Orographic Effects of the Tibetan Plateau on the East Asian Summer Monsoon: An Energetic Perspective

Abstract

This paper investigates the dynamical processes through which the Tibetan Plateau (TP) influences the East Asian summer monsoon (EASM) within the framework of the moist static energy (MSE) budget, using both observations and atmospheric general circulation model (AGCM) simulations. The focus is on the most prominent feature of the EASM, the so-called meiyu-baiu (MB), which is characterized by a well-defined, southwest-northeast elongated quasi-stationary rainfall band, spanning from eastern China to Japan and into the northwestern Pacific Ocean between mid-June and mid-July.Observational analyses of the MSE budget of the MB front indicate that horizontal advection of moist enthalpy, and primarily of dry enthalpy, sustains the 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 fluxes identifies the longitudinal thermal gradient due to zonal asymmetries and the meridional stationary eddy velocity as the most influential factors determining the pattern of horizontal moist enthalpy advection. Numerical simulations in which the TP is either retained or removed show that the TP influences the stationary enthalpy flux, and hence the MB front, primarily by changing the meridional stationary eddy velocity, with reinforced southerly wind over the MB region and northerly wind to its north. Changes in the longitudinal thermal gradient are mainly confined to the near downstream of the TP, with the resulting changes in zonal warm air advection having a lesser impact on the rainfall in the extended MB region.

Drivers and mechanisms for enhanced summer monsoon precipitation over East Asia during the mid-Pliocene in the IPSL-CM5A

“…Following Chen and Bordoni (2014a), MSE is selected to quantify the relative roles of temperature, moisture, and radiative processes in sustaining the horizontal distribution of the mid-Pliocene EASM precipitation. The highly simplified MSE balance in the atmospheric column is expressed as where MSE is the atmospheric moist static energy, E is the atmospheric moist enthalpy, and F net is the net energy flux into the atmosphere.…”

“…The first term is very small compared with the other terms and can be neglected. A more detailed description of the MSE used to analyze the EASM can be found in Chen and Bordoni (2014a).…”

“…But what is the mechanism that sustains the spatial distribution of this enhanced mid-Pliocene EASM precipitation in both simulations, and the large differences in the mid-Pliocene EASM precipitation between the two simulations? MSE has recently been proposed as an approach to reveal the physical mechanism underlying the observed maintenance of EASM precipitation (Chen and Bordoni 2014a), and has also been used to assess model performance in simulating EASM precipitation (Chen and Bordoni 2014b).…”

“…Following Chen and Bordoni (2014a), MSE budget analysis is used to reveal the possible factor determining the EASM precipitation changes in the mid-Pliocene derived from the AGCM and CGCM simulations. We first examine the JJA-averaged MSE budget integrated from the surface to 100 hPa derived from the AGCM simulation for the PI and mid-Pliocene climate.…”

“…Sampe and Xie (2010) argued that temperature advection from the southeastern flank of the Tibetan Plateau at 500 hPa is important because of its role in inducing vertical motion and because of its control over the position of the rain belt over East Asia. Moist static energy (MSE) has been proposed to gauge the prominent feature of present day EASM precipitation (Chen and Bordoni 2014a). However, no study has investigated the mechanisms controlling the position of the EASM precipitation during the mid-Pliocene.…”