Garden, Pia U. (2022) Southern hemisphere sudden stratospheric warmings in the ECHAM6 model. (Bachelor thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 31 pp.

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Abstract

On rare occasions, a sudden stratospheric warming (SSW) occurs in the Southern Hemisphere (SH), drastically weakening or even reversing the strong winter polar vortex.
During SSW events, circulation changes occur that can have significant effects from the upper stratosphere down to the surface. ECHAM6 model data are used to study these impacts. In the atmosphere-only experiment with a perpetual 2018 conditions setup, 13 SSWs are found in 142 simulated years. Since these events coincide with a negative phase of the southern annular mode (SAM), SAM and temperature indexes are created to better track the downward propagating SSW anomalies. The anomalies in the SAM index also make it possible to divide the SSWs into two groups: those followed by significant, long-lasting effects on the tropospheric circulation and those where this is not the case, although the latter are a minority.
As soon as the anomalies reach the surface, a change in the regional climate of the SH can be found. The Antarctic, for example, experiences significantly higher temperatures and pressures than average. This condition shifts the storm tracks on the SH to the north, resulting in significantly drier and warmer conditions than usual in western South Africa and Australia, whereas an increase in precipitation in southern Australia and New Zealand is simulated. Apart from South America, where no significant results were found, ECHAM6 generally simulates SSW effects on nearsurface climate in the SH very well, consistent with literature on observed negative SAM phases or weak vortex years.

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