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Regional and Seasonal Trends in Tropical Ozone from SHADOZ Profiles: Reference for Models and Satellite Products

A.M. Thompson1, R.M. Stauffer1, K. Wargan2, J.C. Witte3,1, D.E. Kollonige1,3 and J.R. Ziemke4

1NASA Goddard Space Flight Center (GSFC), Atmospheric Chemistry and Dynamics Laboratory, Greenbelt, MD 20771; 301-614-5905, E-mail: anne.m.thompson@nasa.gov
2NASA Goddard Space Flight Center (GSFC), Greenbelt, MD 20771
3Science Systems and Applications, Inc. (SSAI), Lanham, MD 20706
4Morgan State University, Baltimore, MD 21251

Understanding lowermost stratosphere (LMS) ozone variability is an important topic in the trends and climate assessment communities because of feedbacks among changing temperature, dynamics, water vapor and ozone.  LMS evaluations are usually based on satellite observations. Free tropospheric (FT) ozone assessments typically rely on profiles from commercial aircraft. Ozonesonde measurements constitute an independent dataset encompassing both LMS and FT. We used Southern Hemisphere Additional Ozonesondes (SHADOZ) data from 1998-2019 in the GSFC Multiple Linear Regression (MLR) model to analyze monthly mean FT and LMS ozone across five well-distributed tropical sites, three based on combination of nearby individual stations (Figure 1). Our findings: (1) both FT (defined as 5-15 km) and LMS (15-20 km) ozone trends exhibit marked regional and seasonal variability. (2) Stations with the largest FT ozone increases, in the equatorial Americas and western Pacific/east Indian Ocean, show these trends most strongly in February-May when there is convectively-driven wave activity.  (3) LMS ozone losses are greatest in the 2nd half of the year (solid lines in Figure 2) when the losses are correlated with an increase in tropopause height (TH) as derived from SHADOZ radiosonde data.  (4) When the upper FT and LMS are defined by tropopause-relative coordinates, the LMS ozone trend calculated by MLR becomes insignificant, as shown in the dashed lines in Figure 2.  Thus, the 20-year decline in tropical LMS ozone reported in recent satellite- and model-based studies does not point to a chemical loss but is rather a signature of a tropopause that has been perturbed by a changing climate.  Output based on our regional and monthly averaged FT and LMS ozone trends will be available as a reference for satellite- and model-based analyses.

Figure 1

Figure 1. Map of SHADOZ stations used in this study. Stations whose combined records are examined are colored orange (San Cristóbal and Paramaribo), red (Natal and Ascension), and blue (Watukosek and Kuala Lumpur).   Samoa and Nairobi records, colored gray, are analyzed individually.

Figure 2

Figure 2. Monthly mean trends in LMS ozone, solid lines referring to the column amount from 15-20 km and with the LMS defined between as the column starting at the tropopause height (TH) plus 5 km.