Transport in the tropical and subtropical lower stratosphere: Insights from in situ measurements of chemical tracers.

摘要

Understanding the transport pathways and mechanisms that control stratospheric humidity is essential, because of the significant role water plays in the chemical and radiative balance of the stratosphere. These pathways can be elucidated by analyzing the chemical composition of air sampled by aircraft instrumentation. The operation, detection technique, calibration procedure, and performance of the 2-inch Harvard Water Vapor instrument are examined and proven to be suitable for these transport studies.; Aircraft measurements of chemical tracers, Empirical Orthogonal Functions and diabatic backward trajectory calculations are used to identify transport pathways into the subtropical lowermost stratosphere during the CRYSTAL-FACE (July 2002) campaign. High-latitude, convective, and tropical air masses are identified in the chemical composition of subtropical air above the local tropopause. Therefore, equatorward, convective, and poleward transport bring air to the subtropical lowermost stratosphere. The equatorward transport is associated with monsoon circulation over North America. Chemical evidence of the poleward and equatorward branches of the monsoon circulation in the lowermost stratosphere is presented using O3-H 2O correlations. These results suggest that the direction of the large-scale meridional transport in the lowermost stratosphere during the summer is strongly longitude-dependent.; Aircraft measurements of chemical tracers and relative humidity during the CWVCS (August 2001) and Pre-AVE (January 2004) campaigns along with tropical radiosonde data are used to examine mechanisms responsible for setting the observed stratospheric humidity over the Eastern Tropical Pacific. Neither local tropopause temperatures nor the Western Tropical Pacific, i.e., the stratospheric fountain, are found to control the entry value of water vapor into the stratosphere. Summer and winter data show frequent unsaturation and occasional supersaturation with respect to local temperature conditions, and cloud-free air at the tropical tropopause. Horizontal advection through cold pool regions within the TTL is thus hypothesized to be the mechanism controlling stratospheric humidity. Convection is found to hydrate the tropical tropopause layer (TTL) during the winter and to desiccate this layer and hydrate the lower stratosphere during the summer. While complex, identifying the transport mechanism is critical to predicting the response of the atmosphere to thermal and chemical changes in boundary conditions at the tropical tropopause resulting from climate forcing by infrared active species.