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

What is a twister? Why are the majority of twisters observed in areas around the Gulf of Mexico? [15 marks, 2024]

A twister, commonly called a tornado, is a narrow and violently rotating column of air extending from the base of a thunderstorm to the ground. Unlike a tropical cyclone, it is small in spatial extent but may have extremely high wind speeds.

Conditions necessary for the formation of twisters:

1. Warm and moisture-laden air near the surface, which provides heat and water vapour.
2. Cold and relatively dry air above or nearby, creating atmospheric instability.
3. Rapid uplift of warm air, leading to the formation of cumulonimbus clouds and severe thunderstorms.
4. Strong vertical wind shear, that is, variation in wind speed or direction with height.
5. Wind shear creates rotation, which may be tilted vertically and intensified by powerful thunderstorm updrafts.

Thus:

Moisture + instability + lift + wind shear + thunderstorm = twister

Reasons for their concentration around the Gulf of Mexico:

1. The Gulf of Mexico acts as a large reservoir of warm water → abundant moisture and heat to the overlying atmosphere.
2. Meeting of contrasting air masses: Warm and humid air from the Gulf frequently encounters colder and drier continental air.
3. The rapid rise of warm air leads to the development of large cumulonimbus clouds and severe thunderstorms.
4. Surface winds carrying Gulf moisture and stronger winds at higher altitudes often move at different speeds and directions. This produces rotation within thunderstorms.
5. The central and southern parts of the United States contain extensive plains. These allow contrasting air masses to travel and interact without major topographic barriers. This tornado-prone belt of plains north of the Gulf, covering states such as Texas, Oklahoma and Kansas, is popularly known as Tornado Alley.

Therefore, the concentration of twisters around the Gulf of Mexico is mainly due to the simultaneous availability of moisture, heat, atmospheric instability, contrasting air masses and wind shear.