The Fast Break – Victoria

Seasonal climate risk information for Victoria

Volume 18, Issue 10, 25 October 2023

A significant strong low-pressure system delivered high rainfall over much of the eastern half of the state in the first week of the month. This went some way to filling parched soil profiles in Gippsland and setting up a cracker spring for crops and pastures in the eastern Mallee, northern Victoria, central and northeast. This appeared to be a lucky weather system (welcomed by many, but not all…) that flew in the face of broader patterns leading up to and post the event. The southwest and Wimmera have been much drier than normal and consequently soil moisture has declined rapidly in crops and pastures in those districts. Some crops and pastures are unusually only half full. Modelled soil moisture across the southwest is ranked decile 1.

A fully functioning positive Indian Ocean Dipole is in full swing. Ocean temperatures have set up a classic cold pattern off Sumatra and warmer off Africa. The atmosphere rapidly coupled, in sync with the ocean, with higher pressure, less cloud and stronger easterly winds off Sumatra. This is quite late forming but would normally break down in November whenever the northern wet season kicks in. Most models predict the +IOD to break down in November. The model signals for drier are strongest for November with December/January predictions being more neutral.

The El Niño limps along further, with the atmosphere still struggling to stay permanently coupled to the ocean. Cloud and pressure patterns are partially behaving El Niño like, but a current strong burst of westerly wind in the western Pacific (due to cyclone Lola) is likely to reinvigorate the event. Models predict the El Niño to continue into autumn before it decays. It must be remembered that summer El Nino’s have historically been warmer but not necessarily drier.

The Southern Annual Mode (SAM) experienced a brief but moderate positive pulse in the first week of October that probably assisted with the rain in East Gippsland. In coming weeks, the SAM is predicted to be neutral.

Pressure patterns are often one of the key mechanisms though which the drier climate drivers manifest themselves. Stronger pressure over the whole country is indicative of +IOD and/or El Niño and is chasing rainfall triggers away.

My assessment of 12 climate models for Victoria is split between neutral and likely drier rainfall and likely warmer temperatures for the next 3 months.

Soil moisture

Compared to 30 days earlier than 24 September 2023

Probes from Agriculture Victoria, Dookie Land Management Group, Gecko Clan, Perennial Pasture Systems, Gippsland Agriculture Group, Murrayville Landcare and Riverine Plains Inc. networks.

The Australian Water Outlook (AWO) for early October rainfall wet up a diagonal strip east of a line from Mildura to Melbourne. Much of that country is now ranked normal for soil moisture. An area around central Gippsland is ranked wetter at decile 10. The east Northeast, far east Gippsland, western Mallee as well as most of the Wimmera are ranked drier at decile 2 to 3. Most of the southwest is ranked decile 1. Moisture probes rose where rainfall was greater than plant needs, but remained stable in some where water use is high. Many southwest crop and pastures decreased rapidly. The coastal dairy paddocks are unusually dry at less than half full.

The Gifford canola crop rose 62 percentage points from 13 to 75% and the Yalla-Y-Poora crop decreased 30 percentage points from 47 to 17%.

Soil moisture probe data can be accessed in real time at Agriculture Victoria’s soil moisture monitoring website. For pasture insights from soil-probe host farmers, see Agriculture Victoria’s October issue of the Soil Moisture Monitoring of Pastures newsletter.

Model distribution summary for the next 3 months

Predicted rainfall – November to January 2024

Predictions for November to January 2024, the outlook from 12 global model forecasts is split between neutral and likely drier across Victoria.

Model distribution summary for Victoria

Predicted temperature – November to January 2024

Predictions for November to January 2024, the outlook from 12 global model forecasts is split between neutral and likely drier across Victoria.

Model distribution summary for the next four-to-six months

Predicted rainfall – February to April 2024

Predictions for February to April 2024, the outlook from 9 global model forecasts is neutral for Victorian rainfall.

Predicted temperature – February to April 2024

Predictions for February to April 2024, the outlook from 9 global model forecasts is for likely warmer across Victoria.

Model consensus forecast for the next 6 months

Current outlook (24 October 2023)

Previous outlook (25 September 2023)

Sea surface temperature (SST) anomalies

National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch Daily 5 km SST Anomalies (Version 3.1) 22 October 2023.

The Equatorial Pacific Ocean surface has warmed further by a small amount in October. The oceanic values for NINO3 and NINO3.4 are +1.92^oC and +1.59^oC respectively (24 October). Weak to moderate El Niño conditions exist in the central Pacific above the threshold of +0.8^oC. Ocean temperatures to the north and northeast remain normal not in keeping with classic El Niño’s. A weak cooling along the coastal Northern Territory and Queensland is late forming. A strong +IOD oceanic pattern has set up in the Indian Ocean with very warm water off the African coast and cooler water off Sumatra. The Dipole Mode Index which measures the differential between the 2 monitored boxes is +1.79^oC, well over the +0.4 ^oC threshold.

Sea surface temperatures are the key to the world’s rainfall. For more information on how they are measured, maps created and how to read them, check out our eLearn.

Equatorial Pacific sub-sea temperature anomalies

Analysis done 23 October 2023

The Pacific Ocean Equatorial sub-surface temperature is very similar to previous months in October. The western Pacific is increasing in coolness which historically occurs with an El Niño but would be larger and cooler if reversed trade winds were to continue for a longer period.

Southern Oscillation Index (SOI)

30-day moving SOI

The SOI has been increasing and returning to border line El Niño values during October, currently at -7.5 (as of 21 October 2023). El Niño like values are lower than -8.0. While there is high pressure at Darwin, the pressure at Tahiti has been returning to normal or higher but should be lower.

© Copyright Commonwealth of Australia 2023. Bureau of Meteorology

Dipole Mode Index (DMI)

The DMI is the difference between the temperature of the west and east boxes of ocean monitored for the IOD. The DMI has stabilised at a high positive IOD value during October currently at +1.79 ^oC. The +IOD threshold is + 0.4 ^oC. Very warm water off Africa and cooler water off Sumatra and particularly in the monitored boxes is why the value is so positive.

© Copyright Commonwealth of Australia 2023. Bureau of Meteorology.

Pacific Ocean surface wind anomalies

Wind direction and strength anomalies for the last 30 days

Operational data: Surface winds (m/s) 30-day anomaly for: Sunday 24 September 2023 to Monday 23 October 2023.

The Pacific Ocean has experienced a number of small westerly wind bursts during the month but none that have been sustained. Currently the largest westerly wind burst for the year is underway, being forced by the rotation of cyclone Lola of Vanuatu. Stronger westerly wind in the western Pacific is needed to develop, strengthen and finally decay an El Niño but has been rarely apparent for most of this event. Winds in the eastern Indian Ocean have finally changed from southeast to much stronger easterly. This is helping the cold water to upwell off Sumatra and holding warm water off the coast of Africa.

(NCEP Operational climatology data: 1985 -1996, smoothed with five-day running mean) Source: NOAA

World cloudiness anomalies

OLR Anomalies: Average of 21 September 2023 to 21 October 2023

Cloud over the international dateline junction with the equator has increased to more convincing El Niño like levels. A greater cloud pattern still extends over to Papua New Guinea and would more normally be east of the Solomon Islands. The Coral Sea does not show the usual lack of cloud response to El Niño. A massive ‘eye’ of a lack of cloud is a characteristic +IOD footprint due to the cooler water off Sumatra.

Source: Bureau of Meterology

Southern Annular Mode (SAM)

AAO: Observed and GFS forecasts

Observations 26 June 2023 to 23 October 2023

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) has been neutral, bar a brief burst of moderate positivity in the first week of October. Historically positive SAM conditions in spring increase the chance of greater rainfall in far eastern Victoria. This was probably a cofactor in the high rainfall that week. The Bureau of Meteorology and NOAA models are in agreement to stay neutral in the next 2 weeks.

Source: NOAA

Air pressure

Last 30 days air pressure

Operational data: Sea level pressure (mb) 30-day mean for: Saturday 26 August 2023 to Sunday 24 September 2023.

In the past 30 days the Sub Tropical Ridge of High Pressure (STR) is further north than is normal for spring, where Adelaide would be a classic position. In spring this has a habit of preventing access to the tropical air further north. The absolute position of the high is from eastern Australia through to New Zealand which is pushing fronts and lows away from Victoria.

Source: NOAA

Air pressure anomalies

Last 30 days air pressure anomaly

Operational data: Sea level pressure (mb) 30-day anomaly for: Sunday 24 September 2023 to Monday 23 October 2023.

The Sub Tropical Ridge of high pressure remained much higher, particularly over western Victoria. This meant pressure systems did more blocking by moving slower. Pressure is higher at Darwin and marginally higher at Tahiti. The reason for a climbing almost neutral SOI is because the pressure is not lower at Tahiti.

(NCEP Operational climatology data: 1985 – 1996, smoothed with five-day running mean). Source: NOAA

Climate definitions

Read a list of climate acronyms and explanations.

Modelled climate and ocean predictions for Victoria from October 2023 run models

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

View a colour-coded version of this table: Modelled climate and ocean predictions for Victoria from October 2023 run models (WORD - 67.9 KB)

Our new e-learn explains this table.

Four coupled global circulation model forecasts

* Abbreviations:

NDJ =November, December, January

JFM = January, February, March

FMA = February, March, April

Four coupled global circulation model forecasts

* Abbreviations:

NDJ =November, December, January

JFM = January, February, March

FMA = February, March, April

Three ensembles and a statistical model forecast

* Abbreviations:

NDJ =November, December, January

JFM = January, February, March

FMA = February, March, April

Image references

Original images used in this document are sourced from the Bureau of Meteorology under a Creative Commons 3.0 licence and from the NOAA who have a Public Domain policy. Annotations highlighting areas of interest have been added by Dale Grey.