Tropospheric ozone above the southwestern Indian Ocean is strongly linked to dynamical conditions prevailing in the tropics

Abstract

A large ozone profile data set obtained at Réunion Island (21° S, 55° E) since 1992 is presented and linked to observed dynamical conditions in the region. The average seasonal variation of column integrated tropospheric ozone reaches 25 Dobson Units and peaks at 55 Dobson Units during the month of October. Two thermal inversions and high values of ozone mixing ratio during biomass burning activity in southeastern Africa are observed. The marine boundary layer (MBL) is characterized by high relative humidity values and low ozone mixing ratios. The decrease in the water vapor content above 2 km is strongly linked to seasonally evolving dynamical conditions. The first inversion largely hinders vertical exchanges, and low water vapor values in the free troposphere favor the photochemical production of ozone. The second thermal inversion is linked to the zonal wind inversion. Above this second inversion a large ozone enhancement is observed that occurs in parallel the biomass-burning season in southeastern Africa and Madagascar. The season extents from August to December and peaks in October. The ozone maximum concomitantly results from the transport of ozone and its precursors westward from the biomass-burning zones. High ozone values recorded in the upper troposphere during austral winter are likely to originate from additional stratosphere - troposphere exchanges. Winter anticyclonic conditions further favor the photochemical production of ozone. The low tropospheric ozone mixing ratios observed across the entire troposphere during austral summer could result from the heterogeneous destruction of ozone and from less favorable conditions for its photochemical production, because of the development of deep convection. Copyright 1999 by the American Geophysical Union.