Can rain disperse a hurricane

Super storms and climate change

Mid-October 2012. A low pressure system is brewing over the western Caribbean and quickly growing into a full-blown hurricane - "Sandy" is born. A few days later, the cyclone passes over Cuba - the worst hurricane since 1963. Then something unexpected happens: Instead of staying in the Gulf of Mexico, Sandy turns northeast and moves further north parallel to the US east coast - the hurricane becomes the "Runaway".

Superstorm Sandy over the northeast coast of the United States on October 30, 2012. © NASA

The aftermath of a super storm

On October 28, Sandy unites with a winter storm and grows into a true giant: Its vortex of clouds now stretches from South Carolina in the south to the Canadian border in the north - it is more than 1,850 kilometers in diameter. This makes it one of the largest hurricanes in US history - and one of the most momentous. Because the super storm is heading straight for the metropolis of New York.

A few days later, the balance of the storm is devastating: the entire east coast of the USA has suffered severe damage, entire stretches of beach have been completely washed away by waves up to seven meters high, cities are under water. In New York City alone, thousands of buildings are being damaged or destroyed, subway tunnels are full, streets are impassable and power lines are being torn down. The force of wind and waves creates measurable seismic tremors across North America.

Track of the "super storm" Sandy in October 2012. The colors indicate the intensity: beige and orange mark the strength of the hurricane, turquoise the stadium as a tropical storm. © public domain

No longer an exception

For the US east coast and New York, such storms have so far been the absolute exception - after all, they are north of the usual hurricane tracks. But it won't stay that way. The risk of storm surges in New York from such superstorms has already tripled compared to the year 1800, according to researchers in 2016.

And for two reasons. On the one hand, with climate change, the areas in the sea that are warm enough to serve as “breeding areas” for hurricanes and typhoons are shifting. On the other hand, however, the air currents in the atmosphere also change - and with them the forces that help determine the behavior of storms. How this specifically affects the trajectories of cyclones has been investigated by several research teams in recent years.

Turned north

The result: the hurricanes' trajectories have already changed measurably. The Caribbean hurricanes now move westward and across Central America less often than they did a few hundred years ago. Instead, they usually turn north beforehand. As a result, Florida and other stretches of coast on the US southeast coast are increasingly affected by cyclones, as Lisa Baldini of Durham University and her colleagues found.

The researchers see the reason for this “diversion” in climate change: due to the warming, the intertropical convergence zone and the zone of the so-called Hadley cells have shifted further north - and they are taking the hurricanes with them, so to speak. This northward trend could intensify in the future. The researchers warn that the risk of storms also increases dramatically for the northeast coast of the USA. Storms like "Sandy" would then no longer be an exception.

Hotspots migrate towards the poles

And not only that: The places where the cyclones reach their greatest intensity are also shifting further and further towards the poles. In the northern hemisphere, these storm hotspots are already advancing 53 kilometers per decade to the north, and in the southern hemisphere 62 kilometers to the south, as James Kossin of the National Climatic Data Center in Madison and his colleagues have found out. This trend is strongest in the Northwest Pacific, somewhat weaker in the North Atlantic and East Pacific.

Two factors could be responsible for this polar migration of the cyclone hotspots: On the one hand, the expansion of the hurricane “cradles” due to the warming seas has an effect here as well. On the other hand, due to climate change, the wind shear has increased in the tropics, but decreased in the adjacent areas. Because too strong a crosswind hinders the formation of storm eddies, this could also explain the relocation of the storm hotspots.

It seems clear that in the future, the coastal regions will also have to adapt to cyclones that were previously outside the danger zone.

Nadja Podbregar
Status: 01.09.2017