Ocean water is in a constant state of movement. Horizontal movements are called currents while vertical movements are called upwelling and downwelling. Wind, tidal motion, and differences in density due to temperature or salinity are the main causes of ocean circulation. Temperature differentials arise from the equatorial water being warmer than water in the polar regions. In the Northern Hemisphere the currents circulate in a clockwise direction while in the Southern Hemisphere the currents circulate in a counterclockwise direction. In the equatorial regions the currents move in opposite directions—from left to right in the north and from right to left in the south. Currents moving north and south from equatorial regions carry warm water while those in polar regions carry cold water.