Modulations of the Indian summer monsoon by the hot subtropical deserts: insights from coupled sensitivity experiments

This study revisits the role of subtropical deserts in the Indian Summer Monsoon (ISM) system by perturbing surface albedo over the subtropical deserts, to the west of the ISM domain in different ways, using a state-of-the-art coupled model. The analysis of up-to-date satellite datasets, atmospheric re-analyses and our control coupled simulation suggests that the model broadly reproduces the radiation budgets close to re-analyses and observed datasets. However, there are large uncertainties in the top-of-atmosphere radiation budget over the Northern Hemishere (NH) subtropical desert region during boreal summer; while the model has a rather neutral radiation budget during boreal summer over the Sahara Desert, the European Centre for Medium Range Weather Forecasts Interim reanalysis show in contrast a radiative excess throughout the NH desert region and the up-to-date satellite dataset has a clear negative radiation budget over north-eastern Sahara region and over Arabian Peninsula. Taking into account these incertitudes, our key finding is that by darkening the deserts and arid regions to the west of ISM through a negative albedo perturbation in our coupled model, the length and intensity of the rainy season over the Indian region are both significantly increased with two well-defined rainfall anomaly maxima in May-June and September-October. The ISM onset is advanced by 1month and is characterized by a rapid northward propagation of the rainfall band over the Indian domain. Reversing the sign of our artificial albedo perturbation over the deserts in the model gives an opposite response, highlighting the robust role of the subtropical deserts in the ISM system, but the amplitude of the ISM response is also significantly larger, demonstrating nonlinearity in the monsoon-desert relationship. Additional albedo perturbation experiments further demonstrate that the whole hot subtropical deserts extending across Afro-Asian continents, and including the Sahara, plays a key-role in the ISM response. Finally, the modulations of the meridional tropospheric temperature gradient along with stronger equatorial asymmetry of mean easterly shear and moisture distribution over the Indian domain are key-factors for explaining the ISM response and its nonlinearity to the albedo perturbations over the NH subtropical deserts. Further insights from moisture budget show that the nonlinearity in advection moisture tendencies manifests in nonlinearity of the ISM response.