The west-flowing trade winds of the Northern and Southern hemispheres converge at the equator. The trade winds push water to the west within a narrow zone of the ocean surface. This water piles against the western side of the basin, turns to flow toward the pole, and then returns with the east-flowing westerlies. The easterly winds of the hemispheres are not convergent, so there is not such an organized flow of water on the return trip of the gyre, and the eastern boundary currents are not as strong. We've used subtropical gyres to illustrate what gyres are, but they are not the only ones we have.
Subpolar gyres are present in each hemisphere, too.
The position of the continents allows well developed subpolar gyres in the Northern Hemisphere but not in the Southern Hemisphere, although a few small ones do exist near Antarctica. The Antarctic Circumpolar Current flows around the Antarctic continent, essentially unimpeded by deflecting land masses to break it up. The interaction of the currents with the positions of the land results in a series of basins with roughly circular water-flow patterns. Compare this map to the one showing present currents on the ocean surface. Path of a plane flying from the North Pole and one flying from the South Pole toward the equator.