The Fast Break - Victoria

Seasonal climate risk information for Victoria

Volume 14 | Issue 11 | 27 November 2018

The Pacific Ocean surface and undersea temperatures remain in an El Niño state, but little else is interested in making this a fully-fledged event. Trade winds, cloud patterns at the Dateline and pressure patterns around the equator remain at normal levels. This indicates an uncoupled ocean and atmosphere. In this case, local influences on climate are more likely than classical El Niño effects. Models are keen on the temperature of the Pacific to last at El Niño thresholds for the next five months.

Historically, when El Niño’s have occurred over summer, the rainfall has been evenly split between wet, average or dry across Victoria. Autumn El Niño conditions would not necessarily be cause for alarm either, as the Pacific would normally be resetting back to neutral sometime around then. It would be good to see a proper coupled El Niño over summer, to disperse some of the heat out of the Pacific Ocean leaving less around for next season.

Summer rainfall predictions are spread around wetter, average and drier, but most models are favouring average. Such a wide mix is not uncommon from the models in summer.

Historically, the most likely effect of a summer El Niño is warmer temperatures and in this the models are in strong agreement.

Soil moisture

The BoM AWRA modelled soil moisture shows very little in the North-West and greater amounts in the east. The PPS and our soil moisture probes show the northern probes have barely moved for November with limited rainfall and senesced crops. On the foothills of the divide there was rapid water use to remove the remainder of the profile. In the south, probes have not moved or gone up, due to sufficient rainfall to keep the profile unchanged. The exception is Giffard which is still bone dry.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for December to February, with more models suggesting rainfall as average with warmer temperatures.

Model distribution summary for the next four to six months

Graphs showing the distribution of March to May modelled split between rainfall as average or drier, and likely temperature as warmer.

Model consensus forecast for the next six months

Sea Surface Temperature Anomalies

Sea surface temperatures (SST) along the Equatorial Pacific have remained stable over November. NINO3 is at +0.81^oC and NINO3.4 is +0.78^oC (as of 30 Nov), just at El Niño thresholds (+0.8^oC). The warming is currently along classical lines starting from Ecuador. Most models are predicting a Modoki type warming in the NINO 3.4 region. The Indian Ocean remains at a weak Positive IOD, being driven by Kenyan warming rather than Sumatran cooling. The Dipole Mode Index, (DMI) has remained above the threshold of +0.4^oC for more than two months and has officially been classified as +IOD event. The Coral Sea is average and a good moisture source to Australia’s North East but has a cooling trend.

Equatorial Pacific Sub-Sea Temperature Anomalies

The Pacific Ocean Equatorial sub-surface temperatures have cooled a little in November but remain in an El Niño pattern with warmth to depth.

Southern Oscillation Index

The SOI value is currently at -0.1. During November, the pressure patterns around the Equator have shown no interest in behaving in an El Niño fashion yet. The ocean and atmosphere remain uncoupled.

Pacific Ocean Surface Wind Anomalies

The Equatorial Pacific easterly Trade Winds were normal across most of the region during November. The reversed trades near PNG would need to extend to the middle of the Pacific to intensify and develop the El Niño over summer.

World Cloudiness Anomalies

Cloud at the International Dateline (180^oW) junction with the Equator is now normal (white colour). An El Niño would be expected to show an abundance of cloud (blue colours) in this region. The +IOD lack of cloud off Sumatra has dissipated but a lack of cloud across the whole of the northern region indicates the wet season is yet to fire up.

Southern Annular Mode

The Southern Annular Mode (SAM) started November in neutral, but then moved into a moderately strong positive phase. In summer, a +SAM can be beneficial to rainfall in the eastern half of Victoria. NOAA predicts the SAM to remain moderately positive for the next fortnight.

Air Pressure

In the past 30 days, the Sub Tropical Ridge of High Pressure’s Position has moved a bit further north than usual. During summer though, its influence over Victoria’s climate is not as great as in winter. Of greater interest has been the high-pressure positioning in the Tasman Sea and off Western Australia, causing a frequent low-pressure trough region over Victoria. Therefore, there has been northerly winds, humidity and some thunderstorm rainfall during November.

Air Pressure Anomalies

The Sub Tropical Ridge of High Pressure’s Strength was lower over Victoria during November, allowing some connections to the tropical moisture. Pressure at Darwin and Tahiti are both slightly higher which is why the SOI is neutral.

Modelled Climate and Ocean Predictions for Victoria from November 2018 run models

Click on image for a larger version.

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