The Fast Break - Victoria

Seasonal climate risk information for Victoria

Volume 16 | Issue 1 | 11 February 2020

Welcome back Break readers to the 2020 instalments.

The +IOD and the -SAM that conspired to give eastern Australia very dry and hot weather in December both died around new year. The northern wet season which had been inhibited by these drivers at that time was then able to kick off. Since then, summer weather has been a bit erratic.

At the moment, both the El Nino Southern Oscillation and the Indian Ocean Dipole are in neutral phases as is normal for now. We reach the dodgy time of the year where random things can happen to push them in a direction for winter, with limited predictability. Ocean temperatures to our north are generally much warmer, which instigates greater evaporation. Models agree that the Indian and Pacific Oceans are likely to stay warmer for the next three months to the north of Australia.

We currently have enhanced cloud formation over the Coral Sea to the north-east of Australia. This is providing a good moisture source but only if the right triggers drift past. Decreased cloud to our north and increased pressure, are a hang-over from mid-January, where more recent measurements of cloud at least, are closer to normal. The higher pressure north of Darwin is normally not a good omen for moisture transport.

The Southern Annular Mode is now behaving itself, bouncing around from weakly positive to weakly negative. This would be expected to have little influence on Victoria’s climate.

Pressure has been lower due to the positioning of large stable highs to our west and east. If this pattern continues, we could expect some more humid and unstable weather.

Models are in agreement that no major climate factors are influencing the rainfall predictions at the moment. They are firmly sitting on the fence for any outcome is possible for the next three months, so plan for drier, average or wetter. Such predictions are common at this time of the year. Temperature predictions are split towards warmer or average.

Do you want to hear the latest science, insights and innovations happening in the climate and agriculture space? Then the 2020 Climate Webinar series is where you will hear from the experts from Agriculture Victoria, BoM, CSIRO and other agencies.

The series will run during lunchtime or you also have the option to listen to recordings at a time that suits.

For more information and to subscribe to keep up-to-date with these upcoming climate webinars, see Climate webinars or email heather.field@agriculture.vic.gov.au.

Soil moisture

The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture shows increased levels across the whole state, but the Mallee, Wimmera and Central Gippsland have changed the most. This has been due to patchy stormy rainfall.

The Agriculture Victoria soil probe network shows a few sites having picked up some stored moisture due to these storms and one that has gone down with some summer plant growth.

Raywood, Baynton and Pigeon ponds have all increased by around 44% and Taylors lake has dropped by 15%.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for February-April, with models showing equal chances of wetter, average or drier and a split between average or warmer temperatures.

Model distribution summary for the next four to six months

Graphs showing the distribution of May-July forecasts with models showing equal chances of wetter, average or drier and greater chances of warmer temperatures.

Model consensus forecast for the next six months

Sea surface temperature anomalies

The +IOD finally broke down in the first week of January which was historically very late

Current values of the Dipole Mode Index are -0.06°C (as of 11 February) which are totally neutral. This is to be expected for this time of the year. Sea surface temperature (SST) anomalies along the Equatorial Pacific have slightly cooled over summer to remain at neutral temperatures.

NINO3 is at +0.11°C and NINO3.4 is +0.24°C (as of 11 February). The Coral, Arafura and Timor Seas to our north are warmer than normal and evaporating more moisture. If the right trigger can be received summer rainfall is possible.

Equatorial Pacific sub-sea temperature anomalies

The Pacific Ocean Equatorial sub-surface temperatures anomalies have changed little over the last three months.

A warm patch exists below the western Pacific, which could be vulnerable to reversed trade winds pushing it under and over to South America.

Southern Oscillation Index

The SOI has been in  neutral territory for the last three months. The value is currently neutral at +0.1 (as at 11 February). In summer, the SOI is a less reliable indicator, due to large swings in local pressure at Darwin and Tahiti when cyclones occur.

Pacific Ocean surface wind anomalies

Paradoxically there is some reversal to westerly off Papua New Guinea. The convergence of these two is causing greater cloud formation.

World cloudiness anomalies

Cloud at the International Dateline (180°W) junction with the Equator is much greater (blue colour) which is suggestive of El Niño, but the projection of greater cloud further west into the Coral Sea is not in keeping with El Niño.

The lack of cloud (brown colour) over Indonesia still shows some hang over from the +IOD pattern of last year. Greater cloud over eastern Australia has been from moisture streaming in from the warm Coral Sea.

Southern Annular Mode

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) was negative into December but has remained close to normal during January and into February.

In summer a negative SAM would classically cause drying in the eastern half of Victoria. The finger can be pointed at SAM for aiding the dry spring and December in far eastern Victoria.

NOAA predicts that for the next 14 days the SAM will have a small negative burst. Once we reach autumn SAM has a variable effect on our climate.

Air pressure

In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been slightly north of the normal summer position of Melbourne. This has been allowing a few rain events through south of the Divide and particularly for West Gippsland.

We have been situated between two high pressure systems which has been moderating the extremes of climate. There has been more troughing of moisture from the north.

Air pressure anomalies

The Sub Tropical Ridge of High Pressure has been lower over SE Australia, indicating more opportunity for rainfall and unstable weather.

Pressure at Darwin and Tahiti is slightly higher which is why the SOI is neutral. Slightly higher pressure to our tropical north is linked to the lack of cloud in that region too.

Modelled climate and ocean predictions for Victoria from January 2020 run models

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