Increasing destructive potential of extratropical transition events in response to higher CO2 concentration in global climate model

When tropical cyclones (TCs) move to the mid-latitudes, they encounter the baroclinic environment where many of them experience extratropical transition (ET) by which they lose the symmetric and warm-core characteristics and transform into extratropical cyclones (ETCs). ETCs are usually faster than TCs and oftentimes destructive to coastal cities with strong wind and heavy precipitation. Climate models predict that the mean intensity of TCs would become stronger fundamentally due to the increase in atmospheric moisture contents in response to global warming. However, whether the destructiveness of ETCs originated from TCs will change in the future has not been explored with a high-resolution fully-coupled model. To understand the future changes in ET events and the destructive potential of these ETCs, we analyzed the high-resolution Community Earth System Model (CESM) simulations (0.25 degrees for the atmosphere and 0.1 degrees for the ocean) with present-day, doubling, and quadrupling CO₂ concentrations.

The high-resolution model well captures the frequency and annual cycle of the ET events compared to observation with underestimated frequency in the North Atlantic and West Pacific while overestimating them in the East Pacific, South Indian, and South Pacific. Our results show that the frequency and ratio of ET events do not change significantly in both CO₂ doubling and quadrupling experiments. An increase in 10-m wind speed at ET completion is observed mainly in North Atlantic and South Indian. We used the total integrated kinetic energy, which depends on the wind speed and the area covered by the high wind region of a storm, to represent the destructive potential of a storm upon ET completion. It is found that the relative ratio of the strongly destructive ETCs to weaker ETCs increase in response to greenhouse warming.

Our study highlights the destructive potential of transitioned TCs. Since ETCs usually have greater spatial coverage than TCs, the former can impact a larger population and region, albeit with lower intensity. Therefore, accurate prediction of future changes in ET events can have significant socio-economic implications.