Lets take a look at the Indian Ocean first, as there has been so much talk about this being a large influence through Spring, and many areas in SA (rightly so) are becoming inpatient after the guidance was very strong and red hot on rainfall increasing through the region. Other areas have seen widespread rainfall in VIC and NSW with even some parts of QLD picking up on rainfall. The last rain event was influenced more heavily by the Coral Sea than the Indian Ocean.
So what's the deal, what's happening with this influence, and when can we see the Indian Ocean bringing back moisture and rainfall chances for the nation?
Thursday 16th of September.
Area to watch in coming days - this is not being picked up on all modelling but certainly the CMC and GFS below suggest there is some moisture to work with from the Indian Ocean. It will come down to timing of the front and whether it is able to move eastwards and not be forced south by blocking in the Tasman Sea (refer to the medium term forecast).
Moisture in waiting, finally some better signals in the medium term for moisture to start pooling in areas that will allow mid latitude systems to drawn in this moisture and support rainfall increasing for Australia. Time will tell, but it has been consistent in the modelling for the past 4 days.
Sea Surface Temperatures.
Indian Ocean - Current temperatures.
Values slightly elevated over the Indian Ocean basin by about 1C in the IOD region, well short of where the BoM model had suggested where we would be right now, but in line with the Euro and UkMet forecasts. The Basin is generally warmer than normal closer to Australia, but not at the high levels that has a high influence on us just yet. But the basin is expected to warm further as the seasonal shift takes place and this may be the window where the Indian Ocean Dipole rapidly develops, but the window is narrowing.
SST Anomalies - September 1-8 2021.
Values in the last week have come up just south of Java and Bali which is a good sign.
Monthly SSTs - Observed data.
Monthly Anomalies - Observed. The normal values closer to the WA and NT coasts are in response to the persistent southeasterly winds which are expected to wane in the coming weeks, allowing the waters off the coast to warm further.
SSTs Difference between July and August.
Can see a weak warming trend during the past month and this is reflected in the latter stages of August where temperatures fell by 0.5C in response to persistent easterly winds. That cooling has now ceased and recovered by 0.5C, so we shall see how it evolves in the coming weeks if that warming continues.
3 month differences in SSTs - Between May and August 2021.
The basin has warmed by about 1-1.5C in the past 3 months over the northwest of Australia but has stuttered in the past month, but in the last week the temperatures are starting to increase. The basin needs to increase by another 1-2C to have much bigger influence in which the Euro suggests.
Model Spread for the IOD September to November. Remember I am aligning myself with the International Models - Euro and UkMet.
October - The IOD peaks in the coming 6-8 weeks according to these models and I do think we will see increased rainfall chances for many locations.
November - the IOD phase begins to wane, the METEO is way too aggressive and the event should relax and become neutral by December in line with the seasonal shift. The residual impact of the IOD phase does persist into December while the atmosphere responds to the event. That means elevated rainfall will linger over inland areas of the south and east and over the tropics for December and then the nation turns into relying upon the monsoon for deeper incursions of moisture.
So how does that influence rainfall? The big ticket item on most people's mind.
% chance of exceeding of median rainfall for September- many areas in the green have seen above average rainfall already this month.
% chance of exceeding of median rainfall for October,
But there is influence also from the Coral Sea and I want to show you the PW values as this shows the impact quite nicely. Trade winds, they don't get enough attention but they do play a large part in projecting moisture across the country.
Precipitable Water Anomalies - Next 2 weeks
Note in the easterly winds, which are further north this month, the further south they develop, this then pushes a lot of moisture over the eastern states, but also runs it through northern Australia before it is swept up by upper level northwest winds. So note that trend in the first half of this week and then again at the end of the sequence. The moisture can pool over WA and sit there for a number of days before it is dragged southeast as well. So while you may not see much in terms of rainfall over inland areas of northern Australia yet, like we saw last week, the odd stronger trough and front can draw this moisture south and east and lift it into areas of rain.
Moisture Watch and the models - note the projection over northern Australia.
Last Week's system - and the ingredients were there to lift the moisture into areas of soaking rainfall. The atmosphere has responded this week with dry stable air kicking out the moisture but these types of events are likely to appear moreso into October. The event was more reminiscent of what you would see mid to late Spring. And also note the dead dry air over the Indian Ocean northwest of Broome. This shows you the lack of influence from the IOD in this past system.
So plenty to watch over the coming 8 weeks. Tomorrow I will update the drivers for the coming 2-4 weeks and a preliminary look at the La Nina chances moving through Spring into Summer. New data is out next week and I am already in talks with some international agencies and will have more details on those conversations soon.