INDIAN OCEAN AND SOUTHERN OCEAN REVIEW - HOW IS IT LOOKING AS WE MOVE INTO AUTUMN?

The signals are that the Indian Ocean as a whole, remains neutral for the season ahead, which is a fair call. But there are sections of the Indian Ocean that will provide some influence that does not get as much attention.


Also the Southern Ocean is also set to play a part in the distribution and intensity of rainfall across the southern states as well. This in particular as cold fronts return to the charts from around April onwards.


The influence? The warmer than normal waters in both basins.


Sea Surface Temperature Anomalies - Current Data

The SSTs around Australia are still above average in most waters, though with the monsoonal activity over northern Australia in recent weeks, this has caused temperatures to moderate which is seasonally expected. The waters have turned very warm west of the nation through the Central Indian but more interestingly the waters south of the nation have turned much warmer than normal compared to previous years during this time of year. The signal continues to be that waters south and west of the nation will remain seasonal to above seasonal throughout Autumn.

Change in the Sea Surface Temperatures past month.

The biggest shift in conditions has been over the Southern Ocean where values are up to 2C above normal and continuing to warm up. It will be interesting to see how the latest phase of stronger westerly winds will interrupt the warming waters over the Southern Ocean. But the moisture to be advected from the region could be above normal, especially as the westerly wind belt begins to transition north through Autumn leading to more widespread rainfall along and ahead of cold fronts passing through Southern Australia. So that is something to consider as we move through the coming few weeks and will be watching closely and bringing you the details. But the influence on rainfall could be significant if all the climate drivers work in phase with these above average SSTs.

The Southern Annular Mode has been persistently positive in recent months and that is a sign that the La Nina phase is in place, but we have seen it dip into negative territory for the first time since September 2021. That dip is short lived and a positive SAM phase is expected to continue over the coming 6-8 weeks before the influence from the Pacific wanes through April. As the influence begins to wane from the Pacific, the Southern Annular Mode does have a tendency to return to neutral values and more readily move from one phase to the other, rather than being stuck in one phase for months at a time which we have seen (see below). With more negative SAM events moving into the back half of Autumn, this could increase the frontal activity, again over warmer than normal waters, leading to more rainfall in areas exposed to a westerly wind regime.

The Indian Ocean is of great interest. With the warmer than normal waters sitting out over the Central Basin, this can influence the rainfall odds for WA, through SA and into the eastern inland. This is IF the mid latitude jet stream sits over the warmer waters and interacts with cold fronts/troughs passing from west to east, leading to widespread rainfall and enhanced northwest cloud bands. That is a possibility through March to May, especially with the lingering monsoonal weather over northern waters.

You can see above the above average rainfall forecast northwest and west of the nation and the area I have highlighted is the warmer than normal SSTs west of the nation. So there is two ports of higher moisture content and if the short term climate drivers can be in phase to combine with this above average moisture/precipitation areas, then rainfall across the nation could be very much above average through early Autumn and persist to about April.


Indian Ocean Dipole Forecast Guidance International Models - April

The residual wet season and heat levels from the Summer Season will see slightly positive values in the Indian Ocean nearer to Australia but remember above, the SSTs are above average in the Central Indian Ocean which could play a part in lifting moisture content in the mid latitude westerly winds and see this increase rainfall chances with cold fronts passing through the southern part of the basin before moving into WA. So there could be some above average rainfall chances for southern WA, through SA and into TAS, VIC and NSW. Something to watch. Much of eastern inland Australia may not have much influence from the Indian Ocean at this time.

Indian Ocean Dipole Forecast Guidance International Models - May

Modelling including the BoM, is suggesting the Indian Ocean returning to cooler neutral values through May which would support a weak influence on rainfall odds. At this stage seasonal expectation across the nation possible as the mixing out of the above average SSTs over the Central Basin is expected so the influence should wane (if it develops at all) during this time. This is a month of transition for high pressure systems moving over the nation and the westerly wind belt underneath that ridge moving north, enough to influence Australia's weather. The more negative the SAM (more cold fronts) the more rainfall expected over southern Australia, especially given the SSTs in place south of the nation.

Indian Ocean Dipole Forecast Guidance International Models - June

Steady strengthening of the cooler signal through early Winter is expected with some agencies suggesting a negative IOD possible by July into October before that wanes again, the values here suggest weak influence through June from the Indian Ocean. The rainfall likely to remain at seasonal values, but IF the waters south and east of Australia remain above average, then enhanced rainfall from the Southern Ocean via cold fronts and southeasterly winds a top high pressure, leading to above average rainfall. Northern areas of the nation would see a more humid build down, with the wet season lingering into May and not drying out totally until June.

A REMINDER OF MARCH AND THE AUTUMN OUTLOOKS IN CONTEXT OF THE ABOVE INFORMATION

Rainfall Outlook March 2022

The wet end to February over northern and eastern portions of Australia will continue to increase as we kick off March with heavy rainfall from the monsoonal flow over the north and the positive SAM phase in the east drifting further south and west through the nation, with increasing rainfall signals for the beginning of the month for NSW, VIC and parts of SA. The far west will be near seasonal, however the rainfall odds are dictated by any landfalling cyclone activity throughout the interior with heavy inland rainfall to stem from any of those events. The east coast looks especially damp, with persistent onshore winds, widespread showers and thunderstorms. This moisture is likely to travel west of the divide and then link up with inland troughs leading to more rainfall and humid weather through the first half of the month in particular. The southern and eastern states need to be on alert for any weakening cyclone/tropical low activity stemming from the north. Overall, a damp signal is persisting and this will likely continue, specifics will become clearer as we get towards the end of this month. But once again, be aware of the wetter signal as we enter March 2022.

Temperature Outlook March 2022

Mixed signals from the modelling nationally, the cooler than normal bias will be in place for the eastern parts of the nation with persistent easterly winds and higher than average rainfall with the La Nina continuing to take hold. For the north, the MJO should be working it's way through northern Australia and landing up in the western Pacific, so some chance of cooler than normal weather for chunks of the north and northeast. The weather for the southwest and south of the nation, near seasonal under long fetch easterly winds and ridging being further south, the winds dry out and warm up running over land, so a higher chance of above average temperatures over the west and south of the nation for now, but as mentioned above, tropical systems may change that very quickly, with inland rainfall and thick cloud cover leading to below average temperatures.

A refresher on AUTUMN 2022

Autumn Rainfall Outlook

% chance of exceeding median rainfall for Autumn 2022

As mentioned through the video, the overall confidence is growing for a wetter than average season across much of the nation, with that said, the higher chances exist over the northwest and the east coast where climate drivers currently in play, coupled with elevated SSTs will support more rainfall than usual during this time. The rest of the nation is leaning towards wetter than normal values, but the overall strength of those values has waned a little. Still, seasonal rainfall should be received for most during this time, but perhaps we can pencil in a few more drier spells than what we have seen recently, with some luck. That may ultimately reduce the flood risk. The lingering La Nina over in the Pacific may lead to a prolonged wet season for northern Australia. The east coast will see enhanced rainfall for the period with easterly winds feeding off above average SSTs to bring higher rainfall for longer duration. Wild cards to bring significant rainfall will be late season cyclone activity over northern Australia and the east coast low season off the southern QLD and NSW/VIC coastal areas between April and June.

Autumn Temperature Outlook

% chance of exceeding median temperatures for Autumn 2022

The cooler than average conditions over eastern Australia likely to continue, though I have contracted the cooler bias to on and east of the divide since the last update. Seasonal weather overall is expected for much of the nation with some areas leaning warmer than normal. The north of the nation with elevated humidity values with the build down over the course of April and into May. This will assist in keeping temperatures up. Otherwise, most elsewhere, seasonal conditions can be expected.

Key Notes

This is the take away from the data sets - more analysis can be found in the video. It is going to be a dynamic season - I think make the most of the quieter period in the coming few weeks in southern and eastern areas.

I will have the next 6 week outlook update coming tomorrow - which will be very telling as to the influence of those warmer waters over southern and western parts of the nation.



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