The record of extreme droughts at Manilla, NSW, relates to the Southern Oscillation only now and then, and relates to global warming not at all.
This graph shows some of the same data as I presented earlier in the post “Manilla’s Record of Droughts”. The graph there showed precise dates, but it was hard to tell when extreme droughts were more or less frequent. This graph adds up the number of months of extreme drought in each decade. (See Note below: How I count drought months.)
There are separate columns (getting progressively redder) for extreme droughts of duration 3 months, 1 year, 3 years, and 10 years.
Extreme droughts of 10-year duration occurred only in the 1920’s and 1940’s.
Extreme droughts of 3-year duration occurred in the 1910’s, 1940’s, and 1960’s.
Extreme droughts of 1-year duration occurred in the 1900’s, 1940’s, 1960’s and 2000’s.
Extreme droughts of 3-month duration occurred in the 1880’s, 1900’s, 1910’s, 1920’s, 1940’s, 1970’s and 2000’s.
No extreme droughts at all occurred in five of the fourteen decades: the 1890’s, 1930’s, 1980’s, 1990’s, and 2010’s.
Relation to the Southern Oscillation Index
I posted this graph of cumulative values of the SOI earlier.
The record of the Southern Oscillation Index relates to the Manilla record of extreme rainfall deficiency only now and then. Persistent El Niños from 1911 to 1915 seem to relate to four months in the decade of the 1910’s having extreme 3-year droughts, carrying forward to two months in the 1920’s having extreme 10-year droughts. Similarly, the catastrophic droughts of short to very long duration in the 1940’s relate to El Niños that persisted from 1939 to 1942.
Other major El Niño events did not produce extreme droughts at Manilla: those of 1896, 1982, and 1997.
Long term trends in the Southern Oscillation Index do not predict Manilla’s extreme droughts at all. The 1940’s droughts came in the middle of the 60-year period from 1915 to 1976 when La Niña (suppressing droughts) was dominant. In the period of El Niño dominance (promoting droughts) from 1976 to 2000, there were almost no droughts at Manilla.
Relation to global warming
Manilla’s extreme droughts came at the (cool) beginning and (warm) end of the period of rapid global warming from 1909 to 1943, as well as in the middle (1960’s) of the cooling period from 1943 to 1975. The most recent period of rapid warming from 1975 to 2005 was almost free of extreme droughts.
The record for this site provides no support for any relation at all between global temperature and drought.
Note: How I count drought months
The y-axis shows “Months in Extreme Drought”. It may not be clear how they were counted.
For each month of record, I calculated the total rainfall in the n months ending in that month. From this I calculated the percentile rank of that total. I counted as “extreme droughts” only those values at or below the first percentile in the record. For simplicity, I showed only counts of extreme droughts of 3- 12- 36- and 120-months duration.
As an example:
In the decade beginning 1940, the graph shows that I counted six months of one-year extreme droughts. That is to say, in each of six months out of the 120 months in that decade the 12-month rainfall total ending in that month was at or below the first percentile rank.
Despite the use of a single criterion for “extreme” (first percentile), the average monthly rainfall in an extreme drought increases remarkably with the duration of the drought. From Manilla’s 130-year record, in which the mean monthly rainfall is 54.3 mm, first percentile monthly values for the selected durations are:
3-month extreme drought: 8 mm per month (15% of average);
12-month extreme drought: 28 mm per month (52% of average);
36-month extreme drought: 39 mm per month (72% of average);
120-month extreme drought: 47 mm per month (87% of average).