Energetic Theory and Hadley cells at a seasonal scale: how will ITCZ respond to a warming climate

Doctoral Candidate Name: 
Xiaoyu Bai
Infrastructure and Environmental Systems

The Intertropical Convergence Zone (ITCZ), a belt of convective systems around the equator with showers and thunderstorms, is an important feature not only to the tropical societies whose water budget depends on it, but also to the atmospheric science field to understand how will the Earth respond to a warming climate. Former studies found that annual and zonal mean ITCZ position is related to interhemispheric atmospheric heat transport (AHTtotal). The radiative imbalance at the top of the atmosphere (TOA) transported across the equator to the cooler hemisphere explains the ITCZ position and its shift. Using idealized model simulations with a ``slab'' ocean, researchers found that an increase in the interhemispheric TOA radiation contrast causes an increase in cross-equatorial energy flux by the Hadley circulation and a shift of the ITCZ towards the warmer hemisphere. The theory that relates AHTtotal and ITCZ position is called energetic theory.

In this dissertation, we analyze Tropical rain belts with an Annual cycle and a Continent-Model Intercomparison Project (TRACMIP) model simulations to test the energetic theory. TRACMIP is a project of idealized models that fill the gap between Couple Model Intercomparison Project Phase 5 (CMIP5) idealized aquaplanet projects and fully-coupled projects. TRACMIP models are thermodynamically coupled to a slab ocean. TRACMIP has idealized tropical continent setups with both present-day and quadruple CO2 (4xCO2) concentration experiments, which can help us understand ITCZ shift and potential precipitation changes over land under a warming scenario. Our findings suggested that TRACMIP simulations do not support energetic theory's expectations under a warming climate.

All of our models simulated a northward shift of ITCZ and mass transport under a warming scenario. Our models disagreed on the changes of the energy transported by Hadley cells and the total energy transported by the atmosphere. In general, the link between mass transport changes and energy transported by the Hadley cells changes broke down the most during Northern Hemisphere tropical wet season. The link between changes of the energy transported by the Hadley cells and total energy transported by the atmosphere broke down the most during Northern Hemisphere tropical dry season. Breakdown of one or both of these links caused the overall link between ITCZ shifts and total energy transport changes to break down.

We encourage more studies to be done on energetic theory and climate change. We look forward to combining energetic theory with monsoon theories to develop a self-contained tropical climate model.

Defense Date and Time: 
Friday, April 8, 2022 - 9:00am
Defense Location: 
McEniry 123
Committee Chair's Name: 
Dr. Jacob Scheff
Committee Members: 
Dr. Brian Magi, Dr. Casey Davenport, Dr. Matthew Eastin, Dr. James Amburgey