Tasmania

Seasonal climate risk information for Tasmania

Volume 3 | Issue 1 | 12 February 2020

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The +IOD and the -SAM that conspired to give the NE 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 Tasmania’s climate.

Pressure has been normal but you have been sandwiched between large stable highs to the west and east. If this pattern continues, we could expect some more humid and unstable weather, particularly in the north

For the next three months 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, so plan for drier, average or wetter and cooler, average or warmer. Such predictions are common at this time of the year.

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 visit: http://agriculture.vic.gov.au/agriculture/weather-and-climate/climate-webinars or email heather.field@agriculture.vic.gov.au

Soil moisture

The BoM Australian Water Resources Assessment (AWRA) modelled soil moisture map shows plant available soil moisture (10-100cm).  The Midlands is now rated very dry for dryland paddocks. The western half is wet and normal the eastern half of Tasmania is rated average to drier.

We would like to plot some publicly available dryland soil moisture probe values on this map for comparison, please contact us if you have remote telemetry, an upper and lower limit and are interested.

Model distribution summary for the next three months

Model distribution summary for the next four to six months

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^oC (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^oC and NINO3.4 is +0.24^oC (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

The Equatorial Pacific Easterly Trade Winds have shown stronger easterly behaviour in the Central Pacific.  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^oW) 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 NE of Tasmania. The finger can be pointed at SAM for aiding the dry December in the NE. 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 frontal events through. 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 normal over Tasmania, and Tasmania has missed some of the rainfall and unstable weather associated with the lower pressure to the north. 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 Tasmania from January 2020 run models