Abstract/Summary

Arctic cyclones pose the major weather hazard to the increasing human activity in the summer-time Arctic, producing strong winds that can have large impacts on the depleting sea ice cover. Whilst the dynamics of mid-latitude cyclones have been studied for decades, summer-time Arctic cyclones have received less attention. This thesis advances our understanding of summer-time Arctic cyclone dynamics and their interaction with sea ice. In this work cyclone evolution in the summer-time Arctic is split into two classes. A climatological analysis demonstrates that ∼65% of cyclones have a low-level-dominant (LLD) vorticity structure during growth, whilst the remaining ∼35% have an upper-level-dominant (ULD) vorticity structure and commonly develop with axisymmetric vortices on the tropopause. These two subsets of cyclones have different characteristics, and hence, this work represents a starting point for developing conceptual models in the future. However, regardless of structure during growth, summer-time Arctic cyclones tend to transition to a persistent columnar vortex structure after maturity, unlike mid-latitude cyclones. The fundamental mechanisms by which friction and sensible heat fluxes over sea ice impact summer-time Arctic cyclones are diagnosed using a potential vorticity framework, with frictional processes identified as having different effects on the dynamics of LLD and ULD cyclones. The results indicate that the columnar vortex structure of mature summer-time Arctic cyclones with friction may be dynamically unstable, and an idealised quasi-geostrophic model is used to characterise this instability. Finally, the sensitivity of summer-time Arctic forecasts to sea ice coupling representation in a numerical weather prediction model is examined. It is found that cyclones have a large impact on sea ice, but that cyclone forecasts are largely insensitive to sea ice coupling choice. This thesis provides novel insights into summer-time Arctic cyclones, with regards to their atmospheric dynamics and interaction with sea ice, providing a platform for further research.