# Will tropical cyclones form and be sustained in areas they previously were not due to global climate change?

With global climate change, is it possible that tropical cyclones (as seen in the Atlantic basin for example) are going to form and be sustained in regions where they previously did not, for example near Europe?

Tropical cyclones (TC) develop and persist in conditions favorable to them:

1) Low Coriolis parameter $f = 2\Omega \sin{\theta}$, where $\Omega$ is the angular velocity of Earth and $\theta$ is latitude. If $f$ is small, the centripetal force balances the pressure gradient force in the gradient wind balance, and the radius of curvature becomes small. While this is one of the reasons why TCs form in the tropics, that still does not prevent the storm to move North and re-curve into Western Europe or Northern California. This limits the latitude extent of the region of TC genesis to approximately 5-30 N (or S). This factor is most certain and not subject to climate change.

2) TCs get the energy from sensible and latent heat fluxes at the surface. Thus, they form over the ocean and it needs to be warm. Historically popular threshold of sea surface temperature (SST) necessary for a TC to develop was 26$^{\circ}$C. Nowadays, we know that this is not enough - TCs need a significant thickness of the upper ocean to be warm in order to get enough heat to be sustainable. Another reason why the ocean is more favorable for TCs than land is surface friction - the air flow over the ocean surface is typically much more aerodynamically smooth compared to over land. Low wind stress and high heat fluxes are key. Though the wind stress is not going to change, heat fluxes may increase if the ocean heat content in the tropics increases (for example, by weakening of the Meridional Overturning Circulation).

3) TCs also need favorable atmospheric conditions in their environment - namely, high humidity (water vapor content) in the environmental air, and low wind shear (difference between wind in the upper and lower troposphere) facilitate TC genesis and longer life. These factors are, in my opinion, least understood of the three, especially in terms of their contribution to the vortex dynamics and vortex-mean flow interaction. This aspect is more likely to be subject to climate change compared to the others.

All this being said, we may see a change in the occurrence frequency of TCs in the future, and/or a change in the distribution of the extreme weather events. However, the mechanisms in the climate that would lead to this change are still not well understood. It is unlikely though that the TCs will start forming in new regions around the world.

As suggested by Kossin et al., "The poleward migration of the location of tropical cyclone maximum intensity," (Nature, 2014, dx.doi.org/10.1038/nature13278), the answer could be "maybe".

As the tropical region increases in size (tropical expansion: the increasing area of the world where tropical phenomena is observed has expanded in time) due in part to anthropogenic reasons, there might be a migration of tropical cyclones poleward.

The migration might not change the cyclone formation area, but it does seem to affect the location of maximum intensity. The migration seems to be occurring in most areas and the trend seems significant in most regions of the world except the North Atlantic, for instance.