Damaging hurricanes are familiar along the US east coast, with the recent hurricane Sandy a dramatic example. In Europe we are unused to such dramatic weather and the widespread destruction that hurricanes can, and do, cause. However, our new research suggests that this is likely to change as Earth’s climate warms over the next century.
Hurricanes are powered by warm sea water and characterised by heavy rainfall. The energy that is released during this rainfall is the thriving force of hurricanes. They originate during late summer in the western part of the tropical Atlantic where the sea water is sufficiently warm.
Due to the prevailing pattern of surface winds moving from east to west - trade winds - over the North Atlantic, hurricanes move westward and can hit the east coast of North America. Apart from the westward movement they also drift towards the poles.
This movement is related to the rotation of the earth, which exerts a poleward force on hurricanes. If they reach the mid-latitudes before hitting land they are caught by the eastward winds that prevail at those latitudes. Remnants of hurricanes can therefore hit Western Europe.
In the present climate these remnants are weak because the cooler sea water in the mid-latitudes reduces the hurricane’s strength.
If hurricanes reach the mid-latitudes before they have died out they can re-intensify again, even if the sea surface water is cold. They can do so because they now also can draw energy from heightened variations in temperature from North to South. This is the source of mid-latitudes storms, but the hurricanes are also still filled with warm moist air.
The combination of these two sources of energy can transform these hybrid storms into super-storms, like the recent Sandy. For the present climate this re-intensification is possible along the east coast of America, but virtually impossible when they reach Western Europe because the hurricanes have virtually died out.
This could change in a warmer climate. Warmer sea water will provide more energy for hurricanes, making them stronger. Stronger hurricanes will have a larger chance to reach Western Europe before they die out. In addition, they will have to travel over a less hostile environment with warmer sea water before they reach Europe.
Another important change is that the region where the sea water is sufficiently warm for the generation of hurricanes will extend eastward. Whereas in the present climate hurricanes only form in the western part of the tropical Atlantic, this area would shift eastward in a warmer climate. This makes their travel distance to Europe shorter, with a larger possibility of retaining their structure and strength when arriving there.
Simulating the weather of 2094
Until now these ideas could not be tested because present day climate models cannot adequately simulate hurricanes due to their relatively small scale of about hundred kilometres. Due to limitations of computer power, climate models can only simulate large scale structures like mid-latitude storms.
However, by using a very high resolution weather forecast model for climate simulations we have tested the possibility that hurricanes will reach Europe in a warmer climate. The weather forecast model that we have used is the European Centre of Medium Range Weather Forecast (ECMWF) model. This one of the best weather forecast models in the world and is also routinely used for hurricane forecasts. It correctly simulated the evolution of Sandy.
We simulated the beginning and end of 21st century conditions and compared them to assess 21st century trends. The future simulations factored in the expected rise of sea water temperature - by 2 to 3°C - due to the increase of greenhouse gases.
These simulations showed a significant increase in the possibility of Sandy like super-storms for Europe. Over Norway, the North Sea, and the Gulf of Biscay, the number of hurricane-force storms between August and October increases from two to 13. The probability of extreme events over the North Sea increases by a factor of five, and over the Gulf of Biscay by 25.
Due to this re-intensification of hurricanes the main storm season shifts from winter to autumn, with far-reaching consequences for flora and fauna. The shift of the hurricane breeding ground towards the eastern tropical Atlantic, and the resulting decrease in path length to Europe is the main cause for the increased number of super storms hitting Europe.
These results are based on the simulations of a single model, and to test their robustness we are now comparing our results with those of other similar simulations done in other climate centres. However, the implications remain clear: Europe will see more hurricanes as a result of climate change.