Victorian seasonal climate outlook
The current Victorian summer climate outlook (December to February 2019) issued 29 November 2018, indicates there is no strong push towards a wetter or drier than average season. Although, warmer than average days and nights are likely.
The existing positive Indian Ocean Dipole (IOD) has been a significant contributor to dry conditions in Victoria, however models, on-balance, expect the positive IOD to follow its normal cycle and decay in the early part of summer.
Development towards El Niño in the tropical Pacific Ocean continues, with outlooks signifying El Niño conditions are likely through summer. However, El Niño typically has a weaker influence on Victorian rainfall during summer than it does in winter and spring.
There’s a lot heat in the Pacific Ocean undersea which has come to the surface and formed an El Niño. But no other atmospheric indicators suggest an El Niño. A proper El Niño would help remove this heat before winter, helping to mitigate the chances of an El Nino forming next year.
Climate models are mixed for Victoria, but this is not unusual for models predicting summer conditions. Mainly, average rainfall’s predicted but with a high chance of warmer temperatures. So, plan for any possibility.
Victorian rainfall and temperature summaries for spring 2018
The 2018 spring rainfall deciles across the state’s main dairy regions ranged between 1 and 6 but most were below average, in the range of 2 to 3 (light-red on the map on the left-hand side). Terang and Birregurra in the south west and Orbost in the coastal east were decile 1 (mid-red on the map on the LHS). Corryong, with a decile 6, and Rochester, with a decile of 4, both in the north of the state were in the “average” range.
The Victorian rainfall totals for the major dairying areas during spring 2018 were between 50-100mm and 200-300 mm. This is indicated by the three shades of green on the map on the right-hand side. The highest rainfall was mostly received along the Great Dividing Range extending into West and South Gippsland (indicated by the deepest shade of green on the map).
Average temperatures for spring 2018 were 1 to 2 degrees warmer for most of the dairying areas in the state (indicated by the large bright yellow shaded area) south west Victoria and a fair proportion of the South and West Gippsland dairying area was average to warmer (indicated by the cream-coloured area).
The map below shows the soil moisture at depth (up to 1m) across Victoria for December 2018, relative to a soil moisture of an ‘average year’. Across the major Victorian dairying areas, the northern irrigation region, the north east, Western Victoria and most of West and South Gippsland have above average “lower soil moisture” - indicated by the blue shading. Below average “lower soil moisture” was recorded in the coastal east region, Macalister Irrigation District (MID) and a small-pocket of South Gippsland.
This is indicated by the orange/red shaded areas and most of the coastal east, around Bairnsdale and Orbost, have “very much below average” (mid-orange shaded area) or in the lowest 1 per cent of lower soil moisture (indicated by the darkest-red shading) in the map below.
Sea Surface Temperatures
Sea surface temperatures in the tropical Pacific Ocean indicate El Niño (likely below average rainfall) or La Niña (likely above average rainfall) conditions for South Eastern Australia (including Victoria). The latest sea surface temperature anomaly map, below, shows warmer water along the Equatorial Pacific which has become an El Niño. The Equatorial Pacific Ocean surface is in an El Niño state, but the atmospheric conditions above aren’t. This is referred to as an uncoupled El Niño and therefore may not behave in the traditional El Niño-way.
Currently the NINO3 is equal to +0.92 °C and NINO3.4 is equal to +0.97 °C. This means both are above the El Niño threshold of +0.80°C. The Coral Sea is surface temperatures are average to warmer and this isn’t like an El Niño. If it were an El Niño it would normally be cooler.
Southern Oscillation Index (SOI)
The SOI is equal to the difference in air pressure between Tahiti and Darwin. The current pattern of high and a pressure around the Equator indicates neutral conditions. It is currently at +4.8 (indicated by the blue circle in the graph).
The Sub-Tropical Ridge (STR)
The position of the sub-tropical ridge plays an important part in how Australian weather varies from season- to- season. During the warmer-half of the year in southern Australia (November to April), the sub-tropical ridge is generally located to the south of the continent. High pressure systems, also called anticyclones, are associated with stable and dry conditions and generally move east along the ridge.
Currently, the centre of high pressure is a touch lower than the normal summer “Melbourne position”; indicated by the red line between the two high pressure systems located near the bottom of Australia in the air pressure map to your right. In summer, this positioning of the STR can result in the tropics coming-down a bit closer to Victoria, delivering moisture and greater humidity.
The SAM is the measurement of the frontal systems spinning around Antarctica and how close they are coming to southern Victoria and Tasmania. Also known as the Antarctic Oscillation (AAO), it describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the Southern Hemisphere. The changing position of the westerly wind belt influences the strength and position of cold fronts and mid-latitude storm systems and is an important driver of rainfall variability in southern Australia. In a nutshell, it’s a measure of the strength of polar westerlies in the Southern Ocean.
The SAM index is calculated by the differences in pressure between 40° and 60° latitude. The SAM pushes or pulls rain-bearing triggers away from southern Victoria. In winter, stronger westerly polar winds pull fronts away and slower polar winds push fronts closer. During summer, the outcome is reversed.
The SAM has been positive (refer to section circle in red in the graph below), which means little for western Victoria during summer; but a positive SAM during summer can increase rainfall in eastern Victoria, south east NSW and east Tasmania. The November rainfall map shows a clear +SAM like fingerprint in these regions. The NOAA predicts that the SAM will return to neutral in the next fortnight; perhaps ending the wet spell in the east.
Modelled climate and ocean predictions for Victorian run models
Agriculture Victoria’s assessment of 12 climate models shows a clear majority predicting an El Niño in the ocean surface to continue during summer. El Niño’s during summer have historically had a variable effect on Victoria’s summer rainfall, but usually result in warmer temperatures. The consensus of summer models currently predict average rainfall, but sniffs of wetter and drier as well. Planning for any rainfall possibility would be wise. Most models predict a warmer summer.
For a summary of all 12 current model predictions for the forecast months of December, January and February as well as March, April and May, refer to: http://agriculture.vic.gov.au/agriculture/weather-and-climate/newsletters/the-fast-break-victoria
The Very Fast Break Video
To view the most recent Very Fast Break video on seasonal climate and risk information for Victoria (produced 7 December 2018) go to: