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Abstract
Storm tracks consist of the migrating weather systems in the midlatitudes and play a central role in redistributing energy within the Earth system and governing day-to-day weather variability. Considering its influence on the climate, predicting and understanding how the storm tracks will change in response to elevated anthropogenic forcing in the far future has gained much attention. In the absence of an observed signal, previous work focused on storm track changes in the climate models. However, significant storm track trends have emerged in the observational data in the satellite era since 1979. The two outstanding observed trends are weakening storm tracks during Northern Hemisphere (NH) summer and strengthening storm tracks during Southern Hemisphere (SH) winter. The processes that contribute to these trends are not fully understood currently. In this thesis, I unravel the physical mechanisms underlying the storm track weakening during NH summer and strengthening during SH winter in the satellite era, utilizing observational data and climate model experiments. Approaches for studying the two storm track trends are different since the existing state-of-the-art climate models capture the NH summer weakening but not the SH winter strengthening. To study the weakening of the NH summer storm track, I use mechanism-denial and single forcing experiments. I show that the land-to-ocean energy contrast changes due to anthropogenic aerosol forcing are more important for NH summer storm track weakening than the equator-to-pole temperature contrast changes from Arctic sea ice loss. To study the strengthening of the SH winter storm track, I use sea surface temperature (SST) nudging experiments to assist the climate model in simulating the SST trends. I show that the Southern Ocean cooling and tropical SST warming pattern are important factors contributing to SH winter storm track strengthening, and climate models would fail to reproduce the storm track trend if they struggle to simulate the SST trends. The results of the thesis improve our understanding of storm track mechanisms and enhance confidence in projected storm track changes in the future.