The Fast Break - Victoria
Seasonal climate risk information for Victoria
Volume 15 | Issue 4 | 29 April 2019
It has been a quiet month on the climate scene. The weak El Niño continues just at the threshold. There has been no reversed trade wind support, so surface warming has stayed the same and the undersea heat in the Pacific has decreased.
The pressure patterns as measured by the SOI have been normal. The increased cloud at the Dateline, whilst reminiscent of El Niño, is still presenting westwards as well, not in keeping with El Niño.
At this stage, there is nothing classical about it. In the Indian Ocean conditions are normal to warmer across the whole basin with some cool patches resulting from cyclone disturbance. The majority of models predict the warm ocean temperatures to hang around for the next three months, and then models are split between them continuing on or collapsing.
What is more uncertain is whether the atmosphere will couple with the warm ocean to from a proper El Niño. The majority of models also keep the Indian Ocean warmer for the next three months and then are completely mixed as to the spring outcome, either a positive Indian Ocean Dipole, neutral or warmer.
Pasture soil moisture is low across the whole state with the exception of far North-East Victoria and far East Gippsland which have had a break. Cropping paddocks that received the large December rain have maintained good soil moisture to depth, where there has been good summer weed control.
The Southern Annular Mode (SAM) has been benign over April, neither pushing nor pulling weather systems away from the mainland.
Pressure patterns made an encouraging move north this month, now in a more typical autumn latitude across Adelaide. The problem was that the absolute pressure rose higher across Victoria and the position of the high worked as a block over the state preventing many rain trigger passages.
The consensus of 12 models is favouring average rainfall and warmer temperatures over the next three months, code for any rainfall outcome is likely. Model rainfall skill is split between low and moderate for this prediction period.
The BoM AWRA modelled plant available moisture shows some increases in the NE, West and far East Gippsland but the majority of pasture paddocks could be expected to be very dry to depth.
The Perennial Pasture Systems and Agriculture Victoria soil probe network decreased by single figure evaporation losses for the month. Useful soil moisture exists to depth in northern cropping paddocks, that benefited from heavy December rains.
Model distribution summary for the next three months
Graphs showing the distribution of global model forecasts for May–July, with models leaning towards average rainfall and warmer temperatures.
Model distribution summary for the next four to six months
Graphs showing the distribution of August to October forecasts with models leaning towards average rainfall and warmer temperatures.
Model consensus forecast for the next six months
|Current outlook (to 29 April)||Previous outlook (to 28 March)|
|May–Jul outlook||Aug–Oct outlook||Apr–Jun outlook||Jul–Sep outlook|
|Pacific Ocean||El Niño||Slightly warm/El Niño||El Niño||El Niño|
|Indian Ocean||Slightly warm||Mixed||Slightly warm||Neutral/Slightly warm|
|Rainfall||Average||Average||Average/slightly drier||Average/slightly drier|
|Temperature||Slightly warmer||Slightly warmer||Slightly warmer||Slightly warmer|
Sea surface temperature anomalies
Sea surface temperatures (SST) along the Equatorial Pacific stabilised over April to remain at weak El Niño temperatures. NINO3 is at +0.87oC and NINO3.4 is +0.76oC (as of 29 April), just at the 0.8oC El Niño threshold.
The Pacific Ocean is warm across the whole Equatorial region and the Coral Sea remains average or warmer as a good moisture source.
The eastern Indian Ocean returned to normal after cyclone Veronica stirred it up, but remains warmer as a whole.
The Dipole Mode Index (DMI) measure of the IOD is neutral (+0.09oC as of 29 April), as is usual over April. Nothing appears “broken” in the tropics.
Equatorial Pacific sub-sea temperature anomalies
The Pacific Ocean Equatorial sub-surface temperatures have decreased further in the amount of heat to depth. This further decreases the chance of a stronger El Niño, with no warmer pool of water to feed the surface. An unusual pattern of warmer anomalies over cooler exists across the whole Equatorial Basin.
Southern Oscillation Index
The SOI value is currently at -3.0 and has changed little over the month. El Niño like behaviour might be indicated by a value less than -8 in winter-spring, where current values indicate normal pressure conditions around the Equator.
The SOI will start to be a more reliable indicator of tropical climate as we come to the end of the wet season.
Pacific Ocean surface wind anomalies
The Equatorial Pacific Easterly Trade Winds have spent the last 10 weeks close to normal after the strong reversal in February. This has not allowed the weak El Niño to gain greater strength.
Close to PNG there has been some strengthening of the trades, which is the opposite of what would be expected for an El Niño.
World cloudiness anomalies
Cloud at the International Dateline (180oW) junction with the Equator is much greater (blue colour). This is usually a classic sign of El Niño, but the westward extent of this extra cloud is not in keeping with a classic El Niño.
Cloud to the north of Australia has been normal to slightly less, as no strong bursts of the monsoon occurred in April. Cloud over Victoria has been normal.
Southern Annular Mode
The Southern Annular Mode (SAM) came out of a moderate positive phase in early April and since then has weakly bounced around neutral. In May, SAM has some influence in southern Victoria, but not as great as in winter.
NOAA predicts the SAM to rise into weak positivity and fall to neutral over the next fortnight. Positive SAM can mean weather systems are pulled further south of Australia, negative SAM means they can be pushed northwards.
In the past 30 days, the Sub Tropical Ridge of High Pressure’s position has been centred close to Adelaide, a normal position for autumn.
This is hopefully a good sign as it will allow rainfall triggers like lows and fronts to come across southern Australia. The positioning of the average high pressure during April was right over Victoria pushing triggers south across to NZ.
Air pressure anomalies
The Sub Tropical Ridge of High Pressure’s strength was higher over Victoria during April, finally breaking a five-month low pressure pattern over summer.
Higher pressure meant slower moving weather systems that often block rain triggers from passing across Victoria.
Pressure is a more dominating factor over Victoria during winter. Darwin pressure is slightly stronger, and Tahiti is stronger, which is why the SOI is close to normal.