Yesterday in the comments section I gave a brief analysis of "Peninsular" rainfall for the period 1871 to 2014 during the monsoon months, June to September.
Today we look at the same set of data ((via IITM under the All India and Macro Regional data set. (It includes six subdivisions namely Coastal Andhra Pradesh, Rayalseema, TN & Pondicherry, Coastal AP ,S I Karnataka and Kerala)) of the "Peninsular" rainfall for the period October to December.
This period is normally classified as the NEM and this is what IMD Chennai says about it..
"The period October to December is referred to as Northeast Monsoon season over peninsular India. Earlier this period was also referred to as 'Post-Monsoon Season' or 'Retreating southwest Monsoon Season'.
Northeast Monsoon season is the major period of rainfall activity over south peninsula, particularly in the eastern half comprising of the meteorological subdivisions of Coastal Andhra Pradesh, Rayalaseema and Tamilnadu-Pondicherry. For Tamilnadu this is the main rainy season accounting for about 48% of the annual rainfall. Coastal districts of the State get nearly 60% of the annual rainfall and the interior districts get about 40-50% of the annual rainfall.
Though the principal rainy season for Interior Karnataka, Kerala and Lakshadweep is the Southwest Monsoon season, rainfall continues till December in these sub-divisions, the period October-December (Northeast Monsoon ) contributing about 20 % of the annual total. "
First we look at a graphical representation of the raw data from 1871 to 2014..
As can be seen the rainfall varies from as low as 93mm in 1876 to 585mm in 1946. Those are the extremes. The average for the entire 144 years is 345mm.
Now we look at a smoother version of the above data which has been averaged every three years..
As can be seen there is a cyclical behavior that seems to swing between highs and lows every few decades. The lows are formed every 35 to 45 years and since the last one was in 1990 the next can be expected around 2025 to 2035 (since it is averaged for the previous three years the actual lows might form from 2022-25 or 2032-35).
An even more smoother version (9 years) gives us this ;
Here too we get the same 35 to 45 year lows in rainfall.
We now compare the Peninsular Oct to Dec rainfall to the Annual Peninsular rainfall for all the years;
(smoothed every nine years)
Same conclusions as above!!! Also the range varies from 35% (1888) to 25%(1955).
And finally we compare the Peninsular Oct to Dec rainfall with the Peninsular Monsoonal rainfall;
(smoothed every nine years)
Again the same conclusions as far as the lows are concerned. The range varies in this case from 65%(1888) to below 45%(1912,1955).
Conclusions: The Peninsular rainfall from Oct to Dec shows a huge variation from 93mm to 600mm and ranges between 25% to 35% of its annual rainfall averaging around 29.5%. As compared to the south west Monsoon it varies from below 45% to almost 65% averaging around 54%. A cyclical behavior is very evident in the charts above and the current century shows nothing unusual.
The fact that some kind of regularity has been seen in the trend for the last 144 years does not suggest at all that a new trend might not emerge in the future..weather is too complicated for any one to truly predict accurately a hundred per cent all the time. I personally feel a time period of 150 years is too short for any kind of definitive trends to be emerge.