Victorian seasonal climate outlook
Adapted from: Bureau of Meteorology websitehttp://www.bom.gov.au
Milking the Weather Webinar 3, 6th June 2018 http://agriculture.vic.gov.au/agriculture/weather-and-climate/climate-webinars
The Fast Break - May 2018 update http://agriculture.vic.gov.au/agriculture/weather-and-climate/newsletters
The El Niño–Southern Oscillation (ENSO) is currently in a neutral state - neither El Niño nor La Niña. Over the coming months, most of the surveyed climate models predict that the central tropical Pacific Ocean will slowly warm. A few models are suggesting that El Niño thresholds may be reached by August or October. The Indian Ocean Dipole (IOD) is also currently neutral, with Model outlooks suggesting the Indian Ocean dipole (IOD) will remain neutral during the winter.
Rainfall and temperature summaries for autumn
It's really only the south west quarter of the state that was in the average decile range (4 -7) over autumn, as indicated by the white section in the map on the left. Everywhere else was at decile 2-3 (below average) or 1 (very much below average – in the lowest 10 % of records) indicated by the darker shaded red areas.
Rainfall and temperature summaries for Autumn
The lowest rainfall totals over autumn were mostly received in the northern and eastern parts of the state, particularly around the irrigation districts (indicated by the yellow coloured areas in the map to the right).
Average temperatures for March to May were 2 to 3 degrees warmer in the northern part of the state (indicated by the light brown shaded area in the map to the left). For the rest of the state, they were 1 to 2 degrees warmer (indicated by the yellow shaded area).
From the map below, we can see that coming into the winter season, there isn't much soil moisture north of the divide (particularly around the Goulburn Murray areas). Around the Macalister Irrigation District and coastal east region, it's very much below average (indicated by the mid orange shaded areas). It's looking a bit better in the south west of the state and around south and west Gippsland, indicated by the cream and pale blue shaded areas.
Sea Surface Temperatures
Looking at the surface of the Ocean at the moment, the NINO3 box (orange in the diagram below) and the NINO3.4 box (which is brown), NINO3 is 0.28°C warmer than normal and NINO3.4 is 0.13 °C warmer. The threshold is +0.8°C forEl Niño or -0.8°C forLa Niña. Since the summer La Niña, there has been some warming at the surface, but still in neutral range.
For more information about the NINO regions (as depicted in the diagram below), go to: www.ncdc.noaa.gov/teleconnections/enso/indicators/sst.php
Southern Annular Mode (SAM)
The SAM is essentially 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 essentially 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. In summer the outcome is reversed.
The SAM only really starts to make sense from winter onwards. We had some strong negativity back in April which didn't really lead to much. That means the fronts in theory were coming closer to us but they certainly had an effect in Tasmania. In recent months, SAM has been doing very little. The trend in the next week or so is to come up in a little positive blip and then back down to neutrality again. When it sits at normal and in negativity it means that those fronts are closer in coming through.
The Southern Tropical Ridge (STR)
The position of the sub-tropical ridge plays an important part in the way the weather in Australia 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), which are associated with stable and dry conditions; generally move eastwards along the ridge.
Regarding the diagram above, in terms of the pressure patterns around the Sub Tropical Ridge, the latitude of the high-pressure system (indicated by the red line) is around normal. In the middle of winter, we would expect high pressures to be positioned centrally around the top of the Great Australian Bite. The real problem over May has not been the latitude but the longitude influence. The east-west position of that high was just a classic blocking position for rain in most of Victoria, with exceptions in the western part of the state and around parts of west Gippsland.
Southern Oscillation Index (SOI)
Sustained positive values of the SOI above +7 typically indicate La Niña, while sustained negative values below −7 typically indicate El Niño. Values between +7 and −7 generally indicate neutral conditions.
When the SOI is between +5 and – 5, which is where it is at the moment (as indicated by the graph below) there's nothing going on. It really needs to be between +10 and –10 for something significant to take place. It's worth monitoring the SOI in winter and spring for Victorian rainfall outlooks.
Skill level of climate models at this time of year for the next three months is really low to moderate. This is one of the poorer times of the year for models to be predicting rainfall. Some models are saying slightly warmer and a couple are saying slightly cooler but essentially no great change is predicted. For the Indian Ocean we have the vast number of the models predicting the temperature to be neutral as well. Regarding rainfall, the vast majority of models are sitting on average.