I have been grumbling for months about how dry Townsville has been in the last year, and the annual figures are now in: 2015 was the driest year on record, with just under 400mm compared to our average of 1135mm and our highest-ever of 2400mm. We have just continued the pattern by completing a dry January, 77mm compared to an average of 270.
My figures are from the BoM’s Climate Data Online for Townsville airport, here (I have rounded them off to the nearest whole millimetre). Looking at the annual rainfall totals in that table, a pattern pops out: four-digit and three-digit totals don’t alternate randomly but come in clusters, 3-5 of each at a time. For instance, 2001-06 were all below 1000mm, while 2007-12 were all above it. Extracting the first and last columns and colouring the totals blue for wet years and brown for dry years (see table at left; click on it for a larger version) makes the pattern more obvious, and highlights one other quirk: a run of dry years is sometimes interrupted by a single wet year (e.g. 1968 and 1981) but the converse isn’t true.
I also divided the range of annual totals into three: close to average (1000 – 1300 mm), wet years (more than 1300) and dry years (less than 1000). Colouring them appropriately brings out another feature, showing just how rare an “average” year is: only one year in five is within that zone.
Monthly rainfalls also vary wildly. I could repeat the same exercise for (e.g.) January rainfalls, where the average is 270mm but the actual figures vary from a mere 9mm to 1142. We can’t even say that a wet January is particularly likely to be followed by a wet February and March. Sequences like 85 – 549 – 53mm (2002) and 19 – 316 – 73mm (2003) are not uncommon.
In fact, we have to realise that an average month or year is not at all normal.
Climate is what we expect, weather is what we get
So can the statistics tell us anything useful at all? Well, yes. They can obviously say things like, “Yes, 2015 was much drier than usual. It wasn’t your imagination.” They can also warn us not to expect an average month or year, something that long-term residents are vaguely aware of but many newcomers are not.
More subtly, they can warn us that we are getting a run of extremes and should be concerned by them. As Hansen warned us some time ago, every broken record carries the fingerprint of climate change – not necessarily caused by climate change, but made more likely by climate change.
They can show us, too, how our climate is changing over the long term – and those trends are likely to continue into the future, over at least a similar time-frame. “Climate is what we expect, weather is what we get,” (the quote is well known but its origin is obscure) and in those terms it’s clear that we need to look at periods of more than ten years.
Fortunately, we have done that – well, the BoM has, and has made the data available to all of us online. In particular, their trend maps make long-term changes beautifully (scarily?) clear. Here, for instance are the national rainfall and average temperature trends since 1970.
Food for thought – especially the extreme drying trend down the East coast, where most Aussies live. For more detail, follow my link to look into different regions, seasons and data sets.
As for Townsville, all we know is that we don’t know what to expect. The BoM made some educated guesses for the Townsville Bulletin’s January 4 article about our meagre 2015 rainfall but most of the article was concerned with very short-term forecasts. The BoM’s Outlook page suggests that the North will be drier and hotter than average in the next few months, Feb-April, but we shall have to wait and see.