Abstracts:
Susann Tegtmeier – Long-term changes of the Brewer-Dobson circulation and their impact on stratospheric composition
Stratospheric trace gases like HCl and N2O show a hemispheric asymmetry, with trends over the last decades having opposing signs in the Northern Hemisphere (NH) and Southern Hemisphere (SH). Some of this difference is due to hemispherically asymmetric changes in the rate of transport by the Brewer–Dobson circulation (BDC). These changes also impact stratospheric ozone, which has recovered more in the SH compared to the NH. Long term changes in transport further complicate the analysis of trends in stratospheric chlorine when compared to changes in surface emissions of ozone depleting substances (ODSs) regulated by the Montreal Protocol.
Here we use N2O as a proxy for the stratospheric circulation to calculate long-term trends in mean age of air (AoA), which is a common proxy for BDC transport. First, N2O observations from ACE-FTS are combined with results from CLaMS simulations to derive AoA anomalies. We find that air in the NH aged by up to 0.3 years per decade relative to the SH over 2004–2017 with the maximum hemispheric difference occurring near 30 hPa. The N2O proxy is then used to analyze how much of the decrease in stratospheric chlorine is related to transport variations and how much is driven by the reduction in chlorinated long-lived ODSs. We show that the observed decrease in stratospheric chlorine is 25%−30% smaller than expected based on trends of long-lived ODSs alone. This can be explained by the increase in chlorinated very short-lived substances, which offsets the long-term reduction of stratospheric chlorine by up to 30%
Matt Toohey – How long does stuff stay in the stratosphere? Recent progress on an age-old question
The time that material spends in the stratosphere is a key consideration to understanding the cumulative environmental impact of things like high-altitude aircraft exhaust, space debris, volcanic eruptions and hypothetical solar radiation management schemes. Volcanic eruptions which inject large amounts of gases to the stratosphere constitute natural “pulse injection” experiments, which can be used to estimate the mean lifetime or residence time of stratospheric aerosols, which is controlled by both the aerosol’s gravitational settling and stratospheric circulation. The lifetime of aerosol from major tropical eruptions like Pinatubo (1991) is commonly thought to be around 12 months. Using satellite observations and simple conceptual models, I challenge this finding and argue the true lifetime is around 22 months for Pinatubo aerosol, since it must account for the “lag” between injection and the initiation of removal. Using passive tracer experiments, I show how the residence time of air (an important component of the aerosol lifetime) varies strongly with injection latitude and altitude. Observations of water vapour anomalies from the Hunga Tonga eruption of 2022 show strong qualitative agreement with the passive tracer simulations, corroborating gas residence times of over 4 years for tropical stratospheric injections.
What is the Joint Oslo Seminar (JOS):
- Atmospheric and climate sciences have a stronghold in Oslo among the four institutions University of Oslo, the Meteorological Institute, CICERO and NILU.
- This joint seminar invites renowned international experts to contribute to an informal series of lectures, meant to create interaction with the Oslo atmospheric and climate science community on recent highlights and analysis in the field.
- Normally seminars will be held on Thursdays (12:15 pm -1pm).