The Amazing Variance of Indian Rainfall

This year’s Climate and Society class is out in the field (or lab or office) completing a summer internship or thesis. They’ll be documenting their experiences one blog post at a time. Read on to see what they’re up to.

James Oliver, C+S ’16

Every year, somewhere between April and September, the Indian monsoon arrives. When exactly the rainy season begins depends not only on geography, but also on other meteorological and climatological factors that can alter the monsoon arrival by nearly two months! Such a drastic year-to-year difference holds a large amount of significance to decisions makers across the country.

Picture yourself as a homeowner. In your yard you have a small garden filled with numerous different plants. Tomatoes, basil, beans, peppers, and whatever else you want to grow. One of, if not the top factor for your garden’s health, is the weather. Is it going to rain today? How much? Do you need to water the plants yourself? How much water will you need to provide your garden over the next week? Two weeks? The next month?

Accurate and precise weather forecasts can help you make each and every one of these decisions. But beyond a humble garden, Indian farmers and other decision-makers need trustworthy forecasts throughout the entire year, especially for the monsoon season. The timing of the drastic shift in precipitation is key to make the most responsible decisions for the oncoming monsoon.

CHIRPS daily rainfall over East Champaran, Bihar, India, 2009. Source: IRI

In the above time series, you can see the daily time series of rainfall for East Champaran, one of districts within Bihar, India. The precipitation data is from the CHIRPS dataset and is estimated via satellite. Rainfall charts over the entirety of Bihar would be much more uniform, however, when scaled down to a smaller level we see much more variability across the days. When examining the data we see several days of 20+ mm of rainfall coming in mid-May, early June, and around the ends of July and August. However, in between these wet periods, East Champaran saw many periods of multiple days of no rainfall. This infrequency of precipitation up until July, and even throughout the month of July stresses the importance of good forecasting for decision-making.

It is not just the highly variable years in which people depend on reliable forecasts. It is similarly important to get the uniformly-wet monsoon seasons predicted accurately.

CHIRPS daily rainfall over East Champaran, Bihar, India, 2000. Source: IRI

Going back to 2000, we can see the CHIRPS time series of precipitation for East Champaran looks drastically different than the one from 2009. The frequent and persistent rains occur much earlier in the year and lasts for the majority of the summertime. The dry spells are much less noticeable as they happened much less often and lasted only a few days at the most. While you may assume that a generally rainy monsoonal season is a simpler forecast, this does not take away from the importance of this forecast’s accuracy. An anomalously rainy spring and summer require proper and adequate preparation just as much as highly variable rainy seasons.

There are dozens upon dozens of case studies that I could show of the incredible year-to-year variance of Indian rainfall. East Champaran is just one district within one state of the entire country and the fluctuation in precipitation is massive. And yet, decisions need to be made all the way down to the household or field level. Numerous factors come into play, which result in drastic differences in rainfall year-to-year.

For example, the Indian tropical cyclone season begins on April 1^. In years in which a cyclone landfalls in India in the months of April, May, or early June, this historically tends to delay the monsoonal onset. It is not just meteorological drivers that impact the monsoons however. Climatological oscillations such as El Nino and the Madden-Julian Oscillation play large roles in how the spring and summer months play out across India.

In the near future, it is important that we come to better understand the oscillations better and how they impact the Indian monsoon. One of the ultimate goals through monsoon research is to discern how we can improve the forecasting of the monsoon season. Not only is it essential to better forecasting on short scales of one to ten days, but also the improvement of sub-seasonal scale predictability of two weeks to two months can greatly assist those affected in these areas, as this is the optimal time period for forecast-based decisions to be made.