Role of mid-tropospheric stability in the simulation of monsoon low pressure systems

The north-west propagating Low Pressure Systems (LPS) are an important component of the Indian Summer Monsoon (ISM). The objective detection and tracking of LPS in reanalysis products and climate model simulations are challenging due to the weak structure of the LPS compared to tropical cyclones. A robust method is presented here to objectively identify and track LPS, which mimics the conventional identification and tracking algorithm based on detecting closed isobars on surface pressure charts. The new LPS tracking technique allows a fair comparison between the observed and simulated LPS. Further, the LPS tracking algorithm is applied to the climate model simulations from the CMIP5 historical, RCP scenarios and AMIP-4K simulations. The CMIP5 models show considerable spread in terms of their skill in LPS simulation. About 60% of the observed total summer monsoon precipitation over east-central India is found to be associated with LPS activities, while that in model simulations this ratio varies between 5 – 60%. Those models which simulate synoptic activity realistically, have better skill in simulating seasonal mean monsoon precipitation. The model-to-model variability in the simulated synoptic activity is found to be linked to the mid-tropospheric stability over the Bay of Bengal region, with the models having more stable mid-tropospheric layer (~500 hPa) poorly simulate monsoon LPS. These findings lay bare the mechanisms behind the model simulation of ISM precipitation, synoptic activity and their interdependence. The changes in the nature of synoptic activity in a warming scenario is accessed using the tracking algorithm. The LPS tracks and hence the synoptic activity is found to be shifted southward in line with the shift in the mean precipitation pattern in RCP simulations. The relationship between synoptic activity and precipitation patterns in a warming scenario will be discussed.