Simple Models for Complex Tropical Dynamics

Abstract

Studying Earth's tropics is an essential part of understanding the climate, simulating the Earth system, and predicting the societal impacts of weather. In this thesis, we use a hierarchy of models -- including analytically tractable equations, simplified simulations, and full general circulation models -- to study tropical phenomena including the Hadley Circulation, the Inter-Tropical Convergence Zone (ITCZ), the South Asian monsoon, Pacific and ENSO seasonality, the Walker Circulation, and the modeling of the tropical energy budget. We begin with an examination of tropical SSTs and the ITCZ under warming, finding that the Hadley cells weaken and tropical SST gradients decrease in a warmer climate. The ocean's subtropical cells strengthen and transport more energy in a warmer climate, further flattening SST gradients. The ITCZ, meanwhile, increases in strength with warming because of the exponential relationship between humidity and temperature, and the presence of a dynamic ocean changes a single-ITCZ with a sinusoidal seasonal cycle to a double-ITCZ with a square wave seasonal cycle. Next, we study the ``monsoonal mode,'' an energy and precipitation anomaly triggered by the South Asian Monsoon that moves into the West Pacific during Northern Hemisphere autumn. The monsoonal mode is discussed as a possible underlying cause of the seasonality of the Pacific, i.e., that the West Pacific and ENSO both have seasonalities that favor one season despite being on the equator. To show this, ENSO seasonality is examined using simplified simulations and an energy budget of the Central-Eastern Equatorial Pacific. Similar techniques are then used to study ENSO events in warmer climates, and it is found that the Pacific zonal SST gradient and the Walker circulation, which are the sources of ENSO instability, weaken with warming, decreasing the magnitude of ENSO events. Lastly, we assess the energy budget of CMIP6 models. It is shown that all CMIP6 models have more energy input to the deep tropics than ERA5 reanalysis, and this bias is bigger in the Southern Hemisphere. The hemispheric asymmetry in this bias can be traced back to radiation absorbed by the atmosphere, which is associated with dust (for shortwave radiation) and total column water (for longwave radiation). As a whole, this thesis demonstrates the utility of studying complex problems with simple models and deepens our understanding of Earth's tropics.