Royal Netherlands Meteorological Institute
Section of Atmospheric Composition
Ozone and relative humidity (RH) soundings, launched over the Indian Ocean during the 1998 winter monsoon (February-March), were analyzed. In the marine boundary layer (MBL), ozone mixing ratios were relatively low (10-20 ppbv) except close to the Indian subcontinent (40-50 ppbv) where profiles were strongly influenced by pollution. Sometimes, relatively low ozone levels were observed in the upper troposphere. These were associated with deep convection in regions where MBL ozone levels were also low. In the mid-troposphere (300-500 hPa), ozone maxima (60-90 ppbv) were often found with low RH. A remarkable new finding of this study is that in more than a third of the profiles, laminae with very high ozone mixing ratios (up to 120 ppbv) were observed just below the tropical tropopause (between 100-200 hPa). Back trajectory analyses showed that these layers originated in the vicinity of the subtropical jet stream (STJ). We hypothesize that stratosphere-troposphere exchange (STE) near the STJ by either shear-induced differential advection or clear-air turbulence (CAT) caused the mid-tropospheric maxima (STE followed by descent) and the upper tropospheric laminae. Another new finding is that stratospheric intrusions were not only found near the STJ but also deep within the tropics. Given the thickness of the mid-tropospheric intrusions (typically 3-5 km) and the very high ozone mixing ratios of the upper tropospheric laminae, it seems that STE plays an important role in the tropical tropospheric ozone budget, at least over the Indian Ocean during the winter monsoon.
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