Cooling trend of the tropical cold point tropopause temperatures and its implications

Operational sounding data (1973-1998) were used to determine cold point tropopause (CPT) characteristics. A cooling trend (-0.57±0.06 K/Decade during 1973-1998) in tropical CPT temperatures has been found, which is opposite to what has been hypothesized to explain the trend in stratospheric water vapor. Given this trend, the annual averages of the CPT saturation mixing ratios (SMRs) inferred from the analysis of the 1994-1997 data in the work of Dessler [1998] are substantially smaller than those from data before the mid-1990s. This implies that while Newell and Gould-Stewart's "stratospheric fountain" might not have been necessary to explain stratospheric water vapor during the mid-1990s, it was necessary in most years before that. Changes in tropical convection occurrence frequency and / or strength are suggested to explain the cooling trend in the CPT temperatures. It is suggested that the observed positive trend in stratospheric water vapor is probably due to changes in the residual circulation.