orographic Influence of the Tibetan Plateau on the Asiatic Winter Monsoon Circulation Part L Large-Scale Aspects¹

Abstract

Some of the characteristic features of the 1978-79 winter mean circulation over and around the Tibetan Plateau were investigated by using twice-daily wind, temperature and geopotential height data at eight standard pressure levels. Surface pressures and winds were estimated over a smoothed topography (determined by Berkofsky and Bertoni, 1955) from the 2.5° latitude by 2.5° longitude resolution, standard pressure level data for the period from 1 December 1978 through 28 February 1979.
Winter mean surface winds tend to flow around, rather than cross over, the high mountains of the Tibetan Plateau. Thus, forced vertical motions do not appear to be very strong along the periphery of these mountains. A local direct Hadley cell is present in this region with updrafts (downdrafts) to the south (north) of the Tibetan Plateau.
West of the Tibetan Plateau, the winter mean flow patterns at 700 and 500mb tend to split into two major streams with axes at 25°N and 45°N. These two westerly flows then converge approximately 1000km downstream (east) of the Tibetan Plateau. In association with this circulation pattern, pairs of anticyclonic and cyclonic vorticity cells occur at the western and eastern borders of the Tibetan Plateau. These vorticity pairs are dominant in the lower troposphere below about 500mb and are indicative of strong surface frictional effects. A comparison of observed and geostrophic winds suggests that the planetary bound- ary layer extends to a height of about 1.5km above the earth's surface.
At 200mb, a strong jet stream flows along the southern boundary of the Tibetan Plateau. This jet stream is characterized by pronounced eastward acceleration that is primarily associated with strong, nongeostrophic, southerly winds. Momentum fluxes due to transient disturbances are only a minor factor in the maintenance of this 200mb jet stream.
Along any given latitude, 500mb winter mean temperatures are warmer over the Tibetan Plateau. Consequently, the north-south temperature gradient over the Tibetan Plateau is weaker than it is to the north or south, which is congruent with the splitting of the westerly flow at this level. There is no indication of a northward sensible heat flux due to transient eddies directed from the tropics (India-Bay of Bengal), across the Tibetan Plateau, to the Mongolia-Siberia region. In contrast, sensible heat fluxes due to transient eddies are northward and substantial over central and northern China where low-level northerly surges bursting out of Siberia are associated with cold temperatures. Fairly strong northward sensible heat fluxes due to transient eddies are also noted over the Kazakh region as the prevailing southwesterlies transport large amounts of moisture and heat from the Mediterranean-Caspian Seas region.