The tropical tropopause over the western Pacific: Wave driving, convection, and the annual cycle

The annual cycles in height and temperature of the tropopause over the tropical western Pacific island of Truk (7.5 degrees N, 151.8 degrees E) are derived for the period 1980-1988 from daily radiosonde profiles. Earlier work has shown that tropopause properties are fairly uniform over large areas of the tropics, so that conclusions drawn from the Truk observations can be taken as representative of the western Pacific warm pool region, and can be applied with caution to wider regions of the tropics. The relative roles of tropical convection and extratropical wave driving in determining the tropopause properties are discussed, and it is suggested that the chief role of wave driving is to lower the temperature in the uppermost part of the troposphere during the northern winter months, causing a decrease in local stability, and allowing penetrative overshooting of convective turrets to take place more readily than during the rest of the year. As the winter progresses, the overshooting convective turrets create widespread cirrus anvils, incorporating stratospheric air with a high potential temperature, and establishing a new higher tropopause. Anomalously high tropopause potential temperatures during the northern spring months are tentatively attributed to the combined wave-driving action of the two hemispheres, as wave activity increases with the approach of winter in the southern hemisphere, but still remains at a high level during northern spring. The day-to-day variability in tropopause height and temperature is discussed, and is shown to have implications for troposphere-stratosphere exchange. The use of monthly mean tropopause temperatures to calculate the saturation vapor pressure of water entering the stratosphere may give misleading results, since injection is likely to take place primarily in association with the transient rises in tropopause height and decreases in temperature that occur more or less randomly from day to day, while the monthly means also include the drops in height and increases in temperature that do not represent injections into the stratosphere.