Influences of the Tibetan Plateau on the Asian Monsoon in Different Seasons

The influence of a large-scale orography on climate depends not only on its mechanical forcing and thermal forcing which it exerts on the atmosphere, but also on the background atmospheric circulation. In winter the Tibetan Plateau possesses two leading heating modes resulted from the relevant dominant atmospheric circulations, in particular the Northern Atlantic Oscillation and the North Pacific Oscillation. The prevailing effect of the mechanical forcing of the Tibetan Plateau in wintertime generates a dipole type of circulation, in which the anticyclonic gyre in middle and high latitudes contributes to the warm air of the inland area to the west, and the cold air of the seashore area to the east, of the northeast Asia. Whereas the cyclonic gyre in low latitudes contributes to the formation of prolonged dry season over middle and south Asia and moist climate over southeastern Asia. Such a dipole circulation also generates a unique persistent rainfall in early spring (PRES) over South China.

In 1980's Yanai and his colleagues identified the boundary layer structure and the thermal states over the Tibetan Plateau before the summer monsoon onset. Our study supports these findings, and demonstrate that such a boundary layer structure is a consequence of the atmospheric thermal adaptation to the surface sensible heating which evanishes quickly with increasing height. The overshooting of rising air which is induced by surface sensible heating then can form a layer of constant potential temperature with a thickness of several hundreds of mb. The thermal forcing of the Tibetan Plateau on the lower tropospheric circulation looks like a Sensible Heat driven Air Pump (SHAP). It is the surface sensible heating on the sloping sides of the Plateau that the SHAP can effectively influence the Asian monsoon circulation. In spring the SHAP contributes to the seasonal abrupt change of the Asian circulation, and anchors the earliest Asian summer monsoon onset over the eastern Bay of Bengal. In summer, this pumping, together with the thermal forcing over the Iran Plateau, produces a bimodality in the South Asian High activity in the upper troposphere, which is closely related with the climate anomaly patterns over South and East Asia. Because the isentropic surfaces in the middle and lower troposphere intersect with the Tibetan Plateau, in summertime, the Plateau becomes a strong negative vorticity source of the atmosphere, and affects the surrounding climate and even the northern hemispheric circulation via energy dispersion of the Rossby Wave.

Future prospect:

The modulation of the Tibetan Plateau on air-sea interaction is a newly recognized monsoon dynamics. In spring over the southern Bay of Bengal (BOB), a vortex commonly develops, followed by the Asian summer monsoon onset. A case study reveals that the BOB monsoon onset vortex is formed as a consequence of air–sea interaction over BOB, which is modulated by Tibetan Plateau forcing and the land–sea thermal contrast over the South Asian area during the spring season.

The surface characteristics are extremely complex over the Tibet Plateau and its surround area. The qualified surface as well as upper air data are far beyond the needs to our research purposes although there have been some station data and field experiments, for example, the CAMPTibet. Future work should set up more observation sites on the Plateau especially over its western part. Another issue is to improve the description of the boundary processes, cloud, radiation and precipitation etc. in the Plateau monsoon region. Work will focus on identifying and explaining potentially significant teleconnections, such as the influence of Tibetan snow cover on the Asian monsoon and Northern Hemispheric conditions to improve the fidelity of global coupled climate models by bringing together theoreticians, observation experts, process modelers and modeling centers.