Age Spectra and Transport Rates in the UTLS over the Tropics and the North American Monsoon Region

Abstract:

The Upper Troposphere / Lower Stratosphere (UTLS) plays a significant role in controlling the chemical composition and the radiative property of the stratosphere. Transport and mixing processes with time scales ranging from a few hours (e.g., convection) to several months (e.g., slow ascent) have been previously observed in this region of the atmosphere. While the tropics act as the dominant gateway for air entering the stratosphere, monsoon regions have recently gained significant interest as sources of direct injection of surface air into the extratropical stratosphere, bypassing the thermal and chemical control of the tropics.

In this study, we use high-altitude aircraft measurements of chemical tracers with different atmospheric lifetimes to investigate the aforementioned processes. These measurements were recently collected over the tropics during the NASA ATTREX campaign and over the North American Monsoon (NAM) region during the NASA SEAC4RS campaign. In addition, we use surface station data and trajectory analysis in order to elucidate the chemical and dynamical context of the aircraft measurements. Our goals are twofold: (i) examine the age of air and age spectra in the UTLS over the tropics and the NAM region, and (ii) examine the transport rates linking the tropics and midlatitudes at UTLS altitudes. This study is primarily based upon the analysis of greenhouse gas data, in particular CO2. This tracer has a well-defined seasonal cycle driven by biosphere-atmosphere interactions and an annual trend driven by anthropogenic activity. This ‘clock-like’ property makes it a useful tracer for studying transport timescales. This information is then complemented by process-specific information given by additional chemical tracers such as H2O, O3, CO, and short-lived hydrocarbons. Multi-tracer aircraft data from more than 150 vertical profiles over the tropical Pacific and over 50 vertical profiles over the NAM region between Jan 2013 and Mar 2014 provide unprecedented statistics for elucidating specific processes and timescales controlling the chemical and dynamical properties of the UTLS. Understanding these properties in the UTLS is important for accurately forecasting the long-term impact of surface emissions and convective activity on the climate system.