Lesson 8: Currents Physical Oceanography

Last class we learned about ocean layers What are the three main ocean layers? How does temperature change with depth in the thermocline? How does it change below the thermocline? Teacher’s Notes: Three main layers are surface (mixed) layer, thermocline, deep ocean In the thermocline, temperature changes rapidly with depth Below the thermocline, temperature is relatively constant

Today we’re going to explore ocean currents An ocean current is a regular movement of large amounts of water along defined paths. There are two primary types of ocean currents: Surface Currents (to a depth of about 400 m) Driving factor: Wind Deep Currents (entirely below the effect of wind) Driving factor: Density differences Thermohaline circulation: Ocean circulation driven by differences in density caused by temperature (“thermo”) and salinity (“haline”) variations Deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation.

Why are currents so important? Photo: NOAA Influence world climate and weather Ocean navigation and transportation Support marine life (transport mechanism, food source) Transport of materials (both helpful and harmful) and energy to different regions and depths of the ocean Teacher’s Note: Below are additional examples of currents supporting marine life: -The eel uses the Gulf Stream to get to the Sargasso Sea where spawning occurs. -Upwelling off the coast of Peru supports one of the most productive fishing areas in the world – nutrients brought to the surface attract large populations of anchovy. Photo: http://www.nmfs.noaa.gov/pr/species/mammals/cetaceans/rightwhale_southern.htm; Accessed: November 2010 Marine organisms like the Southern right whale (above) depend upon currents to circulate the nutrients that support their food sources

What drives ocean currents? Density gradients (differences) drive deep ocean currents Upwelling brings cold, nutrient-rich water from the depths up to the surface Wind is one of the primary drivers of surface currents

Density Currents Density Currents are a type of vertical current that carries water from the surface to deeper parts of the ocean. Density currents circulate thermal energy, nutrients and gases.

Upwelling Upwelling is the vertical movement of water toward the ocean’s surface. occurs when wind blows across the ocean’s surface and pushes water away from an area. Deeper colder water then rises to replace it. often occurs along coastlines. brings cold, nutrient-rich water from deep in the ocean to the ocean’s surface.

Major Ocean Currents Surface currents extend to about 400 m below the surface, and they move as fast as 100 km/day. Earth’s major wind belts, called prevailing winds, influence the formation of ocean currents and the direction they move.

Coriolis Effect The Coriolis Effect is the movement of wind and water to the right or left that is caused by Earth’s rotation. It causes fluids such as air and water to curve to the right in the Northern hemisphere, in a clockwise direction. The Coriolis effect also cases fluids to curve to the left in the southern hemisphere, in a counterclockwise direction.

Other Impacts The shapes of continents and other land masses affect the flow and speed of currents. Currents form small or large loops and move at different speeds, depending on the land masses they contact.

An important ‘current’ event: Thermohaline Circulation (THC) THC creates a world wide current system called the “global conveyor belt" The global conveyor belt begins with sinking of cold, dense water near the North Pole in North Atlantic Cold temps + Sea ice = cold, salty, dense water that sinks Then water moves south and circulates around Antarctica, where cold salty conditions “recharge” it The water then moves northward to the Indian, Pacific and Atlantic ocean basins It can take around 1,000 years for water to complete one cycle of the entire global conveyor belt! Teacher’s Note: Sea ice formation increases salinity because, the salt is left behind in the water as the sea ice forms. As the cold, salty dense water sinks, surface water moves in to replace it thereby starting a current. When water reaches Antarctica, the cold, salty conditions lead to more sinking, thus “recharging the current”.

A map of the global conveyor belt Photo: NASA Photo: http://www.jpl.nasa.gov/news/news.cfm?release=2010-101; Accessed: November 2010

Student activity In today’s activity, we will play a game to learn the names and locations of the ocean’s currents