Role of Narrow Mountains in Large-Scale Organization of Asian Monsoon Convection*

Abstract

The Asian summer monsoon is organized into distinct convection centers, but the mechanism for this organization is not well understood. Analysis of new satellite observations reveals that narrow mountain ranges are an important organizing agent anchoring monsoon convection centers on the windward side. The Bay of Bengal convection, in particular, features the heaviest precipitation on its eastern coast because of orographic lifting as the southwest monsoon impinges on the coastal mountains of Myanmar (also known as Burma). This is in contrast to the widely held view that this convection is centered over the open ocean as implied by coarse-resolution datasets, a view that would require an entirely different explanation for its formation. Narrow in width and modest in height (≤1 km), these mountains are hardly mentioned in conceptual depictions of the large-scale monsoon and poorly represented in global climate models. The numerical simulations of this study show that orographic rainbands are not a local phenomenon but exert far-reaching effects on the continental-scale monsoon. The realization that these overlooked geographical features are an important element of the Asian monsoon has important implications for studying the monsoon in the past, present, and future.

Convectively coupled Kelvin and easterly waves in a regional climate simulation of the tropics

“…1a and b shows that the WRF model is able to capture the general pattern of observed time-mean rainfall, including the relatively narrow strip of rain across the central north Pacific. Several notable smaller-scale features are also apparent, such as local rainfall enhancements near the Western Ghats of India and the highlands of Burma, both associated with the Asian summer monsoon (Xie et al 2006). These monsoon features are much too strong, however, as illustrated by the difference map in Fig.…”

The resolution sensitivity of the South Asian monsoon and Indo-Pacific in a global 0.35° AGCM

“…In this section, we draw upon sensitivity experiments performed in other studies to understand how the improved resolution of different orographic features influences the monsoon in the MetUM. Xie et al (2006) use tropical rainfall measuring mission (TRMM) observations to argue that when monsoon precipitation is examined on a fine scale, it is comprised of a distinct set of rain bands, each anchored to an orographic feature: the Western Ghats and Himalayas in India; the Arakan Yoma, Bilauktang, Cardamom, and Annaman Cordillera in Indochina and the Andaman and Philippine Islands in the surrounding seas. They also show, using sensitivity tests, that the correct orographically forced diabatic heating over the monsoon domain can improve the precipitation and large-scale circulation of the monsoon, especially in the Bay of Bengal.…”

“…Another important source of resolution sensitivity is the representation of orography. Several orographic features are fundamental drivers of the South Asian monsoon through multiple mechanisms: orographically forced precipitation and the resulting diabatic heating (Xie et al 2006), providing a barrier between extratropical and tropical air Kuang 2010, 2013;Tang et al 2013), elevated surface heating (e.g. Li and Yanai 1996) and confinement and modification of the cross-equatorial monsoon circulation Rodwell and Hoskins 1995;Slingo et al 2005).…”

Downscaling over Vietnam using the stretched-grid CCAM: verification of the mean and interannual variability of rainfall

“…Vietnam has a very narrow middle section (hereafter Central Vietnam) with a complex terrain. The central region is dominated by the Truong Son Range with elevation exceeding 1,200 m; it is described as a mesoscale mountain and its role is important in determining rainfall over Central Vietnam (Xie et al 2006). In South Vietnam, the region of the Mekong River Delta is fairly flat.…”

“…The orography plays an important role in rainfall (e.g. Sarker 1966;Xie et al 2006;Wang and Chang 2012). As the southwesterly air approaches the mountains, moisture-laden air is forced to rise, producing intense convection on the windward side.…”