Tropical stratospheric upwelling impacts the tropical equilibrium
climate sensitivity by reducing the effective forcing
Abstract
An atmospheric composition feedback mechanism modulates the global
equilibrium climate sensitivity (ECS) through changes in the tropical
upper-tropospheric and lower-stratospheric (UTLS) water vapor. The
feedback mechanism is caused by the acceleration of the Brewer-Dobson
circulation. This process changes the ozone (O$_{3}$)
concentration, resulting in a drier and cooler UTLS region than without
O$_{3}$ changes. Thus, the planetary long-wave emissivity
increases, and the ECS decreases. However, the BDC alone already
provides dynamical cooling through the tropical stratospheric upwelling,
potentially impacting the ECS. Here, we analyze the implications of this
effect from a tropical clear-sky perspective, applying a one-dimensional
radiative-convective equilibrium (RCE) model that explicitly accounts
for the adiabatic cooling by the BDC and includes an interactive
representation of O$_{3}$. We study how increasing upwelling
modifies the change of the tropical energy budget resulting from a
doubling of CO$_{2}$. An increase in upwelling reduces the tropical
ECS mainly through an increased tropical energy export related to the
adiabatic cooling. The atmospheric composition feedback through
O$_{3}$ contributes less than 30\% to the tropical
ECS reduction. Due to the dominance of the energy export, any impact on
the global ECS will depend on the redistribution of the energy in the
extratropics. We show that GCMs simulate similar changes of the tropical
energy export under increased upwelling which corroborates that the
findings obtained with the RCE approach bear relevance for the global
climate.