Orographic Influence of the Tibetan Plateau on the Asiatic Winter Monsoon Circulation

Part III. Short-Period Oscillations

Abstract

Empirical orthogonal function analysis was applied to 3.5-5.5 day filtered wind, temperature, and geopotential height data for the 1978-79 winter (1 December 1978 to 28 February 1979) at seven levels (the surface, 850, 700, 500, 300; 200 and 100mb) over the Tibetan Plateau region (20° to 50°N and 57.5 ° to 117.5 °E). The resulting eigenvector series were then truncated at eigenmode 12 to remove unwanted, small-scale noise which is of questionable accuracy. This was followed by the compositing of each meteorological variable at every level and grid point, with reference to changes in the first eigenvector coefficients for 3.5-5.5 day filtered meridional wind data at 200mb.
Composite charts of 3.5-5.5 day filtered winds and geopotential heights at 200mb reveal that northeast-southwest oriented trough and ridge systems systematically propagate eastward with nearly the same phase speed (-12m•s-1) throughout the entire Tibetan Plateau region. However, the amplitudes of these systems decrease as they pass over the Tibetan Plateau. Thus, this area of high mountains appears to exert some measure of control on the activity of 3.5-5.5 day wave perturbations even at the 200mb level.
At 500mb, wind changes associated with the eastward passage of major trough and ridge systems are much less pronounced over the Tibetan Plateau than those to the north. These weak wind variations over the Plateau are probably a result of the 500mb level being located within the planetary boundary layer above this highly elevated area. Similarly, midlatitude systems do not appear to extend across the Tibetan Plateau into the subtropical troposphere below 500mb due to the frictional (or mechanical blocking) effects of these high mountains. This is demonstrated by the rapid weakening of cyclonic (anticyclonic) cells as they propagate northeastward over northern India toward the southern border of the Himalaya Mountains.
Interestingly, 700mb composite charts suggest that small-scale low (high) pressure cells develop near the northeastern periphery of the Tibetan Plateau. These orographically induced, small-scale disturbances then move southward along the eastern border of the Tibetan Plateau, and become most intense over the Red Basin before dissipating near the southeastern corner of the Plateau. Over central China, 700mb northerlies (cold monsoonal surges) are strongest when an orographically induced anticyclonic cell in the lee of the Tibetan Plateau approaches the Red Basin and a major trough is over northeastern China. However, these northerly surges do not appear to penetrate into the South China Sea region. In contrast, 700mb northerly surges with periods of 6-9 days may extend as far south as 20°N, and possibly beyond.may extend as far south as 20°N, and possibly beyond.