The Fast Break - Victoria
Seasonal climate risk information for Victoria
Volume 13 | Issue 7 | 30 July 2018
North of the divide crops are alive, but growth is slow if they haven’t connected with stored soil moisture. Pasture growth is also poor. The South West is quite good but central Gippsland remains dry.
There are still no concrete signs of an actual El Niño in surface temperature, cloud, wind or pressure patterns, only the warm slug of water under the eastern Pacific, which this month cooled off a bit.
All eyes are on the Indian Ocean, which is kind of playing up. A sort of “+IOD” exists in the cool water anomaly off Java and the cloud patterns off Sumatra. Measurements of the IOD are neutral though, due to the Kenyan and Sumatran ocean regions being both warm and cool. This month there has been a rapid change, with most models jumping on the +IOD band wagon. Models are predicting the cool area off Java to progress further westwards to the Sumatran region. Whether the African region can rewarm is the key question.
In the last 12 years when Indian Ocean patterns around Indonesia have been like this, the spring rainfall has usually been lower. Our only saving grace is that this is a very abnormal “+IOD” at the moment. With a weak connection between ocean and atmosphere and the Indian Ocean Equatorial wind patterns are normal. Ocean temperatures to depth off Sumatra are normal to warmer, and are not cooler, as you would expect to see for a classical +IOD. Time is going to tell whether any of these differences matter or not. Historically a +IOD often suppresses North West cloud band activity.
Through May and June, pressure patterns were very high and in an unfavourable position. This has partially improved during July. Hope to see those pressure patterns keep moving through, rather than hanging around like unwanted house guests.
A majority of models still suggest a late forming El Niño (BoM ENSO Outlook still at El Niño WATCH) and now, most are gunning for a +IOD. This month models swapped from average, to higher chances of drier rainfall for spring. My limited 11-year history of doing monthly model assessments, has shown that when more than half the models in July show drier for August-October, that has nearly always happened. If this was proven incorrect this year, I don’t think anyone would be unhappy!
Model skill is about as good as it gets this month.
The BoM AWRA modelled soil moisture map shows building plant available soil moisture (10–100 cm) south of the Divide, and poor soil moisture elsewhere. There has been little movement in most subsoil probes for the month (30–100 cm).
Model distribution summary for the next three months
Graphs showing the distribution of August-October modelled rainfall as drier and temperature as average.
Model distribution summary for the next four to six months
Graphs showing the distribution of November-January modelled rainfall as average, and temperature as warmer.
Model consensus forecast for the next six months
Current outlook (27 July)
Previous outlook (28 June)
Neutral (possible El Niño)
Sea surface temperature anomalies
Sea surface temperatures (SST) along the Equatorial Pacific have stalled during July. NINO3 is at +0.44oC and NINO3.4 is +0.42oC (as of 30 July), both at neutral ENSO conditions. The Coral Sea is still a bit warmer, a good sign. In the Indian Ocean the cool patch off Java has grown larger and cooler and like a +IOD.
The monitoring regions highlighted are both partly cooler and warmer off both Africa and Indonesia. Not a classic +IOD in any way shape or form. Warmer tropical ocean anomalies nearer to Australia can provide more evaporation as a moisture source, whereas cooler ocean anomalies can kill the moisture source.
Equatorial Pacific sub-sea temperature anomalies
The Pacific Ocean Equatorial sub surface temperatures went off the boil in July, with the extent of warm anomalies decreasing. This is probably why a few models got off the El Niño band wagon this month.
This warmth at depth is still the only indicator of an El Niño across the globe, all other indicators are normal. For an El Niño to occur we would need to see trade winds reverse and the SOI to go strongly negative.
Southern oscillation index
The SOI is currently at +0.1 and rising (as of 27 July), as close to neutral as you get. Pressure patterns around the Equator as measured at Darwin and Tahiti are completely normal. Sustained values of the SOI greater or less than 8 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 been normal through July (shown by the small arrows). In the last week there has been a strong reversal close to PNG, which will be worth watching to see if it propagates further East.
Whilst the short-term reversal in February kicked off the chance of an El Niño, sustained reversal is needed to make one happen. Central Pacific trade wind reversals can calm the ocean and cause the sun to warm the surface further.
World cloudiness anomalies
Cloud at the International Dateline (180oW) junction with the Equator has finally returned to normal (white colour) in the last 30 days. Greater cloud above PNG is more like La Niña. In the Indian Ocean there is a classic +IOD lack of cloud off Sumatra but no corresponding greater cloud off Africa.
Southern annular mode
The Southern Annular Mode (SAM) has bobbed up and down during July and has been no great influence on winter climate so far. The BoM and NOAA models suggest a moderately strong negative phase of the SAM in coming weeks. This might hopefully mean a few more frontal rains.
SAM has its greatest influence over frontal system positioning in south of the Divide regions during winter. Positive SAM pulls fronts away from southern Australia and negative SAM pushes them towards us.
In the past 30 days, the Sub Tropical Ridge of High Pressure is still sitting higher than normal allowing fronts through. The positioning of the high over NSW has been blocking them from reaching much of SE Australia, not ideal north of the Divide.
Air pressure anomalies
The Sub Tropical Ridge of High Pressure was lower in pressure over SE Australia. Meaning that frequent frontal systems were passing through, sadly they have been very weak at connecting to tropical moisture to the north.
The overall pressure levels over Australia have improved. Pressure at Darwin and Tahiti are both normal which is why the SOI is close to zero.
Modelled climate and ocean predictions for Victoria from July 2018 run models
12 climate models show their predictions for the next six months for the Pacific Ocean, Indian Ocean, rainfall and temperature for Victoria.