Manilla’s Yearly Rainfall History

Lately, Manilla’s rainfall is normal, and more reliable
than it ever was.

Manilla yearly rainfall record, 21-yr smoothed

Yearly rainfall totals

The first graph helps to make sense of the history of Manilla’s rainfall, using the totals for each year. The actual figures make little sense, jumping up or down from one year to the next. The figures here have been calmed down. First, I replaced each yearly figure by an average of twenty-one years, ten years before and ten years after the date. Then I smoothed that figure some more.
The pattern is plain. There were periods in the past when there was much more or less rain than usual.
In four decades the rainfall was some 30 mm higher than normal: the 1890’s, 1950’s, 1960’s and 1970’s. In four other decades, the rainfall was some 30 mm lower than normal: the 1900’s, 1910’s, 1920’s and 1930’s.
Rainfall here collapsed about 1900. The collapse was was widespread, as was recognised half a century ago.

Using the average line drawn across the graph (at 652 mm), you can see that rainfall was below average from 1902 to 1951: almost exactly the first half of the twentieth century. After 1951, rainfall was above average for the 44 years to 1995. Since then, the annual rainfall (as plotted) has been remarkably close to the 132-year average.
Present rainfall will seem low to those who remember the 1970’s, but the 1970’s were wet times and now is normal. Few alive now will remember that Manilla’s rainfall really was much lower in the 1930’s.

The pattern in Manilla’s history of annual rainfalls is better shown in a graph in a later post. The new graph, using 21-year median values, has a clear pattern of collapse, growth and collapse.

Manilla yearly rainfall scatters.

Yearly rainfall scatter

The second graph also groups the data twenty-one years at a time. It shows the scatter of yearly rainfalls in each group. More scatter or spread means the rainfall was less reliable. Comparing the graphs, times of high scatter (very unreliable rainfall) were not times of low rainfall, as one might think. Annual rainfall scatter and rainfall amount were not related.
Times of very unreliable rainfall came in 1919 (dry), 1949 (normal) and 1958 (wet). Times of reliable rainfall came in 1908 and 1936 (both dry). However, by far the most reliable rainfall came since 1992, extending to 2004 and likely up to this year.

Global warming

It has been argued that human-induced climate change will cause climatic extremes to happen more often in future. Already, when any extreme climate event is reported, someone will say that climate change has caused it.

The present steady rise in global temperature began about 1975. Does this Manilla rainfall record show more extreme events since that date? Definitely not! Quite the contrary.
Annual rainfall began unusually high in 1975 and decreased, to converge on its long-term average. Now it could be said to be more extreme only in the sense of “more extremely normal”.
From 1975, the scatter of annual rainfalls began near its normal value, then rapidly decreased. Annual rainfall is now more reliable than it has ever been. The rainfall in one year is usually much the same as in the year before. Again, that is “more extremely non-extreme”!
This Manilla rainfall record is one counter-example to the snow-balling catalogue of reported extreme climatic events.


Note 1: Handling the data

The data is the list of yearly rainfall totals for Manilla, beginning in 1883. An updated list is published in the “Manilla Express” each year.
The data is very “noisy”, with each value different from the last. I applied a “window” of 21 years, extending ten years before and after a given date.
For the first graph, I took 21-year averages, which smoothed the data, calming it down. To further smooth it, I applied the formula (1:2:1)/4 twice.
For the second graph, I analysed each 21-year group to find the 1st and 3rd quartile values. I used the Inter-Quartile Range (IQR), Q3-Q1, as a measure of the scatter in that group. Again, I smoothed the result by applying the formula (1:2:1)/4 twice.

Note 2. Similar graphs

A. 7-year totals

I prepared graphs similar to this one previously, by doing 7-year sums of rainfall. They are in two threads of “weatherzone” forums:
(i) “General Weather/Lake George”

“Modelling Lake George Historic Levels”

(ii) “Inter-decadal climate cycles or shifts” (closed).

“Seven-year rainfall trends in Eastern NSW uplands”

(a post relating 7-year rainfall to the Inter-Decadal Pacific Oscillation)

(a similar post).

B. Manilla summer rainfall using 6-year Gaussian smoothing

“Decadal and Inter-decadal changes in rainfall:III.”

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