Empirical orthogonal function analysis was applied to 12-20 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 6 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 12-20 day filtered meridional wind data at 200mb.
Composite charts of 12-20 day filtered vorticity perturbations at 200mb exhibit prominent standing oscillations anchored in the vicinity of northeastern India and Burma. Nonlinear interaction between the winter mean absolute vorticity field and 12-20 day disturbances is primarily responsible for the development of 200mb vorticity perturbations over this region. These standing vorticity perturbations are superimposed upon weak eastward propagating modes, which move slowly (-4 m•s^-1) in the subtropical westerlies (-45 m•s^-1) south of the Tibetan Plateau. Velocity potential fields at 200mb are greatly enhanced by local topographic effects and tend to reduce the eastward advection effect of the strong westerly flow.
Over Indochina and the northern South China Sea along about 100° to 110°E, northsouth oriented, divergent vertical overturning (departure from winter mean) is thermally indirect (direct) when an upper level trough (ridge) system intensifies over the northeastern India-Burma region. Substantial temperature changes (±1.5°) in the lower troposphere below about 700mb over southern China are associated with these intraseasonal variations in local vertical overturnings.