The circular movement of synchronous extreme precipitation preceding Kerala floods in 2018 and 2019

In 2018 and 2019, Kerala, the southernmost state in India, experienced extreme precipitation, leading to appallingly devastating floods that damaged life and property. Kerala is vulnerable to flooding due to its topography, geographical location, and meteorology. Several phenomena have been attributed to these extreme precipitations; however, no single explanation suffices to explain such complex climate phenomena. We view the occurrence of extreme precipitation that leads to floods, such as an emerging phenomenon through the lens of complex system theory. We analyze the patterns of synchrony of extreme fluctuations in precipitation, outgoing longwave radiation, and water vapor transport. We construct time-varying functional climate networks, in which the statistical similarity between the time series of extreme precipitation at different spatial locations is estimated using event synchronization. The network topology reveals that excessive precipitation during the Kerala floods was associated with a coherent pattern of synchronized extreme rainfall. In the coherent phenomena discovered, the extreme rainfall was synchronized across a wide range of length scales spanning 100-1000 km. Furthermore, it traverses a synoptic scale path. After originating in the equatorial Indian Ocean, the coherent pattern moves eastward across the Bay of Bengal. The pattern stops over the Maritime Continent and changes its direction. It moves westward toward the Indian peninsula and accumulates over southwest India. We find that the extreme precipitation was driven by enhanced convective activity, leading to cloudiness and high-vapor transport in the atmosphere. Our findings improve the understanding of intraseasonal variability in the Indian monsoon and extreme precipitation events.