A review of recent progress on Tibet’s role in the South Asian monsoon

The Tibetan Plateau exerts a profound influence on winds in boreal winter primarily through mechanical means, blocking flow to create waves in the jet stream that extend around Earth’s full circumference (e.g. Held et al., 2002). In contrast, this plateau was thought to influence boreal summer winds primarily through its thermal effects, providing a heat source over 4 km high and 2,000 km wide that generates the interhemispheric flow of the South Asian monsoon. But recent work has shown that although orography greatly strengthens the South Asian summer monsoon, it operates via a different mechanism that requires only a relatively narrow but continuous chain of mountains around the northern edge of the monsoon, rather than a broad plateau. This review presents a brief history of research on the role of orography in the South Asian summer monsoon, with a focus on recent work that frames monsoon dynamics in terms of modern theories for precipitating large-scale circulations. This review does not address the mechanical forcing by orography that seems important for East Asian climate (e.g. Wu et al., 2007; Wang et al., 2008; Molnar et al., 2010; Park et al., 2012). I. Classic view of Tibet as an elevated heat source The rapid geographic expansion of Chinese meteorological observations in the 1950s provided the first view of the Tibetan Plateau as a heat source for the boreal summer troposphere. Estimates of the atmospheric heat budget showed that the plateau must emit abundant sensible heat in boreal summer, sufficient to warm much of the overlying troposphere by about 2 K day−1 (Yeh et al., 1957; Staff Members of Academia Sinica, 1958). Together with the existence of a warm upper-tropospheric anticyclone in the vicinity of Tibet, these estimates were taken as evidence that direct heating of the middle to upper troposphere by plateau surface fluxes forced a large part of the South Asian monsoon (e.g. Koteswaram, 1958). However, it was soon realized that condensation and precipitation of water just south of the plateau instead constituted the dominant diabatic heat source for the South Asian monsoon; the plateau’s sensible heat fluxes were argued to indirectly drive the monsoon by causing this latent heating (Flohn, 1968). In other words, Tibet was claimed to be part of a “heat engine”, with its sensible heating causing ascent and horizontal moisture convergence, that in turn produces the latent heating that drives monsoon flow. Decades later, Li and Yanai (1996) used gridded analyses of observations to confirm that diabatic heating by precipitation over the Bay of Bengal is the dominant heat source in the South Asian monsoon, and this diabatic heating is balanced by adiabatic cooling in the ascending branch of the monsoon circulation. This balance between moist convective heating and adiabatic cooling due to ascent is a general characteristic of tropical circulations (e.g. Sobel and Bretherton, 2000), and makes inferring the causal heat source difficult because any net temperature change results from the small residual of large opposing terms in the thermodynamic equation. Nevertheless, Li and Yanai (1996) argued that observations of large sensible heat fluxes over the plateau supported “the thermal influence of the Tibetan Plateau as a dominant factor driving the planetary-scale