Southern NSW

Seasonal climate risk information for Southern NSW

Volume 1 | Issue 10 | 29 October 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.

The positive Indian Ocean Dipole continues with ocean temperature to depth and at the surface, wind, cloud and pressure patterns all singing from the +IOD hymn sheet. All models surveyed predict the IOD to continue into December which would be very late historically for an IOD breakdown (in the last 10 years most died in late October). This points towards a late start to the northern wet season as this is the trigger to the IOD’s demise.

In the Pacific Ocean there is very little of note going on, with most indicators in neutral territory.

The Coral Sea is warmer and evolving a bit more cloud, which could be a good moisture source if a connection could be made to it.

The Southern Annular Mode has dived into strongly negative territory due to coupling with the Sudden Stratospheric Warming that occurred in September over Antarctica. This is unlikely to mean much for western NSW. Historically a negative SAM has had greatest effect on eastern NSW. This is due to the predominant wind flow in this area not being from the rain bearing east.

Pressure patterns have changed little being dominated by stronger than normal high-pressure systems with a centre that is further north than is usual for spring. This is continuing to block rain triggers from the south and moisture transport from the north-west.

The majority of models predict that a drier and warmer November-January is the most likely outcome

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.

You can use the new Local Climate Tool to identify how historical +IOD events have affected rainfall in your area.

Soil moisture

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

The BoM Australian Water Resources Assessment (AWRA) modelled soil moisture shows very little of the cropping areas of NSW with much soil moisture left. With the exception of the southern Riverina at decile two to three soil moisture, all other areas are on decile one or lower.

Model distribution summary for the next three months

Graphs showing the distribution of global model forecasts for July-September, with models leaning towards drier rainfall and warmer/average temperatures.

Model distribution summary for the next four to six months

Graphs showing the distribution of October-December forecasts with models leaning towards average-drier rainfall with warmer temperatures.

Model consensus forecast for the next six months

Current outlook (to 29 October)

Previous outlook (29 September)

Nov-Jan

Feb-Apr

Oct-Dec

Jan-Mar

Pacific Ocean

Neutral

Neutral

Neutral

Neutral

Indian Ocean

Cold (+IOD)

Warmer/Neutral

Cold (+IOD)

Neutral

Rainfall

Slightly drier

Average/slightly drier

Slightly drier

Average/slightly drier

Temperature

Slightly warmer

Slightly warmer

Slightly warmer

Slightly warmer

Sea surface temperature anomalies

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

In the Indian Ocean the Dipole Mode Index (DMI) remains at strong positive IOD levels (+2.06oC as of 29 October). Cooler water off Indonesia and warmer water off Kenya. The threshold for +IOD is +0.4oC. Sea surface temperatures (SST) anomalies along the Equatorial Pacific have warmed during October but remain at neutral temperatures. NINO3 is at +0.41oC and NINO3.4 is +0.68oC (as of 29 October). In the Coral Sea temperatures are warmer as a good moisture source.

Equatorial pacific sub-sea temperature anomalies

Equatorial undersea temperature anomalies in the Pacific show some weak warming at depth.

The Pacific Ocean Equatorial sub-surface temperatures have warmed during October replacing the coolness. As a result, the water above has also warmed. At this stage of the year the Pacific undersea is of little use to be monitoring, as the window for serious El Niño and La Niña activity has passed.

Southern oscillation index

The SOI value is currently at -8.5 and stable (as at 28 June)

The SOI rose rapidly up to neutral levels during October. The value is currently at -5.2 (as at 29 October). This indicates normal pressure patterns around the Equator.

Pacific ocean surface wind anomalies

The Equatorial Pacific trade wind anomalies

The Equatorial Pacific Easterly Trade Winds are normal. In the Indian Ocean strong Trade Wind reversals off Indonesia continue consistent with a +IOD. This is pushing warmer water to Africa and keeping cooler water upwelling off Sumatra.

World cloudiness anomalies

Cloud is greater at the junction of the Equator with the Dateline.

Cloud at the International Dateline (180oW) junction with the Equator is slightly less (brown colour) which is weakly suggestive of La Niña, but this would be the only indicator backing that horse. The lack of cloud (brown colour) off Sumatra and over Indonesia shows the typical +IOD pattern due to reduced evaporation off the cooler ocean in that region. There has be reduced cloud over most of Eastern Australia and tropical connections to the NW have been weak.

Southern annular mode

the SAM has spent much of June in moderate to weak positive territory.

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent the first half of October near normal and has then dove into strong negativity. NOAA predicts the SAM to stay strongly negative for the next 14 days. This Sudden Stratospheric Warming that occurred over Antarctica in September has finally coupled with lower altitudes causing the SAM to go negative. Spring and Summer is when the SAM has its greatest influence on rainfall in Eastern NSW. In spring and summer, a negative SAM would be expected to cause drying along eastern NSW.

Air pressure

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

In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been just above the Bight, much further north of a spring position of Adelaide. The pressure ridge further north has been a feature for the last five months. In winter this was helpful allowing fronts through the Riverina but blocking rain triggers from more northerly regions. In spring this remains a hindrance to tropical moisture connections to the NW, consistent with the drying mechanism of a +IOD.

Air pressure anomalies

Pressure at Darwin is slightly higher than Tahiti, pressure over SE Australia has been much higher.

The Sub Tropical Ridge of High Pressure has been much stronger over the whole of Australia although here has been some weakening of this high pressure in tropical areas this month. Higher pressure to the south of Australia has been chasing frontal and low-pressure rainfall triggers away.  Higher pressure in the tropics has been making it hard to get moisture down from the north and north west.  Pressure at Darwin is higher and Tahiti is normal, which is why the SOI is slightly negative.

Modelled Climate and Ocean Predictions for Southern NSW from October 2019 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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