Although surface currents along the equator typically flow east to west, the net transport of water in the equator occurs deeper and is away from the equator as shown above. This is called the Ekman transport, and is named after the oceanographer who discovered it. The transport is caused by the winds and the Coriolis Force,which is caused by the rotation of the Earth. Ekman found that the net water transport was below the surface and that it was perpendicular to the wind direction; to the right of the wind in the north, and to the left of the wind in the south. Thus, since the trade winds tend to blow east to west in the equatorial region, the net transport is away from the equator. Notice the much deeper thermocline, or the boundary between the warm and cold water, in the west than in the east. Most of the sea-level variations measured by altimetry are caused by changes of the thickness in this layer of warm water.