ENSO impact on mid-tropospheric composition above the Tropical Western Pacific via air mass transport

The Tropical West Pacific (TWP) is the key entry point of air into the stratosphere during Northern Hemispheric winter. Thus, the local air composition can influence global atmospheric chemistry and dynamics. Its interannual variability is itself affected by the El Niño Southern Oscillation (ENSO). The transport history of tropospheric air masses above the TWP is in particular reflected by the local ozone (O3) and relative humidity (RH) characteristics (Müller et al., 2024b).

We use regular balloon-borne profile measurements of these quantities from the Palau Atmospheric Observatory (PAO) to assess the (interannual) variability of TWP air masses and controlling processes (Müller et al., 2024a). Located in the centre of the tropical warm pool (7°N, 134°E) in Koror, Palau, the PAO has been filling a previous observational gap in this region since 2016 and has recently become a member of the SHADOZ (Southern Hemisphere Additional Ozonesondes) network. Our latest analysis of the PAO ozonesonde record showed how transport to the TWP mid-troposphere (5-10 km altitude) is modulated by the movement of the Intertropical Convergence Zone, allowing transport of polluted air masses from tropical Asia mainly between February and April into the otherwise clean air column (Müller et al. 2024b, Sun et al. 2023).

Here, we present an extended view on air mass transport to the region with a focus on the impact of ENSO on its interannual variability using an updated PAO time series (2016-2024), interhemispheric transport modelling using GEOS-Chem (Sun et al. 2023) and trajectory calculations from the Lagrangian Chemistry and transport model ATLAS (Wohltmann and Rex 2009). We found that very humid and ozone-poor air masses are suppressed in the free troposphere during El Niño. For La Niña conditions, the O3/RH distribution is shifted towards higher RH indicating enhanced convection compared to neutral conditions, but seasonal observations of dry ozone-rich air masses and long-range transport from Asia still occur.

The high convective activity in the TWP induces and maintains an ozone-poor humid tropospheric background. Its modulation by ENSO via sea-surface temperature and dynamical changes consequently either suppresses (El Niño) or strengthens (La Niña) the background composition. However, for the given time series the seasonal transport patterns prevail, which relate to a background composition of low O3 and high RH between July and October and dynamic disturbances of this background in form of dry ozone-rich layers between November and April.