MECHANISMS GOVERNING THE EFFECT OF PLATEAUS ON LOW-LATITUDE REGIONAL CLIMATE (Invited Presentation)

Heating of the atmosphere by broad, elevated terrain is thought to alter regional climate in Asia, the Americas, and even on Mars. But the mechanisms that cause the troposphere to equilibrate at warmer temperatures over elevated terrain are not well understood. This is particularly true in South Asia, where the strength of the summer monsoon is now known to be created by the mechanical and thermal effects of narrow mountain chains rather than by the broad heating of the Tibetan Plateau; it has remained unclear why the Tibetan Plateau is not the dominant thermal forcing in the Asian region. Here we quantitatively describe the radiative-convective physics that controls temperatures over elevated terrain. We use cloud system-resolving model simulations to show that elevation-induced warming over high plateaus is typically counteracted by cooling due to the large surface albedo of those plateaus. We present a theory for the influence of surface elevation on temperatures in radiative-convective equilibrium, and show that the magnitude of the elevated heating effect is set by top-of-atmosphere radiative changes dominated by the sensitivity of the moist adiabatic lapse rate to surface height. Elevated heating is weaker in warmer climates and can even change sign to become elevated cooling. Furthermore, non-elevated surfaces with the albedo of modern India would enter a local runaway greenhouse regime without ventilation by monsoon winds. These results indicate that changes in surface albedo are just as important as more commonly studied changes in surface elevation for the evolution of low-latitude regional climate throughout Earth’s history.