Break in the Break?
It's early May in the Hills, and we've had The Break. There's no formula or set of expert-defined preconditions for this, like say an economic recession or a disease state. You just know it - the turning red & yellow leaves, moist ground with green shoots, woodsmoke, and a stretch of sub-20 days ahead with no end in sight. You know the summer spell is broken.
People in southern Australia, particularly the older generations, talk about the Break in slightly mythical terms, and it's great to listen to. One rule of thumb my Mum used to tell me, probably passed on from her mum, is that it usually rains on Easter or Anzac Day. Local folk-knowledge of climatic cycles based on the wisdom of years, passed down via oral tradition.
I was fascinated by Break theories from a young age. I started rain records at my family home as a kid in the mid 80s, and sure enough my first year confirmed everything I'd been told. I remember being at a Year 8 school camp in April, the soil powdery-dry from almost zero rain in three months, and it bucketed down (60+ mm in a week) turning everything into a brown eroding slush. No wonder I'd seen so many sodden Anzac Day parades on TV, I thought, and found it hard to get outdoors on Easter weekends. It was The Break.
But as the years passed Breaks didn't come as I'd imagined. Some years they'd run late - May or even early June. And they'd run late more often than early. And they wouldn't always come in that dramatic, decisive way like on Year 8 camp. It could sputter in pathetically - a few mm here and there, maybe a 5 or 10, then a couple of dry weeks, then god knows what. Sometimes the rain could come with a bang but with humid, warm, ex-tropical systems and then further warm days. Nothing seemed certain. Sometimes it could be cold, dry & frosty like in Canberra or the NSW Tablelands.
We still get normal Breaks for sure. This year (2020) we seem to have gotten lucky with early April rains. But for decades I've been convinced something's not right. Was I onto something or not? If so, was it always this way or is it climate change?
Recently I decided to stop the wondering & put my theory - and Break mythology - to the test. The Bureau has some deep data within its Climate Data Online section. Hundreds of spreadsheets of daily and monthly rain records, some going back over 100 years.
My hometown of Hahndorf, SA, has monthly records going back to 1886. There are a few gaps but I could put in proxy amounts based on records from nearby stations. The town has a Mediterranean climate (Köppen classification Csb) with some cool temperate and continental influences coming from its position in the Adelaide Hills, 350m above sea level in a valley 40 kms from the coast. Winter rain is about four times summer rain.
So I ran a simple stats exercise: what are the monthly average rain amounts for Hahndorf (mean and median) over the last 30 years compared to the previous 100? Here are the results:
| _____________________ | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mean average (pre-86) (mm) | 28.5 | 26.6 | 32.8 | 66.7 | 99.4 | 118.0 | 120.8 | 118.1 | 97.4 | 76.1 | 44.6 | 35.5 | 864.5 |
| Mean average (1986-) (mm) | 22.2 | 23.1 | 28.2 | 47.2 | 86.8 | 113.1 | 127.7 | 108.4 | 91.4 | 58.1 | 38.0 | 42.9 | 787.0 |
| Difference (mm) | -6.3 | -3.5 | -4.6 | -19.6 | -12.6 | -4.8 | 6.9 | -9.7 | -5.9 | -18.0 | -6.6 | 7.5 | -77.5 |
| Difference (%) | -22.2% | -13.3% | -14.1% | -29.3% | -12.7% | -4.1% | 5.7% | -8.2% | -6.1% | -23.7% | -14.9% | 21.1% | -9.0% |
| _____________________ | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Median average (pre-86) | 22.1 | 15.5 | 22.6 | 52.6 | 84.1 | 107.6 | 121.2 | 109.7 | 86.3 | 72.3 | 37.6 | 31.2 | 864.6 |
| Median average (1986-) | 17.2 | 15.4 | 25.0 | 40.2 | 81.0 | 118.8 | 119.2 | 107.6 | 83.8 | 52.8 | 28.6 | 28.8 | 766.8 |
| Difference (mm) | -4.9 | -0.1 | 2.5 | -12.4 | -3.1 | 11.3 | -2.0 | -2.1 | -2.5 | -19.5 | -9.0 | -2.4 | -97.8 |
| Difference (%) | -22.2% | -0.6% | 10.9% | -23.6% | -3.6% | 10.5% | -1.6% | -1.9% | -2.8% | -27.0% | -23.9% | -7.5% | -11.3% |
The figures say a lot. To start with the annual rain has dropped about 10%. This is consistent with most of southern Australia from the east coast to WA, maybe not quite as bad (most of southwest WA has dropped 15-20%). A quick look at other SA towns tells a similar story - drops in the 8-15% range.
Turning to individual months is the interesting bit. The block of four wet months from June to September hung in there - just - with a drop of just under 3% on the mean average. July was up and December even more (definitely noticed those late spring storms).
But the transition periods of mid-autumn and mid-spring were shockers. April was the worst of all with a 30% drop on the mean, and October was not far behind. The entire late summer and autumn period (Jan-May) saw monthly declines of 13-30%, and confirm the spring drying trend November was down 15%.
The median averages tell a similar tale. Some differences, but this was mainly just the median being so far below the mean in the first place (eg March for example). The spring and autumn drying trends were still there.
So I reckon I'm right. The Break has broken down. And if it's not happening in April it isn't always happening in May either. With late summers also being drier, this is leaving many gardeners desperate for subsoil moisture deep into autumn - not good.
My explanation for the trend is not new or out-there at all. The wisdom is that, due to climate change, high and low pressure systems are on average passing across southern Australia at gradually higher (more southerly) latitudes, meaning rain-bearing cold fronts are arriving later on in autumn, and fading away earlier in spring, than before.
Now I know why my water bill has been recently highest in the Apr-Jun quarter.
