Victorian seasonal climate outlook
Bureau of Meteorology website - September 2018, http://bom.gov.au
Spring Seasonal Outlook - August 2018, http://agriculture.vic.gov.au/agriculture/weather-and-climate/climate-webinars
The Fast Break Vic: Vol 13, Iss 8 - http://agriculture.vic.gov.au/agriculture/weather-and-climate/newsletters/the-fast-break-victoria
The 2018 attempt at an El Niño has still failed to arrive thus far. Pacific Equatorial surface temperature and temperature to depth, cloud and pressure are still not interested in El Niño. Recent reversal of the trade winds suggests a second attempt with further warming of the undersea. The BoM ENSO Outlook is still at El Niño WATCH, but the timeline has now been pushed out to November. This contrasts with 2015, where the ENSO outlook had declared an El Niño by June.
Despite most models predicting a +IOD, this hasn’t happened either. Water is normal off Africa and trade winds are also normal across the Equatorial Indian Ocean. The +IOD area of cold water off Indonesia has led to a +IOD like lack of cloud in that region. Pressure over the tropical Indian Ocean is generally higher, which isn’t helping moisture transfer from the North west.
The Southern Annular Mode was negative for most of August, leading to frequent fronts, but a lack of tropical connection has hampered the north of the state. A lack of East Coast and cut-off lows is not helping East Gippsland.
Most models still predict a late spring forming El Niño but predictions are mixed about whether we will get a +IOD now.
Rainfall predictions, while not as emphatic as in July, are still balanced between drier and average. Temperature predictions are also split between average and warmer.
Planning for the event of a drier or shorter spring could be time well spent.
Rainfall and temperature summaries for winter
South west Victoria and West and South Gippsland experienced rainfall within the average decile range over winter, indicated by the white on the map. With some small patches in the South west experiences above average rainfall (blue). Other parts of the state, as indicated by the red shades, was at decile 2–3, with or 1 (very much below average – in the lowest 10 per cent of records) indicated by the darker shaded red areas.
The highest rainfall totals over winter were mostly received in the South west and along the Great Dividing Range extending into West and South Gippsland. The lighter green areas of the map indicate areas that received less rainfall.
Average temperatures for June to August were one degree warmer for most of the state. There was a small area around Mildura that was two degrees warmer than the average. There was also a small patch around Portland that was one degree cooler than the average for winter.
The map below is showing the soil moisture at depth (up to 1m) across the State relative to a soil moisture of an ‘average year’. The map is showing that areas of Eastern and Northern Victoria are showing below average soil moisture. Some parts of the South west and South Gippsland are showing average to above average soil moisture levels at depth as illustrated by the blue and white areas.
Sea surface temperatures
Sea surface temperatures in the tropical Pacific Ocean are used to predict if El Niño (below average spring rainfall) or La Niña (above average spring rainfall) conditions for South eastern Australia are likely. Sea surface temperatures in the tropical Pacific Ocean have been slightly warmer than average above the equator and around average on the equator. The threshold for El Nino conditions is when the sea surface temperature is greater than 0.8°C warmer.
The indicators of sea surface temperature are currently 0.2°C NIN03, 0.3°C Nino3.4 and 0.6°C NINO4 warmer. These temperatures are less than 0.8°C, indicating a neutral ENSO state, meaning it’s neither El Niño or La Niña conditions. According to the Bureau of Meteorology most models are predicting that the ocean will continue to slowly warm with a weak El Niño event occurring in November.’
Southern Annular Mode (SAM)
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. In summer the outcome is reversed.
A negative SAM indicates more frequent winter rain-fronts. There was some strong negativity throughout August which was also supported by more frequent fronts, resulting in rainfall throughout the South west and South Gippsland. SAM has recently trended strongly positive but starts to lose its influence over much of Victoria in spring. The exception is, a positive SAM can be conducive to rain in the far east of the state over spring and summer.
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.
The Southern Oscillation Index (SOI) has an average value of -3.9 for the last 90 days, indicating that there is neither El Niño or La Niña.
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.
The subtropical ridge was in a higher than normal position during winter (refer to the red line over Australia marked in the map on the left). The location of the ridge further north of the Great Australia Bight can result in frequent fronts, which has occurred, but there has been limited connection with the moisture coming in from the tropics. While some parts of Western Australia, Tasmania and south-western Victoria have benefitted with frequent fronts of rainfall, other parts particularly North of the Great Dividing Range have missed out.
Indian Ocean Dipole (IOD)
The Indian Ocean Dipole (IOD) is a measurement of the difference in Sea Surface temperatures in the Tropical Western and Eastern Indian Ocean. According to the Bureau of Meteorology, the IOD is currently neutral with three out of seven climate models predicting a positive IOD may be likely. There has been a large volume of colder water off the coast of Indonesia during winter, which has been behaving positive IOD like. This has been adversely affecting the north west cloud band connection to tropical moisture down through central Australia. A positive IOD in spring could lead to decreased rainfall.
Climate and Ocean Predictions for Victoria – August 2018 run models
Twelve climate model predictions over the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Victoria. Nine of the twelve climate models are predicting a slightly drier Spring, with average to slightly warmer temperatures.