Biases in model simulations of Indian Summer monsoon low-pressure systems

Monsoon low-pressure systems (LPSs) during the Indian Summer monsoon season are crucial synoptic-scale phenomena over the Indian subcontinent.  An average of about 14 LPSs per summer monsoon season form contributing approximately 60-70 percent of rainfall over large parts of India. Many previous modelling studies have shown a systematic bias of a southward shift in LPS activity and a dry bias over the monsoon core zone of ISM.  This study is an attempt to account for these biases in the NCAR Community Earth System Model (CESM2.1.3) simulation. Our present-day simulation using the fully coupled configuration of the model shows a southward shift in LPS activity compared to ERA5 reanalyses, consistent with prior studies showing similar biases in CMIP5 (Coupled Model Intercomparison Project Phase 5) model simulations relative to various reanalysis datasets.

We show that biases in simulating LPS activity in CESM2.1.3 are related to a southward bias in the latitude of the low-level westerly jet as well as a dry bias over northwestern India. The simulated monsoon low-level jet, typically located poleward of the zero absolute vorticity contour, is more east-west oriented and biased southward in the model simulation compared to ERA5. This is due to enhanced meridional advection of negative absolute vorticity over the central Indian Ocean below the zero absolute vorticity contour from increased cross-equatorial flow and reduced meridional advection of positive absolute vorticity over the western Arabian Sea above the zero absolute vorticity contour. Further, it is likely that a bias towards larger dry air intrusion from the north and west of Arabian Sea into India leads to a southward displacement of the monsoon low-level jet. This shift adversely affects LPS genesis and growth in northern and northwestern India, leading to a southward bias in LPS activity in model simulations as compared to reanalysis data. We also offer evidence to show that the southward shift in LPS activity and the dry air intrusion into northwest India are nearly common biases across the three generations (CMIP3, CMIP5 and CMIP6) of climate models, contributing to a dry bias over the monsoon core zone of the Indian summer monsoon.