Fast Break: Climate risk information for South Australia

Volume 1 | Issue 3

Read the Tasmanian issue

Most northern agricultural areas could do with more soil moisture, with only the SE region being closer to normal.

Things are delicately poised at the moment. The Pacific Ocean surface is slowly warming but still at neutral values. The undersea is much warmer in the eastern Pacific, which is where many models are getting their predictions for a winter or spring forming El Niño. Despite the undersea primed for El Niño, all other indicators such as, trade winds, SOI and Pacific surface temperatures are still at neutral levels. Indicators like the cloud at the dateline have quite a way to move as they are still more like La Niña.

These things suggest that if an El Niño is to occur, it’s still some time off. It’s important to remember that El Niño doesn’t guarantee drought in SA. Historically El Niño years have resulted in South Australian spring rainfall being in the lowest third around 50-60 per cent of the time. Most of these drier years can still be agronomically OK, but soil moisture reserves play a big role. Low rainfall drought years have occurred less than 10 per cent of the time.

The SAM is still behaving normally and the latitude of the Sub Tropical Ridge of high pressure is good. Unfortunately like in May, the pressure has situated itself over SA which means all but the most coastal and higher altitude regions have been in a blocked pattern. Absolute pressure is still higher, meaning highs are moving slower and pushing rainfall triggers south. Higher pressure at Darwin is making moisture harder to transport south.

More model predictions are leaning towards a spring forming El Niño, but trade wind reversals near PNG need to occur to lock this in. Predictions for the Indian Ocean over winter are all over the shop, anything could happen. Most model’s rainfall and temperature predictions are still sitting on the fence for average, code for anything is possible.

Soil Moisture

map-of-SA-showing-dry-soil-to-depth-but-moisture-probes-showing-higher-values
The BoM AWRA modelled plant available soil moisture (10-100cm) shows reasonable moisture in the high rainfall districts and improving in the southern Mallee, but other areas are far from exciting. A selection of moisture probes (courtesy of NR-SAMDB, EPARF, SARDI and Agbyte) shows some paddocks with higher moisture levels to depth (30-100cm). The Hoyleton, Parndana and Moorlands probes have wet up further this month, but all others have stayed the same or dried.

Model distribution summary for the next three months

Graphs showing the distribution of June-August modelled rainfall as average, and temperature as warmer to average
Graphs showing the distribution of June-August modelled rainfall as average, and temperature as warmer to average

Model distribution summary for the next four to six months

graphs showing the distribution of September-November modelled rainfall and temperature is for average
graphs showing the distribution of September-November modelled rainfall and temperature is for average

Model consensus forecast for the next six months

Current outlook (28 June)

Current outlook (28 May)

Jul-Sep outlook

Oct-Dec outlook

Jun-Aug outlook

Sept-Nov outlook

Pacific Ocean

Neutral (possible El Niño)

El Niño

Neutral

Neutral

Indian Ocean

Mixed

Neutral

Neutral

Slightly warmer/neutral

Rainfall

Average

Average

Average

Average

Temperature

Average

Slightly warmer/average

Slightly warmer/average

Average

Equatorial Pacific and Indian Ocean Sea surface temperature anomalies are normal. Seas to the north of Australia are slightly cool
Sea surface temperatures (SST) along the Equatorial Pacific have continued the slow warming trend. NINO3 is at +0.53oC and NINO3.4 is +0.48oC (as of 27 June), both slightly warmer, but still at neutral ENSO conditions. The Coral Sea continues to stay warmer as a hangover from the La Nina over summer. Temperatures in the Timor Sea have re-warmed closer to normal after the May cooling. The tropical Indian Ocean is benign. Warmer tropical ocean anomalies nearer to Australia can provide more evaporation as a moisture source, whereas cooler ocean anomalies can kill the moisture source
Undersea warm temperatures have increased in the eastern Pacific and are close to breaking out at the surface
The Pacific Ocean Equatorial sub surface temperatures made further warm progress eastwards and is now starting to break out at the surface. The models predicting an El Niño are getting that signal from here. This is the only indicator of an El Niño at this stage, all other indicators are normal. For an El Niño to occur we would need to see trade winds reverse and the SOI to go negative. The relatively slow mid-year progress would suggest a greater chance of a spring El Niño rather than a winter one.
The SOI value is currently at minus 4.6 and falling
The SOI is currently at -4.6 and falling (as of 27 June). Pressure patterns around the Equator as measured at Darwin and Tahiti are both slightly higher so the SOI is neutral. Sustained values of the SOI greater or less than 8 are meaningful and can indicate El Niño (when negative) and La Niña (when positive).
The Equatorial Pacific trade winds have been normal throughout May
The Equatorial Pacific easterly Trade Winds have been essentially normal through June (shown by the small arrows). In the last week there has been a small amount of reversal in the centre, but it’s not very convincing. Central trade wind reversals can calm the ocean and cause the sun to warm it further. In coming months watch for reversed trade winds (westerlies) around PNG (El Niño), or stronger easterlies in the central Pacific (La Niña), or just normal trade winds (neutral).
there has been less cloud at the junction of the Equator with the Dateline, which is consistent with La Niña, other areas are more normal
Cloud at the International Dateline (180oW) junction with the Equator has been less (brown colour) in the last 30 days. The total area of less cloud in the western Pacific has shrunk. Cloud to the north of Australia has been slightly less but some evidence of weak NW cloud band activity exists
the SAM has spent June weakly positive and negative
The Southern Annular Mode (SAM) has done little during June, spending time weakly negative and weakly positive. The winter period is where SAM has its greatest influence over frontal system positioning in southern regions. Positive SAM pulls fronts away from southern Australia and negative SAM pushes them towards us. The BoM and NOAA models suggest a return to neutral SAM in coming weeks.
the STR of high pressure has been at a winter position at the top of the Bight
In the past 30 days, the Sub Tropical Ridge of High Pressure has remained in the slightly better position centred just north of the Bight, letting the odd front through. The position of the high centre over South Australia is not ideal for SE Australia, but just OK for Tasmania.
pressure at Darwin and Tahiti are both higher, where pressure over SE Australia has also been higher
The Sub Tropical Ridge of High Pressure was higher in pressure over SE Australia. Meaning that high pressure systems moved slower and remained a blocking position for longer. Pressure at Darwin and Tahiti are both higher which is why the SOI is close to zero. Higher pressure to the north of Australia can make it harder to get moisture down to the south.
A table showing 12 climate models which show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and Temperature for South Australia
12 climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and Temperature for South Australia (Go to html table version of this image)