Rainfall Deficiencies II: 12-months Duration

Log of severe and extreme rainfall deficiency of 12-month duration at Manilla

This is the second of four graphs that show Manilla’s history of rainfall deficiencies (rainfall droughts), for periods of duration 3 months, 12 months, 36 months, and 120 months.

This second graph includes those periods of severe or extreme rainfall deficiency that last twelve months. They are rainfall droughts that affect four successive seasons, sometimes making for two failures a year apart.
In Manilla’s climate, a time of severe 12-month rainfall deficiency has a rainfall total less than 400 mm, when it normally would be 640 mm.
The graph shows that such one-year droughts were very common around 1945-1950 and 1965-1970 (in 8% of months) and also 1905 (in 7% of months). They were not common (only 2% of months) around 1885, 1890, and 1980. Recently, around 2015, there have been none at all.
Remarkably, extreme 12-month rainfall droughts (in blue) were almost as common as severe ones in the long period from 1940 to 1975.

Note added June 2015

I have analysed a remarkable and unexpected relation between days of heavy rainfall and the frequency of year-long droughts at Manilla (as graphed here) in a series of posts:
More droughts After Heavier Rains I.
More droughts After Heavier Rains II.
More droughts After Heavier Rains III.

Areas shown on the graph

Rainfall deficiencies are called “severe” when they are lower than are recorded for five percent of the months. I have called deficiencies “extreme” when they are lower than are recorded for one percent of the months.
In this graph, I have coloured extreme deficiencies in blue. The maroon colour is deficiencies that are severe, but not extreme. The top edge of the maroon area marks the proportion of severe deficiencies including extreme deficiencies. As an average, this line is at five percent.

Data analysis

I have described the data and the method in “Notes” to my post “Manilla’s Record of Droughts”, of 25 November 2014.

More readable graphs

In my post “Extreme Droughts by Decade at Manilla”, dated 23 December 2014, I graphed the occurrence of extreme rainfall droughts of different duration.
That graph suffers from a problem common in showing extreme events: they are so rare it is hard to see any pattern!
This time, I have used three “tricks” to improve the display:

  • adding to the few extreme rainfall deficiency events the more numerous severe rainfall deficiency events, and displaying the two together;
  • making the resolution finer by overlapping by five years the decades that are used for counting; and
  • applying some smoothing ((1:2:1)/4).

At the beginning and end of the record, decades are incomplete. I have assumed that the rate of occurrence for the part-decade would continue for the full decade.

Rather than simply counting the rainfall deficiency events in each decade, I have expressed the result (on the y-axis) as a percentage occurrence. Since there are 120 months in a decade, the percentage is 5/6 of the count of months meeting the condition.

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