Tasmania

Seasonal climate risk information for Tasmania

Volume 3 | Issue 6 | 25 June 2020

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Soil moisture remains wet across the north and increased dramatically over the South East. The Midlands are predicted to be about half full which is normal for this time of year. A drier month has allowed wet profiles to drain in the northern half.

In the Pacific Ocean, around a third of the models surveyed predict a weak “just at threshold” La Niña can form in coming months and why the BoM moved to La Niña watch. The eastern Pacific surface cooled further, but the central Pacific is unchanged, both slightly cooler but neutral. The amount of colder water to depth has reduced, particularly in the central Pacific. The atmosphere is where the success of this La Niña attempt lies. At the moment, only less cloud at the Dateline matches the ocean. It needs more support from the trade winds and the pressure patterns, as without those, it will struggle to form.

The Indian Ocean is even more delicately poised. This region is worth watching intently in coming weeks and months. About half of the models surveyed are predicting a weak to moderate negative IOD in coming months. Major changes are needed off Africa for this to occur. In the last month there has been no change to the SST anomalies to our NW after they neutralised from warm to normal in May. Current ocean temperatures are much warmer off Africa and normal around Sumatra. This has caused the Dipole Mode Index to be at around the threshold for +IOD in recent weeks. Warmer water to depth, lower pressure and more cloud off Kenya is also more reminiscent of a +IOD. Fortunately, normal water at the surface and warmer at depth, normal pressure and cloud off Sumatra are not features of the eastern part of a positive dipole. Despite this, a north west “lack of cloud” anomaly streaming across southern Australia is also more like a +IOD.

The Southern Annular Mode has been in a persistent moderate positive phase for June. This would be expected to pull fronts away from Tasmania and the lower rainfall for most of June would suggest this has been at least partly to blame. The SAM is currently normal and models are mixed as to which direction it’s going to head in coming weeks.

Pressure patterns are a mixed bag, the absolute latitude of the sub-tropical ridge is at a helpful more northern winter position. The position of the high should have seen some fronts sneaking across Tasmania. Opposing this has been higher than normal pressure all over the country, chasing fronts further south and not assisting moisture transport from the tropics.

My assessment of 12 climate models for Tasmania shows no departure from a neutral rainfall and temperature forecast for the next three months. Code for plan for anything. Some greater consensus exists for a drier western half of Tasmania.

Soil moisture

The BoM Australian Water Resources Assessment (AWRA) shows modelled plant available soil moisture (10-100cm) has increased in the South East. Most other areas are similar to last month and remain full in the north. The Midlands are around half full. Deciles rank most of the state as wetter than normal, but the northern Midlands are rated as normal.

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

Sea surface temperature anomalies

Sea surface temperature (SST) anomalies along the Equatorial Pacific are cooler in the east, but unchanged in the centre this month and still at normal temperatures. NINO3 cooled most, to be at -0.55oC and NINO3.4 is unchanged at -0.20oC (as of 26 June). A La Niña would be when NINO3.4 got below -0.8oC. The western Indian Ocean remains very warm, and around Sumatra is normal. The Dipole Mode Index (DMI) is +0.4, at the threshold for +IOD. Other indicators in the west are +IOD like, but Indonesian indicators are not. The Indian Ocean is a long way from being in a negative IOD phase.

Equatorial pacific sub-sea temperature anomalies

The Pacific Ocean Equatorial sub-surface temperature anomalies have warmed to more normal levels during June in the central Pacific. The Eastern Pacific remains cooler to depth but has lost some coldness too. This would indicate a loss of enthusiasm for La Niña by the undersea.

Southern oscillation index

The SOI took a dive into negativity in June, currently at -9.0 (as at 29 June). This would indicate El Niño like pressure patterns around the Equator. Despite this no other ENSO indicator is in agreeance. The main reason for the negativity is the long remaining high pressure at Darwin, with no change to the pressure at Tahiti. The ocean and atmosphere are uncoupled.

Pacific ocean surface wind anomalies

Trade Winds have been normal in the Pacific Ocean. Some small stronger anomalies existed in the central Pacific, but not enough to change the underlying ocean situation. This lack of stronger easterly Trade Winds is why the attempted La Niña at the ocean surface is not getting any support. Most of the Indian Ocean has had normal winds. There has been a small stronger burst of south easterly wind near Java. this is not in keeping with a -IOD, as winds would normally be stronger westerly.

World cloudiness anomalies

Cloud at the International Dateline (180oW) junction with the Equator is less indicating La Niña like patterns. What is not La Niña like though, is the strong lack of cloud to the west of the Dateline, perplexingly over the top of warmer water. Cloud patterns finally resemble normality to the north of Australia, after 10 months of less cloud. A lack of north west cloud band activity is visible and like the increased activity in March and April, this is a much lower trajectory than normal cloud bands. Cloud has been normal across Tasmania. The large amounts of cloud over the warm water north of Madagascar are more in keeping with +IOD than -IOD.

Southern annular mode

The Southern Annular Mode (SAM) or Antarctic Oscillation (AAO) spent most of June in a moderate positive phase. SAM has its most influence over Tasmania in winter and Summer. A positive SAM would usually mean frontal systems are pulled further away from the island, something that has probably been influencing the lower rainfall in the north. The SAM is currently slightly negative and models are mixed as to which direction it’s going to head in coming weeks.

Air pressure

In the past 30 days, the Sub Tropical Ridge of High Pressure (STR) has been at a normal winter position centred over the Bight. This happened in late April many weeks earlier than normal. This has been a positive influence allowing fronts to come closer to southern Australia. The large high pressure over southern Australia has been just enough north to allow some fronts across Tasmania.

Air pressure anomalies

The Sub Tropical Ridge of High Pressure was higher over Tasmania in June. This would mean slower moving high-pressure systems chasing fronts away. Pressure has its greatest effects in winter over Tasmania. Pressure was higher at Darwin and normal at Tahiti which is why the SOI is negative. Pressure is higher to our north which is not ideal for moisture transport down. La Niña would normally see lower pressure at Darwin and a -IOD would see lower pressure at Sumatra. At the moment, pressure patterns are far from interested in these two phenomena.

Modelled Climate and Ocean Predictions for Tasmania from June 2020 run models