Impact of Convection and Dynamics on Greenhouse Gas Distribution in the TTL

The Tropical Tropopause layer (TTL) plays an important role in setting the chemical composition of the global stratosphere, hence affecting Earth’s climate. In this study, we use recent aircraft data collected aboard the NASA Global Hawk during the ATTREX campaign, which targeted the TTL across the equatorial Pacific during the boreal winters of 2013 and 2014. This unprecedented dataset provides hundreds of vertical profiles in the TTL. Our analysis focuses on investigating how tropical convection and dynamics affect the distribution of greenhouse gases in the TTL and through the TTL the global stratosphere. We focus on measurements of CO2 and O3. CO2, given its sensitivity to time of year, latitude, and local origin of air (e.g., land versus ocean), provides valuable information on age and geographical origin of surface air sampled in the TTL. O3, given its sensitivity to altitude (e.g., troposphere versus stratosphere) and surface type (e.g., clean marine versus pollution from land), allows us to further constrain the origin of air sampled in the TTL. We find that the cold point tropopause, more than the lapse rate tropopause, provides an important dynamical separation between the convectively-driven and the radiatively-controlled regimes in the TTL. This is evidenced by the vertical distribution and variability of even CO2 and O3, tracers that are not temperature sensitive like ice and water vapor are, for instance. We complement the tracer measurements with trajectory calculations that are traced back in time and space to geographical regions of recent convection. Our analysis corroborates findings from previous studies on the dominant role of the western Pacific Warm Pool as the main source of convective air to the TTL across the Pacific. Equatorial Africa is found to be the second most significant contributor, by a factor of seven smaller than the Warm Pool. CO2 measurements also revealed that both the ITCZ and SPCZ, important tropical convergence zones over the Pacific, serve as conveyor belts for surface air from northern and southern hemispheres to not only populate the TTL within each hemisphere, but also to deliver southern hemisphere air across the Equator and deep into the northern hemisphere (e.g., 10 – 15 N). Understanding the processes that determine not only the humidity and cloud cycles in the TTL, but also its overall chemical composition, is critical for more accurate assessments and forecasts of changes in Earth’s climate system.