The Fast Break - Victoria
Seasonal climate risk information for Victoria
Volume 15 | Issue 10 | 29 October 2019
The positive Indian Ocean Dipole continues with ocean temperature to depth and at the surface, wind, cloud and pressure patterns all singing from the +IOD hymn sheet.
All models surveyed predict the IOD to continue into December which would be very late historically for an IOD breakdown (in the last 10 years most died in late October). This points towards a late start to the northern wet season as this is the trigger to the IOD’s demise.
In the Pacific Ocean there is very little of note going on, with most indicators in neutral territory.
The Coral Sea is warmer and evolving a bit more cloud, which could be a good moisture source if a connection could be made to it.
The Southern Annular Mode has dived into strongly negative territory due to coupling with the Sudden Stratospheric Warming that occurred in September over Antarctica. This is unlikely to mean much for Victoria in spring.
The SAM has erratic behaviour at this time, but historically -ve SAM springs have greatest effect on the eastern seaboard, a drier east Gippsland is possible. This is due to the predominant wind flow in this area not being from the rain bearing east.
Pressure patterns have changed little being dominated by stronger than normal high-pressure systems with a centre that is further north than is usual for spring. This is continuing to block moisture transport from the north-west.
The majority of models predict that a drier and warmer November-January is the most likely outcome.
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 the South West and West Gippsland remaining wetter where other areas of the state are drier.
Pasture paddocks at Moyston, Baynton and Greta all used in excess of 50% of a full profile and cropping paddocks at Hamilton, Yalla-Y-Poora and Giffard all used in excess of 40% for the month.
Some cropping paddocks stopped water use due to hay cutting, fallow or crop maturity. Longwarry remains very wet.
Model distribution summary for the next three months
Graphs showing the distribution of global model forecasts for November-January, with models showing increased chances of drier rainfall and warmer temperatures.
Model consensus forecast for the next four to six months
Graphs showing the distribution of February – April forecasts with models showing increased chances of average rainfall and warmer temperatures.
Model consensus forecast for the next six months
|Current outlook (to 27 October)||Current outlook (to 27 September)|
|Indian Ocean||Cold (+IOD)||Warmer/neutral||Cold (+IOD)||Neutral|
|Rainfall||Slightly drier||Average||Slightly drier||Average|
|Temperature||Slightly warmer||Slightly warmer||Slightly warmer||Slightly warmer|
Sea Surface Temperature Anomalies
In the Indian Ocean the Dipole Mode Index (DMI) remains at strong positive IOD levels (+2.06°C as of 29 October). Cooler water off Indonesia and warmer water off Kenya.
The threshold for +IOD is +0.4°C. Sea surface temperatures (SST) anomalies along the Equatorial Pacific have warmed during October but remain at neutral temperatures.
NINO3 is at +0.41°C and NINO3.4 is +0.68°C (as of 29 October). 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 warmed during October replacing the coolness.
As a result, the water above has also warmed. 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 rose rapidly up to neutral levels during September. The value is currently at -5.2 (as at 29 October).
This indicates normal pressure patterns around the Equator.
Pacific Ocean Surface Wind Anomalies
The Equatorial Pacific Easterly Trade Winds are normal. In the Indian Ocean strong Trade Wind reversals off Indonesia continue consistent with a +IOD.
This is pushing warmer water to Africa and keeping cooler water upwelling off Sumatra.
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 this would be the only indicator backing that horse.
The lack of cloud (brown colour) off Sumatra and over Indonesia shows the typical +IOD pattern due to reduced evaporation off the cooler ocean in that region.
There has be reduced cloud over most of Eastern Australia and tropical connections to the NW have been weak.
Southern Annular Mode
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent the first half of October near normal and has then dove into strong negativity.
NOAA predicts the SAM to stay strongly negative for the next 14 days. This Sudden Stratospheric Warming that occurred over Antarctica in September has finally coupled with lower altitudes causing the SAM to go negative.
Winter is when the SAM has its greatest influence on rainfall over southern Victoria. In spring, a negative SAM would be expected to cause drying along the eastern NSW into far eastern 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.
The pressure ridge further north has been a feature for the last five months.
In winter this was helpful allowing fronts through south of the Divide, but in spring starts to be a hindrance to tropical moisture connections to the NW, consistent with the drying mechanism of a +IOD.
Air Pressure Anomalies
The Sub Tropical Ridge of High Pressure has been much stronger over the whole of Australia although here has been some weakening of this high pressure in tropical areas this month.
Higher pressure to the south of Australia has been chasing frontal and low-pressure rainfall triggers away. Higher pressure in the tropics has been making it hard to get moisture down from the north and north west.
Pressure at Darwin is higher and Tahiti is normal, which is why the SOI is slightly negative.
Modelled Climate and Ocean Predictions for Victoria from October 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.