The Indian summer monsoon
A seasonal reversal of the winds that delivers most of the year's rain in four months, and varies enough within and between seasons to matter for everyone who depends on it.
Onset and withdrawal
The monsoon arrives over Kerala around the first of June, with a spread of about a week either side, and advances northwest over the following six weeks until it covers the country by the middle of July. Withdrawal starts from northwest India in September and retreats in the reverse direction, with the last stages over the southern peninsula bringing the northeast monsoon rains to Tamil Nadu.
Declaring an onset is harder than it sounds. Rainfall alone is ambiguous, because pre-monsoon thunderstorms can produce several wet days that do not persist; operational criteria therefore combine rainfall at a set of coastal stations with the depth of the westerly wind field and the outgoing longwave radiation over the eastern Arabian Sea.
Active and break spells
Rain within the season comes in spells. The band of deep convection that normally sits over central India weakens and moves either towards the equator or towards the Himalayan foothills, and the country sees a break of a week or more before the band re-establishes 1. The northward propagation is a robust feature of the boreal summer intraseasonal oscillation and links the monsoon to the equatorial Indian Ocean.
Because the seasonal total is a sum over these spells, the number and length of breaks in a given year explains a good part of why the year turns out wet or dry 2.
From one year to the next
The standard deviation of all-India summer rainfall is roughly a tenth of its mean. That modest spread hides the fact that the largest departures cluster in years when the tropical Pacific or the Indian Ocean is in an unusual state, and that a substantial part of the remaining variance appears to be internal to the atmosphere and therefore not predictable from boundary conditions alone 3.
This is the practical limit on seasonal forecasting: skill comes from the slowly varying ocean, and the fraction of monsoon variance the ocean controls is real but partial.
Data
Daily gridded rainfall over India from the India Meteorological Department 4, reanalysis winds and moisture for the circulation, and satellite precipitation for the ocean. Note the change in station density through the record before reading trends from any gridded rainfall product.
- Sikka, D. R., and S. Gadgil, 1980: On the maximum cloud zone and the ITCZ over Indian longitudes during the southwest monsoon. Mon. Wea. Rev., 108, 1840–1853.
- Goswami, B. N., and R. S. Ajaya Mohan, 2001: Intraseasonal oscillations and interannual variability of the Indian summer monsoon. J. Climate, 14, 1180–1198.
- Gadgil, S., 2003: The Indian monsoon and its variability. Annu. Rev. Earth Planet. Sci., 31, 429–467.
- Rajeevan, M., J. Bhate, J. D. Kale, and B. Lal, 2006: High resolution daily gridded rainfall data for the Indian region. Curr. Sci., 91, 296–306.