Dynamics of Breaks in the Indian Summer Monsoon
R. Krishnan
Frontier Research System for Global Change, Tsukuba, Japan
and
M.Sugi
Meteorological Research Institute, Tsukuba, Japan
Diagnostic analysis of observations and complementary experiments with a simple numerical model have enabled us to synthesize the morphology and dynamics of "breaks" in the Indian Summer Monsoon. Almost one week ahead of the onset of a break spell over India, a monotonically decreasing trend in convective activity is found to occur over Bay of Bengal in response to a steady eastward spreading of dry convectively stable anomalies from the equatorial Indian ocean. A major intensification of the convectively stable anomalies over Bay of Bengal is seen about 2-3 days prior to commencement of a monsoon break. Both observations and modelling experiments reveal that rapid northwest propagating Rossby waves are triggered in response to such a large strengthening of the convectively stable anomalies. It is shown that an abrupt movement of anomalous Rossby waves from Bay of Bengal into northwest and central India marks the initiation of a break monsoon spell. Typically the Rossby waves are found to traverse from central Bay of Bengal to northwest India in about 2-3 days time. With the establishment of a break phase, the eastward spreading low-latitude anomaly decouples from the rapid northwest propagating anomaly. This decoupling effect paves way for the emergence of a convectively unstable anomaly over the equatorial Indian ocean. It is proposed that the dynamics of the rapid northwest propagating anomalous Rossby waves from central Bay of Bengal towards northwest India and decoupling of the eastward propagating anomaly are two extremely vital elements that determine the transition from an above normal phase to a break phase of the monsoon and also help maintain the mutual competition between convection over the Indian subcontinent and that over equatorial Indian ocean. Through modeling experiments it is demonstrated that low-latitude Rossby wave dynamics in the presence of a monsoon basic flow, which is driven by a steady north-south differential heating, is a primary physical mechanism that controls the so-called monsoon breaks.
