Interhemispheric Coupling in the Chemistry-Climate Model EMAC and Its Modulation by the QBO / Gutachter: Prof. Dr. Ulrike Langematz, Dr. Blanca Ayarzagüena
Franziska Schmidt / Masterarbeit – 2016
The interhemispheric coupling (IHC) in the middle atmosphere is a process that connects the meridional circulation in both hemispheres via wave interactions. An anomalous planetary wave forcing in the winter hemisphere modifies the stratospheric and mesospheric residual circulation (RC) in winter. The change in circulation in turn is accompanied by temperature anomalies in various locations in the winter hemisphere. The modification of the temperature in the equatorial mesosphere leads to a propagation of the signal into the summer hemisphere. Finally, a change in the mesospheric RC induces a temperature anomaly at the summer pole in the lower thermosphere. This thesis examines the IHC in the chemistry-climate model EMAC and assesses the impact of the quasi-biennial oscillation (QBO) on the process. Previous research has characterized the mechanism behind the IHC in detail, but far too little attention has been paid to the role of the QBO. A transient EMAC simulation with realistic conditions for the past is used to gain an understanding of the IHC in the model. Several correlation and composite analyses are carried out in this study. In the first instance, the extent of the IHC signal is ascertained in EMAC. The modification of the circulation and temperature can be tracked as far as the subtropics in the summer mesosphere and is comparable to other climate models. The equator crossing of the signal is evident in various meteorological parameters. However, the propagation to the summer pole can not be assessed in EMAC, because the change in temperature is expected to be in higher altitudes above the top of the model. Nonetheless, the modification by the QBO in the tropical middle atmosphere is examined. It was hypothesized that the stratospheric QBO impacts the equator crossing of the IHC signal substantially. This proposed mechanism could not be found in EMAC. Instead, a modification of the temperature induced by the mesospheric quasi-biennial oscillation (MQBO) stood out and a further analysis of the impact of the MQBO was accomplished. A slight modification of the IHC signal became apparent at the equator. Particularly, an enhanced signal was detected for a strong planetary wave event and a QBO east phase and for a weak planetary wave event and a QBO west phase. However, these results were not consistently significant. Nevertheless, the findings suggest that the temperature signal of the MQBO modifies the equator crossing of the IHC. This modification may account for a variability at the summer pole proposed in other studies.