What is a twister? Why are the majority of twisters observed in areas around the Gulf of Mexico?

What a twister is

A tornado is a small-scale vortex, hundreds of metres to a few kilometres across, with extreme winds. It is often nested in a mesocyclone of a supercell thunderstorm. It is not a cyclone of the Bay of Bengal. That is a warm-core synoptic storm. It is not a dust devil. A dust devil is too shallow and has no parent thunderstorm. Waterspouts are related vortices over water. Intensity is rated on scales such as Enhanced Fujita in the US, by damage, because measuring the core is hard.

Triggering needs strong vertical wind shear to tilt and stretch rotation, a capping inversion that then breaks, and a lifting mechanism such as a dryline, a front, or outflow. The funnel is the condensed visible core. The wind field is wider.

• Scale: A tornado is local, intense, and short-lived compared with a tropical cyclone.
• Parent: The usual parent is a rotating supercell. Some weaker tornadoes have other parents.
• Hazard: Extreme wind, debris, and a narrow but catastrophic damage swath follow.

Why the Gulf of Mexico neighbourhood

The Gulf is a large source of maritime tropical (mT) air. In spring especially, that air streams north into the Mississippi valley and southern Great Plains. Aloft, dry desert air from the Mexican plateau and cold polar (cP) air from Canada create an unstable stack. Moisture sits below, dry air sits above, and a jet stream provides shear. The dryline in Texas and Oklahoma is a classic lifting line. This is Tornado Alley on the popular map. Dixie Alley nearer the Gulf has more nocturnal, rain-wrapped tornadoes in forests. Those storms are deadly because they are harder to see.

Other continents have tornadoes, including India, rarely, in thunderstorm belts. They lack this regular clash of Gulf moisture, Rocky lee trough, and polar jet at mid-latitudes, together with a large observing network. Majority observed is also a detection fact. The US has radar, spotters, and a long record. Even allowing for that, the physical concentration is real.

• Moisture fuel: Gulf mT air is the warm low-level source.
• Shear and clash: Jet, dryline, and cP fronts create rotating supercells.
• Geography: Flat plains let storms organise. The Gulf keeps supplying moisture.

The Gulf is therefore the main moisture source of the North American tornado maximum. It is not the same as a tropical cyclone basin.

What the Gulf is not

The Gulf of Mexico is also a hurricane basin. That is a different process. A hurricane is a warm-core tropical cyclone hundreds of kilometres wide. Landfalling hurricanes can spawn tornadoes in outer bands, which adds some Gulf-coast twisters to the tally. That does not explain Oklahoma in April. Moisture from a warm sea plus mid-latitude shear is the main cause. The hurricane is a smaller extra source. Other warm gulfs without that jet-and-dryline geometry do not host a Tornado Alley.