A moist model monsoon

A substantial literature takes it for granted that the prime reason for the great strength of the South Asian monsoon is the heating effect of the Tibetan plateau. On page 218 of this issue, Boos and Kuang show that what counts is the barrier posed by the mountains at the southern edge of the plateau, not the heating. This view has deep implications for our mechanistic understanding of this monsoon system, for our projections of how it might be altered in a warmer world, and for our understanding of Earth history. The prevailing wisdom is based on plausible and venerable ideas. First, there is the idea of the monsoon as a giant sea-breeze (onshore) circulation. Land heats faster than the ocean: in summer, or on a diurnal basis, the temperature rises faster in the lower levels of the atmosphere above land, and so the pressure falls. The resulting pressure gradient drives the atmospheric flow onshore and, in the case of the monsoon, moisture-laden winds blow towards land and the rains begin. It is a reasonable expectation that enhancing the heating should increase the power of the monsoon — which brings us to the extra heating expected from the looming presence of the world’s largest elevated region, the Tibetan plateau. The solar energy driving Earth’s climate system is absorbed far better by land than by air. Hence, an elevated land surface should be warmer than the nearby atmosphere at the same height above sea level. So we should expect the presence of the plateau to put an extra charge into the heating differential driving the monsoon. The previous paragraph set up a straw man, but one still esteemed in research papers and textbooks. It is perhaps adequate to explain monsoonal wind shifts, but not the rains. A heat low forms over land well before the monsoon begins, but still it does not rain. In fact, the land is hotter in May, before it rains, than at the height of the monsoon season in July. A more useful view, nicely presented by Gadgil, unifies monsoon rains with other tropical rainfall. In this view, tropical rainy seasons are largely a matter of when the Intertropical Convergence Zone, a near-global belt of deep atmospheric convection, passes overhead as it migrates north–south with the seasons. This perspective brings moisture to the fore as a driver. Deep convection requires that surface air has enough buoyancy to rise to the top of the troposphere (the lowest 10–15 km of Earth’s atmosphere). Water vapour is a crucial ingredient; a rising parcel of air can acquire additional buoyancy from the heat that is released as water vapour condenses. To see if a region is ripe for deep convection, we invoke a parameter called moist entropy, which tells us how the temperature of a parcel of air will change as it rises, accounting both for the decrease due to expansion as the pressure falls with height, and for the increase as the water vapour it contains condenses, releasing heat.Thus, we compare a measure of moist entropy at the surface, such as the equivalent potential temperature (θe ), which is indicative of the density a parcel will have as it rises, with a measure of the density in the upper troposphere. In the tropics, we expect mean conditions to reflect a statistical equilibrium between convection and radiation. Where a parcel of moist air is able to rise to great heights from near the surface, that is, where there is deep convection, it will pass a level at which it becomes saturated, because temperature decreases with height. Thereafter, its temperature is enhanced by the latent heat released as water vapour condenses. Thus, in convective regions, upper-level temperatures are largely determined by the temperature and humidity of near-surface air. Boos and Kuang show that, below cloud level, the maximum moist entropy is located south of the Himalayas, indicating that the important surface entropy supply is in the moist air over the Indian continent, not the Tibetan plateau to the north. The peak upper-tropospheric temperatures are over the continent, although direct heating of the Tibetan plateau is evident in a local dry boundary layer. It is telling that the straw-man reasoning above considered radiative processes, but not moist convection. Noting the sharp gradients in θe at the Himalayas, Boos and Kuang hypothesize that the mountains enable a stronger monsoon by preventing the high-entropy warm and moist air to the south from being diluted by mixing with the low-entropy cold and dry air to the north Monsoon circulation