Ocean CirculationCurrent Notes

Key Points• Warming and expansion of the

atmosphere causes winds which drag the water along

• The effects of temperature and the rotation of the earth on its’ axis and the positions of the continents lead to the complex patterns found on the sea surface.

• Currents have considerable effect on human activities. From transportation to moderation of local climates, humans depend on ocean currents.

Surface Circulation

• Surface currents are a response to the uneven heating of the earth by the sun.

• Temperature differences cause warmer water near the equator to swell and move toward the poles.

• Wind energy is converted to water movements called "currents" by friction between the wind and the water surface. The surface currents resemble the surface winds. Once these surface currents are set in motion they are influenced by three other factors: Coriolis effect, presence of coasts, and horizontal pressure gradients.

The Coriolis Effect• Objects, or ocean currents, moving through the Northern

hemisphere are deflected to the right because of changes in angular momentum that occur with the changes in latitude.

• Because of the earth's rotation, any freely moving object or fluid will appear to: – Turn to the right of its direction of motion in the

Northern Hemisphere and – Turn to the left of its direction of motion in the Southern

Hemisphere. • This causes currents to travel clockwise around high

pressure systems in the Northern Hemisphere, and counter-clockwise in the Southern Hemisphere.

Wind Patterns• Surface currents are driven by winds. • The wind is again influenced by the Earth's

rotation and in the latitudinal band of 30-60 degrees north and south, the Westerlies move currents towards the northeast (in the Northern Hemisphere) and southeast in the Southern Hemisphere.

• Between the equator and 30 degrees north and south, the trade winds blow toward the southwest in the Northern Hemisphere and the northwest in the Southern Hemisphere.

• The air currents moving along the surface of the Earth from the poles (90 degrees) to 60 degrees north and south of the equator, flow from east to west and are referred to as the polar easterlies

Surface Wind Patterns

Gyres• Winds together with the direct action of the

Coriolis force move tremendous volumes of surface water in a large circular patterns known as gyres which are centered around 30 degrees latitude both north and south. These gyres move clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere.

The Ekman Current• In 1905, Eckmann determined a theoretical value

for this phenomenon known as the Eckmann spiral. It contributes to the creation of the gyres.

• Ekman currents always carry water to the right of the wind in the northern hemisphere. The Ekman currents push warm water to the center of the ocean. This surface convergence creates a mound of surface water that is elevated up to one meter above the equilibrium sea level.

The Effect of Currents

The currents regulate earth’s climate by transporting heat from tropical to polar

regions and greatly affect the geographic distribution of organisms.

Thermohaline CirculationAnd the Great Ocean Conveyor

The Three Layer Ocean (A review)• Surface layer• Intermediate Layer (\cline)• Deep or Bottom Layer

Mixing (Overturn)• Downwelling

– Surface water sinks and displaces/mixes with deep water

– Temp and Density profile is consistent and straight down.

Upwelling• Where offshore surface water movements

transport water away from an area, this area is compensated by the upward movement of deeper waters. The process of upwelling brings nutrient rich waters to the surface allowing large phytoplankton blooms to develop and feed which in turn support major fisheries.

• This is a common process near coastlines of continents when the water carried out to the open ocean is replaced with water coming from depths between 100 and 200m.

The Great Ocean Conveyor• Global Thermohaline circulation• Critical in regulating the earth’s

climate• Brings dissolved oxygen to the

deep ocean.

Cold water that flows north in the Atlantic Ocean forms around Antarctica. Antarctic Bottom Water forms on the continental shelf and sinks to spread through the bottom of the world's oceans. It is the coldest, deepest water in the ocean. Antarctic Intermediate Water forms further north and flows at a shallower depth.

Marine Biology Text ReadingPages 53-61