Quantifying tracer transport in the tropical lower stratosphere using WACCM 1Universidad Complutense de Madrid, Depto. de Meteorología y Geofísica, Madrid, Spain
01 Nov 2013
2National Center for Atmospheric Research, Boulder, Colorado, USA
Received: 24 Apr 2013 – Published in Atmos. Chem. Phys. Discuss.: 21 May 2013 Abstract. The zonal mean transport of ozone and carbon monoxide (CO) near the tropical
tropopause is investigated using the Whole-Atmosphere Community Climate
Model version 4 (WACCM4). The variability in temperature, ozone and CO in
the model shows good agreement with satellite and balloon observations.
Modeled temperature and tracers exhibit large and closely coupled annual
cycles in the tropical lower stratosphere, as in the observations. The
thermodynamic and tracer budgets in the model are analyzed based on the
Transformed Eulerian Mean (TEM) framework on log-pressure coordinates and
also using the isentropic formulation. Results show that the coupled
seasonal cycles are mainly forced by tropical upwelling over altitudes with
large vertical tracer gradients, in agreement with previous observational
studies. The model also allows explicit calculation of eddy transport terms,
which make an important contribution to ozone tendencies in the tropical
lower stratosphere. The character of the eddy fluxes changes with altitude.
At higher levels (~2 km above the cold point tropopause),
isentropic eddy transport occurs during winter and spring in each hemisphere
in the sub-tropics, associated with transient Rossby waves acting on strong
background latitudinal gradients. At lower altitudes, close to the tropical
tropopause, there is a maximum in horizontal eddy transport during boreal
summer associated with the Asian monsoon anticyclone. Sub-seasonal
variability in ozone and CO, tied to fluctuations in temperature, is
primarily driven by transient tropical upwelling. In isentropic coordinates,
the overall tracer budgets are similar to the log-pressure results,
highlighting cross-isentropic advection as the main term in the time-mean
balance, with large seasonality above the tropopause. However, in isentropic
coordinates the tracer variability is largely reduced on both seasonal and
sub-seasonal timescales, because tracer fluctuations are highly correlated
with temperature (as a response to upwelling).
Revised: 03 Oct 2013 – Accepted: 05 Oct 2013 – Published: 01 Nov 2013
Citation: Abalos, M., Randel, W. J., Kinnison, D. E., and Serrano, E.: Quantifying tracer transport in the tropical lower stratosphere using WACCM, Atmos. Chem. Phys., 13, 10591-10607, doi:10.5194/acp-13-10591-2013, 2013.