Unit 7 lesson 4 notes!

The ocean moves from place to place with currents. Currents are essentially rivers of water within the ocean.

Some currents travel along the ocean surface, while others flow in the middle of the ocean, or near the ocean floor. Because they carry water from place to place, they also carry energy.

Current depends on a number of factors, including the direction of the wind, the earth’s rotation, earth’s geography, and the temperature and salinity of the water.

Surface Currents Surface currents are driven mostly by prevailing winds – or winds that tend to always blow in a specific direction.

When air blows across water, some of it crashes into and brushes along the surface water, pushing and pulling the water along. This is one way that energy in earth’s atmosphere is transferred to the oceans.

Wind = White Ocean = Black

When the Wind Stops…

Think about what happens when you push a toy sailboat across the water. The sailboat will keep moving across the water even after you stop pushing. This is because the sailboat has inertia. If an object is already in motion, it will tend to stay in motion. The same is true for ocean water. Even if the wind stops blowing for a little while, the water keeps moving. However, if the wind does not blow for a long time, or if it blows in a different direction for a long time, the currents will eventually change according to the new situation.

Factors of Ocean Currents

The other factors that can affect the flow and patterns of ocean currents are:

• the topography of the seafloor

• sea surface height

• the shape of the coastline

• water temperature

Deepwater Currents

Currents not only flow on the water surface, they also move deep down in the ocean - Thermohaline Circulation Thermohaline circulation depends on temperature and salinity. When ocean water freezes, salinity and density increase. The colder, denser, saltier water tends to sink and since it has nowhere else to go, it moves outward and away from the poles – thus creating a deepwater current. The exact path of the current depends on a number of things, including the shape of the seafloor, the positions of the continents, and the Coriolis effect.

Deepwater Currents

Unlike surface currents, deepwater currents are not influenced directly by the wind. Instead, deepwater currents are driven by density. As warm surface water from the equator travels toward the poles, the water loses heat. As the temperature continues to drop, the freshwater will freeze, leaving the saltwater even saltier. As a result, much of the water near the poles is both cold and salty. • Think back. How do temperature and salinity affect the

density of the seawater? [6] • What can happen to water when its density increases? [7]

Downwelling

In some places water travels along the surface. In others, it sinks and travels near the bottom. The movement of sinking water is called downwelling. Downwelling is common in areas where surface currents meet or along coastlines.

Upwelling

The amount of water in the oceans is not changing significantly. So, if water is sinking in some places, it must be rising in others. The rising of ocean water is called upwelling. Upwelling is important not only for the flow of energy and water in the oceans, but it also carries very rich nutrients from the ocean bottom to the surface. These nutrients are essential for healthy fish populations.

Cold water rising??

It may seem strange that in some places, large masses of cold ocean water are upwelling from the deep sea. Why would cold water rise? Think about a body of water along a coastline. If the prevailing wind is blowing out to sea, it will push water along with it. Since there is no surface water to replace the water that is moving away from the land, cold water will rise up from the bottom bringing along with it rich nutrients from the ocean floor. Upwelling also occurs where two opposing wind systems meet.

Currents and Convection

Remember we learned that convection is the circulation heat. A convection current is when hot air rises and cool air falls. Convection currents occur throughout the world's oceans too. Convection happens because of the mixing and energy transfer between cold and warm waters during thermohaline circulation, upwelling, and downwelling.

El Niño

Once in a while, prevailing winds slow down or change direction. If they slow down for long enough, they affect the ocean currents, as well as the atmosphere. Every few years, the Westerlies (prevailing winds blowing west) stop blowing for months at a time. The surface currents slow down, and less of the cold, nutrient-rich water can well up from below. This phenomenon is known as El Niño, which can be devastating for the fishing industry in Peru and Chile.

Studying Ocean Currents

Scientists have been investigating ocean currents since the 1700s, when traders moving goods between Europe and the Americas became interested in the Gulf Stream.

Today, scientists use complex instruments to study ocean currents. One device to measure temperature and salinity levels is called a CTD (conductivity, temperature, and depth). Radar technology and video surveillance is also used to track where currents flow and how they affect shorelines.