Rules of the (tropical) road: how hurricanes chart their course

The Atlantic hurricane season is now past its climatological peak. As such, we should start to see fewer new disturbances in the Atlantic turning into tropical storms and hurricanes. Historically speaking, by October 1st, the number of named storms is about half what it was at the peak just a few short weeks prior. 

Hurricane and tropical storm climatology, from the National Hurricane Center

In early September, Hurricane Lee was forecast to make a beeline toward the coast of the Carolinas, then turn north without making landfall in the US. A couple of my relatives asked me at our football tailgate (Go Penn State!) why the storm would change course like that. It seems like the kind of question that would lend itself to this blog! So, without further ado, here are the peculiar paths that tropical storms follow- and the rules governing their movement.

Rule #1: Tropical systems follow the path of least resistance. When it comes to tropical storms and hurricanes, this is the path with the warmest water and the least wind. You can see this when looking at historical storm track maps in comparison to persistent ocean currents. For example, Hurricane Idalia's movement closely followed the Loop Current in the Gulf of Mexico.

Above: the wind history of Hurricane Idalia, August 26- Sept 2,  2023. 

Below: satellite imagery with color coded temperature data in the Gulf of Mexico, showing the Loop Current.

Rule #2: Tropical systems go where the (trade)winds take them. Without the Earth's rotation, there wouldn't be hurricanes. As thunderstorms form in the tropics, the Earth's rotation imparts the Coriolis effect on these clusters, allowing them to organize and spin around each other. Since tropical storms form over the ocean, there's nothing below them to steer their direction. 

This map from the National Hurricane Center shows the point of origin (red dot) and track (grey line) of tropical storms 

that have formed in late August. Notice that the lines emanating from the dots on the right side of the map, 

near the west coast of Africa, primarily move due East.

This is why almost every tropical storm that forms off the Cape Verde islands initially moves almost due east. Storms that form off the western coast of Africa move from east to west with the trade winds, unless or until another weather feature takes over. Speaking of which: 

Rule #3: Large-scale weather systems easily steer tropical storms. It may be surprising, since the average hurricane expends more energy than that in the entire nuclear arsenal on Earth, but these storms are pretty easy to move. If you've ever seen the sport of curling, it may be helpful to envision the curling stone- which weighs over 40lbs!- sliding effortlessly on the low-friction surface of ice. Tropical storms have a similarly frictionless surface below them, which means even "low energy" weather features like a dome of high pressure will change a hurricane's path. When a Bermuda High is strong, for example, you can almost guarantee that the small island nation will not take a direct hit from a tropical storm; instead, the storm track will curve near the East coast, sometimes making landfall in a spot that juts out into the ocean, such as the Outer Banks of North Carolina. 

Bermuda highs often steer tropical storms nearly parallel to the Eastern seaboard of the United States. 

Full animation available at NASA Scientific Visualization Studio.

Rule #4: Tropical systems can get caught up in each other. When two storms get close to each other over the ocean, they can rotate around each other, creating what's known as the Fujiwhara effect. Eventually, the dueling vortices will go their separate ways, or if one storm is smaller, it can get absorbed into the bigger one. 

GIF of Hurricane Irwin and Hurricane Hilary interacting to create the Fujiwhara effect.
Source: National Weather Service "News Around NOAA" site

For more on tropical storms and hurricanes, check out this post about Hurricane Idalia from August, and my thoughts back in May about the start of the tropical season in the Atlantic Basin.