Professor Peter Webster Invited to Halley Lectures

Abstract: Each year the monsoons bring rainfall to nearly half the population of the planet. Small
variations in monsoon rainfall can lead to flood or drought, feast or famine. Therefore, explaining
the physics driving the monsoon and turning this knowledge into predictions is one of the great
problems in science. In 1686 Sir Edmund Halley, with trade and navigation on his mind, suggested
that the monsoon was driven by the buoyancy induced by the differential heating between the
Indian Ocean and the landmass of South Asia. With a few embellishments, such as noting the
importance of the rotation of Earth, his theory has stood the test of time. However, during the last
20 years, advances in our understanding of global fluid dynamics, suggest that a land-sea heating
contrast is not sufficient. In fact, at the same latitudes of maximum monsoon summer rainfall, in
other parts of the world there are deserts. Here we will develop an alternative, albeit simple,
general theory of the monsoons and discuss how this may be translated into useful predictions
and a greater understanding of how the monsoons will fair in a changing climate.