Date: 02 Jul 2020
Seasonal climate risk information for Southern NSW
Volume 2 | Issue 6 | 26 June 2020
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We are describing SNSW as a line south of Dubbo. In the predictions, I divide this region into quarters, hence N, S, E, W, NW, NE, SW, SE, defines zones in this southern half. If the models are more precise, I might use terms such as coast, Riverina, Central West, etc.
Some higher rainfalls in the north and far south of SNSW have seen increases to soil moisture but in most areas, rainfall has kept up with crop use with small to no increases. The Mallee and western Riverina has plenty of spare moisture capacity.
In the Pacific Ocean, around a third of the models surveyed predict a weak “just at threshold” La Niña can form in coming months and why the BoM moved to La Niña watch. The eastern Pacific surface cooled further, but the central Pacific is unchanged, both slightly cooler but neutral. The amount of colder water to depth has reduced, particularly in the central Pacific. The atmosphere is where the success of this La Niña attempt lies. At the moment, only less cloud at the Dateline matches the ocean. It needs more support from the trade winds and the pressure patterns, as without those, it will struggle to form.
The Indian Ocean is even more delicately poised. This region is worth watching intently in coming weeks and months. About half of the models surveyed are predicting a weak to moderate negative IOD in coming months. Major changes are needed off Africa for this to occur. In the last month there has been no change to the SST anomalies to our NW after they neutralised from warm to normal in May. Current ocean temperatures are much warmer off Africa and normal around Sumatra. This has caused the Dipole Mode Index to be at around the threshold for +IOD in recent weeks. Warmer water to depth, lower pressure and more cloud off Kenya is also more reminiscent of a +IOD. Fortunately, normal water at the surface and warmer at depth, normal pressure and cloud off Sumatra are not features of the eastern part of a positive dipole. Despite this, a north west “lack of cloud” anomaly streaming across southern Australia is also more like a +IOD.
The Southern Annular Mode has been in a persistent moderate positive phase for June. This would be expected to pull fronts away from southern Victoria. Historically SAM has had an erratic effect on SNSW in winter. The SAM is currently normal and models are mixed as to which direction it’s going to head in coming weeks.
Pressure patterns are a mixed bag, the absolute latitude of the sub-tropical ridge is at a helpful more northern winter position. The position of the high has seen SNSW on average in the dead zone, just to the west of the high’s centre. The other major problem has been higher than normal pressure all over the country, chasing fronts further south and not assisting moisture transport from the tropics.
My assessment of 12 climate models for SNSW shows a consensus for wetter rainfall and neutral temperatures for the next three months.
The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture shows modest increases for the month in the Central West and the southern Riverina. Most other regions were stable. Profiles are modelled to be half full or lower in most places. The Riverina is on the lower end, with plenty of spare capacity. This month’s decile ranking has decreased for many areas, with the Riverina and west of the Newell Hwy now ranked normal, and the slopes at decile eight to nine. Only a region around Forbes remains at decile 10. The Mallee has now dropped to decile two to three.
Model distribution summary for the next three months
Model distribution summary for the next four to six months
Model consensus forecast for the next six months
Current outlook (to 25 June)
Previous outlook (to 25 May)
Slightly cool (possible weak La Niña)
Slightly cool (possible weak La Niña)
Cool (possible La Niña)
Slightly warm (possible weak -IOD)
Warm (possible weak -IOD)
Neutral/ slightly cooler
Sea surface temperature anomalies
Sea surface temperature (SST) anomalies along the Equatorial Pacific are cooler in the east, but unchanged in the centre this month and still at normal temperatures. NINO3 cooled most, to be at -0.55oC and NINO3.4 is unchanged at -0.20oC (as of 26 June). A La Niña would be when NINO3.4 got below -0.8oC. The western Indian Ocean remains very warm, and around Sumatra is normal. The Dipole Mode Index (DMI) is +0.4, at the threshold for +IOD. Other indicators in the west are +IOD like, but Indonesian indicators are not. The Indian Ocean is a long way from being in a negative IOD phase.
Equatorial pacific sub-sea temperature anomalies
The Pacific Ocean Equatorial sub-surface temperature anomalies have warmed to more normal levels during June in the central Pacific. The Eastern Pacific remains cooler to depth but has lost some coldness too. This would indicate a loss of enthusiasm for La Niña by the undersea.
Southern oscillation index
The SOI took a dive into negativity in June, currently at -9.0 (as at 29 June). This would indicate El Niño like pressure patterns around the Equator. Despite this no other ENSO indicator is in agreeance. The main reason for the negativity is the long remaining high pressure at Darwin, with no change to the pressure at Tahiti. The ocean and atmosphere are uncoupled.
Pacific ocean surface wind anomalies
Trade Winds have been normal in the Pacific Ocean. Some small stronger anomalies existed in the central Pacific, but not enough to change the underlying ocean situation. This lack of stronger easterly Trade Winds is why the attempted La Niña at the ocean surface is not getting any support. Most of the Indian Ocean has had normal winds. There has been a small stronger burst of south easterly wind near Java. this is not in keeping with a -IOD, as winds would normally be stronger westerly.
World cloudiness anomalies
Cloud at the International Dateline (180oW) junction with the Equator is less indicating La Niña like patterns. What is not La Niña like though, is the strong lack of cloud to the west of the Dateline, perplexingly over the top of warmer water. Cloud patterns finally resemble normality to the north of Australia, after 10 months of less cloud. A lack of north west cloud band activity is visible and like the increased activity in March and April, this is a much lower trajectory than normal cloud bands. The large amounts of cloud over the warm water north of Madagascar are more in keeping with +IOD than -IOD.
Southern annular mode
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent most of June in a moderate positive phase. SAM has an erratic influence over SNSW in winter. A positive SAM would usually mean frontal systems are pulled further away from southern Victoria. The SAM is currently slightly negative and models are mixed as to which direction it’s going to head in coming weeks.
In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been at a normal winter position centred over the Bight. This happened in late April many weeks earlier than normal. This has been a positive influence allowing fronts to come closer to southern Australia. To counter this the large high pressure over southern Australia has put SNSW over a dead zone, just to the west of the pressure centre.
Air pressure anomalies
The Sub Tropical Ridge of High Pressure was higher over SNSW in June. This would mean slower moving high-pressure systems chasing fronts away. Pressure has its greatest effects in winter over SNSW. Pressure was higher at Darwin and normal at Tahiti which is why the SOI is negative. Pressure is higher to our north which is not ideal for moisture transport down. La Niña would normally see lower pressure at Darwin and a -IOD would see lower pressure at Sumatra. At the moment, pressure patterns are far from interested in these two phenomena.
Modelled Climate and Ocean Predictions for Southern NSW from June 2020 run models
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