Abstract. The tropical western Pacific (TWP) plays an important role in global stratosphere–troposphere exchange and is an active region of the interhemispheric transport (IHT). Common indicators for transport between the hemispheres like the tropical rain belt are too broad or lack precision in the TWP. In this paper, we provide a method to determine the atmospheric chemical equator (CE), which is a boundary for air mass transport between the two hemispheres in the tropics. This method used the model output from an artificial passive tracer simulated by the chemical transport model GEOS-Chem in the troposphere. We investigated the movement of the CE in the tropics, which indicates the migration of atmospheric circulation systems and air mass origins. Our results show the CE on different timescales, suggesting that the different features of the IHT in different regions are highly related to the variation in the circulation systems. We compared the CE with the tropical wind fields, indicating that the region of IHT does not coincide with the convergence of the 10 m wind fields in the tropical land sectors and the TWP region. We compared the CE with the atmospheric composition such as satellite data of CH4 and model simulation of sulfur hexafluoride (SF6). The results show that the CE and north–south gradient of CH4 in the Indian Ocean in January are well consistent with each other, which indicates the CE has good potential to estimate the IHT inferred by observations. We discussed the vertical extent and the meridional extent of the IHT. We find that the vertical structure above 2 km has a slight northern tilt in the Northern Hemisphere (NH) winter season and a southern tilt in the NH summer, meaning the seasonality of the migration of the CE at the lower altitude is larger than that at the higher altitude. The meridional extent of the CE indicates a narrow transition zone where IHT happens throughout the year. We find that the meridional extent above South America is larger compared to other regions. The distribution of the land–sea contrast plays an important role in the meridional extent of IHT. We focus on the TWP region and further compared the tropical rain belt with the CE. There is a broad region of high precipitation occurring in the TWP region, and it is difficult to determine the IHT by the rain belt. In the NH winter, the CE is not consistent with the tropical rain belt in the TWP but is confined to the southern branch of the peak of the rain belt. For the other seasons, both indicators of IHT agree.

<p>A Compact Cloud and Aerosol LIDAR (ComCAL) is operated in Koror, Palau (7.34°N, 134.47°E) since 2018. Palau is located in the Pacific warm pool, which plays an important role in global stratosphere-troposphere exchange in the upper troposphere and the lower stratosphere (UTLS). ComCAL is operated during nighttime, carried out observations of atmospheric profiles of aerosols and clouds, and the lidar profile extends from 8 km to 30 km. Cirrus clouds were detected with very high occurrence in the upper troposphere (above 12 km). The subvisible clouds (with an optical thickness of less than 0.3) often occur in the higher region of the tropical tropopause layer (TTL) above about 16 km which is close to the cold point. The transport of air in this layer with thin cirrus and subvisible clouds was investigated by the TRACZILLA Lagrangian model, a variation of FLEXPART. The back-trajectory analysis gives insight into the origins of cirrus clouds in the TTL whether it is related to the convection or the in situ uplifting of the air masses.</p>

Abstract. The Tropical Western Pacific (TWP) is an active interhemispheric transport region that contributes significantly to global stratosphere-troposphere exchange. We developed a method called Chemical Equator based on model simulations of a virtual passive tracer to analyze atmospheric transport in the tropics, with a focus on the TWP region. We compare the chemical equator to common indicators of transportation such as tropical rain belts and wind fields. We obtained a vertical pattern of interhemispheric transport processes from the model's three-dimensional output.

<p>The Western Pacific Region has some of the highest sea surface temperatures in the world, described as the Tropical Warm Pool (TWP). It plays a major role in the troposphere-stratosphere exchange and, the chemical composition in the TWP will greatly affect that in the Tropical Tropopause Layer (TTL) and therefore the stratosphere. The FTIR station in Koror, Palau (7.5°N, 134°E) is the only FTIR site in the Warm Pool, which was installed as part of the EU-project StratoClim in 2016. The FTIR station in Paramaribo, Suriname (5.8°N, 55°W) was established as part of the EU-program STAR in 2004. The measurement site in Burgos, Philippines (18.5°N, 120.65°E) (Velazco et al., 2017a) just beside the Warm Pool was installed in 2016. Our analysis of FTIR methane measurements at Palau from 11/2018 – 06/2021 and at Suriname from 01/2017 – 05/2021 with the GEOS-Chem model simulations give some insights into transport processes and the origin of air mass in the TWP. The NDACC retrieved CH<sub>4</sub> has good sensitivity to the troposphere and stratosphere. Tropospheric and stratospheric X<sub>CH4</sub> are analyzed separately based on the FTIR measurements. Simulations of CH<sub>4</sub> from the GEOS-Chem model are used to be compared with the measurements from two tropical sites. The position of the Chemical Equator (Hamilton et al., 2008) calculated from the GEOS-Chem model simulations and FLEXPART are used to investigate the seasonal variations of the CH<sub>4</sub> measurements from FTIR.</p>

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