South Australia

Date: 30 Nov 2021

Seasonal climate risk information for South Australia

Volume 4 | Issue 11 | 30 November 2021

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We are describing the cropping areas of South Australia broadly as south of a line starting at Ceduna, east to Port Augusta and down to Renmark. Use of the term’s east and west in this region refer to Port Augusta as a reference, or north and south of Adelaide. Local regions will be used if the models are more specific.

A wetter month over much of the state has not been kind to grain harvesters and hay makers alike, but the southern crops are maturing well. Lower rainfall in the south east saw significant in-crop water use. High rainfall totals in the northern EP and upper north, have at best put some useful soil moisture away for next year, if the weeds can be kept at bay.

The Bureau of Meteorology declared a La Niña event mid-month with a sufficient number of the indicators demonstrating the existence of a coupled ocean and atmosphere in the Pacific. This followed earlier declarations by NOAA and the WMO. The ocean temperatures are not reaching the classic threshold of -0.8oC, but the differential between warm and cool water north of Australia shows classic La Niña behaviour regardless. Undersea temperatures, cloud patterns, pressure and wind patterns are all concurring with a La Niña. Most models predict this event to last until the end of February. Historically, La Niña’s over summer have resulted in greater rainfall across the Mid and Upper North and the north west EP (but with great variability), other regions have been more erratic.

Want to hear more about what a La Nina is and does? Listen to My Rain Gauge is Busted podcast series on the Agriculture Victoria website.

The Indian Ocean has not got the memo that it should be over, having a last gasp effort to couple and behave in a classic -IOD fashion. While ocean temperatures are not strongly indicating -IOD, the cloud, wind and pressure patterns are showing the strongest -IOD behaviour all season. Models predict the -IOD to be normal over summer, but this will only start to happen once the wet season starts in the Eastern Indian Ocean. This should be soon, which will instantly neutralise the Indian Ocean.  The combination of weak La Niña and weak -IOD is leading to plenty of moisture availability in the north when the right triggers exist in the south.

The Southern Annular Mode was positive all month, but this classically has had an erratic effect over summer in southern SA. There are multiple models showing evidence of a positive SAM over summer, which often goes together with a La Niña.

Pressure patterns throughout November were conducive to troughing moisture down from the north, with a gap between high pressures into SA in the first fortnight of the month. Pressure was higher but the existence of a number of weak low-pressure systems though the month was enough to drag moisture down. Higher pressure and a southerly aspect to the pressure position off SA’s southeast, was not conducive to rainfall triggers joining with the moisture, until they passed into central Victoria.

My assessment of 12 climate models for South Australia shows a split between wetter and neutral rainfall and a split between likely warmer and neutral temperatures for the next three months.

Soil moisture

map of SA showing soil moisture probes measurements.
The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture (10-100cm) decile ranks much of the state wetter than normal with some of the north west Eyre Peninsula ranked in the highest one per cent of years. The lower Yorke, eastern KI, southern Mallee and the south east are ranked more normal. The probe network (courtesy of AIREP, AgByte and MFMG) shows many significant increases across the cropping regions with storm lotto paying an unwelcome visit. Only the two most southern probes decreased, with rainfall not keeping up with moisture usage. Jamestown increased by a whopping 61 percentage points from 8 to 69 per cent and Conmurra decreased by 37, from 50 to 13 per cent.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for December-February, with models showing increased chances of drier rainfall and warmer temperature.
Graphs showing the distribution of 12 global model forecasts for December to February 2022, with a split between likely wetter and neutral rainfall and a split between likely warmer and neutral temperatures for the next three months.

Model distribution summary for the next four to six months

Graphs showing the distribution of March-May forecasts with models showing increased chances of average rainfall and warmer/average temperatures.
Graphs showing the distribution of nine global model forecasts for March to May 2022, with neutral forecasts for rainfall and temperature.

Model consensus forecast for the next six months

Current outlook (29 November)

Previous outlook (27 October)

Dec-Feb

Mar-May

Nov-Jan

Feb-Apr

Pacific Ocean

Cool (La Niña)

Slightly cool (possible La Niña)

Cool (La Niña)

Slightly cool (possible La Niña)

Indian Ocean

Normal

Normal

Slightly warm (weak -IOD)

Normal

Rainfall

Neutral/wetter

Neutral

Wetter/neutral

Neutral

Temperature

Warmer/neutral

Neutral

Mixed

Neutral

Sea surface temperature anomalies

Equatorial Pacific Ocean Sea surface temperatures and the IOD are at neutral levels.
The Equatorial Pacific Ocean stalled in November, with NINO3 and NINO3.4 values at -0.65oC and -0.52oC respectively (as of 30 November). This would normally be below the threshold to call a La Niña but the BoM assesses that the differential between the warm western water and the central Pacific, exhibits the correct behaviour. The ocean to the north of Australia is much warmer than normal and capable of evaporating more moisture. The classic horseshoe shape pattern of warmer water in the northern and southern Pacific is a strong signature of La Niña. Most models predict the La Niña to last for summer. In the Indian Ocean the decaying negative IOD has taken its time about it and is still weakly exhibiting warmer water around Sumatra compared to Africa. The Dipole Mode Index (DMI) is currently at -0.32oC just below the -0.4oC threshold. Most models predict the Indian Ocean to return to normality, which would be expected as soon as the northern monsoon season starts.

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: https://rise.articulate.com/share/kJceF16KasOxoeypJi1UWsAz0IzKKUF1

Equatorial pacific sub-sea temperature anomalies

Equatorial undersea temperature anomalies in the Pacific have shown little change for some months.
The Pacific Ocean Equatorial sub-surface temperatures have weakened in the central Pacific but cooled in the far eastern Pacific. This supports the model forecasts of a short-lived La Niña event.

Southern oscillation index

The SOI value is currently at -1.7 (as at 2 March).
The SOI showed some weakness below the +8.0 threshold for La Niña this month but has rocketed up recently to a strongly positive value, currently at +11.1 (at 30 November 2021). Pressure patterns around the equator are like La Niña with much higher pressure at Tahiti and normal pressure north of Australia.

Dipole Mode Index (DMI)

image of DMI-july
The DMI is the difference between the west and east boxes of ocean monitored for the IOD. The index has taken longer to reach neutral due to the fact the northern Monsoon has not yet started in the Indian Ocean to neutralise the weak -IOD. The current value is -0.32oC (as of 30 November 2021) with a threshold for -IOD of -0.4oC. Warming off Sumatra in currently slightly greater in the east box compared to the west box.

Pacific ocean surface wind anomalies

The Equatorial trade wind are reversed off PNG but are stronger in the Central Pacific.
Easterly trade winds in the Pacific have continued to remain very strong pushing warmer water to Australia’s north. This is classic La Niña behaviour. In the Indian Ocean the stronger westerly trade winds have been sustained for another month in keeping with a -IOD. This is pushing the warmer water to the Sumatran coast.

World cloudiness anomalies

Cloud is abundant over the Dateline and lacking to the north of Australia.
A lack of cloud over the international dateline junction with the equator has persisted for another month. This is a classic La Niña-like cloud pattern. For the first time this year the -IOD atmosphere has started to look like a -IOD. Much greater cloud over the eastern box of the IOD is in keeping with greater convection of moisture in that zone. A negative IOD normally increases north west cloud band activity and there is some evidence of that. Greater cloud over eastern Australia has resulted in more rainfall for eastern Australia and cooler maximum but warmer minimum temperatures.

Southern annular mode

the SAM had spent most of February in neutral.
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent November in a weak to moderate positive phase. Currently moderately strong, the NOAA and BoM models are expecting the SAM to fall and bounce back to moderate positivity for the next 14 days. A positive SAM has historically had a variable effect on coastal SA.
the STR of high pressure has been at a close to normal summer position of Melbourne.
In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been at a normal latitude centred around Adelaide. For much of the month the state was in between the two high pressures allowing troughing northerly moisture to come down. The positioning of the high to the west and south of SA put downward pressure on rainfall triggers to miss the South East and KI.
Pressure is slightly higher at Darwin and normal at Tahiti, the SOI is neutral. Tasmania has seen close to average pressure for February
The sub-tropical ridge of high pressure was higher in pressure during November. A small low centred over New South Wales was indicative of a few rainfall triggers that sat there providing some moisture feed on the easterly wrap around. Pressure is higher at Tahiti and normal over Darwin which is why the SOI is positive like La Niña. A large area of low pressure over the IOD east box in the eastern Indian Ocean is the best evidence to date of strong ocean atmosphere coupling of the -IOD, all this as it’s supposed to be decaying!

Modelled Climate and Ocean Predictions for South Australia from November 2021 run models

12 climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and Temperature for South Australia.
12 climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and Temperature for South Australia.
Click here to download this table in MS Word format
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