The Phenomenon Of The Tropopause Changes In Northern Hemisphere Summer And Mechanism Study

Using the monthly mean data from NCEP/NCAR reanalysis, we have investigated the summertime tropopause variations. The results are summarized as follows:(l)Both the tropopause pressure and temperature increase poleward, in general, in both the northern and southern hemisphere during boreal summer, except in latitudes higher than 45° S where both the tropopause pressure and temperature decrease poleward. Both the air pressure and temperature at tropopause distribute uniformly in tropics rather than in extratropics. Both the global mean tropopause pressure and temperature have been decreasing since 1979, indicating that our tropopause has been moving higher and higher. The interannual/interdecadal variabilities of tropopause reach their maximum in both the middle latitudes in northern hemisphere and the extratropical and polar regions in the southern hemisphere, whereas the tropopause variabilities reach their minimum in tropics. The dominant periodicities as detected in the index of either pressure or temperature at tropopause are the 11 year and 3-5 years. The variations of tropopause have been found well correlated with ENSO variabilities, volcanic activities and some other meteorological factors in troposphere. The prominently positive correlation between the tropopause pressure and temperature is observed almost all the world.(2)Over Tibetan plateau, the variations of tropopause pressure have been found well correlated with respectively the anomalous temperature and geopotential height in both troposphere and stratosphere. The anomalous tropopause pressure correlates well with the anomalous skin temperature in Tibetan plateau. The vertically integrated heat budget anomalies are responsible for explaining the pressure anomalies over Tibetan plateau. Over Tibetan plateau, the correlation of tropopause pressure and OLR is found positive. In 1992, the values of OLR are anomalously larger; the convection is restrained, which results in that the tropopause pressure (altitude) is anomalously higher (lower). Whereas in 1998, the values of OLR are anomalously smaller; the convection is intensified, which results in that the tropopause pressure (altitude) is anomalously lower (higher). The South Asia High weakens so as to tropopause pressure over Tibetan plateau increases in 1992. However, the South Asia High intensifies which results in that the tropopause pressure over Tibetan plateau decreases in 1998.(3)In Tibetan plateau region, an anomalous cyclonic circulation is observed in theupper troposphere when an anticyclone circulation is found in the lower troposphere during the summertime of 1992. These circulation anomalies lead to decent of the air in the troposphere, causing the tropopause lower than normal. The opposite scenarios are observed in summer of 1998.(4) A remarkable interdecadal catastrophe in temperature at tropical tropopause had taken place around year of 1978. The temperature at tropical tropopause decreases during both the cold period and warm period. However, the trend is found to be 0.67 ℃/10a from 1958 to through to 2000. The periods as detected in temperature at tropical tropopause during cold period are dominantly the quasi-three years while that during warm period are dominantly the 10-12 years. The pattern of standard errors of tropical tropopause temperature shows great differences between the cold and warm periods.(5) The whole tropical tropopause has been warmed up from cold period to warm period. The differences of tropical tropopause temperature between these two periods are intrinsically related to the anomalous variations of air temperature and the geopotential height in the troposphere. The decrease (increase) of the tropical tropopause temperature during cold (warm) period can be explained by the vertically integrated heat budget anomalies. The tropical tropopause temperature decreases in both cold and warm periods, which possibly relates to the increase of the tropical SST in the following falls. Whether the tropical tropopause temperature is high or low during cold period can be used...