MetSer­vice Up­date

Pre­pare for heat & hu­mid­ity

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

For New Zealand this means a clear shift in the odds, favour­ing above av­er­age sum­mer tem­per­a­tures for most re­gions, bar the eastern South Is­land. In the case of the north and east of the North Is­land, there is also a clear sig­nal for higher hu­mid­ity.

Be­low av­er­age sea tem­per­a­tures were ev­i­dent across much of the cen­tral and eastern equa­to­rial Pa­cific Ocean by late Oc­to­ber (the clas­sic “cold tongue”), and sea sur­face tem­per­a­tures had cooled suf­fi­ciently to meet La Nina thresh­olds by mid Novem­ber.

Be­low av­er­age sea sur­face tem­per­a­tures, in a nar­row band along the equa­tor, are the pri­mary hall­mark of La Nina (Fig­ure 1). Other in­di­ca­tors, re­lated to the at­mo­spheric com­po­nent of this weather pat­tern, are en­hanced east­erly trade winds, sup­pressed con­vec­tion (rainfall) in the trop­ics near the Date­line (180), and a sus­tained pos­i­tive South­ern Os­cil­la­tion In­dex (SOI).

LATE-DE­VEL­OP­ING EVENT

The 2017 La Nina is a late-de­vel­oper. Usu­ally, La Nina con­di­tions will form dur­ing the south­ern hemi­sphere au­tumn, strengthen dur­ing win­ter and spring, and peak around Christ­mas time. A typ­i­cal La Nina event will con­tinue in the first few months of the new cal­en­dar year, be­fore dy­ing out again dur­ing the au­tumn pe­riod. This La Nina was very slow to form, and is cur­rently rel­a­tively weak. Fig­ure 2 shows a time se­ries of pre­vi­ous La Nina events, as mea­sured by the NINO3.4 In­dex.

WHAT DOES THIS MEAN FOR NEW ZEALAND?

New Zealand weather maps don’t al­ways play out to the “stan­dard” La Nina recipe dur­ing weak or late-de­vel­op­ing events. Some­times, we see the New Zealand re­gion be­ing in­flu­enced by sev­eral com­pet­ing in­flu­ences. His­tor­i­cally, late and/or weak La Nina events have his­tor­i­cally had mixed (vari­able) re­sults on New Zealand sum­mer (De­cem­ber – Fe­bru­ary) rainfall.

To il­lus­trate, Christchurch sum­mer (De­cem­ber – Fe­bru­ary) rainfall to­tals, as a per­cent­age of sum­mer nor­mal, are shown be­low (Ta­ble 1). It is ev­i­dent that, in Can­ter­bury at least, La Nina con­di­tions are no guar­an­tee for a clearcut recipe for sum­mer rainfall:

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