Influence of stratosphere-troposphere exchange on tropospheric ozone over the tropical Indian Ocean during the winter monsoon

Ozone (O-3) 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), O-3 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 O-3 levels were observed in the upper troposphere. These were associated with deep convection in regions where MBL O-3 levels were also low. In the midtroposphere (300-500 hPa), O-3 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 O-3 mixing ratios (up to 120 ppbv) were observed just below the tropical tropopause (between 100 and 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 subtropical jet by either shear-induced differential advection or clear-air turbulence (CAT) caused the midtropospheric 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 midtroposphere intrusions (typically 3-5 km) and the very high O-3 mixing ratios of the upper tropospheric laminae, it seems that STE plays an important role in the tropical tropospheric O-3 budget, at least over the Indian Ocean during the winter monsoon.