Very Wet Days at Manilla: Decade Excesses

Log of decade totals of rainfall excess, Manilla, NSW Last month I posted a complete log of days at Manilla that had more than 50 mm of rainfall.
I call days that have more than 50 mm of rainfall “very wet days”. At Manilla, on the average, these have come only once per year. Days with more than 50 mm of rainfall have no special meaning, but they can be taken as a rough indication that local flooding, or even general flooding, is likely: the “Flooding Rains” of Dorothea Mackellar.*
The graph I posted did not show whether these very wet days, likely to cause floods, had a bigger effect at some times than at others. This graph shows that.

Since it is only the excess rainfall that runs off, leading to flooding, I have subtracted 50 mm from each “very wet day” rainfall amount. Then I have summed all such excesses for each half-decade. I summed the half-decades in pairs to give a decade sum (in mm) centered on the years 1885, 1890, 1895, etc. For example, the decade centered on 1925 had a total of daily rainfall excesses of 157 mm. (Values for 1880-84 were estimated from those for 1883 and 1884.)
Some decades had very high values of excess rainfall: there was about 250 mm in the decades centered on 1900, 1960, 1965, 1980, and 2000. There were very low values, below 100 mm, in the decades centered on 1885, 1890, 1950, and 1990. There appears to be no trend.

Note added June 2015.

The close similarity of two graphs, the one of heavy rainfalls in this post, and the one of year-long droughts in an earlier post led me to write a further series of three posts:
More droughts After Heavier Rains I.
More droughts After Heavier Rains II.
More droughts After Heavier Rains III.

* By arrangement with the Licensor, The Dorothea Mackellar Estate, c/- Curtis Brown (Aust) Pty Ltd.

Log of Very Wet Days at Manilla.

Graphical log of days with over 50mm rain

In the 130-year record of very wet days at Manilla, NSW, extreme rainfalls have not become more common recently.

Data

I arranged all daily rainfall readings for Manilla, NSW, from March 1883 to December 2014 in order of rainfall amount, and selected only the 125 readings greater than 50 mm. I plotted the values against the date, expressed in years, to two decimal places. (See Note below.)

Result

The five highest readings

The five highest readings, greater than 110 mm per day, include events that gave rise to two floods and the filling of a reservoir newly-built to store water for irrigation. The highest daily reading, 142.7 mm, came with the highest flood known at Manilla, in 14/01/1964. Thus, the highest flood matches the highest daily rainfall. That is because nearly all the flood-water came down the Manilla River, which flows in a semi-circle, with none of the catchment area far away from the rain-gauge.
These five highest readings seem to fly in an arc above the rest, with a peak near the middle of the graph. The rise and fall of this arc may have no meaning, for there are very long gaps between the events. All the same, it is a fact that there were no readings above 110 mm per day in the decades before 1910 or after 1998.

Periods with no daily readings over 80 mm

Continue reading

Southern Oscillation Index: CUSUM plot

SOI CUSUM plot

This graph is a log of cumulative values of the monthly Southern Oscillation Index for the last 139 years. (See Note added 25th August 2014 below.)

(See also Note Added 19 December 2015 regarding the mis-match between this SOI record and the climate record at Manilla.)

(See Note added 27/3/2016 below, for a prior construction of this graph.)

High values of the SOI (contrary to NINO3.4 values for the ENSO index) relate to deluges in Australia and low values relate to droughts.

This is the CUSUM technique, invented in 1954 by E.S.Page. Pay attention to the slopes on the graph.

I have identified major El Niño and La Niña events on the graph. La Niñas have extreme upward slopes and El Niños exteme downward slopes.
The main feature of the graph, which is obscure in graphs that do not use CUSUM, is that La Niñas dominated the 60-year period from 1917 to 1976, and El Niños dominated the 25-year period from 1976 to 2000. I have drawn linear trend lines to make this clear. The first trend line (La Niña dominant) begins at an SOI CUSUM value of -30 in May 1917 and ends at a value of +960 in February 1976, yielding a slope of +1.4 SOI units per month. The second trend line (El Niño dominant) ends at a value of -40 in December 1999, yielding a slope of -3.5 SOI units per month.
The tendency to El Niños in the second period was greater than the tendency to La Niñas in the first period by a factor of more than two.
Although the period since 2000 is very short, the trend seems to slope upward at about +1.0 SOI units per month.

These decadal changes in the short-term mean value of the SOI are graphed in a later post. That graph does not use the CUSUM concept, and the changes in the mean value are overwhelmed by month-to-month variation.


I posted discussion of an earlier version of this graph in “Weatherzone” Forums >> Weather >> Climate and Climate Change >> ENSO Discussion 2012 Post #1103736


Note added 25th August 2014

Another CUSUM SOI graph

By searching the net for “cusum soi” I find that a plot of the cumulative sum of values of the Southern Oscillation Index was published by Cordery and Yao in 1993: “Non stationarity of phenomena related to drought”.

Neither the data nor the approach of Cordery and Yao are the same as mine.

Data

Cordery and Yao used monthly normalised SOI anomaly data supplied by the Bureau of Meteorology, as I did. They mention that “Prior to 1933 there are 7 gaps in the SOI sequence resulting from a total of 102 months of data missing from the Papeete pressure record.” I have not found any note of this with the Bureau’s current data table.
Apart from an (unexplained) reduction in the scale of CUSUM values by a factor of 500, there are important differences in detail. During the time of La Niña dominance, I find that the major CUSUM peaks (La Niña turning to El Niño) in 1918, 1939, and 1976 lie almost in one line. Cordery and Yao’s plot has the 1939 peak relatively much higher: the second highest on the record after the 1976 peak, and almost as high.

Approach

Cordery and Yao used CUSUM to show that the SOI series was not stationary for a part of the time. I used it to identify persistent shifts in the SOI mean value.


Note added 19 December 2015

The influence of the Southern Oscillation index on the climate of Manilla, NSW is cryptic at best.
In particular, the inter-decadal changes shown on this graph are not expressed at all in the episodes of drought at this station. Extreme droughts were concentrated in the period from 1900 to 1950 as shown here.

I have discussed the mis-match with the SOI in another post.

In that post, I also pointed out:
“The record for this site provides no support for any relation at all between global temperature and drought.”

Note added 22 March 2018, amended 21 May 2018.
I have since found a relation of that kind, described in the post “Rainfall kurtosis vs. HadCRUT4, revised”.
Patterns of rainfall kurtosis and global temperature.Leptokurtosis of Manilla 12-monthly rainfall totals, which indicates extremes of rainfall – both positive and negative – has a pattern that matches that of global temperature anomalies when detrended.


Note added 27 March 2016

Prior construction of this graph.
I constructed this CUSUM SOI graph in 2012 on my own initiative, without knowledge of a prior construction by David Archibald in 2010. His graph (yellow background colour) appears to have an identical trace. Without adding linear trend lines, Archibald identifies the same end points of the long period of La Nina dominance followed in 1976 by a period of El Nino dominance.
Archibald published his graph in a guest post in “Watt’s Up With That”:

My graph and Archibald’s can both by found in a search of images for “southern oscillation index”.