Past changes in the vertical distribution of ozone – Part 3: Analysis and interpretation of trendsReport as inadecuate

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1 Department of Chemistry Cambridge, UK 2 ESRL - NOAA Earth System Research Laboratory 3 CIRES - Cooperative Institute for Research in Environmental Sciences 4 IAC - Institute for Atmospheric and Climate Science Zürich 5 Bodeker Scientific 6 BIRA-IASB - Belgian Institute for Space Aeronomy - Institut d-Aéronomie Spatiale de Belgique 7 DWD - Meteorological Observatory, Deutsche Wetterdienst 8 Hampton University 9 GSFC - NASA Goddard Space Flight Center 10 Department of Chemistry Waterloo 11 ISAS - Institute of Space and Atmospheric Studies Saskatoon 12 IRM - Institut Royal Météorologique de Belgique Bruxelles 13 SSAI - Science Systems and Applications, Inc. Lanham 14 JPL - Jet Propulsion Laboratory 15 SHTI - LATMOS LATMOS - Laboratoire Atmosphères, Milieux, Observations Spatiales 16 School of Chemistry Wollongong 17 FMI - Finnish Meteorological Institute 18 National Institute of Water and Atmospheric Research NIWA, Lauder, New Zealand 19 Institute of Astrophysics and Geophysics, University of Liège, B-4000 Liège, Belgium 20 University of Massachusetts, Department of Astronomy 21 JHU - Johns Hopkins University 22 Federal Office of Metrology 23 University of Toronto 24 Georgia Institute of Technology Atlanta 25 NOAA-NWS-NCEP Climate Prediction Center 26 LaRC - NASA Langley Research Center Hampton

Abstract : Trends in the vertical distribution of ozone are reported and compared for a number of new and recently revised datasets. The amount of ozone-depleting compounds in the stratosphere as measured by Equivalent Effective Stratospheric Chlorine – EESC maximised in the second half of the 1990s. We therefore examine the trends in the periods before and after that peak to see if any change in trend is discernible in the ozone record. Prior to 1998, trends in the upper stratosphere ~ 45 km, 4 hPa are found to be −5 to −10% per decade at mid-latitudes and closer to −5% per decade in the tropics. No trends are found in the mid-stratosphere 28 km, 30 hPa. Negative trends are seen in the lower stratosphere at mid-latitudes in both hemispheres and in the deep tropics. However it is hard to be categorical about the trends in the lower stratosphere for three reasons: i there are fewer measurements, ii the data quality is poorer, and iii the measurements in the 1990s are perturbed by aerosols from the Mt. Pinatubo eruption in 1991. These findings are similar to those reported previously even though the measurements for the two main satellite instruments SBUV and SAGE II and the ground-based Umkehr and ozonesonde stations have been revised. There is no sign of a continued negative trend in the upper stratosphere since 1998: instead there is a hint of an average positive trend of ~ 2% per decade in mid-latitudes and ~ 3% per decade in the tropics. The significance of these upward trends is investigated using different assumptions of the independence of the trend estimates found from different datasets. The averaged upward trends are significant if the trends derived from various datasets are assumed to be independent, but are generally not significant if the trends are not independent. This arises because many of the underlying measurement records are used in more than one merged dataset. At this point it is not possible to say which assumption is best. Including an estimate of the drift of the overall ozone observing system decreases the significance of the trends. The significance will become clearer as i more years are added to the observational record, ii further improvements are made to the historic ozone record e.g. through algorithm development, and iii the data merging techniques are refined, particularly through a more rigorous treatment of uncertainties.

Author: N. R. P. Harris - B. Hassler - F. Tummon - G. E. Bodeker - D. Hubert - I. Petropavlovskikh - W. Steinbrecht - J. Anderson - P. K.



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