Synoptic-scale monsoon disturbances: a review and recent progress

Abstract:

A large fraction of the annual mean continental precipitation in monsoon regions falls in synoptic scale cyclonic vortices known simply as monsoon low pressure systems. Intense occurrences of these storms are classified as monsoon depressions and are responsible for many monsoon precipitation extremes. Despite the importance of monsoon low pressure systems, their dynamics are incompletely understood and there is little understanding of their sensitivity to larger scale climate forcings such as changing greenhouse gas concentrations. In this talk I present an overview of monsoon low pressure systems, their dynamics, and their connection with meteorological extremes. I discuss the possible relevance of both moist baroclinic growth and tropical cyclogenesis mechanisms to disturbance amplification, and review how these systems cluster in association with larger-scale intraseasonal modes such as the Madden-Julian Oscillation. I summarize recent results indicating that, contrary to previously established thinking, Indian monsoon depressions propagate westward against the low-level mean monsoon flow because of nonlinear, adiabatic self-advection (i.e. beta drift). This propagation mechanism can be easily understood using potential vorticity (PV) as a diagnostic, and I illustrate how PV can be used to more generally understand the dynamical similarities of low pressure systems that exist in Earth’s various regional monsoons.