Monsoon Systems and Variability

The seasonally reversing winds and heavy rains that bring the wet season to billions of people across Asia, Africa, the Americas, and Australia.

Definition

A monsoon is a seasonally reversing wind and rainfall regime produced by the differential heating of land and ocean and the seasonal march of the tropical convergence zone, delivering a pronounced wet and dry season.

Scope

This topic covers the world's monsoon systems, the seasonally reversing circulations driven by the contrast in heating between land and ocean and by the migration of the tropical rain belt. It treats the dynamics of monsoon onset, the South Asian, East Asian, West African, Australian, and American monsoons, their interannual and intraseasonal variability and links to El Nino, and how monsoon rainfall may respond to a warming and more aerosol-laden atmosphere.

Core questions

• What drives the seasonal reversal of monsoon winds and rains?
• What controls the timing and strength of monsoon onset?
• How does monsoon rainfall vary from year to year and within a season?
• How will monsoons respond to greenhouse warming and aerosols?

Key theories

Land-sea thermal contrast and moisture transport

Stronger summer heating of land than ocean draws moist maritime air inland, and the latent heat released by the resulting rainfall reinforces the circulation that sustains the monsoon.

Migration of the tropical rain belt

Monsoon rains follow the seasonal movement of the intertropical convergence zone toward the summer hemisphere, so the monsoon can be viewed as a regional expression of the global tropical rain belt.

Mechanisms

In summer the land heats faster than the surrounding ocean, lowering surface pressure over the continent and drawing in moist oceanic air; as this air rises and rains, the released latent heat strengthens the inflow, while the tropical convergence zone migrates poleward into the heated hemisphere. Interannual swings in this system are modulated by El Nino, sea-surface temperatures, and land-surface conditions, and aerosols and greenhouse gases alter the heating that drives it.

Clinical relevance

Monsoon rainfall sustains the agriculture and water supply of billions of people, so variability in its onset and strength has profound consequences for food security, water resources, and flood and drought risk.

Evidence & guidelines

The IPCC Sixth Assessment Report assesses that global monsoon precipitation is likely to increase overall with warming while regional changes vary, and that aerosols have historically suppressed some monsoon rainfall.

History

Edmond Halley first attributed the monsoon to differential land-sea heating in the seventeenth century, a view later refined to include moisture and latent heating; modern field campaigns and modeling clarified the dynamics, the role of El Nino, and the prospects for seasonal prediction of monsoon rainfall.

Debates

Future of monsoon rainfall under warming

Whether and where monsoon rains will intensify or weaken under competing influences of greenhouse warming, which adds moisture, and aerosols, which can suppress rainfall, remains uncertain.

Key figures

• Peter Webster
• Jagadish Shukla
• Tim Palmer
• Tetsuzo Yasunari

Seminal works

• webster1998
• hartmann2016

Frequently asked questions

What causes the monsoon?

It is driven mainly by the seasonal contrast in heating between land and ocean, which reverses the winds and draws moist air inland, reinforced by the latent heat of the resulting rains.

How is the monsoon linked to El Nino?

El Nino tends to weaken the South Asian summer monsoon and shift rainfall, which is why monitoring the tropical Pacific helps forecast monsoon strength.