The Fast Break - Victoria

Seasonal climate risk information for Victoria

Volume 15 | Issue 5 | 28 May 2019

Adequate opening rains fell across most of the state, but some regions could do with more. Soil moisture has started to recharge in the higher rainfall areas.

The current weak El Niño like conditions in the central Pacific has moved little for the month. The undersea warm water support further weakened, as did the cloud patterns at the dateline. The pressure patterns also remain normal, so it is an uncoupled ocean-atmosphere event.

The Trade Winds are the one aspect of an El Niño which have reinvigorated and could be expected to maintain the current weak El Niño or to activate it a bit further. The majority of the surveyed models predict the Pacific sea surface temperatures will hang around El Niño thresholds for the next three months and then decay away for the spring.

All eyes have turned to the Indian Ocean this month. Half the models surveyed predict a positive Indian Ocean Dipole (+IOD) in the coming three months. At the moment excessive heat has built up off the African Coast and things are cool around the northern coast of Australia. This is worth watching for further westward propagation of the cooler water towards Sumatra.

There is less cloud cover to the west of the Indonesian box, and hopefully this doesn’t travel further eastwards. Pressure is higher across the whole Indian Ocean, which is not assisting moisture transport to the south. For the month, trade wind behaviour across the Equatorial Indian Ocean has been normal. Historically, +IOD’s lead to greater chances of a drier spring in Victoria.

Pressure patterns and the Southern Annular Mode (SAM) have been favourable this month for increased frontal activity. This has led to frequent light rainfall events particularly in the SW quarter of the state.

A few more models have shifted slightly drier, with models currently split between an average to drier winter with likely warmer temperatures.

Soil moisture

The BoM AWRA modelled plant available soil moisture shows increases over Victoria, with percentage increases really depending on rainfall totals over the past month.

The NW, central Gippsland and between Benalla and Yarrawonga in the NE have not had the good start that other regions have had.

The Perennial Pasture Systems and Agriculture Victoria soil probe network shows 12 sites increased by 10% over the month with moisture detected at 30cm depth, while all other probes stayed the same or with a small increase.

There were slight decreases of less than 10% in the NW, probably due to evaporation and indicating that rainfall has been insufficient to reach 30cm. Useful soil moisture exists to depth in those northern cropping paddocks that received the heavy December rains.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for June-August, with models leaning towards average-drier rainfall and warmer temperatures.

Model distribution summary for the next four to six months

Graphs showing the distribution of September to November forecasts with an increased number of models leaning towards drier rainfall and warmer temperatures.

Model consensus forecast for the next six months

Sea surface temperature anomalies

Sea surface temperatures (SST) along the Equatorial Pacific changed little over May and are just at or below El Niño temperatures. NINO3 is at +0.74°C and NINO3.4 is +0.76°C (as of 28 May), the threshold for an El Niño is +0.8°C.

The Dipole Mode Index (DMI) measure of the IOD has just gone into positive territory (+0.53°C as of 28 May), the threshold for +IOD is +0.4°C. The clear driver for this at the moment is the Western Indian Ocean heat, not Indonesian coolness.

It is also interesting to note that the only cool ocean in the tropics is off NW Australia. This is going to be worth watching in coming months.

Equatorial Pacific sub-sea temperature anomalies

The Pacific Ocean Equatorial sub-surface temperatures have lost a little heat at the surface and cooled at depth.

This is a very weak El Niño signature that would require more reversed trade wind activity off PNG if it was to warm further to depth.

Southern Oscillation Index

The SOI value is currently at -6.8 (as at 28 May) and has changed little over the month. This is within the threshold of normality between plus and minus eight.

At this time of year, the SOI becomes a more useful indicator of pressure patterns around the Equator, since the northern wet season is over. Once again, worth watching from now on.

Pacific Ocean surface wind anomalies

The Equatorial Pacific Easterly Trade Winds had been weakly reversed for the first two weeks of May, but have increased in strength for the last two weeks. It would be expected that this should start to warm the NINO3.4 region up further.

It should also have some effect in warming up the Western Pacific to depth.

World cloudiness anomalies

Cloud at the International Dateline (180^oW) junction with the Equator is slightly greater (blue colour). This is usually a classic sign of El Niño, but as it was in April, the westward extent of this extra cloud is not in keeping with a classic El Niño.

Cloud to the north of Australia was because of a late finish to the monsoon. There has been slightly less cloud in eastern Victoria (brown colour). The lack of cloud west of Indonesia and the NW into WA shows all is not well with the Indian Ocean.

This pattern is similarly shaped to a +IOD but is currently too far west of the classic position starting over Sumatra and Java. It’s worth watching in coming months.

Southern Annular Mode

The Southern Annular Mode (SAM) went strongly positive in early May and then dove into moderate negativity. In May, SAM has some influence in southern Victoria, but not as great as in winter.

NOAA predicts the SAM to stay negative for the next fortnight. Negative SAM means weather systems can be pushed closer to Victoria, due to a relaxing of the westerly winds spinning around Antarctica.

Air pressure

In the past 30 days, the Sub Tropical Ridge of High Pressure’s position has been higher than its normal autumn position close to Adelaide. This has allowed frequent fronts and storm systems across Victoria with numerous small tally rain events.

The predominant westerly stream has been blocked by the ranges, rain shadowing central Gippsland.

Air pressure anomalies

The Sub Tropical Ridge of High Pressure’s strength was slightly higher over mainland Victoria during May, but lower to the south.

Pressure has been particularly higher over WA which has allowed more rain triggers across the SE of Australia.

Darwin pressure is slightly stronger than at Tahiti which is why the SOI is slightly negative, but still within the neutral zone.

Modelled climate and ocean predictions for Victoria from May 2019 run models

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

The Fast Break is produced by Agriculture Victoria as part of the GRDC's (Southern and Northern Region) Using Seasons Forecasts Partnership Project.