Troposphere to stratosphere transport at low latitudes as studied using HALOE observations of water vapour 1992-1997

In this paper we present observations of water vapour (version 18 retrievals) in the equatorial tropopause region made by the Halogen Occultation Experiment (HALOE) instrument, which flies on the Upper Atmosphere Research Satellite. These data make an important new contribution to the observational knowledge of this region, since water vapour here has largely been observed to date using in situ instruments, and the only global longitudinally-resolving satellite observations to be reported are those from the Stratospheric Aerosol and Gas Experiment Il (SAGE II). Firstly, the HALOE water vapour climatology is presented. Comparison of HALOE and SAGE II climatologies shows considerable agreement. An exception is over Asia in June, July and August (JJA), where the HALOE fields show a distinct region of moist air (associated with convection due to the Asian summer monsoon), whereas no such feature was reported in the SAGE II observations. This suggests that HALOE is sensitive to water vapour variability near the tropopause over regions of strong convection. Secondly, the troposphere to stratosphere transport of air at low latitudes is investigated. Pressure/longitude sections near the equator show that dry air with water vapour mixing ratios of less than 2.4 parts per million by volume (p.p.m.v.) enters the stratosphere in December, January and February, then travels upwards and polewards in the subsequent seasons. A more detailed examination of pressure/longitude sections indicates episodes when dry air also enters the equatorial stratosphere in March, April and May and JJA at times of strong convection. There is also evidence of entry of air with higher water vapour mixing ratios, of between 3.6 and 4.0 p.p.m.v, into the stratosphere in September, October and November, chiefly at 10 degrees N and 20 degrees N. Further examination of the region of moist air over Asia in JJA (mentioned above) shows that there is clear cross-isentropic upward transport in the 375-410 K region of unsaturated air. The HALOE water vapour data set reported here provides an important new insight into low latitude processes near the equatorial tropopause.