Monsoons sustain billions of people but also drive some of the world’s most severe floods. Understanding how they form and vary is critical for resilience. In this article, JBA Climate Scientist Jack Giddings explores the science behind monsoons and how their dynamics shape global flood risk.
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Monsoons are one of the most powerful and widespread climate systems, delivering seasonal rainfall that sustains agriculture, ecosystems, and water supplies for billions of people. They are also a major driver of flood risk, with intense downpours leading to population displacement, economic disruption, and loss of life across the tropics and subtropics.
The location and scale of monsoons lie in their unique dynamics – driven by contrasts between land and ocean, large-scale atmospheric circulation, and complex ocean–atmosphere interactions. Understanding how monsoons form, evolve, and vary is essential for anticipating flood events, developing early warning systems, and strengthening long-term resilience. This article explains the science behind the monsoon, showing how a system that underpins life and livelihoods can also create some of the world’s most severe flood hazards.
The word monsoon comes from the Arabic mausim, meaning “season”. Monsoons are thermally driven circulations that bring alternating wet and dry periods to tropical and subtropical regions. At their foundation lies the Intertropical Convergence Zone (ITCZ) –a dynamic belt of rising air and convective storms that circles the Earth near the Equator.
As the ITCZ shifts north in boreal summer and south in austral summer, it interacts with continents, where strong heating draws the zone inland. This intensifies pressure gradients, pulls in warm, moist ocean air, and marks the onset of monsoonal rainfall. At its northern and southern extremes, the ITCZ takes on its most monsoonal character, creating the world’s major monsoon regions (Figure 1).
Key drivers of monsoon formation include:
Driven by a combination of these factors, monsoons around the world display their own characteristics in terms of location, movement, and intensity. How these drivers interplay can be seen most clearly in the South and Southeast Asian monsoon.
Figure 1: Animation illustrating the northernmost (boreal summer) and southernmost (austral summer) position of the ITCZ and associated global monsoon regions.
Among the world’s monsoon regions, the South and Southeast Asian monsoon is the largest and most economically critical. Each summer it brings a reversal of winds and a surge in rainfall across Pakistan, India, Sri Lanka, Bangladesh, Nepal and Myanmar.
For India alone, the monsoon provides about 80% of annual rainfall (Turner & Slingo, 2009). Its timing, strength, and duration often mark the difference between drought and flooding, with direct impacts on agriculture, rural livelihoods, and the national economy.
The system is fuelled by the warm waters of the Indian Ocean. As trade winds cross the Equator, they pick up moisture. Reaching the Indian subcontinent, this air rises, cools, and produces intense rainfall across southern and western India, spreading eastwards into Bangladesh and Myanmar.
Topography amplifies these effects:
Despite their seasonal regularity – sustained by large-scale moisture sources and shaped by regional topography – monsoon systems vary considerably from year to year. Two major climate systems strongly influence this variability: the ENSO (characterised by El Niño events, the warming phase of the cycle, and La Niña events, the cooling phase) and the Indian Ocean Dipole.
Together, these systems show how conditions thousands of kilometres away can shape rainfall extremes.
Figure 2: Animation illustrating the effect of ENSO on the interannual variability of the monsoon across Southeast Asia.
This variability is not limited to year-to-year shifts. Within a single season, monsoons can alternate between active phases (intense rainfall) and break phases (weakened rainfall). These swings are influenced by large-scale oscillations such as the Madden–Julian Oscillation and the Boreal Summer Intra-Seasonal Oscillation:
Recent research also highlights the role of phytoplankton blooms, which enhance sea surface warming and moisture availability. This process has been shown to intensify rainfall during the onset and retreat of the South Asian monsoon (Giddings et al., 2020).
Forecasting monsoons remains a significant challenge. The India Meteorological Department has issued seasonal outlooks since its foundation in 1875, evolving from simple statistical approaches to sophisticated climate models that now incorporate variables such as Eurasian snow cover and Northern Hemisphere jet stream patterns.
These forecasts matter for farmers, governments, disaster managers, and even financial markets. In India, monsoon outlooks can influence stock prices. Across Africa and the Pacific, seasonal guidance shapes planting decisions and water management strategies.
Yet notable uncertainty remains. The complexity of ocean–atmosphere interactions, combined with the influence of climate change, makes it difficult to predict the exact timing, strength, and impacts of monsoon rains. This is where tools such as Flood Foresight provide added value. In addition to near real-time monitoring of flood risk during extreme rainfall events, Flood Foresight has also been applied to seasonal flood outlooks, translating rainfall forecasts into expected flood hazard. By doing so, it gives governments, development banks, and humanitarian agencies the means to anticipate both imminent events and the broader season ahead.
Monsoons are among Earth’s most powerful climate systems. From South Asia to West Africa, from northern Australia to the Americas, they sustain agriculture, ecosystems, and livelihoods for billions of people – yet their variability also brings destructive floods and droughts.
As the risk of extremes grows, integrated, people-centred approaches to flood risk management will be critical. Regional cooperation, sustainable land management, and inclusive planning can help protect lives and build long-term resilience in a climate where monsoon extremes are expected to intensify.
Giddings, J., Matthews, A. J., Klingaman, N. P., Heywood, K. J., Joshi, M., and Webber, B. G. M. (2020). The effect of seasonally and spatially varying chlorophyll on Bay of Bengal surface ocean properties and the South Asian monsoon. Weather and Climate Dynamics, 1, 635–655. Available here.
Richardson (2020). Understanding and quantifying extreme precipitation events in South Asia, Part I –Understanding climate drivers through case studies. CARISSA Activity 4: Climate services for the water and hydropower sectors in South Asia. [Online]. Available here. [Accessed 11 August 2025].
Turner, A. G., & Slingo, J. M. (2009). Uncertainties in future projections of extreme precipitation in the Indian monsoon region. Atmospheric Science Letters, 10, 152–158. Available here.
WMO (2025). Above normal rainfall is forecast for southwest monsoon in Asia. [Online]. Available here. [Accessed 28 September 2025].
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