SAM’s in charge

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The South­ern An­nu­lar Mode (SAM) is one of New Zealand’s main cli­mate driv­ers, and its in­flu­ence over win­ter has been clear to see if you know what to look for.

Head­ing into spring, it looks set to re­main the key player driv­ing our weather pat­terns in the short-term. The SAM not only plays a huge role in the weather in NZ, but also in cli­mate vari­abil­ity through­out the South­ern Hemi­sphere. The word an­nu­lar means ring-shaped, and in the con­text of the South­ern An­nu­lar Mode refers to a belt of west­erly winds which cir­cle the South Pole. The strength and dis­place­ment of this belt north­wards or south­wards from its nor­mal po­si­tion, the mode, in­di­cate the phase of the SAM and can help us de­duce trends in the weather and cli­mate.

In NZ, when SAM is in its pos­i­tive phase, the belt of west­er­lies con­tracts to the South Pole. Fre­quent highs sit over the South Is­land and near the Chatham Is­lands, pro­duc­ing fairly set­tled weather over the coun­try, over­all, with pre­vail­ing northerly air­flows and warmer tem­per­a­tures.

In its neg­a­tive phase, the belt of west­er­lies is dis­placed fur­ther north­ward and washes over NZ; wet­ter, windier and cooler south­west­erly con­di­tions pre­vail, with fre­quent fronts and lows.

THE IN­FLU­ENCE OF SAM OVER WIN­TER

A sus­tained pe­riod of pos­i­tive SAM oc­curred in Jan­uary, Fe­bru­ary and March 2018 as seen in Fig­ure 1; it was no ac­ci­dent that this co­in­cided with our heat­wave sum­mer.

Dur­ing late au­tumn and through win­ter, semi-reg­u­lar switch­ing of the SAM from its pos­i­tive to neg­a­tive phase has been ev­i­dent. This led to change­able weather maps in the NZ re­gion; in turn we have seen fluc­tu­a­tions in both rain­fall and tem­per­a­ture across the coun­try.

Whilst gen­er­ally quite a mild win­ter, we have had sev­eral win­tery blasts from the south. The tem­per­a­ture in Waikato has reg­u­larly fluc­tu­ated through­out the win­ter months, and you can no­tice a loose cor­re­spon­dence be­tween sus­tained fluc­tu­a­tions in the SAM and the tem­per­a­ture anom­aly at Hamil­ton as seen in Fig­ure 2. Whilst not nec­es­sar­ily warmer when in a pos­i­tive phase, and cooler when in a neg­a­tive phase, the shape of the tem­per­a­ture graph does ‘re­spond’ to the tra­jec­tory of the SAM.

A no­table cold snap with a sus­tained southerly air flow over the North Is­land oc­curred at the start of Septem­ber. From Septem­ber 3 un­til 12 Welling­ton had only a cou­ple hours of northerly winds on the morn­ing of Septem­ber 10, with the max­i­mum tem­per­a­ture recorded there over the nine-day stretch a miserly 11.6°C. This com­pared with its av­er­age max­i­mum tem­per­a­ture of 13.4 de­grees for Septem­ber and ran pretty close to Welling­ton’s record spell of souther­lies of 10 days and 10 hours which oc­curred in 1960. A de­cent cold snap in mid-Septem­ber also brought low and deep snow­fall to Fiord­land, South­land and Cen­tral Otago, with snow ly­ing in both Queen­stown and Ar­row­town. At Cardrona ski field, 54cm of fresh snow was re­ported in 24 hours. No­tice­ably, the SAM took a sharp tum­ble from strongly pos­i­tive in the pre­ced­ing days be­fore the storm.

WHAT NEXT FOR SPRING?

The SAM will likely con­tinue to dom­i­nate pro­ceed­ings as we head deeper into spring, although the semi-reg­u­lar switch­ing of the SAM looks un­likely to last much be­yond the end of Septem­ber. The long-range mod­els sig­nal po­ten­tial for a pro­longed phase of pos­i­tive SAM and block­ing highs.

El Nino may also play a mod­est part late in the year, although all in­di­ca­tions show any re­sponse here in NZ will be rather muted.

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