Narrow Mountains of Asia Shape Monsoon Circulation

Imagine a world without mountains! Temperature, wind and rainfall patterns would differ greatly from what we know. In Asia, the very high Tibetan Plateau exerts a strong influence on climate. Heated directly by intense solar radiation during summertime, the temperatures in June– August form a regional maximum. Lhasa at 3.65 km above sea level, for example, has an average comfortable 15°C during summer, a far cry from the nearly freezing temperatures found at the same altitude and latitude a few thousand kilometers to the east over the North Pacific. The high-altitude heat source intensifies the summer monsoon of Asia and draws the monsoonal flow towards the plateau. The impact of the Tibetan Plateau on the summer monsoon circulation was recognized in the late 1950s and explicitly demonstrated by Syukuro Manabe and colleagues in the 1970s with a general circulation model of the atmosphere they developed at Princeton (see IPRC Climate, Vol. 5, No. 2). While the effect of the massive Tibetan Plateau on the monsoon is now well known, the role of other, less massive mountains has not received much attention. IPRC team coleader for Indo-Pacific climate Shang-Ping Xie and Haiming Xu and their colleagues at IPRC and NASA’s Jet Propulsion Laboratory decided to study the role of these mountains in images from the first precipitation radar flown in space on the Tropical Rain Measuring Mission (TRMM) satellite. They noticed that, except for the rain band hugging the foothills of the Himalayas, all the major rain bands over South and Southeast Asia during summer are anchored on the windward side of mountain ranges (Figure 1).