The mechanical impact of the Tibetan Plateau on the onset of the South Asian summer monsoon

The impact of the Tibetan Plateau on the onset of the South Asian summer monsoon is examined using a hierarchy of atmospheric general circulation models. During the pre-monsoon season and monsoon onset (April-May-June), when westerly winds over the Southern Tibetan Plateau are still strong, the Tibetan Plateau triggers early monsoon rainfall downstream. The downstream moist convection is accompanied by the earlier onset of monsoonal low-level winds and subsidence upstream of the Tibetan Plateau. In experiments where the Tibetan Plateau is removed, monsoon onset occurs about a month later, but the monsoon circulation becomes gradually stronger to gain comparable strength during the mature phase. During the mature and decaying phase of monsoon (Jul-Aug-Sep), when westerly winds over the Southern Tibetan Plateau almost disappear, monsoon circulation strength is largely unaffected by the presence of the Tibetan Plateau. A dry dynamical core with an idealized mountain in the subtropics consistently simulates upstream divergence and downstream convergence when background zonal wind over the mountain is westerly. The mechanically-driven downstream convergence might be able to drive substantial moisture convergence in the moist atmosphere, which would trigger earlier monsoon onset. We attempt to explain the downstream convergence from the Bernoulli function over elevated topography, where the outcrop of surface isentropes is simplified as a heavy-side step function.