- Alfred-Wegener-Institute, Helmholtz Center for Polar and Marine Research, Potsdam, Germany
The Upper Troposphere/Lower Stratosphere (UT/LS) above the Tropical West Pacific (TWP) is an important pathway into the global stratosphere, influencing stratospheric chemistry and atmospheric dynamics overall. Its composition and dynamics are subject to both seasonal and inter-annual variations like El Niño Southern Oscillation (ENSO). We have studied the effects of ENSO on the composition and transport dynamics of the UT/LS using the variables ozone and relative humidity over ice (RHi). The analysis is based on measurements, made at the Palau Atmospheric Observatory (PAO) from 2016 until 2024 using ECC ozone sondes with Vaisala RS92/RS41 radiosondes (Müller et al., 2024a). The PAO is located on the island-state of Palau (7.3° N, 134.5° E); in the tropical warm pool and is part of the Southern Hemisphere ADditional OZone (SHADOZ) network.
We found that the UT/LS (14-18.5 km) tends to be drier and more ozone-rich during El Niño, compared to La Niña. During El Niño the ascending branch of the Walker Circulation is weakened, resulting in the suppression of local convection and thus less uplift of humid, ozone poor air-masses and accumulation of ozone in the UT/LS. Using Lagrangian-Backtrajectories, calculated with the Lagrangian Chemistry and Transport Model ATLAS (Wohltmann et al., 2009; Wohltmann et al., 2019) we further identified where the air-parcels were last mixed in a convective cell on their way to Palau. We found that under El Niño conditions, during winter and spring the distribution for this point of last mixing shifts eastward with the ascending branch of the Walker Circulation. This is in contrast to summer and autumn, where the point of last mixing distribution shifts northward, with the transport path taking an anticyclonic route from Asia to Palau, indicating the dominant influence of the Asian-Summer-Monsoon (ASM) over the typical El Niño pattern.
We conclude that ENSO is an important mode of variability for the composition and transport dynamics of the UT/LS. However, the seasonal differences for El Niño conditions highlight the importance of the ASM in this region during summer and autumn.
How to cite: Röpke, T., Müller, K., Wohltmann, I., and Rex, M.: How ENSO affects ozone, RHi and transport dynamics at the UTLS above Palau and the Tropical West Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13178, https://doi.org/10.5194/egusphere-egu25-13178, 2025.
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