Mechanisms for the weakening of tropical circulation

As global warming becomes dominant, many climate changes are starting to show. One of the well-known climate changes is a weakening of tropical circulation, which could affect rainfall amount and intensity both in regional and global scales. The global-mean water vapor increases at a rate of 7.5%/K, roughly following the Clausius-Clapeyron thermal expansion at a constant relative humidity, while the global-mean precipitation increases at a rate of only around 2%/K. The fact that the precipitation increases more slowly than the water vapor implies a weakening of tropical circulation (Soden and Held 2006; Vecchi and Soden 2007). In our study, however, we will demonstrate this simple relationship between the global-mean precipitation and water vapor cannot guarantee a weakening of tropical circulation. In fact, tropical circulation could strengthen even though the global-mean precipitation increases much more slowly than the global-mean water vapor. We propose here a new mechanism for this robust climate change in the strength of tropical circulation. Based on a more precise column-integrated moisture budget, atmospheric stability associated with convection depth is the main mechanism. Convection tends to extend higher in a warmer climate, due to an uplifting of the tropopause. The higher the convection, the more stable the atmosphere. This leads to a weakening of tropical circulation.