What phenomenon do 'gyres' refer to in oceanography?

Gyres refer to large systems of circular ocean currents that are influenced primarily by the wind patterns and the Earth's rotation. These currents are typically centered over major subtropical high-pressure systems. In the North Pacific and North Atlantic, for example, the gyres move clockwise, while those in the Southern Hemisphere move counterclockwise, due to the Coriolis effect. This circular movement plays a crucial role in regulating the climate and temperatures of ocean waters, as well as affecting weather systems across vast regions.

The distinction of gyres as centered over subtropical highs emphasizes their relationship with global atmospheric pressure systems, making them crucial for understanding both oceanic and atmospheric dynamics. The other answer options, while related to ocean currents, do not specifically define the broader and more systematic phenomenon of gyres. Coastal currents, deep ocean currents, and tidal currents all describe different aspects of ocean behaviors that are not encapsulated by the concept of gyres.