Circulation of the Ocean

Oceanography, An Invitation to Marine Science | 9e Tom Garrison

• Ocean circulation is driven by winds and by differences in water density

• Along with the winds, ocean currents distribute tropical heat worldwide – Surface currents- wind-driven movements of water at or near the ocean’s surface-move in circular circuits—gyres— around peripheries of major ocean basins – Thermohaline currents are the slow, deep currents that affect the vast bulk of seawater beneath the pycnocline

• El Niño and La Niña affect ocean and atmosphere – El Niño is an exception to normal wind and current flow

© 2016 Cengage Learning. All Rights Reserved. Mass Flow of Ocean Water Is Driven by Wind and Gravity • Currents – mass flow of water – Surface currents • Wind-driven • At the ocean surface – Thermohaline currents • Slow, deep currents below the pycnocline • Depend on density differences caused by temperature and salinity

• The primary force driving surface currents is wind-10% of water in the ocean • Surface currents, water flowing horizontally in the uppermost 400 meters, flow around the periphery of ocean basins • The prime movers are the powerful westerlies and the persistent trade winds (easterlies). • 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. © 2016 Cengage Learning. All Rights Reserved. Surface Currents Are Driven by Winds

The North Atlantic gyre, a series of four interconnecting currents with different flow characteristics and temperatures.

The effect of Ekman spiraling and the Coriolis effect cause the water within a gyre to move in a circular pattern.

The movement of water away from point B is influenced by the rightward tendency of the Coriolis effect and the gravity-powered movement of water down the pressure gradient. © 2016 Cengage Learning. All Rights Reserved. Ekman spiral

• The Ekman spiral and the mechanism by which it operates. • The length of the arrows in the diagrams is proportional to the speed of the current in each layer.

Though the direction of movement varies for each layer in the stack, the theoretical net flow of water in the Northern Hemisphere is 90° to the right of the prevailing wind force.

The movement of water away from point 2 is influenced by the rightward tendency of the Coriolis effect and the gravity-powered movement of water down the pressure gradient.

© 2016 Cengage Learning. All Rights Reserved. Water Cannot Flow Uphill • Hills in the mid-oceans • Maintained by wind energy • The surface of the North Atlantic is raised through wind motion and Ekman transport to form a low hill. Water from point 2 turns westward, flows along the side of the hill.

Westward-moving water is balanced between the Coriolis effect (which would turn the water to the right) and flows down the pressure gradient, driven by gravity (which would turn it to the left).

Thus, water in a gyre moves along the outside edge of an ocean basin. © 2016 Cengage Learning. All Rights Reserved. Water Cannot Flow Uphill • The hill is formed by Ekman transport. Water turns clockwise (inward) to form the dome, then descends, depressing the thermocline. The hill is formed by Ekman transport. Water turns clockwise (inward) to form the dome, then descends, depressing the thermocline.

© 2016 Cengage Learning. All Rights Reserved. Water Flows in Six Surface Gyres Geostrophic gyres–in balance between pressure gradient and Coriolis effect Five geostrophic gyres-One exception – Antarctic Circumpolar Current

Names and usual direction of the world ocean’s major surface currents. The powerful western boundary currents flow along the western boundaries of ocean© 2016 Cengage basins Learning. in both All Rights hemispheres. Reserved. Boundary Currents Have Different Characteristics

• Western boundary currents – Narrow, deep, fast currents found at the western boundaries of ocean basins.

– The Gulf Stream, Japan Current, and the Brazil Current

– Agulhas Current and the Eastern Australian Current

• Eastern boundary currents – Cold, shallow and broad, and their boundaries are not well defined.

– The Canary Current, Benguela Current, and the California Current

– The West Australian Current and Peru Current

• Countercurrents – flow on the surface in the opposite direction

• Undercurrents – beneath surface currents

• Calm centers-dead zones in the center of Gyres

The general surface circulation of the North Atlantic. The unit used to express volume in transport in ocean currents is the sverdrup (sv), named in honor of Harald Sverdrup, one of this century’s pioneering oceanographers.

The numbers indicate flow rates in sverdrups (1 sv = 1 million cubic meters of water per second).

General summer air circulation patterns of the US east and west coasts.

Warm ocean currents (red); cold currents, (blue).

Air is chilled as it approaches the west coast and warmed

• Wind-induced vertical circulation – Upwelling-upward motion of water. This motion brings cold, nutrient rich water towards the surface near the equator. Can have coastal upwelling. – Downwelling- downward motion of water. It supplies the deeper ocean with dissolved gases. – Wind can cause coastal downwelling • Langmuir circulation – Long sets of counter-rotating gyres at the ocean surface

Equatorial Upwelling South Equatorial Current, straddles geographical equator. Water north of equator veers to the right, and water to the south veers to the left. Surface water therefore diverges, causing upwelling.

In the Northern Hemisphere, coastal upwelling can be caused by winds from the north blowing along the west coast of a continent. Water moved offshore by Ekman transport is replaced by cold, deep, nutrient laden water © 2016 Cengage Learning. All Rights Reserved. Coastal Upwelling Wind Can Cause Vertical Movement of Ocean Water

Shallow, slowly twisting cells of surface water are known as Langmuir circulation in honor of the researcher who explained their motion.

Wind blowing from the south along a Northern Hemisphere west coast for a prolonged period can result in downwelling. Areas of downwelling are often low in nutrients and therefore relatively low in biological productivity.

© 2016 Cengage Learning. All Rights Reserved. El Niño and La Niña Are Exceptions to Normal Wind and Current Flow • Southern Oscillation – changes in winds across the tropical Pacific • El Niño (ENSO) – Warmer than normal waters in the eastern Pacific

El Niño year Non-El Niño year © 2016 Cengage Learning. All Rights Reserved. El Niño and La Niña Are Exceptions to Normal Wind and Current Flow • La Niña – Colder than normal waters in the eastern Pacific

© 2016 Cengage Learning. All Rights Reserved. Thermohaline Circulation Affects All the Ocean’s Water • The movement of water due to different densities is thermohaline circulation. • Because the ocean is density stratified, the densest (heaviest) water is at the bottom. • There are five common water masses: – Surface water (to 200 meters) – Central water (to bottom of main thermocline) – Intermediate water (to 1500 meters) – Deep water (below intermediate water to 4000 meters)

© 2016– CengageBottom Learning. waterAll Rights Reserved. (in contact with seafloor) Different Combinations of Temperature and Salinity can Yield the Same Density S curve tracks the combination of temperature and salinity(density) with increasing depth. Points 1 and 2 are on the same gently curving isopycnal—line of equal density. Seawater at points 1 and 2 have different temperatures and salinities but the same density. If these two masses of water were to merge, their combined density would be greater than either of their separate densities—a situation represented at point 3. This process is known as caballing and leads to the formation of deep © 2016 Cengage Learning. All Rights Reserved. water. Thermohaline and Surface Flow are Connected • Polar regions – formation of bottom water – Downwelling • Water masses move slowly across ocean bottom, rise slowly (How slowly?) • Convergence zones • Contour currents – bottom currents

• Look up these concepts in the book!

A model of thermocline circulation caused by heating in lower latitudes and cooling in higher latitudes. The thermocline at middle and low latitudes is “held up” by the slow upward movement of cold water.

The water layers and deep circulation of the Atlantic Ocean. Arrows indicate the direction of water movement. Convergence zones are areas where water masses approach one another.

Global pattern of deep circulation resembles a vast “conveyor belt” that carries surface water to the depths and back again.