Rainfall in Bangalore over the last century: Has it really changed?

We the Bangaloreans long for the rains to cool us down once the heat of summer sets in during late February. We are relieved when the first pre-monsoon showers come with the customary thunder and lightning, but then we throw our hands up in dismay (curse the BBMP!) when the occasional intense showers flood our homes, hide the pot holes, and inundate the roads. 

With the constant rhetoric of climate change ringing in our ears, we have all become more sensitive to the weather. Some of us may be curious as to what are general patterns of rainfall over Bangalore in the recent past, perhaps even over the last century. Here is a simple analysis of rainfall data showing some interesting patterns. Next time somebody asks you about Bangalore weather, you can confidently say a few things about Bangalore rain. 

Mean annual rainfall for 100 years along a West-East axis across peninsular India show some interesting patterns. Bangalore in the middle receives lower rainfall in June and July when the South West monsoons break over the western regions (Kodagu) than later in the season showing a historical bimodal pattern.  

There seem to be two peaks – one in May and the other in September-October. Bangalore gets a lot of rainfall from the North East monsoons, but also a fair share from the South West monsoons. There are a lot of clouds and strong winds over Bangalore when the SW monsoons bring rain to the Western coast and the Ghats in June and early July.

Rainfall picks up towards August. Perhaps, initial Southwest monsoon rainfall has to saturate soil in the western regions increasing moisture in the air which then seeds the clouds that bring rainfall to Bangalore later in the season.

Apparently, the forested Western Ghats plays a critical role in rainfall inland. Not sure how else one explains this dip in rainfall in Bangalore in June-July when the Western regions are getting so much rainfall. The rainfall patterns for Chennai are for comparison as they get their rainfall from the NE monsoons.

When one looks at the observed rainfall from 2004-2010 with the yearly variation for each month, the rainfall pattern has not changed since the last 100 years, but the peak rain spreads out through September – October. Variation between years is high in October for example compared to May. It means that pre-monsoon rains in May have been more consistent year on year than in October. 

This is difference is perhaps due to the fact that the pre-monsoon showers are more local convection-driven rains (what goes up must come down!) unlike the Southwest or the Northeast monsoons which depend on warming oceans, and ocean currents thousands of kilometers away.

From 1901 to 2010 when one looks at the total annual rainfall, the peaks and troughs (the squiggly line) seem to display similar patterns over the last century. It appears that after peaks, rainfall always seems to dip to a low.

So it is almost certain that in years we have high rainfall the following year is going to be low. Rainfall ranges between a low of 500 mm and max out at about 1350 mm. Perhaps global warming will make those troughs and peaks more intense in the years ahead leading to more sleepless nights for all the farmers dependent on the Kaveri and the water tribunal that manages the interstate allocation of water.

This increasing bi-polar pattern will require some forethought and planning if we need drinking water and hydropower during the years in the trough. Flooding frequency and intensity may also increase.

Surprisingly, there appears to be an overall marginal increasing trend in rainfall over the last 100 years. However, if its distribution is more erratic every year and across years then that marginal increase perhaps means very little in terms of water for farming or drinking.

Data was sourced from the India water portal (http://www.indiawaterportal.org). Rainfall data from 1901-2002 was interpolated global monthly rainfall (Mitchell and Jones, 2005) also available at the portal which acknowledges the Tyndall Centre for Climate Change Research. Observed rainfall data from 2004-2010 was from stations managed by the Indian Meteorological Department (IMD).

1. Mitchell, T.D., Jone, P.D. (2005). “An improved method of constructing a database of monthly climate observations and associated high resolution grids” International Journal of Climatology 25:693-712

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