The Fast Break - Victoria

Seasonal climate risk information for Victoria

Volume 15 | Issue 1 | 4 February 2019

A weak El Niño like ocean appeared in November and December 2018. Despite the central Pacific Ocean and surface interested in the concept of El Niño the atmosphere above in terms of pressure, cloud and wind patterns weren’t having a bar of it. Consequently, the attempt rapidly collapsed to neutral over January. But, a risk still remains that it could come back properly this year.

Current indicators are neutral, or in the case of cloud and trade winds weakly El Niño.

Similarly, to the same time in 2015, a large slug of warm water remains at depth in the western Pacific. Trade wind reversals to the east of PNG will need to be watched in coming months, as we don’t want to see westerly wind bursts force that warm water over to South America and kick off a more serious El Niño. Current model forecasts suggest a weak one can hang around over autumn but most think it will collapse in winter. Bare in mind, model skill at this time is poor as it will be essentially random weather around PNG that may or may not kick off a potential El Niño.

The one climate indicator that has been active over summer, has been a positive phase of the Southern Annular Mode. This can increase the eastern half of Victoria’s rainfall over summer but has only had limited effect on giving some rainfall to East Gippsland so far.

Pressure patterns generally have been behaving themselves, but a persistent high pressure in the Tasman Sea has stopped the passage of fronts and allowed frequent movement of warm air from the north, increasing temperatures.

Modelled rainfall predictions for Victoria are split between drier to average, but there is a strong model consensus for a continuation of warmer temperatures.

Soil moisture

The BoM AWRA modelled plant available moisture shows very little apart from the ranges and West Gippsland. The Agriculture Victoria and PPS soil moisture probes show a very different story in the north, with fuller profiles in many paddocks after the heavy December rain and extensive summer weed spraying.

Trend direction is in relation to late November 2018. Without weed control, cropping paddocks would be closer to the BoM values. Pasture paddocks in the south are also closer to modelled values.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for February to April, 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 May to July 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 through November and December were just at or above the El Niño level of 0.8^oC.

Temperatures have since cooled rapidly in January, such that NINO3 is at +0.38^oC and NINO3.4 is +0.36^oC (as of 3 Jan), both neutral temperatures. The Indian Ocean Dipole is neutral as is usual over summer. The Coral Sea is warmer than normal, but temperatures are a bit cooler off the NW coast

Equatorial Pacific sub-sea temperature anomalies

The Pacific Ocean Equatorial sub-surface temperatures peaked in October and were then on the decline. Support for the El Niño at the surface waned in December and dropped away in January.

Currently two distinct pools exist, a cooler in the east and a warmer in the west. If there is reversed trade wind activity in the west, it could potentially push the warmer water under to South America, which doesn’t rule out a second attempt at an El Niño this year.

Southern Oscillation Index

The SOI value is currently at -0.7. This indicates normal pressure patterns around the Equator as measured at Tahiti and Darwin. During summer the SOI is often affected by tropical weather and is a very unreliable indicator.

Pacific Ocean Surface Wind Anomalies

The Equatorial Pacific Easterly Trade Winds are normal in the east and show some vague reversal activity in the west. Watch for strong trade wind reversals as this could indicate a potential reforming of the El Niño.

World cloudiness anomalies

Cloud at the International Dateline (180^oW) junction with the Equator is slightly greater (blue colour). This is usually a sign of El Niño but in this case the cloud intensifies to the west instead of to the east.

This cloud is more in keeping with the warm Coral Sea. There is a lack of cloud in the Indian Ocean and I am not sure if it means anything over summer.

Southern Annular Mode

The Southern Annular Mode (SAM) has been essentially positive since November. In summer, a +SAM can be beneficial to rainfall in the eastern half of Victoria.

Over most of summer this has meant the far east has got some rain where the rest of the state has got next to none. 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 is marginally higher than its summer position of Melbourne. This is stopping some of the moisture from the north coming down.

The real feature has been the persistent high in the Tasman Sea that has been allowing hot northerly winds to sweep down. It’s also meant a block to the normal movement of the frontal systems that cause cool changes, meaning that extended heat waves were experienced, especially north of the Divide.

Air pressure anomalies

The Sub Tropical Ridge of High Pressure’s strength was normal to lower over Victoria but the extent of the higher than normal pressure in the Tasman Sea can be observed.

Pressure at Darwin and Tahiti are normal which is why the SOI is neutral.

Modelled Climate and Ocean Predictions for Victoria from January 2019 run models

Click image to view a larger verson.

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.