The Fast Break - Victoria
Seasonal climate risk information for Victoria
Volume 16 | Issue 5 | 27 May 2020
It has been drier in May, particularly north of the Divide, but few people are complaining as it’s allowed timely sowing of crops across the state and into perfect moisture. Most of the state’s soil moisture is now ranked as wetter than normal. A number of sites now have 100% full profiles which is very early for this to occur before true winter sets in. With only average rainfall from now some of these paddocks are likely to get very wet.
The Pacific Ocean made further inroads to looking pre-La Niña like with its pretty blue wiggle along the Equator. The surface cooled substantially due to enhanced trade wind activity in the central Pacific. Cloud patterns at the dateline are also indicative of La Niña, but as is often the case, it’s the pressure patterns not playing ball. The pressure at Darwin is still higher than Tahiti rather than being lower. The system will remain uncoupled if the SOI doesn’t go strongly positive.
The whole Indian Ocean remains warm but did cool off somewhat this month. Things are still very warm off the coast of Africa and until this changes there will be no -IOD. Winds have picked up blowing from the west into Sumatra which would be needed to continue to kick a -IOD event off. Cloud has also increased over Indonesia for the first time in six months.
The Southern Annular Mode (SAM) had been negative but has recently gone moderately positive. This has probably had something to do with things being drier in the north.
Pressure patterns while in a favourable position, being further north, were set up as a moisture blocking pattern to our north west. This meant just the fronts sneaking through southern Victoria.
My assessment of 12 climate models for Victoria shows a strong consensus for likely wetter rainfall and a split between likely neutral or cooler temperatures for the next three months.
The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture shows increases in soil moisture over the whole state, with lower values in the North-West.
The North-West is ranked normal and the far east is drier, but the rest of the state is ranked wetter than normal.
The Agriculture Victoria PPS soil probe network shows increases greater than 10 per cent in the south west locations. Many northern sites have no change for the month, with most rainfall effecting the top 30 cm above our sensors.
Birchip and Sheep Hills decreased by 18 and 10 per cent respectively, which is mainly a heavy soil equalisation effect rather than actual moisture loss.
The Dartmoor lucerne increased by 73 per cent and the Pigeon Ponds phalaris increased by 60 per cent.
Model distribution summary for the next three months
Predictions for June to August 2020, from 12 global model forecasts, for Victoria as a whole, with models showing a strong consensus for likely wetter rainfall and a split between neutral and likely cooler temperatures.
Model distribution summary for the next four to six months
Predictions for September to November 2020, from nine global model forecasts, for Victoria as a whole, with models showing a strong consensus for likely wetter rainfall and a split between neutral and likely cooler temperatures.
Model consensus forecast for the next six months
|Current outlook (to 27 May)||Previous outlook (to 27 April)|
|Jun – Aug||Sep – Nov||May – Jul||Aug – Oct|
|Pacific Ocean||Slightly cool||Cool (possible La Niña)||Slightly warm||Slightly cool/neutral|
|Indian Ocean||Warm (-IOD)||Warm (-IOD)||Slightly warm||Slightly warm|
|Rainfall||Slightly wetter||Slightly wetter||Neutral||Neutral/ slightly wetter|
|Temperature||Neutral/ cooler||Neutral/ cooler||Neutral||Neutral|
Sea surface temperature anomalies
Sea surface temperature (SST) anomalies along the Equatorial Pacific cooled significantly this month but are still at normal temperatures. NINO3 is at -0.21°C and NINO3.4 is -0.28°C (as of 25 May). A La Niña would be when NINO3.4 got below -0.8°C.
Oceans are still warmer to the north west and north east, an enhanced moisture source. The Dipole Mode Index is +0.31°C (as of 25 May), which is neutral. Positivity is being driven by the warm African box, but encouragingly the NE corner of this western IOD region cooled this month.
Admittedly there is a lot of cooling that needs to occur in that region before anything reminiscent of a -IOD. The threshold for IOD events is +/- 0.4°C.
Equatorial Pacific sub-sea temperature anomalies
The Pacific Ocean Equatorial sub-surface temperature anomalies have cooled further during May. Upwelling in the eastern Pacific is now occurring and is visible as the wavy blue region in the SST anomaly map.
Such behaviour is consistent with developing La Niñas, but it doesn’t guarantee they happen.
Southern Oscillation Index
The SOI spent most of May in neutral territory, currently at +1.5 (as at 25 May). More convincing evidence for a La Niña would be a significantly positive SOI.
This would be when pressure is lower at Darwin and higher at Tahiti, with a sustained SOI value above +7. Until the pressure patterns and the ocean surface are reinforcing each other, the system will be uncoupled.
Pacific Ocean surface wind anomalies
Trade Winds have been blowing slightly stronger in the central Pacific helping to upwell the cooler water at depth. This would need to keep going, to cool off the Pacific further.
From the SST anomaly chart it can be seen that the stronger trade winds have pushed warmer water around PNG. In the Equatorial Indian Ocean, winds were normal in the middle of the month, but have recently started blowing stronger westerly towards Sumatra.
This positioning is more in keeping with a -IOD and would be needed to continue for that to occur.
World cloudiness anomalies
Cloud at the International Dateline (180°W) junction with the Equator is reduced, this is more in keeping with La Niña (brown colour). A more convincing La Niña pattern would be if the cloud anomaly was more centred over the Equator.
For the last six months cloud over Indonesia and north of Australia has been less. A welcome change to this pattern has finally occurred with greater cloud off Sumatra and normal to greater cloud in the Arafura Sea.
The north west cloud band pattern visible last month has disappeared.
Southern Annular Mode
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent May split between weakly negative and moderately positive.
A positive SAM means the westerly winds around Antarctica have sped up and are pulling fronts further south of Australia.
SAM would traditionally have a greater impact on Victoria during winter. The NOAA 14-day prediction for SAM is trending towards staying weakly to moderately positive.
In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been positioned at a normal winter position over the Bight, some two months earlier than normal. This would suggest that frontal systems were able to come closer to Victoria.
While rainfall has been erratic in May temperatures have been moderated by the persistent cool south westerly winds, especially south of the Divide.
Air pressure anomalies
The Sub Tropical Ridge of High Pressure was normal in pressure over Victoria, but a large high over Western Australia curtailed the north west cloud band activity of the previous month.
The pressure is slightly higher at Darwin and at Tahiti which is why the SOI is normal. More convincing La Niña pressure patterns would see the pressure drop at Darwin.