Search

There has been increasing recognition of the role of the tropical tropopause layer (TTL) in determining stratospheric water vapor concentrations—the TTL being a layer of transition between air showing tropospheric properties below and stratospheric properties above. This study investigates the spatial structure of temperatures in the TTL. A dehydration index based on the atmospheric region with temperatures colder than a specific reference temperature was defined to examine the TTL temperature structure and possible influences on stratospheric water vapor. The results indicate that dehydration regions with cold temperatures (e.g.

Clouds cool the climate system by reflecting shortwave radiation and warm it by increasing the atmospheric greenhouse. Previous studies have shown that in tropical regions of deep convection there is a near cancellation between cloud-induced shortwave cooling and longwave warming. The present study investigates the possible influence of the 1998 El Nino upon this near cancellation for the tropical western Pacific's warm pool; this was accomplished by employing satellite radiometric measurements (Earth Radiation Budget Experiment, and Clouds and the Earth's Radiant Energy System). With the exclusion of the 1998 El Nino, this study also finds near cancellation between the shortwave and longwave cloud forcings and demonstrates that it refers to the average of different cloud types rather than being indicative of a single cloud type. The shortwave cooling slightly dominates the longwave warming, and there is considerable interannual variability in this modest dominance that appears attributable to interannual variability of tropopause temperature. For the strong 1998 El Nino, however, there is a substantially greater tendency toward net radiative cooling, and the physical mechanism for this appears to be a change in cloud vertical structure. For normal years, as well as for the weaker 1987 El Nino, high clouds dominate the radiation budget over the warm pool. In 1998, however, the measurements indicate the radiation budget is partially governed by middle-level clouds, thus explaining the net cooling over the warm pool during the 1998 El Nino as well as emphasizing differences between this event and the weaker 1987 El Nino.

Tropical cold point tropopause (CPT) characteristics have been calculated using ECMWF reanalyses and high resolution radiosonde soundings obtained in TOGA COARE as well as operational sounding data. It is found that ECMWF reanalyses are suitable for investigating the morphology of the tropical CPT and the variabilities of the tropopause over the entire Tropics, despite an almost constant one-side warm bias. The daily locations of the tropical coldest CPT are clustered over the western Pacific warm pool region in January and spread out longitudinally as the year progresses. During the Indian summer monsoon, the locations of the coldest CPT are deflected northward. The influences of the quasi-biennial oscillation (QBO) and the El Nino–Southern Oscillation (ENSO) on the tropical CPT have been separated using bivariate regression. The stratospheric zonal wind shear at 50 mb leads the variation in the tropical CPT temperatures by about 6 months. The QBO signature in the tropical CPT is mainly zonally ...

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.