Abstract
Abstract. We explore the potential of ozone observations to constrain transport processes in the tropical tropopause layer (TTL), and contrast it with insights that can be obtained from water vapour. Global fields from Halogen Occultation Experiment (HALOE) and in-situ observations are predicted using a backtrajectory approach that captures advection, instantaneous freeze-drying and photolytical ozone production. Two different representations of transport (kinematic and diabatic 3-month backtrajectories based on ERA-Interim data) are used to evaluate the sensitivity to differences in transport. Results show that mean profiles and seasonality of both tracers can be reasonably reconstructed. Water vapour predictions are similar for both transport representations, but predictions for ozone are systematically higher for kinematic transport. Compared to global HALOE observations, the diabatic model prediction underestimates the vertical ozone gradient. Comparison of the kinematic prediction with observations obtained during the tropical SCOUT-O3 campaign shows a large high bias above 390 K potential temperature. We show that ozone predictions and vertical dispersion of the trajectories are highly correlated, rendering ozone an interesting tracer for aspects of transport to which water vapour is not sensitive. We show that dispersion and mean upwelling have similar effects on ozone profiles, with slower upwelling and larger dispersion both leading to higher ozone concentrations. Analyses of tropical upwelling based on mean transport characteristics, and model validation have to take into account this ambiguity between tropical ozone production and in-mixing from the stratosphere. In turn, ozone provides constraints on transport in the TTL and lower stratosphere that cannot be obtained from water vapour.
Highlights
The tropical tropopause layer (TTL) plays an important role for climate, as changes therein due to increasing greenhouse gases may affect troposphere-stratosphere exchange of radiatively active trace gases (Highwood and Hoskins, 1998; Gettelman and Forster, 2002; Fueglistaler et al, 2009a)
Halogen Occultation Experiment (HALOE) measurements of water vapour and ozone are binned into zonal mean, monthly means averaged over the period 2001–2005, using the method of Grooß and Russell (2005)
Our analysis suggests that a combination of ozone and water vapour may be able to constrain transport in the TTL and lower stratosphere better than water vapour alone
Summary
The tropical tropopause layer (TTL) plays an important role for climate, as changes therein due to increasing greenhouse gases may affect troposphere-stratosphere exchange of radiatively active trace gases (Highwood and Hoskins, 1998; Gettelman and Forster, 2002; Fueglistaler et al, 2009a). Water vapour and ozone in the TTL are both controlled to leading order by relatively simple processes. We combine transport as represented by backtrajectories based on European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim data (Simmons et al, 2006; Uppala et al, 2008) with simple models of physical and chemical processes controlling water vapour and ozone. Fueglistaler et al (2005) and Fueglistaler and Haynes (2005) showed that the mean, the annual cycle and interannual variability of water entering the stratosphere in the tropics can be reconstructed from trajectories, assuming that the lowest temperature in the backward history of an air parcel (here a trajectory) determines its water vapour mixing ratio.
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