Some Theoretical Thinking on the Changing Tropical Climate

Abstract

This dissertation synthesizes long-standing theoretical ideas to develop quantitative constraints on several aspects of tropical climate, namely convection, heat stress, precipitation, and outgoing longwave radiation. A zeroth-order picture of the tropical troposphere is that deep convection maintains a moist adiabatic temperature profile in the vertical and gravity waves quickly smooth any temperature gradients in the horizontal, which is formally known as convective quasi-equilibrium (QE) and the weak-temperature-gradient (WTG) assumption. We expect that strict QE-WTG should yield a uniform moist static energy (MSE) threshold for deep convection. Consistent with the theoretical expectation, we find that deep convection only occurs over the highest subcloud MSEs and that the convective subcloud MSE (daily-mean subcloud MSE weighted by precipitation) is roughly uniform between 20°S and 20°N. QE-WTG forces the highest subcloud MSEs to be equal over land and ocean, not only in the present climate but also in much colder and warmer climates. The annual-maximum wet-bulb temperature, a metric for extreme heat stress, is also controlled by the QE-WTG dynamics due to the functional relationship of wet-bulb temperature with MSE. We provide a theoretical projection that the annual-maximum wet-bulb temperature will increase roughly uniformly by about 1°C for each 1°C of tropical mean warming. This result suggests that limiting the mean surface warming also limits heat stress extremes in the tropics. QE-WTG controls the occurrence of deep convection and thus precipitation. Global climate models consistently predict that tropical precipitation will be distributed more unevenly in space with global warming. We show that the unevenness of precipitation can be traced back to the unevenness of subcloud MSE distribution. We then explain the change in sublcoud MSE distribution with a simple scaling accounting for the Clausius-Clapeyron increases of boundary-layer specific humidity under invariant relative humidities. The invariance of relative humidities also has implications for the linearity of outgoing longwave radiation with surface temperature.