Role of clouds in tropical climate and its variability

Abstract

In this thesis, I study the impacts of convective heating, clouds and cloud-radiativeeffects on the tropical climate. Broadly speaking, these impacts are studied in frame- works where the distribution of heating or clouds is taken as given, and their effects on the resulting circulation are analyzed. In Chapter 2, the distribution of diabatic heating is prescribed in a linear equation for the Walker circulation streamfunction. Through tractable closed-form solutions, I conclude that the strength of the Indo- Pacific zonal overturning is not simply a function of the total convective heating, but that spatially more concentrated heating leads to more vigorous circulations. In Chapter 3, I use a cloud-locking technique in the ESM2G abrupt-4xCO 2 experiments to elucidate the role of the cloud-radiative effects (CREs). Hadley and Walker cir- culation weakening and Hadley expansion are analyzed, and the changing CREs are shown to have considerable impacts depending on the variable and timescale in ques- tion. In Chapter 4, we show that the interaction between longwave CREs, convection and circulation alter the spectrum of the tropical waves. The presence of this inter- action reduces gross-moist stability and leads to stronger moisture modes (like the Madden-Julian Oscillation) at the expense of the gravity-like waves (e.g., Kelvin). These effects are robust in both the AMIP and aquaplanet configurations of the AM4 model. Given the results in this thesis, I argue that analyzing systems where clouds and heating are uncoupled from circulation can yield useful insights of the coupled system.