Dynamical and Chemical Coupling of the Summer Monsoons and the Upper Troposphere-Lower Stratosphere

The upper troposphere-lower stratosphere (UTLS) is a transition region between the troposphere and the stratosphere. During the boreal summer, the UTLS is dominated by large-scale anticyclonic circulations over the Asian and North American monsoon regions, exhibiting complex dynamical and chemical characteristics. Re-cent studies have emphasized the important role of the summer monsoon systemin stratosphere-troposphere exchange of water vapor and chemical species, which strongly influences the atmospheric chemistry and climate system. The transport in the UTLS region occurs in both directions, stratosphere-troposphere transport (STT)and troposphere-stratosphere transport (TST). For example, observational studies indicate localized maxima of tropospheric pollutants and stratospheric water vapor(SWV) in the UTLS, which are controlled by deep convection and large-scale circulation. Meanwhile, stratospheric ozone (O3) can fold into tropospheric air and entrain into the planetary boundary layer (PBL) via deep STT, and thus affect air quality at the surface. In this thesis, we aim at improving the understanding of the transport processes in the UTLS that are linked to monsoon dynamics using observations and modelling tools. First, we investigate the TST transport in association with the Asian summer monsoon. We examine the simulation of SWV in the Community Earth System Model, version 1 with the Whole Atmosphere Community Climate Model as its atmospheric component [CESM1(WACCM)]. CESM1(WACCM) generally tends to simulate a SWV maximum over the central Pacific Ocean instead of over the Asian continent as observed, but this bias is largely improved in the high vertical resolution version. The high vertical resolution model with increased vertical layers in the UTLS is found to have a less stratified UTLS over the central Pacific Ocean compared with the low vertical resolution model. It therefore simulates a steepened potential vorticity gradient over the central Pacific Ocean that better closes the upper-level anticyclone and confines the SWV within the enhanced transport barrier. We further study the transport pathways connecting the Northern Hemisphere sur-face and the North American (NA) UTLS by diagnosing Boundary Impulse Response idealized tracers implemented at the Northern Hemisphere surface during summer. In ensemble average, air masses enter the NA UTLS region above Central America, and then slowly mix into the higher latitudes. However, fast transport pathways with modal age around two weeks are evident in some tracer ensembles. For these rapid transport pathways, the tracers first reach the UTLS region over the eastern Pacific and the Gulf of Mexico as a result of enhanced deep convection and vertical advection, followed by horizontal transport over the United States by a strengthened UTLS anticyclone circulation. To the end, we evaluate the downward transport of stratospheric O3 via STT using simulation from a state-of-the-art chemistry climate model implemented with an artificial stratospheric ozone tracer (O3S). We find that O3 transported from the stratosphere makes a significant contribution to the surface O3 variability where back-ground surface O3exceeds 95th percentile, especially over the western U.S. Maximum covariance analysis is applied to O3 anomalies paired with stratospheric O3traceranomalies to identify the stratospheric intrusion and the underlying dynamical mechanism. The first leading mode corresponds to deep stratospheric intrusions in the western and northern tier of the U.S., and intensified north easterlies in the mid-to-lower troposphere along the west coast, which also facilitate the transport to the eastern Pacific Ocean. The second leading mode corresponds to deep intrusions over the Intermountain Regions. Both modes are associated with eastward propagating baroclinic systems, which are amplified near the end of the North Pacific storm tracks, leading to strong descents over the western United States