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Tuesday 19 Jan 2021Seasonal, regional Hadley cell dynamics and Earth's monsoons

Dr. Ruth Geen - University of Exeter

Zoom 13:30-14:30

Summer monsoons provide most of the annual rainfall to countries throughout the tropics, influencing over one third of the world's population. These systems are critical to global agriculture, but gaps in our understanding of monsoon dynamics limit our ability to understand both their interannual variability and future change. Aquaplanets have been found to reproduce some characteristics of Earth’s monsoon systems, such as abrupt changes in the location of the tropical precipitation maximum and seasonal wind reversal. In this perspective, monsoon onset reflects a change in the dynamical regime of the Hadley cell, from an eddy-dominated equatorial regime to an angular momentum conserving solstitial regime. Here we use a hierarchy of idealised model simulations to deconstruct the basic dynamics of a summer monsoon.

We first explore the controls on the Hadley cell regime change by varying the rotation rate, orbital period and slab ocean depth of an aquaplanet. Our simulations indicate that the transition occurs when the tropical convergence zone reaches a critical latitude that is primarily set by rotation rate. While the basic seasonality can be reproduced in an aquaplanet, zonal asymmetries introduce complex regional behaviour to Earth’s monsoons. Building back up the hierarchy, we add continents and orography to explore how the seasonal Hadley cell evolution interacts with stationary waves to produce the observed spectrum of different monsoon timings and intensities. Last, we demonstrate how insights from this highly idealised work can be applied to real-world problems in seasonal prediction, specifically forecasting the onset of the monsoon over the South China Sea.

Zoom Meeting Details:

Meeting ID: 982 3649 2255

Password: 932957

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