The Indian rainfall is largely governed by the Monsoon season when it rains heavily for 4 months each year. The following graph shows the very strong seasonal cycle:
To see other than seasonal fluctuations and cycles it is necessary to remove the seasonal components. It was found that the variations during the wet and dry seasons have different proportionalities. To make these extremely different seasons have similar amounts of variations it was found that using the square root of monthly rainfall was a good measure. So the following graph shows (monthly rainfall)^0.5 after removing the seasonal cycle.
When the spectrum of rainfall is determined, the seasonal cycles dominate, with 1 year, 1/2 year, 1/3 year cycles being very strong and 1/4 and 1/5 also showing. Other cycles are present, but these are more easily studied after removing the strong seasonal cycles.
The following graph therefore shows the spectrum of (Indian monthly rainfall)^0.5 with seasonal variations removed:
There are some long cycles present, namely 19 years, 10 years, 7.0 years, and 3.86 years. Of these we might suspect that the 19 year cycle is related to any of several lunar cycles that affects ocean and atmospheric tides.
Of seven strong shorter cycles, four have been marked with green triangles at the exact locations of harmonics in repeated 2:1 ratios. The periods found were 627, 303.5, 153.0 and 75.75 days. Each is rather close to half of the previous one. Not only that, but these cycles have very similar periods to a series of cycles found in solar activity and reported extensively in peer reviewed journals. The most commonly reported one of these cycles is generally called the 154 day cycle (or sometimes from 150 to 160 days) and it is noted that the relationship of 308 – 154 – 77 days and 103 – 52 – 26 days are all present in a variety of solar phenomena. Indeed, the 26 day period is the solar siderial rotation period which we see as 28 days because the earth is moving around the Sun a little each solar rotation.
For more about these solar cycles please see: http://www.cyclesresearchinstitute.org/journal/CRI200510-solar154day.pdf