Seasonal climate risk information for Tasmania
Volume 2 | Issue 10 | 29 October 2019
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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. It is uncertain what this might mean for Tasmania. The SAM has erratic behaviour at this time, historically -ve SAM springs have greatest effect on the eastern sea board, a drier east NE would be possible if this continued into summer. This is due to the predominant wind flow in this area not being from the rain bearing east. For now, a continuation of the westerly flow rain shadowing the east is not out of the equation.
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 moisture transport from the north-west, important for larger rainfall dollops in spring.
The majority of models predict that average rainfall and an average to warmer November-January is the most likely outcome. Code for plan for anything.
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 map shows plant available soil moisture (10-100cm) shows that unirrigated areas of the Midlands have dried out to sub 32% amounts. The western half is still wet and apart from it, other areas of Tasmania are rated at decile one to three for soil moisture. The northern agricultural areas are rated decile one.
We would like to plot some publicly available soil moisture probe values on this map for comparison, please contact us if you have remote telemetry, an upper and lower limit and are interested.
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 26 October)
Previous outlook (29 September)
Sea surface temperature anomalies
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
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 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 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 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 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. Winter is when the SAM has its greatest influence on rainfall over Tasmania. In spring, SAM’s influence is erratic but it’s not impossible that a continuation of the westerly flow rain shadowing the east will continue. In summer a negative SAM might be expected to be drier in the NE.
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 allowed the dominant westerly flow of frequent fronts but set up the eastern rain shadow. In spring this starts to be a hindrance to tropical moisture connections to the NW, more necessary as plant water use increases and greater than frontal rain is required. Such behaviour is 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 although here has been some weakening of this high pressure in tropical areas this month. Higher pressure to the south of Australia has been weakening frontal and low-pressure rainfall triggers. 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 Tasmania from October 2019 run models
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