Dry-air intrusion over India during break phases of the Indian summer monsoon in CMIP6 models

Episodes of dry-air intrusion over northern India have been observed during break phases of the Indian summer monsoon (ISM). Previous investigations have provided observational evidence of a significant reservoir of unsaturated air over the northern Arabian Sea, serving as the source of this dry-air intrusion. It was also suggested that the monsoon low-level jet, which typically transports moisture to continental India during the active phase, instead transports dry air during the break phase of the ISM. While the existence of dry-air intrusion is well-documented through observations, its representation in climate models remains uncertain. It is important to enhance our understanding of the process of dry-air advection in climate models to assess their fidelity in simulating the climate over the region. In this study, we quantify the extent of dry-air intrusion and examine its mechanisms in simulations from the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Most CMIP6 models analysed in this study simulate the observed pattern of dry-air advection over continental India realistically during the summer monsoon-break phase. Some models also simulate dry-air transport from West Asia, possibly due to an overly smoothed representation of orography. Furthermore, the majority of CMIP6 models successfully capture the intrinsic modes associated with the dry monsoon phase, as demonstrated by empirical orthogonal function analysis of low-level zonal winds. Our analyses indicate that global climate models exhibit better skill in simulating dry processes of the monsoon compared with moist processes. These findings uncover previously underexplored aspects of the monsoon, which are essential for assessing future regional climate changes accurately. This study demonstrates that the low-level jet plays a key role in transporting dry air from the northern Arabian Sea to continental India. The figure illustrates the regression of saturation-deficit transport vectors on PC2 of daily zonal wind anomalies at 850 hPa for the CESM2 model during July-August from 1995-2014. The shading represents statistically significant regression coefficients of the magnitude of saturation-deficit transport on PC2. image