Final answer:
The absence of tropical cyclone activity at the equator is due to the absence of the Coriolis effect, which causes deflection of winds necessary for cyclonic rotation. This effect is zero at the equator, preventing the formation of spiraling tropical cyclones which observe counterclockwise rotation in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Step-by-step explanation:
The absence of tropical cyclone activity at the equator is due to the absence of the Coriolis effect, which is essential in producing the circulation necessary for storm formation. At the equator, the Coriolis force is effectively zero, hence no deflection occurs, and a key ingredient in the formation of the spiraling motion of tropical cyclones is missing. The significance of the Coriolis effect can be observed in the way tropical cyclones rotate—counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere due to the deflection of winds caused by the Earth's rotation. This crucial impact of the Coriolis force is beautifully demonstrated in Figure 6.16, showing this deflection in a non-inertial frame of reference.
In the Northern Hemisphere, the Coriolis force deflects winds to the right, resulting in a counterclockwise rotation in low-pressure systems, including hurricanes and tropical storms. The opposite is true in the Southern Hemisphere, where the rotation of tropical cyclones is clockwise. The rotating system of the Earth underneath also explains the rotation due to inertia in an inertial frame, rendering the Coriolis force a fictitious force employed to describe phenomena in a non-inertial frame.
Understanding that the Coriolis effect is essential for the initial cyclonic motion ultimately answers why tropical cyclones, including hurricanes and typhoons, do not form at the equator. Rather, they typically form between about 5° and 30° latitude in both hemispheres where the Coriolis force is sufficiently strong to induce rotation.