MASS Seminar: 'Revisiting the role of the Tibetan Plateau on the South and East Asian monsoons'

Abstract
Land-sea thermal contrast and heating of the atmosphere over the Tibetan Plateau (TP) have long been considered fundamental drivers of the large-scale Asian monsoon circulation. Recent studies have challenged this prevailing view by suggesting that monsoons can occur even in the absence of zonal inhomogeneities in the lower boundary, and that the TP might be acting more as a barrier between the midlatitude and tropical air masses than as an elevated heat source.

Here we use reanalysis data and experiments with the AM2.1 atmospheric general circulation model to revisit the role of the TP on the South and East Asian monsoons. Simulations with and without the TP reveal a modest impact of the elevated topography on the South Asian monsoon. However, the East Asian monsoon, and in particular the so-called Meiyu-Baiu (MB) front spanning from East China to Japan and into the central north Pacific, almost disappears in late spring and early summer if the TP is removed.

Analysis of the observed Moist Static Energy budget in the MB region reveals that horizontal advection of moist enthalpy, and primarily dry enthalpy, sustains the MB front in a region of otherwise negative net energy input into the atmospheric column. A decomposition of the horizontal dry enthalpy advection into mean, transient and stationary eddy terms identifies the zonal advection of the zonally asymmetric thermal gradient and the thermal advection by the meridional stationary eddy velocity as dominant factors. The AM2.1 simulations show that the presence of the TP leaves the longitudinal thermal gradient largely unaffected and primarily influences the meridional eddy velocity by creating, through its interaction with the subtropical westerly jet, a region of downstream convergence, with enhanced southerlies over the MB region and northerlies to the north of it. These results suggest that mechanical effects from the interaction between the TP and the mean westerly flow might be at least as important as its thermal influence for the existence of the MB rainfall system.

Speaker's website: http://www.gps.caltech.edu/~bordoni/