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 th e 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 co nvection in regions where MBL O-3 levels were also low. In the midtroposphe re (300-500 hPa), O-3 maxima (60-90 ppbv) were often found with low RH. A r emarkable new finding of this study is that in more than a third of the pro files, laminae with very high O-3 mixing ratios (up to 120 ppbv) were obser ved just below the tropical tropopause (between 100 and 200 hPa). Back traj ectory 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 differentia l 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 b ut 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 up per 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.