The Fast Break - Victoria
Seasonal climate risk information for Victoria
Volume 15 | Issue 8 | 28 August 2019
While winter frontal systems have been keeping many in the game, all eyes are on the +IOD and hoping it won’t spoil the party.
Nothing much happened of note in the Pacific Ocean over August. Ocean temperatures at the surface and at depth are normal, trade winds aren’t doing much and cloud patterns are also normal. The Southern Oscillation Index (SOI) has moved into neutral territory given there was no support from the oceans while the SOI was negative over July.
The Positive Indian Ocean Dipole (IOD) remains the major climate feature. Ocean temperatures have cooled further around Indonesia and are warm enough off Africa for the Dipole Mode Index (DMI) to be above the threshold for +IOD. Lack of cloud over Indonesia is a classic +IOD pattern, as is stronger easterly winds around Java and Sumatra, helping to upwell the cooler water.
Water to depth is also cooler in this region, which suggest it’s not going away soon. All models show agreement in that a +IOD is likely to hang around for the next three months. Most of them predict it to break down in October/November as they usually do. The +IOD has historically increased the chances of a drier spring in much of Victoria.
Closer proximity of frontal systems due to a negative SAM and the Sub Tropical Ridge of high pressure allowing them to sweep across most of Victoria has kept us in the rain game so far. As these weaken into spring the question will then be whether or not the tropics step up to the plate.
As we might expect with a positive IOD, most models assessed predict higher chances of a drier three months ahead. There is less consistency between the models on temperature, however dry conditions with clear skies increase the chance of frost.
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 higher rainfall areas with filling/full soil profiles.
The northern areas are drier, as are parts of East Gippsland. Some of the northern probes have either not moved or gone down over the past month where rainfall is not keeping up with crop use.
South-West soils are full and North-East probes have remained steady, in Gippsland the Yarram and Giffard probes have increased for the month.
Model distribution summary for the next three months
Graphs showing the distribution of global model forecasts for September-November, with models leaning towards drier rainfall and no strong signal for temperatures.
Model distribution summary for the next four to six months
Graphs showing the distribution of December to February forecasts with models leaning towards average/slightly drier rainfall and warmer temperatures.
Model consensus forecast for the next six months
|Current outlook (to 29 August)||Previous outlook (to 29 July)|
|Sept – Nov||Dec – Feb||Aug – Oct||Nov – Jan|
|Pacific Ocean||Neutral||Neutral||Neutral/slightly warm||Neutral|
|Indian Ocean||Cold (+IOD)||Neutral/slightly warm||Cold (+IOD)||Neutral|
|Rainfall||Slightly drier||Average/slightly drier||Slightly drier||Average/slightly drier|
|Temperature||Average/slightly warmer||Slightly warmer||Average||Average/slightly warmer|
Sea Surface Temperature Anomalies
Sea surface temperatures (SST) anomalies along the Equatorial Pacific have cooled a little bit further during August so far and remain at neutral temperatures.
NINO3 is at -0.1°C and NINO3.4 is +0.2°C (as of 18 August), the threshold for an El Niño is +0.8°C.
The Dipole Mode Index (DMI) shows a positive IOD at +0.79°C (as of 20 August) continues in the Indian Ocean. The threshold for +IOD is +0.4°C.
Equatorial Pacific Sub-Sea Temperature Anomalies
The Pacific Ocean Equatorial sub-surface temperatures have done very little during July.
A cooler anomaly in the east is starting to upwell at the surface, but the warmth in the central Pacific is negligible compared to a classic El Niño.
Southern Oscillation Index
The SOI value is currently at -0.9 (as at 26 August) and spent the last week of July and August returning to neutral.
While the pressure patterns during July around the Equator were weakly El Niño like, this has quickly dissipated with neutral conditions now being experienced.
Pacific Ocean Surface Wind Anomalies
The Equatorial Pacific Easterly Trade Winds – as indicated by the red oval - have experienced some westerly (reversed) wind activity, but this doesn’t seem to have had any effect on the Pacific surface or the undersea.
World Cloudiness Anomalies
Cloud at the International Dateline (180°W) junction with the Equator is clear (white colour). The lack of cloud (brown colours) off Indonesia is typical of an +IOD pattern. There has been a lack of cloud over SE Australia.
Southern Annular Mode
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) has spent most of August in the negative phase.
A negative SAM means a weakening of westerly winds around Antarctica and a pushing of frontal systems and storm tracks closer to southern Victoria. Winter is when the SAM has its greatest influence on rainfall.
In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been at or slightly higher than a normal winter position (level with the top of the Bight).
This has been letting frequent fronts through, particularly in southern areas. The higher than normal pressure has been putting stronger than normal easterly winds around Indonesia helping to upwell the cooler water in that region.
Air Pressure Anomalies
The Sub Tropical Ridge of High Pressure was stronger than normal over the whole of the mainland, with anomalies higher for the current period than the previous month.
Higher pressure at Darwin makes it harder to get any northern moisture down to southern areas for making rain. Pressure at Darwin and Tahiti are relatively equal and that is why the SOI is close to zero.
Modelled Climate and Ocean Predictions for Victoria from August 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.