­ing ar­eas have seen record-break­ing rain­fall for the first half of the year. We take a look at wh

NZ Grower - - Agronomist - By James Mill­ward, MetSer­vice Me­te­o­rol­o­gist SPON­SORED BY THE HOR­TI­CEN­TRE GROUP

A stormy start to the year

North­ern ar­eas of both Is­lands started the year with a del­uge, with an in­tense sub-trop­i­cal low travers­ing the coun­try on Jan­uary 4, and an­other on Jan­uary 18, drop­ping 333m of rain in 24 hours over Golden Bay in Nel­son. Otago and South­land had to wait un­til Fe­bru­ary for sig­nif­i­cant rain­fall. This came in the form of ex-trop­i­cal cy­clone Fehi on Fe­bru­ary 1 which dropped 78mm at Cromwell, spik­ing the ac­cu­mu­la­tions well above av­er­age for the year. Ex­trop­i­cal cy­clone Gita then ar­rived on Fe­bru­ary 20-21, tar­get­ing the north­ern South Is­land and helped Nel­son and Blen­heim set the wettest Fe­bru­ary on record for both lo­ca­tions. The rain­fall was 370% and 400% (i.e. four times) of the Fe­bru­ary av­er­age, re­spec­tively.

On March 12 the fi­nal ex-trop­i­cal cy­clone, Hola, of the sea­son passed over the New Zealand re­gion of­fer­ing good to­tals to the north­east of the coun­try. Cli­ma­to­log­i­cally, about one ex-trop­i­cal cy­clone comes within 550km of the coun­try per sea­son. The in­flu­ence of a weak La Nina over this cy­clone sea­son was clear, with three ex-trop­i­cal cy­clones af­fect­ing New Zealand.

Autumn: A breakdown of the heat, and a regime shift

In late March, the South­ern An­nu­lar Mode (SAM) re­turned to a sus­tained neg­a­tive phase for the first time since Oc­to­ber 2017. The SAM de­scribes the west­erly wind belt that cir­cles Antarc­tica and the strength and ex­tent of these west­erly winds dom­i­nate our weather maps. In a neg­a­tive phase, west­erly gales and ac­tive cold fronts that lie over the South­ern Ocean are dis­placed to the north, and wash up and over New Zealand. In the trop­ics, La Nina weak­ened to neu­tral con­di­tions.

Cooler south­west­er­lies helped break down the heat in the oceans around New Zealand and sea sur­face tem­per­a­tures cooled sig­nif­i­cantly as a re­sult over the autumn months.

Stormy west­er­lies in autumn con­tin­ued to see rain­fall rates tick along at near av­er­age rates, with the most notable storm af­fect­ing the en­tire coun­try from April 8 to 10. A to­tal of 70mm in 12 hours was recorded in gauges across the Welling­ton re­gion whilst Auck­land was bat­tered by highly dam­ag­ing winds.

Win­ter: A gloomy start for the East

High pres­sure pre­vailed over the south­ern half of the South Is­land dur­ing June herald­ing the start of win­ter as the SAM swung back to pos­i­tive. Pe­ri­odic wet and cloudy east­er­lies af­fected the North Is­land and up­per South Is­land, pro­duc­ing an ex­tremely wet month for all east­ern re­gions be­tween Christchurch and North­land.

In con­trast, this pat­tern pro­duced an ab­nor­mally dry June for the west­ern and south­ern re­gions of the South Is­land.

Where are we now?

Un­sur­pris­ingly, rain­fall ac­cu­mu­la­tions for the first half of the year in the key grow­ing re­gions are well above av­er­age, with some re­gions break­ing records. Pukekohe saw its wettest Jan­uary to June pe­riod on record - 858.4mm, break­ing the 1996 record - whilst Blen­heim recorded its third wettest Jan­uary to June pe­riod (578.6mm) since records be­gan in 1941. Cromwell was also sit­ting some 70mm above its av­er­age Jan­uary to June ac­cu­mu­la­tion. The con­stant, and some­times ex­treme, rain­fall has been wide reach­ing and soil mois­ture deficits are now na­tion­ally at or be­low av­er­age com­pared with the his­tor­i­cal av­er­age deficit at this time of year, apart from the far south of the coun­try; quite a dras­tic turn-around from late last year.

Look­ing ahead

Con­fi­dence is grow­ing amongst the mod­els that El Nino will form by spring. Dur­ing El Nino springs, New Zealand gen­er­ally sees en­hanced south­west­erly winds. These con­trib­ute to our El Nino spring trade­mark: be­low av­er­age tem­per­a­tures right across the coun­try.

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 - Rain­fall ac­cu­mu­la­tion plots for Pukekohe, Blen­heim and Cromwell for the year-to-date (red line), track­ing well above av­er­age.

▴ Fig­ure 1. Rain­fall ac­cu­mu­la­tion plots for Pukekohe, Blen­heim and Cromwell for the year-to-date (red line), track­ing well above av­er­age.

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