Variability of monsoon inversion over the Arabian Sea and its impact on rainfall

This work investigates the spatial and temporal variability of the monsoon inversion (MI) over the Arabian Sea for the study of 37-years period (1980-2016) using MERRA version2 (MERRA2) reanalysis and downscaled simulations generated with the Weather and Research Forecasting (WRF) model. After validating the downscaled products with the observations from four radiosonde stations (Salalah, Mumbai, Goa and Mangalore), we analysed the variability of MI from diurnal to seasonal scales. The diurnal evolution of the MI suggests that radiative cooling over the Arabian Peninsula at night, together with the onset of boundary layer jets along the coast of Oman, play an important role in amplifying the intensity and spatial extent of the MI during night and into the early morning hours. A seasonal analysis of the simulated winds and MI reveals that WRF reproduced the monsoon characteristics as observed in MERRA2 reanalysis, including the spatial and vertical orientations of the MI. The downscaled winds are slightly overestimated (by 1-2 m center dot s(-1)) over the Arabian Sea, which resulted in an increased simulated wind shear over the western Arabian Sea, ultimately manifested in the form of an enhanced MI. We further investigated the variability of the lower tropospheric wind speed and associated changes in MI over the 37-year period. This reveals a significant decreasing trend during the summer monsoon over the western Arabian Sea. We argue that this led to a decreasing trend in the lower tropospheric wind shear and advection of temperature and hence, reduced the frequency of occurrence of the MI (shown in graphical abstract).