Working notes on how the climate of the Indian region behaves — what sets the seasonal cycle, what makes one year differ from the next, and what is changing. Written for students beginning research in atmospheric science.
Monsoon
The Indian summer monsoon is a seasonal reversal of the low-level winds, driven by the contrast between a rapidly warming continent and the slowly warming ocean beside it. Rain arrives over Kerala around the first of June and covers the country by mid-July; withdrawal begins from northwest India in September. Across most of the subcontinent this single season delivers three quarters of the annual rainfall.
Within the season the rain is far from steady. Active and break spells alternate on time scales of roughly a fortnight to a month, as the band of deep convection moves north from the equatorial Indian Ocean towards the foothills 1. The seasonal total averaged over the country varies by about ten per cent from year to year — a small number that carries a large part of the country's agricultural risk.
ENSO
El Niño and the Southern Oscillation are the ocean and atmosphere halves of one coupled mode of the tropical Pacific. Bjerknes showed that the trade winds, the slope of the thermocline and the sea surface temperature gradient sustain one another, so a small push in either medium grows 2. The system swings between warm and cool states every two to seven years.
Because the Walker circulation spans the tropics, the Pacific state is felt over India: warm events tend to place subsidence over the monsoon region and suppress the seasonal rain. The relationship is a tendency rather than a rule, and its strength has itself drifted over the last century 3.
Indian Ocean Dipole
The Indian Ocean has a coupled mode of its own. In its positive phase the western basin is anomalously warm and the waters off Sumatra and Java anomalously cool, with easterly wind anomalies along the equator holding the pattern in place 4. The index in common use is simply the difference in sea surface temperature anomaly between those two boxes.
Positive events often, but not always, occur alongside El Niño, which makes separating the two influences on Indian rainfall a careful exercise rather than a correlation.
Warming
Global mean surface temperature in the 2011–2020 decade stood about 1.1 °C above the 1850–1900 baseline. Over India the rise in annual mean temperature since 1901 has been smaller than the global figure, while the Indian Ocean has warmed steadily and the frequency of extreme daily rainfall over central India has increased.
For the monsoon the question is not whether the mean total changes — a warmer atmosphere holds more moisture, which argues for more rain — but how that rain is distributed in time. A season with the same total delivered in fewer, heavier events is a different season for a farmer and for a reservoir 5.
Radiative equilibrium
An interactive grey atmosphere: choose the number of layers, the solar constant and the surface albedo, and watch the equilibrium temperature profile solve in the browser. It is the smallest model that produces a warm surface and a cold upper atmosphere for the right reason, and it runs on one page with no installation 6.
- 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.
- Bjerknes, J., 1969: Atmospheric teleconnections from the equatorial Pacific. Mon. Wea. Rev., 97, 163–172.
- Webster, P. J., and S. Yang, 1992: Monsoon and ENSO: Selectively interactive systems. Quart. J. Roy. Meteor. Soc., 118, 877–926.
- Saji, N. H., B. N. Goswami, P. N. Vinayachandran, and T. Yamagata, 1999: A dipole mode in the tropical Indian Ocean. Nature, 401, 360–363.
- Turner, A. G., and H. Annamalai, 2012: Climate change and the South Asian summer monsoon. Nature Clim. Change, 2, 587–595.
- Manabe, S., and R. T. Wetherald, 1967: Thermal equilibrium of the atmosphere with a given distribution of relative humidity. J. Atmos. Sci., 24, 241–259.