The Fast Break - Victoria

Seasonal climate risk information for Victoria

Volume 15 | Issue 3 | 28 March 2019

The Pacific Ocean is hovering around El Niño-like temperature and the warmer deep-sea temperatures, normally associated with an El Niño have made some progress over to South America. At this stage they aren’t providing much support to the ocean surface. Cloud patterns are typical of El Niño, but the SOI and trade winds are only loosely singing that tune.

So, currently the ideal of El Niño is pretty half-baked. This is a very early onset for El Niño, historically, with maybe only one other year where this has happened. Current ocean temperatures to the north of Australia are normal or warmer and aren’t in keeping with the cooler temperatures of a classical El Niño yet. All the 12 models surveyed suggest El Niño will be around for the next three months, two thirds continuing it on through winter.

Certainly, more trade wind reversal will be needed to lock it in and some of that will be random tropical weather. It’s important to remember that historically 75 per cent of growing seasons have been wetter, average or slightly drier when we have had El Niño’s before, and only 25 per cent of years have been very ordinary.

The seasonal break will be an act of weather with a predictability less than eight days. At this time of the year, the big climate drivers aren’t operational yet. Local pressure and rainfall trigger mechanisms are the critical ingredient needed to connect with a usually warm ocean to our north (being the end of summer and warm up north anyway).

SAM and the Sub-Tropical Ridge are not the major drivers in autumn though they are in winter. So, it’s a case of hurry up and wait! Models are split between average (code for anything is possible) and drier rainfall over the next three months but remember, model skill is at its poorest this time of the year. Strong consensus continues to indicate warmer temperatures for the next six months.

Soil moisture

The BoM AWRA modelled plant available moisture shows no change from last month with little soil moisture in any dryland pastures, except for far East Gippsland.

The Perennial Pasture Systems and Agriculture Victoria soil probe network show no significant changes either, with most probes drying by less than single digit percentages for the month. Cropping paddocks in the north that have sprayed out weeds have useful soil moisture at depth.

Having lasted through summer and with autumn cooling and decreased daylength, these amounts could be expected to remain till planting time.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for April-June, with models split between average/drier rainfall with warmer temperatures.

Model distribution summary for the next four to six months

Graphs showing the distribution of July to September forecasts with models split between average/drier rainfall with warmer temperatures.

Model consensus forecast for the next six months

Sea surface temperature anomalies

Sea surface temperatures (SST) along the Equatorial Pacific continued to warm slowly through March to just reach the El Niño temperature of 0.8^oC. NINO3 is at +0.81^oC and NINO3.4 is +0.85^oC (as of 27 Mar).

The Pacific Ocean is warm across the whole Equatorial region and the Coral Sea, which is not a classical El Niño pattern (warm in the east, cool in the west). The Coral Sea quickly returned to warmer after cyclone Oma cooled it down.

It remains to be seen whether the Indian Ocean to the north west also returns to normal temperatures after cyclone Veronica. The IOD is neutral, as is usual over early autumn.

Equatorial Pacific sub-sea temperature anomalies

The Pacific Ocean Equatorial sub-surface temperatures have experienced a slug of warm water making a rapid foray to the east. This was pushed over by a down-welling Kelvin wave set up by the reversed trade winds in February.

This now looks like a weak pre El Niño pattern, but in 2014 (a “failed” El Niño) and 2015 (a “successful” one) there were much larger expanses of +4^oC warmer water to depth in March.

Southern Oscillation Index

The SOI value is currently at -6.7 and has been exhibiting El Niño like behaviour (less than -8), significant if it was the middle of winter. The SOI is still being affected by tropical wet season weather phenomena.

During March there was high pressure at Darwin which has now passed, which is why the SOI is rising. Until the northern wet season is over (usually end of April) the SOI will be an unreliable indicator of El Niño.

Pacific Ocean surface wind anomalies

The Equatorial Pacific Easterly Trade Winds have returned to close to normal after the strong reversal in February. At the moment, the trade winds are flipping between week long bursts of normal and weak reversals in the western Pacific.

A fully formed El Niño would see continuous strong reversals for many months.

World cloudiness anomalies

Cloud at the International Dateline (180^oW) junction with the Equator is much greater (blue colour).  This is usually a classic sign of El Niño, but quite early for it to be locking in.

The warmest ocean in the world is under that spot and is evaporating the most moisture to form extra cloud. El Niño classically moves this pool of warmer water further to the east whilst La Niña moves it closer to Australia.

Decreased cloud to the north of Australia is associated with the passage of tropical weather systems.

Southern Annular Mode

NOAA predicts the SAM to rise into positivity over the next fortnight. Positive SAM can mean weather systems are pulled further south.

Air pressure

In the past 30 days, the Sub Tropical Ridge of High Pressure’s position is marginally lower than normal, centred around Melbourne. The STR should make a transition over autumn between the summer (Melbourne position) and the winter (top of the Bight) location.

It would be good to see the ridge start to move north, as the seasonal break rarely happens until it does. A slow transition has been a common feature in the last 25 years.

Air pressure anomalies

The Sub Tropical Ridge of High Pressure’s strength was lower over Victoria as Australia was sandwiched between two large blocking pressure systems off WA and over NZ.  Darwin pressure is slightly stronger and Tahiti is normal, which is why the SOI is slightly negative.

Pressure is a more dominating factor over Victoria during winter.

Modelled Climate and Ocean Predictions for Victoria from March 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.