While it may feel like our weather of late has been confusing and inconsistent there’s actually some very clear climate drivers behind these patterns, and they’re quite straightforward to unpack. As gardeners, growing our weather literacy beyond a 7-day forecast is key to reading the longer climate patterns and learning more about how to best steward our garden ecosystems towards abundance.
(i) A number of gardeners have found their tomatoes and other summer veg slow to start this growing season… why did we not have a run of consistent warmer weather in the latter part of 2019?
(ii) Melbourne had about 125mm of rain for the four months Sept-Dec 2019 and only 374mm for 2019, followed by 115mm in January 2020 alone – that’s a significant and welcome change, so what’s behind that pattern?
(ii) In early February we’ve now seen the swing from very dry conditions to flooding for much of east coast NSW and QLD with record rainfalls in many areas.
It is a poignant time to be asking questions about our weather patterns.
Below is a quickfire snapshot of the elements combining to produce the conditions we’ve been experiencing, but are by no means the total sum of climatic influences on our recent weather. To unpack that takes longer than a simple blog post, and is loads more fun to do in person! Voilà – the Understanding Weather course 🙂
A few key layers of weather patterning during the last 6 months or so
(i) from October 2019 to early January 2020 we had a very negative Southern Annular Mode (SAM) which means the weather fronts circling the South pole stayed further out from the Pole, clipping SE Australia (& New Zealand) more often, and holding the high pressure systems further North (they move further south in step with the suns’ track towards its southern-most overhead path, the Tropic of Capricorn); this was most apparent with the ongoing cooler weather into December, punctuated by one or occasionally two very hot days together (but no long run of 2-5 days of +35C). *You can watch the current polar Lows circling via the BOM’s synoptic charts.
(ii) For a similar period (Oct-Jan) the Indian Ocean Dipole (IOD) was at record positive strength, which makes for dryer conditions in Australia (and wetter conditions in eastern Africa). Essentially warmer ocean waters in the west – alongside Africa – offer more moisture to the rising warm air (thus higher precipitation potential), while cooler waters in the east – alongside Australia – and sinking cool, dry air offer less moisture to the atmosphere (hence lower rainfall potential). This was one of the key drivers for the heightened fire risk and subsequent volatile fire conditions, and was also evident with the floods and damage in eastern Africa (more on that here & here).
(iii) the strong positive IOD also delayed the monsoon (which contributes to the tropical wet season), thus once the IOD reduced the monsoon moved into the northern Australian region, bringing with it warm, moist air, and when the conditions aligned the subsequent rainfall. This has continued on til present, and has also coupled with some larger, slow moving High pressure air packets offshore to our east (and also impacting New Zealand) to bring the warm, moist airflow off the oceans and onto the east coast – producing all the rainfall we’re currently seeing.
And we may yet see Tropical Cyclone Uesi come down the east coast and push even more moisture into the regions’ atmosphere… time will tell! (more info here)
It is always an interesting time to be watching the weather!