Dynamics of dry air intrusion over India during summer monsoon breaks

The dry air intrusion over India during summer monsoon break phases is well known. It has been argued that this dry air originates over the desert regions of West Asia. Here, we show that a reservoir of saturation deficit air exists over the western and northern Arabian Sea during the summer monsoon season. The monsoon low-level jet (LLJ) that transports the moisture to continental India in the active phase of monsoon, transports the dry air to northern and central India during the break phase. The LLJ undergoes a weakening and broadening before the monsoon break phase in response to increased barotropic instability. The broadening of LLJ leads to an intensification of zonal flow in the poleward flanks and a weakening at the core. The development of a positive meridional SST gradient over the northern Arabian Sea favours an increase in the low-level zonal flow in the north, which advects the moist deficit air across northwest India. The dry air intrusion results in enhanced static stability over northern and central India and strong suppression of convection. Further, the enhanced static stability weakens zonal flow from the northern Arabian Sea region and leads to the demise of the dry air intrusion. Thus, internal mechanisms are responsible for the dry air intrusion over India and its termination during the break phase of the summer monsoon. An index for the dry air intrusion is constructed based on the saturation deficit transport. This dry air intrusion index is used to identify the dry air intrusion events during monsoon breaks during the 1981-2014 period. The statistics show that there were 34 (4) monsoon breaks with (without) dry air intrusion during 1981-2014 period. We also note that the dry air intrusion and the monsoon breaks are happening simultaneously, suggesting that it is difficult to establish a cause-effect relationship.