The Fast Break - Victoria

Seasonal climate risk information for Victoria

Volume 13 | Issue 9 | 27 September 2018

Still no sign of an El Niño and yet the majority of models predict one will occur in late spring/summer. The Pacific Ocean surface is still neutral, the SOI is vaguely interested, the cloud patterns aren’t remotely keen, the deep sea is warm to depth, but only in the western and central Pacific. The major change this month is the easterly trade winds have shown some El Niño like behaviour, with at least two, one-week long reversals of significant strength from PNG to half way across. This is likely to force the warmer water to depth further to the east, and warm the surface in the middle of the Pacific. All this is likely to take months which will just about see out the growing season before it occurs. Historically, summer El Niño’s have been hit and miss for Victoria.

The Coral Sea is still warmer which is most unlike a “classic” El Niño.

All action has been in the Indian Ocean for some months. In the last weeks the Dipole Mode Index (DMI) measurement of the IOD has gone positive. Despite predictions from models for its occurrence all season, it has only just happened. We have had +IOD like cooling in the Timor Sea for many months but the current +IOD condition is being caused by the rapid warming off the African coast. Trade wind patterns are not consistent with a proper +IOD, but the lack of cloud off Sumatra is. Certainly, the lack of NW moisture feed all season had something to do with the Indian Ocean not playing ball. If the +IOD hangs around for two months it will be called an event, but it would normally die in late October early November when the tropical wet season fires up. Historically around 50-60 of springs in +IOD years have been in the driest third of records.

Pressure patterns, which were favourable in August, have deteriorated in September by increasing in pressure over the state and chasing rainfall triggers away. Such behaviour is classic +IOD induced.

The SAM which had been behaving itself in winter will cease to be a major driver in spring. Connections to tropical moisture will be more important than weak fronts.

Most models are still predicting a continuation of drier conditions for spring with warmer temperatures. The major warm signal is for December.

While the season is pretty much finished in the north, in southern areas it would be prudent to be planning for a shorter, rather than longer spring.

Soil moisture

The BoM AWRA modelled soil moisture map shows only the South West with good levels of plant available soil moisture (10 -100cm) remaining. Nearly all probes had significant deep stored water use for the month, which is pertinent as this means the crops are still actively growing. The exception were the probes at Coonooer Bridge and Birchip which have unfortunately been unable to get their roots into the deeper moisture.

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 temperatures (SST) along the Equatorial Pacific hardly changed in September. NINO3 is at +0.17^oC and NINO3.4 is +0.24^oC (as of 26 Sept), both at neutral values. The Indian Ocean has changed dramatically.

The Dipole Mode Index, (DMI) has screamed up to +0.87^oC where the threshold for +IOD is +0.5 ^oC. The cold pool of water near Sumatra has moved a little further north into the east IOD region, but the critical change is that the water of Kenya has significantly warmed up the western region.

Equatorial Pacific Sub-Sea Temperature Anomalies

The Pacific Ocean Equatorial sub surface temperatures haven’t done much this last month. The undersea warm anomaly made limited progress to the east, so an El Niño at the surface is at the earliest, many months away.

Southern Oscillation Index

The SOI is currently at -6.7 and falling, in the neutral range. Sustained values of the SOI greater or less than 7 are meaningful and can indicate El Niño (when negative) and La Niña (when positive).

Pacific Ocean Surface Wind Anomalies

The Equatorial Pacific Easterly Trade Winds have recently reversed (shown by the circled arrows). Early in the month the trade winds experienced another similar reversal for a week. The first real evidence the El Niño is attempting to fire up.

The surface of the ocean should start to warm further as a result. Reversed trade winds are needed to kick off an El Niño and to maintain it. In the Indian Ocean trade wind patterns haven’t dramatically changed from normal, despite the +IOD ocean patterns.

World Cloudiness Anomalies

Cloud at the International Dateline (180^oW) junction with the Equator is slightly less (brown colour) in the last 30 days but has varied greatly from more, normal and less. The cloud building off PNG is unlike an El Niño.

The lack of cloud off Sumatra remains +IOD like and the whole Indian Ocean basin is lacking in cloud.

Southern Annular Mode

The Southern Annular Mode (SAM) has spent the start of spring in a positive phase. Potentially dragging fronts away from Victoria but ceasing to be the dominant force that it was over winter.

In spring and summer, a +SAM can be beneficial to rainfall in the eastern third of Victoria. Both BoM and NOAA predict the SAM to go moderately negative in the coming fortnight.

Air Pressure

In the past 30 days, the Sub Tropical Ridge of High Pressure has moved a bit southward, but at a normal winter position rather than spring. The middle of spring should see the ridge level with Adelaide.

A ridge further north of normal may start to inhibit moisture troughing during spring. A high started to dominate over Victoria again blocking fronts away.

Air Pressure Anomalies

The Sub Tropical Ridge of High Pressure was higher in pressure over SE Australia helping to keep any fronts and lows further south. Pressure at Darwin and at Tahiti is normal which is why the SOI is close to neutral. The higher pressure over the whole of the eastern Indian Ocean is not helpful for getting moisture down to us.

Modelled Climate and Ocean Predictions for Victoria from September 2018 run models

Click on the table to view a larger version.

12 climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Victoria.