Droughts and flooding rains at Manilla NSW were related in a way that is remarkable and unexpected.
Part 1. Graphical logs
As the first graph shows, for most of the 130-year record year-long droughts came in direct proportion to very heavy daily rainfall five years earlier. (For data details, see Note 1, below.)
The match between these two variables is astonishing. Both are based on rainfall readings, but they are scarcely related. Excessive daily rainfalls are transient extreme weather events; 12-month droughts are an aspect of climate.
Mackellar’s “Droughts and flooding rains”
Dorothea Mackellar’s famous line * is more apt for this graph than for other graphs where I use “flooding rains” to mean periods unlike drought. (See Note 2. below.) The rains and droughts that I plot here both bring hardship. Severe droughts lasting one year are among the worst of droughts: long enough to use up reserves, and not so long as to be eased by periods of rain. The daily rainfall events plotted are the ones that cause damaging floods.
Features of the graphical log
This second graph shows the data at the actual dates. Although the data points for the decade excess of heavy daily rainfall and those for frequency % of 12-month droughts have a matching pattern for much of the record, the pattern is offset. Heavy rainfall points come five years earlier than corresponding drought points. Notice that the heavy rainfalls do not (except in 1980) come squarely in gaps between droughts.
Lagging the rainfall points by five years (as in the first graph) makes some matches almost exact. Such matches occur at all data points from 1890 to 1975, except those from 1940 to 1955, where drought frequencies are relatively higher. Both variables show a two-decade-long, slow decline from 1905 to 1925. At the chosen scales, the amplitude of corresponding rises and falls are usually similar as well.
After 1975, daily rainfall oscillates through a wide amplitude with a twenty-year period, while the frequency % of drought varies less and more slowly, including a rise from 1980 to 2000 that looks like the decline in both variables that occurred from 1905 to 1925.
Can droughts vary directly with flood-causing rains?
Common sense would have heavy rainfall less common in times of drought. This need not be true: the desert, a climate zone in permanent drought, has intense rain storms. In this particular data set, the daily rainfall values are specifically those extreme events likely to cause floods. Up to 1975, it seems clear that the decadal sums of these flood-generating rains vary directly with the frequency of periods of 12-month drought, but the droughts come five years later.
This variation is explored in Part II.
Note 1. Data
(See this earlier post.)
The graph of 12-month droughts shows the percentage of months in each decade that have a 12-month rainfall total below the 5th percentile. They have a median value of 5% but range above and below it. Values are plotted against the mid-year of each decade, with decades overlapped by five years. They have been smoothed (1:2:1)/4.
B. “Flooding Rains”
(See this earlier post.)
From the 130-year record of daily rainfalls at Manilla, NSW up to December 2014, I selected all 125 “very wet days” that have readings exceeding 50 mm. I listed them by date.
Since it is only the excess rainfall that runs off, leading to flooding, I have subtracted 50 mm from each rainfall amount. Then I have summed all such excesses for each half-decade. I summed these in pairs to give a decade sum (in mm) centered on the years 1885, 1890, 1895, etc.
Note 2: “Droughts and flooding rains” *
I usually quote Mackellar’s line in reference to the quasi-biennial cycle of smoothed monthly climatic variables between hot and dry climate and cool and wet climate. This cycle, unpredictable in occurrence or period or intensity, prevails at Manilla and other places in Australia. At times it is in phase with the El Nino Southern Oscillation (ENSO).
In my series of posts: “3-year climate trends”, the data that I plot on each of the six standardised graphs usually moves back and forth between the top right corner “Droughts” and the bottom left corner “Flooding Rains”. I have explicitly labelled the corners in this way on the top left graph, which has rainfall anomaly (inverted) on the y-axis and daily maximum temperature anomaly on the x-axis.