The Kuroshio Current and the Mexican Gulf Stream are the two largest currents in the world. These two large currents flow in the west side of oceans for the same reason. The Coriolis force generated by the Earth's rotation has a stronger effect on horizontal movement at higher latitudes. This is called the "ƒÀ effect" and influences large-scale, slow movements.
The ƒÀ effect tends to reduce east-west changes. When an east-west change occurs, a north-south flow is generated, creating a whirlpool. This whirlpool averages the east-west difference. The wave generated at this time is called Rossby wave and is unique in that it only propagates in a westerly direction.
The subtropical circulation system in the North Pacific Ocean flows clockwise and is contained by the Asian Continent and the North American Continent. In this area, the Coriolis effect is stronger at higher latitudes, and theƒÀeffect applies a stronger force to the right for northward flows. For this reason, the subtropical circulation system heads in a northerly direction as the broad Kuroshio Extension and Kuroshio. The North Pacific Current that accumulates in the west is the Kuroshio.