Effects of Local and Non-Local Processes on the Aging of the Tropopause Layer and the Lower Stratosphere
Wednesday, 16 December 2015: 11:20
3010 (Moscone West)
As concentrations of greenhouse gases continue to increase in the troposphere, general circulation models predict an acceleration in the Brewer-Dobson circulation (BDC). A common diagnostic used to evaluate this change in circulation is age of air, which is defined as the time elapsed since stratospheric air was last in contact with the troposphere. An acceleration in the BDC can lead to shorter residence times and hence younger age of air. This change would affect the chemical distribution of pollutants and ozone-depleting substances in the stratosphere and potentially impact the recovery of the stratospheric ozone layer. In this study, we use in-situ measurements and model outputs to derive and compare vertical profiles of age of air and to assess the impact of various dynamical processes on age of air over the tropics and Northern Hemisphere midlatitudes. We focus on measurements of winds and several tracers with varying atmospheric lifetimes recently obtained in the tropopause region and lower stratosphere during the NASA ATTREX and NASA SEAC4RS campaigns between Jan 2013 and Mar 2014. We derive age of air using CO2 measurements at the surface and at altitude. This tracer is nearly chemically inert in the atmosphere and has well-known sources at the surface: an increasing trend over time resulting from anthropogenic activity and a superimposed seasonal cycle resulting from photosynthesis and respiration by the biosphere. During the field deployments, we encountered episodes of contrasting transport and mixing processes compared to background conditions, namely local and non-local convection as identified by trajectories and distinct chemical signatures, nearby typhoons, wave activity, and shifts in large-scale circulation. In addition to aircraft measurements, we also examine data from earlier high-altitude balloon flights, which allowed us to sample deeper into the stratosphere. We complement the observations with model outputs from the NASA GEOS-5 atmospheric global climate model and Lagrangian trajectories driven by ERA-interim circulation. These comparisons indicate good agreement in age of air at and above the tropopause with increasing biases deeper into the stratosphere. The implications of these results will be discussed.