Final answer:
The most common wind direction 15° south of Earth's equator is southeast, due to the southeast trade winds influenced by the Coriolis effect in the Southern Hemisphere.
Step-by-step explanation:
The most common wind direction 15° south of the Earth's equator is governed by the general circulation of the atmosphere and the Coriolis effect. In the Southern Hemisphere, the Coriolis force causes moving air to deflect to the left. Therefore, the predominant winds this close to the equator, but still within the tropics, are the southeast trade winds. These trade winds are part of a larger circulation pattern known as the Hadley Cell, which helps to transport heat from the equator toward the poles. The location 15° south is well within the belt where the southeast trade winds are effective.