Tropical tropospheric ozone (TTO) maps from Nimbus 7 and Earth Probe TOMS by the modified-residual method: Evaluation with sondes, ENSO signals, and trends from Atlantic regional time series

The modified-residual (MR) method for retrieving time-averaged stratospheric ozone and tropospheric ozone column amounts from the Total Ozone Mapping Spectrometer (TOMS) is applied to the 14 complete calendar years of Nimbus 7 observations (1979-1992). These are available as digital data at http://metosrv2.umd.edu/ similar to tropo/14y_data.d. The MR method has also been used to produce real-time maps of tropical tropospheric ozone (TTTO) from TOMS on the Earth-Probe (1996-present) and ADEOS platforms (1996-1997). Evaluation of the TTO time series for 1979-1990 and 1997-1995 is presented here; it is limited to the few tropical ozonesonde stations operational during those years (Ascension Island; Natal, Brazil; Brazzaville). The standard deviation of the differences between TTO and the sondes is +/-(6-7) Dobson units (DU), depending on location. Stratospheric column ozone, which is also derived by the modified-residual method, compares favorably with sondes (to within 6-9 DU) and with stratospheric ozone inferred from other satellites (usually 8-15 DU lower than the latter). TTO time series and the magnitude of the tropospheric wave-one pattern show El Nino-Southern Oscillation (ENSO) signals during the period from 1979-1992. During 1997 the ENSO stands out at some stations, but not at others. Between 12 degrees N and 12 degrees S, zonally averaged TTO shows no significant trend from 1980-1990. Trends are also not significant during this period in localized regions, for example, from just west of South America across to southern Africa. This is consistent with the ozonesonde record at Natal, Brazil (the only tropical ozone data publicly available for the 1980s), which shows no significant trend. The lack of trend in tropospheric ozone agrees with a statistical analysis based on another method for deriving TTO from TOMS, the convective-cloud-differential approach of Ziemke er al. [1998].