When tropical cyclones die
The final fate of a tropical cyclone in the Atlantic depends on what is happening in the ocean and atmosphere at the time. Each of the following situations will cause a cyclone to weaken and eventually die:
The storm’s source of heat and moisture is removed. When a tropical cyclone passes over land or cold water, the basic fuel that drives the storm is cut off--warm ocean water--is cut off. Passing over land will quickly weaken the storm (not because of friction as some believe, but because of the loss of the warm moisture source).
The storm meets vertical wind shear. Low values of wind shear are needed to form a tropical cyclone. When major wind shear is present it interferes with the processes driving the storm, and it begins to weaken and die.
Dry, cool air moves into the storm. A tropical cyclone is like a heat engine that runs on warm, moist air. The sudden presence of cool, dry air (pulled into the storm by a number of ways) is enough to reduce the deep convection that keeps the storm going.
Low pressure develops above the storm in the higher part of the atmosphere. An that an area of high pressure above the storm helps move out the buildup of air above the storm. This allows the storm to continue drawing in new air at the surface. If the high pressure is replaced by a low-pressure area above the storm, this causes inward flow above the storm. The rising air within the storm then has no escape and the storm collapses in on itself.
Tropical cyclones generally move toward the poles as part of their role in keeping the overall temperature balance in the earth-atmosphere system. When that happens, they leave the tropics and move into the extratropical regions (such as the latitudes of Canada). When tropical cyclones move toward the poles, just over half of them (54%) simply weaken and die. The others move into middle latitudes and interact with the weather patterns of those regions such as existing frontal systems or upper atmospheric troughs. In these cases, the tropical cyclones have one of two fates: they weaken and die or they become reinvigorated and transform into something new, but still significant. See extratropical transition.
