Seasonal climate risk information for South Australia
Volume 2 | Issue 9 | 26 September 2019
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Soils are drying out in many areas due to high plant water use, but a few regions jagged some helpful rain to kick things along.
The positive IOD (+IOD) has continued in ‘text book’ fashion. Cold water off Sumatra and warmer water off Africa are leading to decreased evaporation and cloud over Indonesia. Trade winds are reversed off Sumatra which is pushing warm water and moisture away from Australia’s north west and allowing the cooler water to keep upwelling. Pressure is very high over Australia and in the tropical north, typical of +IOD years. The centre of the high was over SA and this has set up stable and occasionally frosty weather. The high-pressure ridge has not moved south as we would expect in spring which is starting to block moisture transport from the tropics.
The Equatorial Pacific Ocean is benign at the surface, with some cooling underneath in the east that’s probably going nowhere. Wind and cloud patterns and pressure patterns at Tahiti are also normal enough in the Pacific. The Coral Sea is warmer and giving off a bit more evaporation and would be a good moisture source if we could get a link to it.
The Southern Annular Mode (SAM) spent the month half negative and half positive, SAM is an unreliable climate indicator in spring for SA.
All climate models think the +IOD is hanging around for the rest of spring, where it would be expected to break down rapidly with the onset of the northern wet season. A majority of models surveyed predict that drier is the most likely outcome for the next three months. There is also a consensus for warmer temperatures.
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 plant available soil moisture (10-100cm) shows darker areas in the SE and Adelaide Hills with a wetter profile. Other cropping areas shows decreases from last month. KI and the Mid North are ranked at decile two to three for this time of the year, with the central EP much wetter than normal, other areas are considered close to average. The soil moisture probes (courtesy of NR-SAMDB, EPARF, SARDI, AgByte and MFMG)show that some timely rains increased some EP sites, with Kyancutta increasing from 28 to 76% and Lock increasing from 52 to 100%. Many probes show high water use, to be expected at this time of the year without significant rainfall. Jamestown fell from 81 to 24% and Elliston from 93 to 46% for the month.
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 (26 September)
Previous outlook (26 August)
Sea surface temperature anomalies
In the Indian Ocean the Dipole Mode Index (DMI) shows a strong positive IOD (+1.5oC as of 25 September). 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 cooled a tiny bit further in September, but remain at neutral temperatures. NINO3 is at -0.2oC and NINO3.4 is +0.11oC (as of 25 September). In the Coral Sea temperatures are warmer as a good moisture source.
Equatorial pacific sub-sea temperature anomalies
The Pacific Ocean Equatorial sub-surface temperatures have cooled in the east during August and September. Some of this coolness is upwelling along the surface of the Equator. No models are predicting any La Niña like activity.
Southern oscillation index
The SOI dipped strongly into negative values during September. The value is currently at -15.3 (as at 25 September). This would normally indicate pressure patterns around the Equator were El Niño like (greater than -8.0). Instead it’s indicative of much higher pressure at Darwin and normal pressure at Tahiti, which in not El Niño at all. Higher pressure at Darwin makes it harder to get moisture down to South Australia.
Pacific ocean surface wind anomalies
The Equatorial Pacific Easterly Trade Winds are essentially normal. In the Indian Ocean however, Trade Winds are strongly reversed off Indonesia, consistent with a +IOD and pushing warmer water to the west and keeping cooler water off Sumatra upwelling.
World cloudiness anomalies
Cloud at the International Dateline (180oW) junction with the Equator is slightly less (brown colour) which is weakly suggestive of La Niña. The lack of cloud (brown colour) off Sumatra and over Indonesia is a typical +IOD pattern due to reduced evaporation off the cooler ocean in that region. There is an abundance of cloud over the Coral Sea.
Southern annular mode
The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent September split between negative and positive. NOAA predicts the SAM to return to neutral and go weakly negative in the next 14 days. In spring SAM has an erratic effect on SA’s rainfall.
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. In winter this is helpful to frontal activity, but in spring, this flips to be unhelpful as it prevents the tropics from moving closer to SA and blocks the flow of moisture to our NW, consistent with the drying mechanism of a +IOD.
Air pressure anomalies
The Sub Tropical Ridge of High Pressure has been much stronger over the whole of Australia. This 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. The good rainfall event over the EP dragged moisture from off the coast of Africa, a long way away! Pressure at Darwin is much higher and Tahiti is normal, which is why the SOI is negative.
Modelled Climate and Ocean Predictions for South Australia from September 2019 run models
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