Studying the impact of different factors — global warming, El Nino and La Nina and local conditions — on the amount of rainfall in a given area
A number of factors affect rainfall over India. We often hear about the negative impact of El Nino, an unusual warming in the Pacific Ocean, on the Indian monsoon. The opposite of El Nino, a condition called La Nina, is known to aid the monsoon rainfall. Global warming is also expected to increase the moisture-holding capacity of the atmosphere, thereby leading to more intense, heavy rains. These are global factors. Rainfall is also impacted by local conditions, like temperature, humidity, pressure and wind systems.
For some time now, while still at the Indian Institute of Science, Bengaluru, I have been trying to understand which of the several factors has a more dominating impact on precipitation patterns. More specifically, I have been trying to understand what has a greater influence on extreme precipitation events over a small area, like the incessant rainfall we saw in Chennai last year, and similar events in Jammu and Kashmir in 2014, and in Mumbai in 2005. Such extremes are rare by their very definition and their behaviour is markedly different from the majority of the data, thereby making studies about them more difficult.
A better understanding of the driving forces behind long-term changes in extreme events is a science quest that will help us improve weather monitoring and predictions.
For the purposes of my research, I defined an extreme precipitation event as a single day amount of rainfall that was above the 80-percentile mark of daily rainfall over a 35-year period between 1969 and 2014. I used the gridded rainfall dataset of the Indian Meteorological Department (IMD) that divides the country into 100×100 km regions or grids. This dataset is prepared from rainfall observations collected from 2,000 stations spread across the country.
In each of these grids, I studied the intensity, frequency and duration of rainfall during these extreme events. So, I considered the number of days when the rainfall amount remained above that 80-percentile threshold in a single spell. This was the duration of the spell. The number of such spells in a season gave the frequency of such extreme events. Within each spell, the maximum amount of rainfall in a single day was the intensity.
For my study, I tried to assess the influence of only three factors that impact rainfall — the ENSO condition (El Nino and La Nina) as a natural mode of climate variability, global average temperatures as indicators of anthropogenic climate change, and local temperatures as indicators of more localised effects. Using statistical modeling tools that are theoretically tailored to study the changing behaviour of extremes, and using actual rainfall and temperature datasets over 35 years, I tried to see how the intensity, duration and frequency of rainfall are impacted by changes in these three factors.
The results showed that for the duration of high rainfall during an extreme event, none of the three factors — ENSO, global temperatures, and local temperature — played a significant role. In fact, the duration of heavy rainfall spells had no major long-term changes over these 35 years. However, the intensity and frequency of extreme rainfall events showed a greater sensitivity to changes in local temperature compared to the other two factors.
Based on these results, my study has suggested that high rainfall during an extreme event over a small area is possibly impacted more by local reasons than global factors. It is important to note that this result is valid only for the definitions and scales chosen for this study. It may change if we change the size of the grids, for example, or if we try to see the impact of some other factors as well, like the Indian Ocean Dipole. Nonetheless, it is a very important indicator towards the kind of impact that different factors have on the amount of rainfall in a given area. More research is required for a better understanding of such impact, which might even enable us to say whether any particular extreme event can be attributed to climate change or was a result of more local and natural variability in weather systems.