Cli­mate

Early on­set rain­fall

Farms & Farm Machinery - - Contents -

The chance of an early north­ern rain­fall on­set for

2016-17 is higher than av­er­age over much of north­ern Aus­tralia, ac­cord­ing to the Bureau of Me­te­o­rol­ogy (BOM) fore­cast­ing models.

The north­ern rain­fall on­set date oc­curs when the rain­fall to­tal in a par­tic­u­lar re­gion reaches 50mm or more af­ter Septem­ber 1.

It is con­sid­ered to be ap­prox­i­mately the amount of rain­fall re­quired to stim­u­late plant growth af­ter the dry sea­son.

The chance of early on­set rain­fall is high­est over the Top End of the North­ern Ter­ri­tory (NT), the Cape York Penin­sula, the Kimberley and south-east Queens­land. Over the Pil­bara and the south­ern NT, chances of an early or late rain­fall on­set are roughly equal. This means most of north­ern Aus­tralia is likely to re­ceive the first rains af­ter the dry sea­son ear­lier than nor­mal.

In gen­eral, parts of coastal Queens­land and the west­ern Top End re­ceive the first use­ful rain­fall by late Oc­to­ber or early Novem­ber, and this spreads fur­ther south and in­land over en­su­ing weeks. The south­ern in­land re­gions of the North­ern Ter­ri­tory and west­ern parts of West­ern Aus­tralia usu­ally have the lat­est north­ern rain­fall on­set, around mid-Jan­uary.

THE LA NINA FAC­TOR

Above-av­er­age In­dian Ocean tem­per­a­tures and an in­creased chance of a La Nina in 2016 raise the like­li­hood of an ear­lier rain­fall on­set in north­ern Aus­tralia. This is due to in­creased mois­ture in the at­mos­phere, which can en­hance rain­fall.

The ENSO (El Nino-South­ern Os­cil­la­tion) Out­look is at La Nina watch, in­di­cat­ing there is a 50 per cent chance a La Nina could de­velop – twice the nor­mal like­li­hood. Dur­ing La Nina, the north­ern rain­fall on­set typ­i­cally oc­curs ear­lier than usual.

The north­ern Aus­tralian rain­fall on­set is greatly af­fected by the phase of the ENSO. The north­ern rain­fall on­set tends to be ear­lier than usual dur­ing La Nina years, while the on­set date tends to be later than nor­mal dur­ing El Nino years.

At the sea sur­face, Aus­tralian wa­ter tem­per­a­tures have cooled slightly, but re­main warmer than av­er­age and still at El Nino lev­els, de­spite other signs in­di­cat­ing La Nina is com­ing.

Aus­tralia’s cli­mate is also be­ing in­flu­enced by record warm tem­per­a­tures in the In­dian Ocean. The warmth in the In­dian Ocean will likely pro­vide ex­tra mois­ture for rain sys­tems as they cross Aus­tralia.

Cli­mate models sug­gest the El Nino will con­tinue to weaken dur­ing win­ter months this year. A La Nina may help im­prove the fi­nan­cial out­look of many Aus­tralian farm­ers, par­tic­u­larly those af­fected by the cur­rent lack of rain­fall.

A DIF­FER­ENT STORY LAST YEAR

The chance of an early north­ern rain­fall on­set for 2015-16 was lower than av­er­age for north­ern Aus­tralia and this turned out to be fairly ac­cu­rate. This change in pat­tern sug­gests we are now ex­pe­ri­enc­ing a fairly dra­matic shift from El Nino to La Nina, ac­cord­ing to the BOM.

Last year, parts of north-west­ern WA, the Top End of the NT and Cape York Penin­sula, in­land QLD, were given a less than 40 per cent chance of early rain­fall on­set. Else­where across the north, the chance of an early or late rain­fall on­set was 50 per cent.

The long-term me­dian for early north­ern rain­fall on­set is cal­cu­lated by av­er­ag­ing the on­set date for each year from 1960-2009. In the more iso­lated re­gions of Aus­tralia, the weather sta­tion den­sity is not suf­fi­cient to sup­port an anal­y­sis, and so is not in­cluded in BOM’s anal­y­sis.

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