The Atmospheric Transport in the Western Pacific Region by Measurements and Model Simulations

Abstract

The major pathway for air entering the stratosphere is over the Tropical Western Pacific (TWP) region, and this key region influences the atmospheric composition in the stratosphere. Motivated by this, we used trace gas measurements by using the Fourier Transform Infrared (FTIR) Spectrometer and cirrus cloud measurements by using a ground-based COMpact Cloud Aerosol Lidar, COMCAL from the atmospheric observatory at Koror, Palau (7.34°N, 134.47°E}, in the heart of the Pacific warm pool) and combined model simulations to study the transport pathways, with a special focus on the stratosphere-troposphere exchange (STE) processes over this key region.

The atmospheric transport dynamics in the TWP region are closely linked to the movements of the circulation system, particularly the Inter-Tropical Convergence Zone (ITCZ) associated with the up-welling branch of the Hadley cell. Given the limitations of traditional ITCZ indicators, such as the maximum tropical rain belt to determine the air mass origins, I have developed a tool termed the Chemical Equator (CE), modified from Hamilton et al., (2008) to study the Inter-hemispheric Transport (IHT). The CE is calculated by the model simulation of an artificial passive tracer by GEOS-Chem to discern the migration patterns of circulation systems and air mass origins. Subsequently, the CE was used to characterize tropospheric carbon monoxide (CO) and ozone (O3) column measurements using the FTIR Spectrometer and the ozone sondes, respectively. The observed low CO and O3 during summer and early autumn, contrasting with maxima in winter and early spring, were outlined by the seasonal meridian movement of the CE. Additionally, comparisons were made between CE and commonly used IHT indicators, such as satellite measurements of methane (CH4) and CO, and model simulations of sulfur hexafluoride (SF6). Particularly, the position of CE demonstrated agreement with the meridional gradient boundary of those trace gases. Consequently, the impact of IHT on the seasonal variation of the trace gases in the tropospheric TWP region suggests that CE holds the potential to differentiate diverse air mass origins influenced by large-scale atmospheric circulation.

Upper-air observations targeting the Upper Troposphere and Lower Stratosphere (UTLS) were performed to detect cirrus cloud layers using Lidar, COMCAL. The annual cycle shows that cloud layer height peaks with the highest Cold Point Tropopause (CPT) in winter and reaches its minimum with the lowest CPT in summer. In comparison with similar cirrus cloud measurements obtained in other tropical sites, our measurements reveal that cirrus clouds detected over TWP are the coldest and highest. The prevalence of the coldest cirrus cloud layer detected over Palau corresponds to the cold trap, a region of exceptionally cold air, in UTLS over the TWP region. In order to build the relationship between STE in the UTLS region and measurements, we conducted trajectory analysis by Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model simulations based on cirrus cloud layer measurements. Our observation results reveal that only in winter with high supersaturation at the measured cirrus clouds, the air masses are further dehydrated and slowly ascend into the stratosphere.

Conclusively, we present an atmospheric transport scheme over the TWP region based on horizontal IHT and vertical STE processes and provide observational and model simulation support for it. In the lower heights, from the surface to the free troposphere, the transport and air mass origins are characterized by the meridian movement of the CE. In the UTLS region, measurements of cloud layers and trajectories validate the pathways of STE. During summer, pristine air from the Pacific Ocean reaches Palau, with oceanic short-lived species injected into the stratosphere through rare and the highest overshooting tops. Conversely, Southeast Asia dominates air mass origins over the TWP region in winter, transporting a high level of anthropogenic species, such as O3 and CO, into the stratosphere via the pathway within the cold trap. This winter-specific cold trap pathway, seasonally persisting over Palau, plays a crucial role in altering the stratospheric atmosphere through the transport of troposphere-originate air masses.