A thermodynamical study of dry air intrusion activity over India during dry phases of summer monsoon in CMIP6 models

It has been argued in recent studies that the source of dry air originating over the desert regions of the West Asia i.e. Middle East intrudes over the continental India during the boreal summer resulting in more prolonged dry spells over India. Singh and Sandeep (2021, Clim Dyn) showed the existence of a huge reservoir of moist deficit air over the northern Arabian Sea at 850-hPa. In addition to this, it has been argued that low level jet undergo weakening and broadening prior to monsoon break phase in feedback to an increased barotropic instability. Furthermore, the monsoon low-level jet which transports the moisture to the continental landmass in the active phase acts as a main carrier in transporting this dry air towards the continental India during the break phase of the summer monsoon. In order to investigate the thermodynamic effects of dry air intrusion activity during dry phases of the Indian Summer Monsoon (ISM), isentropic analysis is performed on climate models simulations of Coupled Model Intercomparison Project Phase 6 (CMIP6). Here, we analyze the specific humidity and wind fields at 316 K isentropic level. The negative specific humidity anomalies of multi models average (MMA) signifies the pattern of dry air advection which shows that a large fraction of the moisture deficit is being transported to the continental India from the northern Arabian Sea, and only a small contribution comes from West Asia. The lead-lag composites of anomalous wind vectors and relative vorticity of MMA at 316 K isentrope clearly show a weakening of the monsoon circulation associated with the break conditions. The anomalous anti-cyclonic circulation pattern propagates westwards from the Bay of Bengal which is a well known feature of the monsoon break spells.  

KEYWORDS: Low level jet; Dry air intrusion; Indian Summer Monsoon