NSF Award
2411500
The tropical tropopause layer (TTL) is the top layer of the tropical troposphere, between about 14km, usually the highest level of convective cloud outflow, and 18km, where the cold point tropopause marks the end of the troposphere and the beginning of the stratosphere above it. The TTL is sometimes called the "gateway to the stratosphere" because the bulk of the water vapor and ozone in the stratosphere travels through the TTL to get there. The gateway terminology is appropriate because TTL temperature regulates the amount of ozone and water vapor entering the stratosphere, with lower amounts entering when the TTL is colder.<br/><br/>An intriguing aspect of the gateway is its connection to tropical sea surface temperature (SST): the TTL is colder, and its gateway narrower, when the tropical oceans are warmer. The relationship is well documented but not well understood, and two competing theories have been proposed. One is that the warming of tropical SSTs leads to deeper and more vigorous convective clouds, producing rising motions in the TTL which cool the layer by adiabatic expansion. The other is that what matters is the pattern of SST change, as SST contrasts induce differences in convection that generate large-scale waves which propagate upward into the TTL, where they induce rising motions and cooling as they dissipate.<br/><br/>Work under this award seeks to reconcile the two theories using idealized modeling experiments in which specially designed patterns of SSTs are used to isolate one mechanism or the other. The simulations are performed with two hierarchies of models of different levels of complexity, one tailored to each theory. Models used for the convection theory are versions of the System for Atmospheric Modeling (SAM) while models used to represent the wave dissipation mechanism include a dry dynamical core model (similar to the Held-Suarez model), and the Model of an idealized Moist Atmosphere (MiMA). Results of the work are then used to examine the role of tropical SSTs in driving long-term TTL trends, one issue being the extent to which the different SST trend patterns found in model simulations and the observational record (see for instance AGS-2203543) matter for the TTL and hence the gateway.<br/><br/>The work is of societal as well as scientific interest due to the global impacts of stratospheric ozone, which shields the earth's surface from harmful ultraviolet radiation, and water vapor, which is a strong greenhouse gas. In addition, the project supports an undergraduate student participating in the NSF-funded Cornell GEOPAths Geoscience Learning Ecosystem (CorGGLE) in summer 2025. CorGGLE is a summer bridge program in which students carry out research projects and are introduced to a broad range of science and science-related careers. The project also provides support and training to a postdoctoral researcher, thereby providing for the future workforce in this research area.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
