South Australia

Date: 31 Jul 2020

Seasonal climate risk information for South Australia

Volume 3 | Issue 8 | 27 July 2020

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We are describing the cropping areas of South Australia broadly as south of a line starting at Ceduna to Pt Augusta and down to Renmark. Uses 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.

Soil moisture decreased across most of the state for the month, but particularly on the Eyre Peninsula, Mid North and Lower Murray, where rainfall below 60% of normal couldn’t keep up with crop use. A number of probes used 11 to 21 percentage points of their soil moisture profile.

The central Pacific further weakened in its attempt to be a La Nina, now at neutral temperatures, and the undersea is a shadow of its former coldness back in May. However, trade winds did pick up in the western Pacific, cloud at the dateline is less and the SOI has finally gone into the positive which keeps the La Nina dream alive. The atmosphere is finally giving this attempt some love, but one wonders if it has left its run too late? Around half of the models surveyed still predict NINO 3.4 could cool sufficiently for a La Nina this spring.

In the Indian Ocean, significant changes occurred during the month. The ever-warm coast of Africa has actually cooled to be partly normal, in accordance with a cooling trend to the undersea in that region, too. There is still a large amount of cloud over this region, which is not in keeping with a -ve IOD. The water off Indonesia has warmed marginally but is still fairly normal and while heat to depth around Sumatra has increased, it would be good to see that surface warm up further. Winds in the equatorial Indian Ocean have been pretty normal and some stronger westerlies are probably key to a -ve IOD developing. The one big change has been the pressure over the eastern equatorial Indian Ocean which has lowered and is also affecting the pressure at Darwin, normal for the first time in at least 12 months. This should be better for moisture transport down from the north. So, while last month a -ve IOD seemed a world away - and was almost a bit +ve IOD - now there is evidence that the Indian Ocean situation has improved. Only one third of models predict a -ve IOD to develop.

The SAM spent July mainly negative, potentially letting fronts across the South East, but this was of no benefit due to the strength and position of the dominant high-pressure system centred over South Australia. Most triggers for rainfall were all being pushed way south of Tasmania. This dominance of pressure over SA must change before wetter forecasts can eventuate.

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

Soil moisture

map of SA showing soil moisture probes measurements.

The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture (10-100cm) shows profiles that are drying. Pasture soils are ranked below 17% full, on much of the EP, Mid North, YP and Mallee. Two thirds of the soil moisture probes (courtesy of NR-SAMDB, EPARF, SARDI, AgByte and MFMG) decreased and those that increased were only by sub five percentage points. Probes that significantly decreased, fell by 11 to 21 percentage points with most falling around 15.

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.

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.

Model consensus forecast for the next six months

 

Current outlook (to 27 July)

Previous outlook (to 25 June)

 

Aug-Oct

Nov-Jan

Jul-Sep

Oct-Dec

Pacific Ocean

Slightly cool (possible weak La Niña)

Slightly cool (possible weak La Niña)

Slightly cool (possible weak La Niña)

Slightly cool (possible weak La Niña)

Indian Ocean

Slightly warm (possible weak -ve IOD)

Slightly warm (possible weak -ve IOD)

Slightly warm (possible weak -ve IOD)

Slightly warm (possible weak -ve IOD)

Rainfall

Neutral/ slightly wetter

Slightly wetter/ neutral

Slightly wetter/ neutral

Slightly wetter/ neutral

Temperature

Neutral

Neutral

Neutral

Neutral

Sea surface temperature anomalies

Equatorial Pacific Ocean Sea surface temperatures are at neutral levels but a +IOD continues in the Indian Ocean.

Sea surface temperature (SST) anomalies along the Equatorial Pacific have marginally warmed to be at normal temperatures and further La Niña development has stalled. NINO3, is at -0.21oC and NINO3.4 is at -0.03oC (as of 28 July). A La Niña would be when NINO3.4 gets below -0.8oC. The Coral Sea to our north east remains much warmer and somewhat reminiscent of a La Niña. The Indian Ocean has changed the most now, at least showing the possibility of a -ve IOD. The warm western box of the IOD now shows some cooler and normal water and the box off Sumatra is normal to warmer. The Dipole Mode Index (DMI) is -0.09 oC, completely neutral. A negative IOD would be more negative than -0.4 oC.

Equatorial pacific sub-sea temperature anomalies

Equatorial undersea temperature anomalies in the Pacific show some warming in the centre.

The Pacific Ocean Equatorial sub-surface temperature warmed further, now looking much more normal. The cold anomaly remains in the far eastern Pacific and is breaking out at the surface. This cool signature pattern looks much weaker than past La Niña events.

Southern oscillation index

The SOI value is currently at -11.1 (as at 27 November).

The SOI has taken a meteoric climb into positivity in July, currently at +6.8 (as at 28 July). The threshold for La Niña like pressure patterns around the Equator is more positive than +8.0.

Pacific ocean surface wind anomalies

The Equatorial trade wind anomalies are strongly reversed off Indonesia.

Trade Winds across the Equatorial Pacific have been normal in the east and stronger in the west. This is helping to pool up the warmer water in the Coral Sea. A conventional La Niña would mean much stronger easterlies across the western half of the Pacific. In the Indian Ocean winds have been normal, a -ve IOD would normally lead to much stronger westerlies streaming into Indonesia.

World cloudiness anomalies

Cloud is lacking off Sumatra and over eastern Australia.

Cloud at the International Dateline (180oW) junction with the Equator has changed little in July. The lack of cloud at the Dateline junction with the Equator is an indicator for La Niña, and probably the strongest indicator to date. The abundance of cloud off the coast of Africa is not in keeping with a -ve IOD, which should mean more cloud off Indonesia. A lack of cloud from the North-West is apparent coming down into South Australia.

Southern annular mode

the SAM had spent most of November in strong negativity.

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) has spent most of July in a negative phase. This would normally have meant more frequent fronts sweeping across South-East SA. This positive winter climate driver for rainfall was nullified by the strong pressure.

Air pressure

the STR of high pressure has been higher than its normal position of Adelaide.

In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been in a normal position for latitude, centred at the top of the Bight. In theory allowing through winter fronts and lows. The longitude of the high was centred over South Australia and the high-pressure extent went below Tasmania. This put a frontal block across south east Australia, effectively sending many passing fronts south.

Air pressure anomalies

Pressure at Darwin is higher and Tahiti is lower, pressure over SA has been normal or lower than average.

The Sub Tropical Ridge of High Pressure was much higher in pressure over south eastern Australia. This meant high pressure systems were stronger and slower moving, chasing fronts and lows further south. In perhaps the biggest change for the month, the pressure to Australia’s north has normalised or become lower over Indonesia, the first time in more than 12 months. This is a positive sign for moisture flow from the north. The pressure at Darwin is normal and Tahiti is slightly higher, which is why the SOI is positive.

Modelled Climate and Ocean Predictions for South Australia from July 2020 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.

Click here to download this table in MS Word format
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