In July 2019, we published an event report exploring the extensive flooding experienced at that time in India, Nepal and Bangladesh. Now that the monsoon season has come to an end in India, we look back at the season as a whole and examine what changes to the climate might mean for monsoonal flooding in the future.
Unusual rainfall
The 2019 monsoon season started with lower than usual rainfall, prompting droughts at the start of the kharif crop season. The total rainfall in June was only 67% of its long-term average (India Meteorological Department, 2019). However, the following months received higher rainfall compared to the long-term average; ultimately, the total rainfall received across India during the 2019 monsoon season was 10% higher than the historical average (India Meteorological Department, 2019). 
Figure 1: The effects of the monsoon flooding in India, Bangladesh and Nepal from 1 July 2019 – 6 October 2019. The map illustrates losses across India, including number of fatalities, houses damaged and evacuations. The total rainfall is based on daily precipitation data collected by NASA’s Precipitation Measurement Missions. (Data source: ReliefWeb, 2019; NASA PPM, 2019) (JBA Risk Management Limited™).*
In addition, the 2019 monsoon lingered around the Indian subcontinent for longer, making it the second-wettest September in history with a total rainfall 52% higher than normal. The wettest September to date was recorded in 1917, with total rainfall reaching 65% higher than normal.
Some major cities were affected by flooding during the monsoon season. Mumbai received a total of 3,670mm of rainfall this monsoon season, which is 56.2% higher than the normal rainfall amount and the highest recorded in 61 years (India Today, 2019). JBA has conducted return period analyses using CHIRPS historical monthly rainfall data from 1981-2019 for Mumbai (Figure 2) and results from the analyses suggest that the return period for the major July rainfall event in Mumbai is 100 years (Figure 3).
Figure 2: Total monthly rainfall for the month of July in Mumbai (19.08N and 72.88E) using CHIRPS historical data from 1981–2019. (Data source: Indian Meteorological Department, 2019) (JBA Risk Management Limited™)
Figure 3: Estimated return period based on monthly rainfall data for the month of July in Mumbai (19.08N and 72.88E) using CHIRPS historical data. A Weibull distribution curve has been fitted to the historic data to give an estimated flood return period. (Data source: Indian Meteorological Department, 2019) (JBA Risk Management Limited™)
Looking at rainfall accumulation over a 14-week period, it is notable that states along the western and southern coast of India received higher than average rainfall totals, compared to the eastern states which experienced a deficit in rainfall volume (Figure 4). A total of 13 states experienced flooding during the monsoon season and more rainfall events were reported this year as compared to 2018 (NASA Earth Observatory, 2019).
Figure 4: Total rainfall from 1 June 2019 – 6 October 2019 normalised against average total rainfall across the same period. The variation in rainfall received is likely due to the influence of the Indian Ocean Dipole on the west coast of India. (Data source: Indian Meteorological Department, 2019) (JBA Risk Management Limited™)*
Floods are typically rare in late September and October during the annual monsoon. The prolonged length of this year’s monsoon and pattern of rainfall along the south and west coast were likely influenced by two weather systems. The continued presence of the Inter Tropical Convergence Zone (ITCZ), a band of low pressure which contributes to the rain belt in the tropics, is believed to have lengthened the season (The Weather Channel, 2019a).
Similarly, the positive phase of the Indian Ocean Dipole (IOD) (where the western Indian Ocean basin has a higher temperature than normal) is likely to have resulted in greater rainfall and more days with monsoon rain, particularly influencing the south and west. The areas of western Madhya Pradesh, Saurashtra and northern parts of the Western Ghats (Goa, Maharashtra and northern Kerala) were particularly impacted by this increased late season precipitation (Rajgopal, 2019; The Weather Channel, 2019b).
The impact of flooding on megacities
This prolonged rainfall caused widespread flooding across India from June to October, including in some major cities. In early July, as previously discussed, Mumbai experienced the heaviest rainfall in a decade, resulting in flooding, while Assam experienced serious floods which resulted in at least two million people being evacuated in early July. Other states, including Maharashtra, Kerala and Karnataka, were inundated in early August. Bihar and Uttar Pradesh states received a high volume of rain within a 24-hour to 48-hour period which resulted in more damage in these states in late October.
Floods cost India an average of USD $7.4 billion a year (Nagar, 2019). As seen in the IPPC’s 2014 report, Asia has a high percentage of population and infrastructure exposed to flood risk, especially in megacities, turning natural catastrophes into major events. Many major cities, including Mumbai, are located close to the sea and have a high tide level, making them more vulnerable to flood risk and climate change-induced sea level rise. Mumbai is amongst the top Asian cities at risk to flood-related losses based on estimated population exposure by 2070, and is also one of the most at-risk cities in Asia based on assets exposed to flood risk (IPCC, 2014).
Influence of future climate change on the monsoon season
Figure 5: Percentage of rainfall deviation from normal in north-east India between 1901 and 2015. The orange bars represent an excess of rainfall as compared to the long-term normal and the blue bars represent a deficit of rainfall as compared to the long-term average. The blue points represent years when a strong El Nino has been observed while the orange points correlate to years with a strong La Nina event. El Nino and La Nina data were obtained only from 1950 onwards. (Data source: Indian Meteorological Department, 2019)
The critical drivers behind monsoon are temperature differences between land and ocean and the amount of moisture available (Amirith, 2018). Climate change is likely to affect the temperature of land and ocean surfaces and thus may alter the present climate conditions and consequently the behaviour of monsoon rainfall (Amirith, 2018). For example, warmer oceans may result in more moisture in the atmosphere and, when the winds pick up the moisture and move towards South Asia, it is likely to result in a “wetter” monsoon.
However, there are many other uncertainties in projecting future changes to the monsoon system as other weather systems like El Nino, La Nina and the IOD may influence the strength of the monsoon. In general, El Nino years are usually associated with monsoon years with less rainfall, whereas La Nina years tend to result in a wetter monsoon season (Kurian, 2019) . With reference to figure 5, strong La Nina events occurred in 1999 and 2007, with these years receiving higher than average rainfall. Conversely, 1992 and 2009 received lower than average monsoon rain, with both years coinciding with El Nino events. However, during a strong El Nino year in 1997, monsoon rain received in India was higher than usual. Although many studies have been carried out, the relationship between ENSO weather events and the intensity of the Indian monsoon has yet to be established (Kurian, 2019).
It is likely that as oceans warm in the next few decades, there may be a smaller temperature difference between land and ocean surfaces. Therefore, monsoonal winds are likely to weaken, carrying less moisture towards the region (Amirith, 2018). In addition, there are other conditions at play which may affect the strength of the monsoon. For instance, aerosols produced as a by-product of human activities may absorb solar radiation in the atmosphere, reducing the amount of radiation received and absorbed by the land mass. Hence, there may be less heating on land resulting in a smaller temperature difference between land and ocean surfaces and weaker monsoon winds (Amirith, 2018).
Despite this uncertainty, South Asia is particularly susceptible to flood risk now and is likely to be more so in the future, especially in major cities. JBA Risk Management can help you manage your risk across the region. We have nationwide return period flood maps at 30m resolution for South Asia, an India Crop Model - which captures multiple extreme weather perils contributing to seasonal crop failure, including flood and tropical cyclone damage – as well as our new Global Flood Model, enabling quantification of flood risk at any location worldwide.
If you are interested in any of our products or services to improve your management of flood risk, please get in touch for more information.
*JBA recognises that various international boundaries, territories, names and the designation of such are the subject of contest and dispute. Their representation on this map is solely for the illustration of natural perils and does not imply any recognition or endorsement of any such contested or disputed issues.
References
Amrith, S. 2018. Unruly Waters: How Rains, Rivers, Coasts and Seas Have Shaped Asia's History. London: Penguin.
Economic Times. 2019. Mumbai fears for homes and lives amid rising seas. [online] Available at: https://economictimes.indiatimes.com/news/politics-and-nation/mumbai-fears-for-homes-and-lives-amid-rising-seas/articleshow/71270565.cms [Accessed 14 Oct. 2019].
India Meteorological Department. 2019. Customized Rainfall Information System. [online] Available at: http://hydro.imd.gov.in/hydrometweb/(S(rgsirh55vost13iylix1elbo))/landing.aspx [Accessed 14 Oct. 2019].
India Today. 2019. Worst monsoon in 25 years kills 148 across India, floods ravage Patna | 10 points. [online] Available at: https://www.indiatoday.in/india/story/worst-monsoon-in-25-years-kills-over-100-across-india-floods-rains-in-patna-10-points-1604967-2019-10-01 [Accessed 14 Oct. 2019].
IPCC. 2014. Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. [online] Cambridge University Press. Available at: https://www.ipcc.ch/site/assets/uploads/2018/02/WGIIAR5-PartB_FINAL.pdf [Accessed 14 Oct. 2019].
Kurian, V. (2019). Will the El Nino factor impact the monsoon?. The Hindu. [online] Available at: https://www.thehindu.com/sci-tech/energy-and-environment/will-the-el-nino-factor-impact-the-monsoon/article26830966.ece [Accessed 29 Oct. 2019].
Nagar, A. 2019. How much do floods cost India?. [online] World Bank. Available at: https://medium.com/world-of-opportunity/how-much-do-floods-cost-india-4a9446117f71 [Accessed 14 Oct. 2019].
NASA Earth Observatory. 2019. Unusual Monsoon Season Causes Flooding in India. [online] Available at: https://earthobservatory.nasa.gov/images/145703/unusual-monsoon-season-causes-flooding-in-india [Accessed 14 Oct. 2019].
NASA PPM. 2019. NASA Precipitation Measurement Missions. [online] Available at: https://pmm.nasa.gov/data-access/pps-ftp#jsimpson-NRTPUB/imerg/gis/early [Accessed 14 Oct. 2019].
Rajgopal, K. 2019. The dipole factor in summer monsoon rainfall. The Hindu. [online] Available at: https://www.thehindu.com/sci-tech/science/The-dipole-factor-in-summer-monsoon-rainfall/article14479847.ece [Accessed 14 Oct. 2019].
ReliefWeb. 2019. Emergency Response Coordination Centre, India Monsoon 2019. [online] ReliefWeb. Available at: https://reliefweb.int/sites/reliefweb.int/files/resources/ECDM_20190920_Monsoon.pdf [Accessed 14 Oct. 2019].
The Weather Channel. 2019a. Why Has Monsoon's Withdrawal Been Delayed? | The Weather Channel. [online] Available at: https://weather.com/en-IN/india/monsoon/news/2019-10-06-why-has-monsoons-withdrawal-been-delayed [Accessed 14 Oct. 2019].
The Weather Channel. 2019b. 2019 to Be One of Strongest Indian Ocean Dipole Years On Record: Expert | The Weather Channel. [online] Available at: https://weather.com/en-IN/india/monsoon/news/2019-09-16-2019-to-be-one-of-strongest-iod-years-on-record-expert [Accessed 14 Oct. 2019].
