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4. Summary

This paper presents a climatological depiction of the tropical tropopause based on radiosonde data from 83 stations covering the period 1961-1990. Our main findings are:

1. The location of the tropopause depends on the level by which it is defined. The climatological (1961-90) annual-mean zonal-mean LRT height varies from ~16.5 km in the equatorial zone to less than 16 km in the subtropics. The CPT resides at ~16.9 km, with very little north-south variability. The annual-mean zonal-mean pressure of the LRT is close to 100 hPa in the equatorial region, but higher in the subtropics. The CPT is generally at lower pressure, with an annual-mean zonal-mean location of about 96 hPa. The CPT is slightly colder than the LRT. In the equatorial zone, the annual-mean zonal-mean CPT temperature is about 81 deg. C, with corresponding water vapor saturation volume mixing ratio of 4 to 6 ppmv.

2. The tropopause is higher, colder, at lower pressure, and with lower saturation mixing ratio, in the Northern Hemisphere than in the Southern Hemisphere particularly in NH winter. The longitudes of minimum temperature (and saturation mixing ratio) differ from the longitudes of maximum height and minimum pressure. The tropopause is highest and at lowest pressure in the region over northern South America and east of Central America during NH winter, and over Africa during other months. However, it is coldest over the western tropical Pacific most of the year.

3. The 100 hPa level is a poor surrogate for the tropical tropopause. The 100 hPa height shows significantly less spatial and temporal variability than the heights of the LRT and CPT. In particular, the 100 hPa level lacks the seasonal variations of the tropopause and resides in the stratosphere during NH summer and in the troposphere during NH winter. Therefore, it is warmer in the mean than the LRT and CPT.

4. Water vapor saturation volume mixing ratios at the CPT and LRT are very close to the minimum values for a given profile (qsmin). However, values at the 100 hPa level are 10-25% higher.

5. Although not discussed above, comparison of our results with tropopause climatologies based on analyses or reanalysis [Highwood and Hoskins, 1998; Hoinka, 1998, 1999] suggests that the analyses depict the tropopause at lower elevation, and with less spatial structure, than the radiosonde data. This is probably due to the relatively poor vertical resolution of the analyses near the tropopause level. Comparison with an earlier climatology based on radiosonde data [Makhover, 1979] shows better agreement.

The dataset presented here is also amenable to studies of the interannual and longer-term variability of the tropical tropopause [Randel et al., 2000, Gaffen et al., submitted]. We hope it will also be useful in other investigations, such as comparisons with model simulations and process studies. We intend to make the data available to the scientific community in the near future.