The Fast Break - Victoria

Seasonal climate risk information for Victoria

Volume 15 | Issue 9 | 26 September 2019

Soils are drying out in the north due to high plant water use, where southern paddocks are mostly still damp.

The positive IOD (+IOD) has continued in ‘text book’ fashion. Cold water off Sumatra and warmer water off Africa are leading to decreased evaporation and cloud over Indonesia. Trade winds are reversed off Sumatra which is pushing warm water and moisture away from Australia’s north west and allowing the cooler water to keep upwelling.

Pressure is very high over Australia and in the tropical north, typical of +IOD years. This has set up stable and occasionally frosty weather, particularly in the north. The high-pressure ridge has not moved south as we would expect in spring which is starting to block moisture transport from the tropics.

The Equatorial Pacific Ocean is benign at the surface, with some cooling underneath in the east that’s probably going nowhere. Wind and cloud patterns and pressure patterns at Tahiti are also normal enough in the Pacific. The Coral Sea is warmer and giving off a bit more evaporation and would be a good moisture source if we could get a link to it.

The Southern Annular Mode (SAM) spent the month half negative and half positive, SAM becomes a much less reliable climate indicator in spring, throughout Victoria.

All climate models think the +IOD is hanging around for the rest of spring, where it would be expected to break down rapidly with the onset of the northern wet season. All models surveyed predict that drier is the most likely outcome for the next three months (something The Break team has never seen in the last 12 years). There is a consensus for warmer temperatures.

You can use the new Local Climate Tool to identify how historical +IOD events have affected rainfall in your area.

Soil moisture

The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture shows the high rainfall areas with wetter profiles. The northern regions have dried progressively towards the ranges this month.

The north is ranked average to drier than normal as is Central and East Gippsland. Much of the SW and West Gippsland is ranked wetter than normal.

Many cropping probes in the north are using significant water, with Youanmite dropping from 77 to 15% and Brim from 82 to 34% for the month.

Moyston, Lake Bolac and Paradise are the only pasture probes to have significantly used water for the month, dropping from 90 to74%, 100 to 69% and 100 to 56% respectively.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for October-December, 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 January-March forecasts with models showing increased chances of average rainfall and warmer temperatures.

Model consensus forecast for the next six months

Sea Surface Temperature Anomalies

In the Indian Ocean the Dipole Mode Index (DMI) shows a strong positive IOD (+1.5°C as of 25 September).

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 cooled a tiny bit further in September, but remain at neutral temperatures.

NINO3 is at -0.2°C and NINO3.4 is +0.11°C (as of 25 September). 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 in the east during August and September.

Some of this coolness is upwelling along the surface of the Equator. No models are predicting any La Niña like activity.

Southern Oscillation Index

The SOI dipped strongly into negative values during September. The value is currently at -15.3 (as at 25 September).

This would normally indicate pressure patterns around the Equator were El Niño like (greater than -8.0).

Instead it’s indicative of much higher pressure at Darwin and normal pressure at Tahiti, which in not El Niño at all.

Higher pressure over Darwin makes it harder to get moisture down to Victoria.

Pacific Ocean Surface Wind Anomalies

The Equatorial Pacific Easterly Trade Winds are essentially normal.

In the Indian Ocean however, Trade Winds are strongly reversed off Indonesia, consistent with a +IOD and pushing warmer water to the west and keeping cooler water off Sumatra upwelling.

World Cloudiness Anomalies

Cloud at the International Dateline (180°W) junction with the Equator is slightly less (brown colour) which is weakly suggestive of La Niña.

The lack of cloud (brown colour) off Sumatra and over Indonesia is a typical +IOD pattern due to reduced evaporation off the cooler ocean in that region. There is an abundance of cloud over the Coral Sea.

Southern Annular Mode

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent September split between negative and positive.

NOAA predicts the SAM to return to neutral and go weakly negative in the next 14 days.

Winter is when the SAM has its greatest influence on rainfall over southern Victoria.

In spring SAM has an erratic effect on Victoria’s rainfall, come summer, a positive SAM could be wetter in the eastern half of Victoria, or a negative SAM drier.

Air Pressure

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.

In winter this is helpful to frontal activity, but in spring, this flips to be unhelpful as it prevents the tropics from moving closer to Victoria and blocks the flow of moisture to our 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.

This 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 much higher and Tahiti is normal, which is why the SOI is negative.

Modelled Climate and Ocean Predictions for Victoria from September 2019 run models

Click image to view a larger version.

12 climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Victoria.

The Fast Break is produced by Agriculture Victoria as part of the GRDC's (Southern and Northern Region) Using Seasons Forecasts Partnership Project.