Understanding the revival of the Indian summer monsoon after breaks

Abstract

In this paper, the authors suggest a dynamical mechanism involved in the revival of the summer monsoon after breaks. In this context, the authors carry out a diagnostic analysis using the datasets from National Centers for Environmental Prediction Reanalysis 2 for the period 1979-2007 to identify a robust mechanism that typifies breaks and subsequent revival of monsoon. The authors find during the peak of significant breaks an anomalous southward shift of the subtropical westerly jet stream, which is invariably accompanied by an anomalous northward shift of a stronger-than-normal easterly jet. These major changes during a break facilitate an instability mechanism, which apparently leads to formation of a synoptic disturbance. Formation of such a disturbance is critical to the subsequent revival of the summer monsoon in 61% of the observed break-to-active revivals. Computations of energetics and correlation analysis carried out suggest an increase in the eddy kinetic energy at the expense of the mean kinetic energy during the breaks, in agreement with the formation of the synoptic disturbance. This demonstrates that barotropic instability in the presence of a monsoon basic flow is the primary physical mechanism that controls the revival of the summer monsoon subsequent to the break events.