Ocean Circulation

Ocean Currents

Ocean currents Surface currents –Affect surface water within and above the pycnocline (10% of ocean water) –Driven by wind belts of the world Deep currents –Affect deep water below pycnocline (90% of ocean water) –Driven by density differences –Larger and slower than surface currents

Deep ocean characteristics Conditions of the deep ocean: –Cold –Still –Dark –Essentially no productivity ( very little food) –Sparse life –Extremely high pressure

Deep currents (Thermohaline Circulation) Deep currents: –Form in subpolar regions at the surface –Are created when high density surface water sinks –Factors affecting density of surface water: Temperature (most important factor) Salinity –Deep currents are known as thermohaline circulation

Identification of deep currents Deep currents are identified by measuring temperature (T) and salinity (S), from which density can be determined

Atlantic Ocean subsurface water masses

Conveyer-belt circulation

Thermohaline Flow and Surface Flow: The Global Heat Connection (Explain in 45 to 50 words)

Global Deep Circulation resembles a vast “conveyor belt” that carries surface water to the depths and back again. This pattern starts with North Atlantic Deep Water north of Iceland. This water flows south through the Atlantic and into deep water Antarctic water. The water mass warms up as it moves north into the Indian and Pacific ocean basins. Eventually upwelling takes place and some of this water returns to the surface at the equator.

Surface Currents Are Driven by the Winds The westerlies and the trade winds are two of the winds that drive the ocean’s surface currents. About 10% of the water in the world ocean is involved in surface currents, water flowing horizontally in the uppermost 400 meters of the ocean’s surface, driven by wind friction. Winds, driven by uneven solar heating and Earth’s spin, drive the movement of the ocean’s surface currents. The prime movers are the powerful westerlies and the persistent trade winds (easterlies).

Three-Cell Model of Atmospheric Circulation

The Coriolis Effect

Ekman Transport

The effect of Ekman spiraling and the Coriolis effect cause the water within a gyre to move in a circular pattern. Surface Currents Flow around the Periphery of Ocean Basins

The surface of the North Atlantic is raised through wind motion and Ekman transport to form a low hill. Water in a gyre moves along the outside edge of an ocean basin. Surface Currents Flow around the Periphery of Ocean Basins

A combination of four forces – surface winds, the sun’s heat, the Coriolis effect, and gravity – circulates the ocean surface clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, forming gyres. Surface Currents Are Driven by the Winds

What are some effects of ocean currents? –Transfer heat from tropical to polar regions –Influence weather and climate –Distribute nutrients and scatter organisms Surface currents are driven by wind: –Most of Earth’s surface wind energy is concentrated in the easterlies and westerlies. –Due to the forces of gravity, the Coriolis effect, solar energy, and solar winds, water often moves in a circular pattern called a gyre. Surface Currents Are Driven by the Winds

Surface Currents Flow around the Periphery of Ocean Basins Surface water blown by the winds at point A will veer to the right of its initial path and continue eastward.

Seawater Flows in Five Great Surface Circuits

Boundary Currents Have Different Characteristics Western boundary currents – These are narrow, deep, fast currents found at the western boundaries of ocean basins. –Gulf Stream, Japan(Kuroshio) Current, Brazil Current –Agulhas Current and the East Australian Current Eastern boundary currents – These currents are cold, shallow and broad, and their boundaries are not well defined. –Canary Current, Benguela Current, Alaska Current, East Greenland Current and Peru Current

Western intensification of subtropical gyres The western boundary currents of all subtropical gyres are: –Fast –Narrow –Deep Eastern boundary currents of subtropical gyres have opposite characteristics

Sverdrup s (1 sv = 1 million cubic meters of water per second). Boundary Currents Have Different Characteristics

North Atlantic Ocean circulation

The Gulf Stream and sea surface temperatures The Gulf Stream is a warm, western boundary current Meanders as it moves into the North Atlantic Creates warm and cold core rings (Eddies)

Eddy formation (C = cold water, W = warm water; blue = cold, red = warm.) Boundary Currents Have Different Characteristics

Pacific Ocean Surface Currents

Antarctic surface circulation

Wind Can Induce Upwelling near Coasts Coastal upwelling: Northern Hemisphere: caused by winds from the north blowing along the west coast of a continent.

Upwelling is the upward motion of water. This motion brings cold, nutrient rich water towards the surface. Nutrient-Rich Water Rises near the Equator

Other types of upwelling Equatorial upwelling Offshore wind Sea floor obstruction Sharp bend in coastal geometry Equatorial upwelling

Wind Can Also Induce Coastal Downwelling Coastal Downwelling : Areas of downwelling are often low in nutrients and therefore relatively low in biological productivity.

Coastal upwelling and downwelling

Surface Currents Affect Weather and Climate

Currents and Climate Warm current  warms air  high water vapor  humid coastal climate Cool current  cools air  low water vapor  dry coastal climate

El Niño-Southern Oscillation (ENSO) El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmas time Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño ENSO describes a combined oceanic- atmospheric disturbance

El Niño and La Niña Are Exceptions to Normal Wind and Current Flow In an El Niño year, the trade winds diminish and then reverse, leading to an eastward movement of warm water along the equator. The surface waters of the central and eastern Pacific become warmer, and storms over land may increase.

El Niño and La Niña Are Exceptions to Normal Wind and Current Flow Non-El Niño year : normally the air and surface water flow westward, the thermocline rises, and upwelling of cold water occurs along the west coast of Central and South America.

Normal conditions in the Pacific Ocean Figure 7-18a

El Niño conditions (ENSO warm phase)

La Niña conditions (ENSO cool phase; opposite of El Niño)

The El Niño

El Niño recurrence interval Pacific has alternated between El Niño and La Niña events since 1950

Effects of severe El Niños

In Perspective Ocean water circulates in currents. Surface currents affect the uppermost 10% of the world ocean. The movement of surface currents is powered by the warmth of the sun and by winds. Water in surface currents tends to flow horizontally, but it can also flow vertically in response to wind blowing near coasts or along the equator. Surface currents transfer heat from tropical to polar regions, influence weather and climate, distribute nutrients, and scatter organisms. They have contributed to the spread of humanity to remote islands, and they are important factors in maritime commerce. Circulation of the 90% of ocean water beneath the surface zone is driven by the force of gravity, as dense water sinks and less dense water rises. Because density is largely a function of temperature and salinity, the movement of deep water due to density differences is called thermohaline circulation. Currents near the seafloor flow as slow, river-like masses in a few places, but the greatest volumes of deep water creep through the ocean at an almost imperceptible pace. The Coriolis effect, gravity, and friction shape the direction and volume of surface currents and thermohaline circulation.