Interhemispheric perspective on the most extreme surface winds in the storm tracks

Abstract. Extratropical cyclones occur most frequently over the oceans, where they cause the strongest extreme surface winds. In this study, we investigate extreme surface winds during winter months in the northern and southern hemisphere, when the wind storms are the strongest. The analysis focuses on storm tracks over the North Atlantic, North Pacific and Southern Ocean. We use ERA5 reanalysis data to make composites of extratropical cyclones that cause the top 100 most extreme surface wind events from 1979 until 2020 ("top 100 extremes"). We focus on large-scale atmospheric processes and find that the most prominent large-scale feature of the top 100 extremes in each basin is the presence of a pre-existing downstream cyclone a few days before the time of the maximum surface winds. Pre-existing cyclones are situated poleward and eastward of the top 100 extremes and their presence is consistent with strong upper-level winds and potential vorticity anomalies. Differences between the basins and hemispheres are quantitative. The top 100 extremes in the northern hemisphere develop in an environment with higher mid-tropospheric Eady growth rates, higher deepening rates and stronger surface wind speeds around the cyclones than in the southern hemisphere. Furthermore, we run the general circulation model ISCA with different boundary conditions to investigate their influence on the basin-wide extreme surface winds. We find a strong positive correlation between basin-wide extremes in mid-tropospheric Eady growth rates and surface winds. Zonalizing sea-surface temperatures across tropical and extratropical regions greatly reduces the differences in extreme surface winds between the North Atlantic and North Pacific; inter-hemispheric differences in extreme surface winds between the North Atlantic and Southern Ocean are greatly reduced when sea-surface temperatures are zonalized globally and made hemispherically symmetric, and orography is flat.