Recent Tangible Natural Variability of Monsoonal Orographic Rainfall in the Eastern Himalayas

Abstract

Himalayas hydroclimate is a lifeline for South Asia's most densely populated region. Every year, flooding in the Himalayan rivers is usual during summer monsoon, which impacts millions of inhabitants of the Himalayas and downstream regions. Recent studies demonstrate the role of melting glaciers and snow in the context of global warming, along with monsoonal rain causing recurrent floods. Here, we highlight the natural variability in the eastern Himalayan hydroclimate over the last 43 years (1979-2021). We found extreme monsoonal rainy years with six dry years and seven wet years after removing the climate change signal. Monsoon rainfall is a significant contributor, and melting snow is not a potential contributor to these anomalous extreme years. The variability of Himalayan monsoonal rainfall is strongly regulated by local monsoonal Hadley circulation associated with tropical sea surface temperature. Our findings demonstrate mechanisms associated with Himalayan wet and dry monsoon. Atmospheric dynamics are attributed as the primary modulating factor, influencing local thermodynamics through moist processes. The insights provided in this study underscore the impact of natural variability-driven challenging events that could be predictable. Thus, this mechanism could improve the predictability of the Himalayas floods. The monsoon rainfall in the Himalayan region is very important for the people living in South Asia. Every year, heavy rainfall during the monsoon season causes floods in the region. We found the strong Himalayan rainfall variability in the eastern side of the Himalayas during the monsoon season and examined the natural rainfall cycle over the past few decades. As a result, we have characterized that there are extreme rainy monsoons and dry monsoons. We also found that the main cause of the extreme flooding is the heavy rain during the monsoon as a part of natural variability rather than the melting snow. Our study supports atmospheric bridge as the primary driver for extreme monsoons over the Eastern Himalayan region. We looked at different factors that influence monsoon rainfall, such as the local landscape, the conditions in the atmosphere, and large-scale features. The eastern Himalayas region includes Bhutan, Nepal, India, and China. Understanding these rainfall patterns can help us predict floods in these countries and better prepare for their impact. Furthermore, as climate change continues to affect the climate system, there is growing concern about how it may influence the natural variability of the Himalayan monsoon. The study highlights the natural variability in the eastern Himalayan hydroclimate over the past 43 years, emphasizing its significance as a recurring natural hazard that affects the regionThe research identifies extreme monsoonal rainy years, with monsoon rainfall as a major contributor to river discharge. Notably, the study rules out the role of melting snow in these extreme eventsThis research underscores the dominant influence of atmospheric dynamics as the primary modulating factor in the Eastern Himalayan monsoon