The Fast Break - Victoria
Seasonal climate risk information for Victoria
Volume 15 | Issue 11 | 29 November 2019
The positive Indian Ocean Dipole is showing slow signs of decay, but all factors that point to its existence are still in play.
Cooler water off Sumatra at the surface and at depth, strong easterly winds, lack of cloud over Indonesia and stronger pressure in that region. Models are mixed as to whether the +IOD will die off in December or January, which would be historically a very late demise.
There is very little of interest going on in the Pacific Ocean, though the Coral Sea is warmer and greater cloud has been evolving from it, all good if you could get a connection to it.
Of greatest interest for the month was the strongly negative Southern Annular Mode that evolved from the Sudden Stratospheric Warming in September, finally connecting with lower altitudes in late October. A negative SAM means storm tracks are moved further north towards the mainland from the Southern Ocean.
Traditionally a negative SAM over spring has had variable outcomes over most of Victoria but can often lead to a drier far East Gippsland. If this was to continue into summer - as some models predict - a drier eastern half of Victoria may ensue.
This year the negative SAM is leading to cooler conditions south of the Divide with plenty of south-westerly winds and some rainfall with it, however north of the Divide and far east of the state has been drier. The predominant westerly wind flow has been causing a classic rain shadowing effect in East Gippsland.
Pressure patterns have shown some change this month, but overall the more northward position of high pressures is still resulting in drier for north of the Divide but allowing some rain in the south. Pressure remains higher to the north of Australia which is not conducive to moisture transport from the north.
The majority of models surveyed predict drier and warmer than average conditions are most likely for summer.
You can use the new Local Climate Tool to identify how historical +IOD events have affected rainfall in your area.
The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture shows most of north of the Divide and Central Gippsland has little moisture left.
There was negligible water use north of the Divide as crops have matured. Some paddocks have residual soil moisture at depth that could be helpful next year. In the south, crop paddocks at Hamilton and Yalla Y Poora used 12-17% for the month.
Most pasture paddocks used 20-30% for the month, a Greta paddock used 43%.
Model distribution summary for the next three months
Graphs showing the distribution of global model forecasts for December-February, with models showing increased chances of drier rainfall and warmer temperatures.
Model distribution summary for the next four to six months
Graphs showing the distribution of March-May forecasts with models showing increased chances of average rainfall and warmer temperatures.
Model consensus forecast for the next six months
|Current outlook (to 27 November)||Current outlook (to 27 October)|
|Indian Ocean||Cold (+IOD)||Warmer||Cold (+IOD)||Warmer/neutral|
|Rainfall||Slightly drier||Average||Slightly drier||Average|
|Temperature||Slightly warmer||Slightly warmer||Slightly warmer||Slightly warmer|
Sea Surface Temperature Anomalies
The +IOD is still in existence, (+1.36°C as of 27 November, threshold for +IOD is +0.4°C) but has backed off somewhat. The Arafura and Timor Seas off Darwin have returned to normal.
Traditionally further westward propagation of this normal temperature water would wipe out the cold patch at Sumatra. Sea surface temperature (SST) anomalies along the Equatorial Pacific have remained stable during October at neutral temperatures.
NINO3 is at +0.54°C and NINO3.4 is +0.61°C (as of 27 November). In the Coral Sea temperatures are warmer as a good moisture source.
Equatorial Pacific Sub-Sea Temperature Anomalies
The Pacific Ocean Equatorial sub-surface temperatures have cooled somewhat and have turned to a confused state of a warmer surface overlying a cooler subsurface.
At this stage of the year the Pacific undersea is of little use to be monitoring, as the window for serious El Niño and La Niña activity has passed.
Southern Oscillation Index
The SOI has fallen rapidly to significant negative values during November.
The value is currently at -11.1 (as at 27 November). This indicates El Niño like pressure patterns around the Equator, flying alone compared to other El Niño indicators.
Once we reach summer, the SOI is not as valuable for watching due to pressure changes at Darwin and Tahiti occurring due to tropical weather systems.
Pacific Ocean Surface Wind Anomalies
The Equatorial Pacific Easterly Trade Winds have shown some westward reversal off Papua New Guinea, but it’s too late to kick off ENSO activity now.
The strong Trade Wind reversals off Indonesia continue due to the +IOD. The +IOD won’t start to decay properly until these winds return to normal or reverse.
World Cloudiness Anomalies
Cloud at the International Dateline (180oW) junction with the Equator is slightly less (brown colour) which is weakly suggestive of La Niña, but no other indicator points towards this.
The lack of cloud (brown colour) off Sumatra and over Indonesia has enlarged greatly this month and the cloud anomaly in a NW direction typical of +IOD pattern has increased.
There has been greater cloud in the Coral Sea, in part due to cyclone Rita.
Southern Annular Mode
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent most of November in strong negativity.
This was due to the Sudden Stratospheric Warming that occurred in September coupling with lower altitudes in October. Such behaviour by SAM is characteristic of this phenomenon.
In spring, a negative SAM would be expected to cause drying along eastern NSW, into far eastern Victoria. In summer a negative SAM would classically cause drying in the eastern half of Victoria.
In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been just above the Bight, much further north of a spring position of Adelaide.
While this can have a variable effect in spring, this year it’s allowing rain to southern areas with strong westerly flow, but rain shadowing East Gippsland where westerly flow isn’t helpful.
It has also been blocking transport of moisture from the north-west. It’s not impossible that the negative SAM - keeping storm fronts further north - is in part responsible for the non-southward migration of the sub-tropical ridge.
Air Pressure Anomalies
The Sub Tropical Ridge of High Pressure has finally changed from stronger to normal/lower.
Stronger pressure over the tropics is still a downer on moisture transport from the north. In conjunction with the lower pressure over Tahiti, this is why the SOI is currently negative.
Higher pressure over Antarctica and a lower pressure belt in the mid-latitudes is a classic signal of a negative SAM.
Modelled Climate and Ocean Predictions for Victoria from November 2019 run models
Twelve climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Victoria.