In this topic, you will learn how the movement of water in the ocean affects the weather, the climate, the land, and you.

Cold water causes warmer water to move. One way ocean water moves is in currents. A current is a large stream of water that flows in the ocean. One kind of ocean current is known as a deep ocean current. In a deep ocean current, water flows more than 650 feet deep, where the water is very cold. Deep ocean currents begin in polar regions, where the water is very cold. Dense cold water sinks below warmer water. Warmer water is not as dense as cold water. The lighter warmer water gets pushed out of the way by dense cold water. This causes a current.

When steady winds blow over the ocean, they move the water in currents called surface currents. Some surface currents move warm water from the equator. Other surface currents move cold water from areas near the poles. Large land masses cause surface currents to change direction. Surface currents that hit against a continent turn and move along the coast.

The rise and fall of ocean water levels is called a tide. Ocean tides are caused by the pull of gravity between Earth and the Moon and the Sun. The pull is greater by the Moon because it is closer to Earth. The pull of gravity causes the oceans to bulge.

Along with currents and tides, ocean waters move in waves. A wave is an up and down movement of water. The wind causes the ocean's surface water to move up and down in a circular path. The wave moves in the direction of the wind.

Ocean currents affect the climate. Some currents carry cold water into areas that might otherwise be warm. Other currents carry warm water into regions that would otherwise be cold. Ocean currents are a very important part of your life. Ocean currents help warm or cool land temperatures.

Some of the ocean waves are so strong, they change the coastlines. The waves hit against the shore and wear it away. This is called erosion. Waves also deposit soil, sand, gravel, and rocks on the shore. This is called deposition. The strong winds that cause big ocean waves can also help form hurricanes. Hurricanes are violent storms with strong winds and heavy rains. The Earth's rotation helps start the storm's whirling winds.