On the occurrence of the coldest region in the stratosphere and tropical tropopause stability: A study using COSMIC/FORMOSAT-3 satellite measurements

The occurrence of coldest region in the lower and middle stratosphere has been investigated using COSMIC/FORMASAT-3 radio occultation measurements. Observations from January 2007 to December 2011, comprising of 2,871,811 numbers of occultations uniformly spread over land and sea, have been used in this study. Using vertical profiles of temperature upto 40 km altitude, zonally averaged at each 5° latitude band between 90°N and 90°S, it is shown that the coldest region in the upper atmosphere occurs during winter in high latitude stratosphere (latitudes >45°) in both the hemispheres with southern hemisphere (temperature less than <−85 °C) cooler than northern hemisphere (temperature ~−75 °C). The spatial extent of the region of low temperature region found between 10 km and 30 km altitude, indicating a 20 km vertical thick layer of cold temperature. In the southern hemisphere, such a region of coldest temperature remains for more than six months (April-October), in the Northern hemispheric polar region (~−75 °C) it is seen mostly during four winter months between October and January. Using NCEP-DOE reanalysis data, we show that cold temperature in the stratospheric region coexists with the jet streams prevalent in those regions. Strong wind jet is surmised to make stratosphere colder. The absence of sunlight in the coldest region is known to cause jet streams. Impact of stratospheric quasi-biennial oscillation (QBO) on the sharpness of tropical tropopause (stability) has also been investigated. Observations suggest that during westerly phase of QBO, the stability of the tropopause increases.