Southern NSW

Date: 27 Aug 2020

Seasonal climate risk information for Southern NSW

Volume 2 | Issue 9 | 27 August 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.

Rainfall was generally above average over the whole of southern New South Wales for August. Welcome rainfall fell in much of the Central West to continue an excellent season so far. All regions of SNSW improved in modelled soil moisture over August which is a welcome change. Riverina and south west districts have plenty of spare capacity to hold more moisture.

The Pacific Ocean surface has moved little for the month but the eastern undersea has re-cooled to look a lot more La Nina-like. Stronger Trade Winds in the western Pacific are pushing and holding warmer water towards Papua New Guinea (PNG) and the Arafura Sea. Cloud patterns at the dateline remain consistent with La Nina, but the pressure patterns as measured by the SOI are yet to shift to La Nina-like. A majority of models predict a La Nina is going to form and hold for spring and early summer. A shift to lower pressure to our north leading to a coupled ocean and atmosphere are key here.

The Indian Ocean has made rapid progress towards being in a negative IOD state in both the surface and sub-surface and in both eastern and western. Significant cooling off Africa and a decrease in the cloud in that region is a good start, but cloud patterns off Sumatra are not matching the warm ocean in that zone, nor are the wind changes swinging stronger westerly towards Indonesia. Most models predict a weak -ve IOD for spring. Enhanced westerly winds occurring, are key here.

The SAM was strongly negative for much of August allowing fronts and lows closer to Victoria, and much of this activity has gone through SNSW as well. SAM is predicted to revert to weak positivity in the next seven days. High pressure systems also moved further north than normal, allowing these systems through. Overall pressure over SNSW was much lower and a welcome change from previous high-pressure dominant months.

My assessment of 12 climate models for SNSW shows a strong consensus for wetter rainfall and a split between neutral and likely cooler temperatures for the next three months. Timing will be critical for the proper formation of the oceanic phenomena, because for south west NSW districts it can’t come quick enough.

Soil moisture

map of Southern NSW showing plant available moisture (%).

The BoM Australian Water Resources Assessment (AWRA) modelled plant available soil moisture shows increases over the whole state for the month. For pasture paddocks the Riverina is now ranked normal where other areas are all ranked wetter than normal. The Central West remains wetter than other cropping districts.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for February-April, with models split between average or drier and a stronger consensus for warmer.

Model distribution summary for the next four to six months

Graphs showing the distribution of May-July forecasts with models showing equal chances of wetter, average or drier and a stronger consensus for warmer temperatures.

Model consensus forecast for the next six months

 

Current outlook (to 27 August)

Previous outlook (to 27 July)

 

Sept-Nov

Dec-Feb

Aug-Oct

Nov-Jan

Pacific Ocean

Cool (La Niña)

Cool (La Niña)

Slightly cool (possible weak La Niña)

Slightly cool (possible weak La Niña)

Indian Ocean

Slightly warm (weak -ve IOD)

Slightly warm

Slightly warm (possible weak -IOD)

Slightly warm (possible weak -IOD)

Rainfall

Slightly wetter

Neutral/slightly cooler

Slightly wetter

Neutral

Temperature

Neutral

Neutral/slightly cooler

Slightly wetter/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 stayed similar in NINO3 (-0.43oC), where NINO3.4 has cooled to -0.46oC (as of 27 August). These temperatures are still not at the -0.8oC threshold for La Niña. The Coral Sea to our north east remains much warmer and somewhat reminiscent of a La Niña. The Indian Ocean has further made movements towards a negative phase. The western box is now more normal and the box off Sumatra has warmed. The Dipole Mode Index (DMI) is -0.69 oC, indicating negative phase ocean surface temperatures. The ocean pattern is not classically negative IOD in the eastern box with a much larger warm area to its west.

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 temperature has rapidly re-cooled, similar to back in May and once again looking like a La Niña signature. The cold anomaly has reformed in the Central Pacific and has worked its way eastwards.

Southern oscillation index

The SOI value is currently at -1.7 (as at 2 March).

The SOI has remained steady in August, currently at +5.4 (as at 27 August). 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 are reversed off PNG but are stronger in the Central Pacific.

Trade winds across the Equatorial Pacific have continued to be stronger easterly for the second month in a row. This is pushing water warmer towards PNG and helping cooler water to upwell in the centre. In the Indian Ocean, winds have remained normal, not consistent with a -ve IOD which would normally have stronger westerlies streaming towards Indonesia.

World cloudiness anomalies

Cloud is abundant over the Dateline and lacking to the north of Australia.

Cloud at the International Dateline (180oW) junction with the Equator has been decreased for many months. This is indictive of La Niña conditions. In the Indian Ocean the large amount of cloud off Africa has dissipated, possibly a good sign. What is perplexing is the lack of cloud off Indonesia over the warmer water, which is more indicative of positive IOD.

Southern annular mode

the SAM had spent most of February in neutral.

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) has spent the last two thirds of August in a strong negative phase. This would not be expected to have a large influence on southern NSW but appears to have resulted in many fronts passing through. Both the NOAA and BoM forecasts predict the SAM to stay negative for five days and return to weakly positive.

Air pressure

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) shifted north of a normal winter position at the top of the Bight. This should have been allowing more fronts across SNSW.

Air pressure anomalies

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 has changed from being higher to lower over Southern New South Wales in the last month. This has meant many more fronts and low-pressure systems have passed by as potential rainfall triggers. The pressure at Darwin is slightly lower and Tahiti is slightly higher, which is why the SOI is slightly positive.

Modelled Climate and Ocean Predictions for Southern NSW from August 2020 run models

12 climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for SNSW

Click here to download this table in MS Word format
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