Asian monsoon rainfall impacts one third of the global population and
predicting its variability and future change is of clear importance.
However, the dynamics of even the climatological monsoon are not fully
understood; seemingly unconnected behaviors and abrupt jumps in rainfall
location occur in different regions through the year. Three independent
subsystems have traditionally been considered: the East Asian, South
Asian, and Western North Pacific monsoons. These are generally viewed as
passive stationary-wave responses to insolation, but this picture cannot
explain the abrupt jumps in rainfall location. Using model simulations,
reanalysis and observations, we show that the complex behavior of all
three subsystems in fact results from active propagation of the
summertime ‘stationary’ wave. A continent-scale cyclone first expands
northwestwards and then propagates eastwards via advective and
evaporative feedbacks. We propose that the monsoon’s response to
forcings may be understood by considering how this wave interacts with
the background state.