Decadal and Inter-decadal changes in rainfall: I.

Log of smoothed summer and winter rainfall anomalies.

Part 1 of 3: The whole 130-year record

(See Notes below for data and plotting details.)

Anomalies of smoothed summer and winter rainfall

Episodes of high or low summer rainfall do not coincide with those of winter rainfall (except in 1891). Nor do they consistently oppose each other, although this is common. The summer rainfall anomaly (red) was extremely low (-101 mm) about 1900, and extremely high (+119 mm) about 1975. The winter rainfall anomaly (blue) had lower extreme values: 1939 (-48 mm) was the lowest of several low values, and 1987 (+63 mm) the highest of several high ones.

Seasonal sums and differences

I plotted the smoothed yearly value of rainfall anomaly as the sum (purple) of a winter anomaly value and that of the following summer. There was an extreme maximum in 1891 (+139 mm!), and minimal values in 1899 (-79 mm) and 1913 (-87 mm), among others.
The difference between summer and winter seasonal anomalies (orange) shows as an extreme summer excess in 1974 (+163 mm), and extreme winter excesses in 1900 (-126 mm) and 1987 (-114 mm).

Log of smoothed sum and difference of summer and winter rainfall anomalies.

“Dreadful Thirst”

Banjo Paterson’s comic verse “City of Dreadful Thirst” refers to the town of Narromine, 300 kilometres west of Manilla.

“Last summer up in Narromine ’twas gettin’ rather warm–
Two hundred in the water bag, and lookin’ like a storm-“

“That cloud that came to Narromine was just a cloud of thirst.”

The poem, published in “The Bulletin” dated 9 December 1899, describes a summer drought beyond comprehension. I suggest that Paterson was writing in response to the most severe summer and annual drought on these graphs, the drought that peaked in 1899-1900. That drought would have seemed even worse by following closely on the extreme rains of 1891, only eight years before.

Linear trends

Over the 130-year record, the linear trend of yearly rainfall (purple) is effectively flat. The gradient of +0.059 mm/yr (less than six millimetres per century) is supported by an R-squared value of only 0.002 (explaining 0.2% of the variation).
However, the summer rainfall linear trend (red) rises at the high rate of 24.7 millimetres per century, and the winter rainfall linear trend (blue) falls at 18.9 millimetres per century. As a result, the difference of summer anomaly versus winter anomaly (orange) rises at the very high rate of 43.7 millimetres per century.

Quartic trends

Quartic trend lines for yearly, summer, and difference smoothed anomalies are similar. They are high in the 19th century then, in the twentieth century, they are low in the first half and high in the second half. For the winter smoothed anomalies, the quartic trend takes the opposite sense, but without much amplitude.

The cause of the trends

Manilla is on the margin between the monsoon belt and the westerly belt. The positive linear trend for summer (monsoon) rainfall and negative linear trend for winter (westerly) rainfall could indicate either:
(i) the monsoon belt rainfall is getting heavier and the westerly belt rainfall is getting lighter, or
(ii) the monsoon belt is extending further south.
The quartic trend pattern of low rainfall in the first half of the twentieth century and high rainfall in the second half is also seen at Lake George, near Canberra. The relation to the IPO is not simple.

Posts on this topic

This post is one of three on “Decadal and Inter-decadal changes in rainfall” based on the 130-year rainfall record at Manilla, NSW, Australia:
I: The whole 130-year record. (This post.)
II: The record restricted to 1891-1982 (92 years).
III: A growth and collapse model for summer rainfall.


Notes

Rainfall at Manilla, NSW, has been observed since 1883. As I posted earlier, there are two distinct rainfall modes, centred on the summer and winter solstices. The summer (monsoon) mode has nearly twice the rainfall of the winter (westerly) mode.

Here, I have assigned to the winter mode the monthly data from April to September, and assigned to the summer mode the monthly data from October, extending to March in the following year.
Each data value has been expressed as an anomaly from the 130-year mean. I found the anomalies not only for the “summer” and “winter” totals, but also for their sum (April to March) and their difference (summer minus winter). To remove high-frequency noise and the effect of ENSO from the plotting, I applied a Gaussian filter of half-width 6 years.

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