Seasonal climate risk information for Southern NSW
Volume 1 | Issue 8 | 28 August 2019
Welcome to the “Fast Break” newsletter for the Southern New South Wales grains industry. We have been constructing these summaries for the last 11 years in Victoria, and started last year in South Australia, New South Wales and Tasmania as part of the GRDC funded “Using Seasonal Forecasts Project” project. If you like it, please consider passing it on through your networks and subscribing.
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.
The SNSW has been much drier in August, with the dividing range the only parts receiving more than 50 per cent of average rainfall. A +IOD isn’t helping the outlook for spring rainfall.
Nothing much happened of note in the Pacific Ocean over August. Ocean temperatures at the surface and at depth are normal, trade winds aren’t doing much and cloud patterns are also normal. The Southern Oscillation Index (SOI) has moved into neutral territory given there was no support from the oceans while the SOI was negative over July.
The Positive Indian Ocean Dipole (IOD) remains the major climate feature. Ocean temperatures have cooled further around Indonesia and are warm enough off Africa for the Dipole Mode Index (DMI) to be above the threshold for +IOD. Lack of cloud over Indonesia is a classic +IOD pattern, as is stronger easterly winds around Java and Sumatra, helping to upwell the cooler water. Water to depth is also cooler in this region, which suggest it’s not going away soon. All models show agreement in that a +IOD is likely to hang around for the next three months. Most of them predict it to break down in October/November as they usually do. The +IOD has historically increased the chances of a drier spring in much of southern NSW.
As we might expect with a positive IOD, most models assessed predict higher chances of a drier three months ahead. Models predict temperatures likely to be warmer to average in southern NSW.
However, dry conditions with clear skies increase the chance of frost.
You can use the new Local Climate Tool to identify how historical +IOD events have affected rainfall in your area.
The BoM Australian Water Resources Assessment (AWRA) modelled soil moisture has remained around the same for August in SNSW, with rainfall barely matching plant use. While soil moisture is ok in parts of the Riverina, things get much drier as you head north.
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 29 August)
Previous outlook (29 July)
Sea surface temperature anomalies
Sea surface temperatures (SST) anomalies along the Equatorial Pacific have cooled a little bit further during August so far and remain at neutral temperatures. NINO3 is at -0.1oC and NINO3.4 is +0.2oC (as of 18 August), the threshold for an El Niño is +0.8oC. The Dipole Mode Index (DMI) shows a positive IOD at +0.79oC (as of 20 August) continues in the Indian Ocean. The threshold for +IOD is +0.4oC.
Equatorial pacific sub-sea temperature anomalies
The Pacific Ocean Equatorial sub-surface temperatures have done very little during July. A cooler anomaly in the east is starting to upwell at the surface, but the warmth in the central Pacific is negligible compared to a classic El Niño.
Southern oscillation index
The SOI value is currently at -0.9 (as at 26 August) and spent the last week of July and August returning to neutral.
Pacific ocean surface wind anomalies
The Equatorial Pacific Easterly Trade Winds - as indicated by the red oval - have experienced some westerly (reversed) wind activity, but this doesn’t seem to have had any effect on the Pacific surface or the undersea.
World cloudiness anomalies
Cloud at the International Dateline (180oW) junction with the Equator is clear (white colour). The lack of cloud (brown colours) off Indonesia is a sign of the +IOD pattern. There has been a lack of cloud over SE Australia.
Southern annular mode
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) has spent most of August in the negative phase. A negative SAM means a weakening of westerly winds around Antarctica and a pushing of frontal systems and storm tracks closer to southern Australia. Winter is when the SAM has its greatest influence on rainfall.
In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been at or slightly higher than a normal winter position (level with the top of the Bight). This has been letting frequent fronts through, which benefits southern areas. The higher than normal pressure has been putting stronger than normal easterly winds around Indonesia helping to upwell the cooler water in that region.
Air pressure anomalies
The Sub Tropical Ridge of High Pressure was stronger than normal over the whole of the mainland, with anomalies higher for the current period than the previous month. Higher pressure at Darwin makes it harder to get any northern moisture down to southern areas for making rain. Pressure at Darwin and Tahiti are relatively equal and that is why the SOI is close to zero.
Modelled Climate and Ocean Predictions for Southern NSW from July 2019 run models
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