Tropical cyclones (TCs) are high-impact weather systems that form over the warm tropical ocean and bring catastrophic damage to coastal areas. However, their impact extends beyond tropical regions. When a TC moves to the mid-latitudes, it undergoes a transformation known as an extratropical transition (ET), where it abruptly adapts to new environmental conditions and becomes an extratropical cyclone. Although extratropical cyclones generally have lower maximum wind speeds compared to TCs, they are larger in size, affecting larger areas and populations. A study revealed that extratropical cyclones originating from tropical areas that impact Europe during the hurricane season can exhibit significantly greater maximum intensity than those formed in the mid-latitudes.
One prominent example of a destructive event caused by a transitioned TC is Hurricane Sandy in 2012. While it made landfall in New Jersey as a Category 1 TC on the Saffir–Simpson scale, it had already transitioned to an extratropical cyclone before the landfall. The storm’s broad outer edge encompassed a vast area of powerful surface winds, leading to damages surpassing $50 billion US dollars.
There has been increasing interest in the projection of future ET activities in recent years, mostly on a regional scale. Several studies have shown that the mean or maximum TC intensity will increase in the future climate under global warming. This implies a higher possibility of TCs surviving longer and persisting into the mid-latitudes increases. It is of interest to predict how ET activities will change in the future to build climate resilience for the population in the mid-latitudes.
In our study, we used a state-of-the-art Earth system model to examine the changes in global ET activities under increased concentrations (doubling and quadrupling) of carbon dioxide (CO2) in the atmosphere. An Earth system model is a set of programs that simulate the states of different components on our Earth (e.g., atmosphere, ocean, land, and ice) and the interactions between them based on physical relationships. The smaller grid size (i.e., the higher resolution) in the model can improve the simulation of TCs and thus a more accurate statistics of TC activities compared to models with coarser grid settings.
We found that the global numbers of ET events under increased CO2 concentrations are similar to that in the climate with the current CO2 level. It is consistent with the lack of changes in the large-scale environment favorable to ET. However, we observe more intense storms upon the completion of the transition, with a more pronounced increase in near-surface wind speeds in the CO2 quadrupling simulation.
What the public cares about the most is the possible damage a weather system can inflict on them, rather than the nature of it (that is, the storm intensity in our case). Typically, TC intensity is represented by the greatest wind speed near the surface. It is based on the common observation that TCs are highly symmetric weather systems with compact structures. A TC with greater wind speed is usually regarded as more destructive. However, due to the asymmetric nature of extratropical cyclones, this assumption may not hold. Therefore, we use a metric called “destructive potential”, which considers both wind speed and spatial extent of a storm, to assess the change in impacts by the storms after the transition is completed. Our findings indicate a decrease in the fraction of storms with lower destructive potential and an increase in storms with highly destructive potential when CO2 levels are quadrupled.
Our study highlights the possibility of a heightened impact from the transitioned TCs in the mid-latitude region due to human-induced global warming. The result will be valuable for policymakers to identify natural disaster hotspots caused by these extreme climate events and develop climate resilience measures in the mid-latitudes in the future.