Extratropical cyclones (ECs) have a crucial importance for shaping the weather and climate of midlatitudes. ECs dominate the daily variability of weather and cause the majority of extreme weather events like extreme surface winds and precipitation. Extreme surface winds caused by ECs cause substantial losses in the insured property when they hit the land and have, therefore, been closely studied. They have been studied both climatologically and in terms of case studies of individual impactful ECs. 

However, ECs that come to land are only a small portion of all ECs. ECs most frequently occur over the midlatitude oceans in the regions called the storm tracks. Even though the highest frequency of occurrence of ECs is over the storm tracks, comparatively less attention was paid to the ECs which cause extreme winds over these areas.

This thesis studies the ECs that cause extreme surface winds over the oceanic storm tracks, in the areas where the occurrence of ECs is the highest. More specifically, the papers in the thesis focus on the ECs and the extreme surface winds over the North Atlantic, North Pacific and Southern Ocean. We analyze various datasets, from observations and reanalysis products, to those produced by our climate modeling experiments. 

One of the key results of the thesis is that the extreme surface winds in the storm track areas are larger in the Northern Hemisphere than in the Southern Hemisphere, even though the Southern Hemisphere is known to have the stronger average surface winds. More specifically, the North Atlantic has the largest extreme surface winds, followed by the North Pacific and the Southern Ocean. The hemispheric difference in extreme surface winds exists because the extreme winds are stronger around winter ECs in the Northern Hemisphere, and the strongest in the North Atlantic.

Another important result from the thesis is that the ECs that cause the most extreme surface winds have a similar large-scale development in all major storm track basins. This development is characterized by the presence of a pre-existing downstream EC, which helps to create an environment conducive to the development of extreme wind-causing ECs. In addition, we connect the differences in extreme surface winds between the basins to differences in the mid-tropospheric Eady growth rates. The extremes of Eady growth rates, as well as the percentage of explosively deepening ECs, are higher in the Northern Hemisphere than in the Southern Hemisphere. Finally, the modeling studies we do show that the main drivers of the differences in the extreme surface winds (and the Eady growth rates) are spatial distributions of sea-surface temperatures and orography.