2004 Volume 82 Issue 6 Pages 1545-1564
Convective variability at submonthly timescales (7-20 days) over the Tibetan Plateau and the associated large-scale atmospheric circulation and convection were examined over regions affected by the Asian Monsoon. The mature phase of the Asian summer monsoon (July-August) was analyzed for those years (1986, 93, 98) in which convective variability on timescales of 14 days was notable over the Tibetan Plateau.
Composite analyses of OLR, based on the filtered Tbb time series over the southern Tibetan Plateau, show that significant convective signals rotate clockwise around 28°N, 90°E, affecting the Tibetan Plateau, Indochina, the Bay of Bengal, and India. Significant signals also appear around the Philippines and the South China Sea. A well-developed wave train extending from North Africa to far-east Asia along the Asian subtropical jet is associated with convective fluctuations over the plateau. The waves are quasi-stationary and have a Rossby wave-like downward wave train with wavenumber 7.
The waves control convective fluctuations over the plateau. During the transition to active (inactive) convection, an upper-level trough (ridge) develops west of the plateau. Simultaneously, cyclonic anomalies strengthen over India between the lower and middle troposphere. The development of the two troughs induces a southerly flow of moist air toward the plateau. Moistening of the lower atmosphere creates favorable conditions for subsequent active moist convection.
Possible processes for forming the wave train over the subtropical jet and a link for convective signals between midlatitudes and the Asian monsoon are discussed.