ausblenden:
Schlagwörter:
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Zusammenfassung:
Major stratospheric sudden warmings are prominent disturbances of the Northern
Hemisphere polar winter stratosphere. Understanding the factors controlling major warmings is required,
since the associated circulation changes can propagate down into the troposphere and affect the surface
climate, suggesting enhanced prediction skill when these processes are accurately represented in models.
In this study we investigate how different natural and anthropogenic factors, namely, the quasi-biennial
oscillation (QBO), sea surface temperatures (SSTs), anthropogenic greenhouse gases, and ozone-depleting
substances, influence the frequency, variability, and life cycle of major warmings. This is done using
sensitivity experiments performed with the National Center for Atmospheric Research’s Community Earth
System Model (CESM). CESM is able to simulate the life cycle of major warmings realistically. The QBO
strengthens the climatological stratospheric polar night jet (PNJ) and significantly reduces the frequency of
major warmings through reduction of planetary wave propagation into the PNJ region. Variability in SSTs
weakens the PNJ and significantly increases the major warming frequency due to enhanced wave forcing.
Even extreme climate change conditions (RCP8.5 scenario) do not influence the total frequency but
determine the prewarming phase of major warmings. The amplitude and duration of major warmings seem
to be mainly determined by internal stratospheric variability. We also suggest that SST variability, two-way
ocean/atmosphere coupling, and hence the memory of the ocean are needed to reproduce the observed
tropospheric negative Northern Annular Mode pattern after major warmings.
