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Chapter 6
Wind and Ocean Circulation
LEARNING OBJECTIVES
1. Understand atmospheric circulation as it relates to ocean circulation.
2. Explain the Coriolis effect and its role in atmospheric and oceanic circulation.
3. Describe surface circulation and geostrophic flow.
4. Explain thermohaline circulation and stratification of the ocean water column.
5. Understand the relationship between surface circulation and deep ocean
circulation.
CHAPTER OUTLINE
6-1. Atmospheric Processes
A. Density of air is controlled by temperature, pressure, and moisture content.
1. Warm air is less dense than cold air and moist air is less dense than dry
air.
2. Air pressure is the weight of the air from Earth’s surface to the top of
the atmosphere and equals 1.04kg/cm2 (standard air pressure, one
atmosphere) at sea level.
3. Low pressure zone is where air density is lower than in surrounding
areas because the air is warmer or has a higher moisture content.
4. High-pressure zone is where air pressure is higher than in surrounding
area because of cooling or lower moisture content.
5. Fluids (air and water) flow from areas of high pressure to areas of low
pressure.
7. Change in pressure across a horizontal distance is a pressure gradient.
- Greater the difference in pressure and the shorter the distance
between them, the steeper the pressure gradient and the stronger
the wind.
8. Movement of air across a pressure gradient parallel to Earth’s surface is
called a wind and winds are named for the direction from which they
come. In contrast, ocean currents are named for the direction towards
which they travel.
B. Rotation of the Earth strongly influences winds.
1. Global winds blow in response to variation in pressure related to
uneven solar heating (insolation) of Earth’s surface.
a. Equatorial low pressure occurs because intense heating and
high moisture content makes the air less dense. Rising air
produces a convergence at the surface into which air flows and
rises.
b. Polar high pressure occurs as cold, dry dense air sinks to the
surface and flows outward forming an area of divergence.
2. Coriolis deflection is the apparent deflection of objects moving across
Earth’s surface to the right of direction of travel in the Northern
Hemisphere and to the left of direction of travel in the Southern
Hemisphere.
a. The deflection is caused by the differential speed of rotation
between Earth’s equator and polar regions, relative to an object
moving across the Earth’s surface.
b. Coriolis deflection increases poleward and as speed increases.
C. Three major convection cells are present in each hemisphere.
1. The Hadley cell extends from the Equator to about 30o latitude.
a. At the equator warm moist air expands, becomes less dense, and
rises, forming a zone of low pressure called the doldrums.
b. As the air continues to rise, it cools and most of its moisture
condenses and falls as tropical rainfall.
c. At high altitude the air flows poleward, becoming cooler and
denser, until it sinks to Earth’s surface at about 30o latitude,
forming a high-pressure zone called the horse latitudes.
d. Most of this air flows southward, as the Coriolis-deflected trade
winds, gradually warming and absorbing moisture. As the air
approaches the equator it again rises and is recycled.
2. The Ferrel Cell extends from 30 o to about 50 o latitude.
a. Some of the air descending at 30o latitude flows poleward as the
Coriolis-deflected westerlies to about 50o where they encounter
cold, dense air moving equatorward from the poles.
b. The less dense warmer air is forced up over the denser cold air at
the polar front and much of it flows equatorward at high altitude
to about 30o, where it enters the horse latitudes.
3. The Polar Cell extends from 90o to about 50o latitude.
a. Some air from the polar front travels poleward becoming cold
and dry.
b. At the pole the air descends, forming a polar high-pressure
zone, and flows outward as the Coriolis-deflected polar
easterlies, which eventually collide with the westerlies at the
polar front.
6-2. Surface Ocean Currents
D. Wind-driven currents are produced by the interaction between the wind and
the water.
1. As wind moves across the water, collision of air molecules with water
molecules inefficiently transfers energy from the air to the water.
- Water moves at about 3-4% of the wind speed.
2. Zonal wind flow is wind moving nearly parallel to latitude as a result
of Coriolis deflection.
3. Westerly-driven ocean currents in the trade winds, easterly-driven
ocean currents in the Westerlies and deflection of the ocean currents by
the continents results in a circular current, called a gyre, which
occupies most of the ocean basin in each hemisphere.
E. Pressure gradients develop in the ocean because the sea surface is warped
into broad mounds and depressions with a relief of about one meter.
1. Mounds are caused by convergences, places where water flows
together and sinks.
2. Depressions are caused by divergences, places from where water rises
to the surface and flows outward.
3. Water flowing down pressure gradients on the ocean’s irregular surface
is deflected by Coriolis and the amount of deflection is a function of
location and speed.
F.
With time, wind-driven surface water motion extends downward into the
water column, but speed decreases and direction changes because of Coriolis
deflection.
1. Eckman Spiral is the spiraling pattern described by changes in water
direction and speed with depth.
2. Eckman transport is the net transport of water by wind-induced
motion.
- Net transport of the water in an Eckman spiral has a Coriolis
deflection of 90o to the direction of the wind.
3. Along coastal areas Eckman transport can induce downwelling or
upwelling by driving water towards or away from the coast,
respectively.
a. Upwelling is when near shore surface water is driven seaward,
resulting in deeper water being pulled to the surface.
- Upwelling also occurs between gyres.
b. Downwelling is when near shore surface water is driven
landward, and its accumulation depresses the deep water.
- Downwelling also occurs in the center of the gyres.
G. Langmuir circulation is a complex horizontal helical (spiral) motion that
extends parallel to the wind.
1. Adjacent helices rotate in opposite directions creating alternating zones
of convergence and divergence.
2. Material floating on the surface becomes concentrated in the zones of
convergence and form sea stripes which parallel the wind direction.
H. Geostrophic flow allows currents to flow long distances with no apparent
Coriolis deflection.
1. Coriolis deflects water into the center of the gyres, forming a low
mound.
2. As height of the mound increases, the pressure gradient steepens
pushing the water outward in an attempt to level the mound.
3. When the pressure gradient equals coriolis deflection, the current flows
parallel to the wind around the mound as a geostrophic current and
this is called geostrophic flow.
4. Gyres in the northern hemisphere rotate clockwise and in the southern
hemispheres counterclockwise.
5. The current flow pattern in gyres is asymmetrical with narrow, deep
and swift currents along the basin’s western edge and broad, shallow
slower currents along the basin’s eastern edge.
- Western boundary current intensification is the result of the
Coriolis effect increasing with distance from the equator, forcing
the eastward-flowing current to turn to the south much more
strongly and quickly than the westward –flowing equatorial
currents that turn to the north.
6. The geostrophic mound is deflected to the western part of the ocean
basin because of the eastward rotation of the Earth on its axis.
- This creates a steeper slope on the western side of the mound
because more water is being forced into the mound by the greater
deflection from the faster flowing current.
7. The Sargasso Sea is a large lens of warm water encircled by the North
Atlantic gyre and separated from cold waters below and laterally by a
strong thermocline.
8. Western boundary currents, such as the Gulf Stream, form a
meandering boundary separating coastal waters from warmer waters in
the gyre’s center.
a. Meanders of the current can break free and form rings, large
swirling migrating eddies, hundreds of kilometers in diameter and
kilometers thick. They can last for years until they are reabsorbed
into the gyres.
b. Warm-core rings are those which enclose a mass of warmer
water compared to the body of water in which they located.
c. Cold-core rings are those which enclose a mass of cooler water
compared to the body of water in which they located.
6-3. Deep-Ocean Circulation
I. Thermohaline circulation is a density driven flow of water generated by
differences in salinity or temperature.
1. Water at the surface is exposed to more rapid changes in salinity
through evaporation or precipitation and in temperature through cooling
or heating.
2. Once water is isolated from the atmospheric influences, salinity and
temperature are largely set for an extended period of time.
3. Based upon depth, surface water masses can be broadly classified as:
a. Central waters—from 0 to 1 km,
b. Intermediate waters—from 1 to 2 km,
c. Deep and bottom waters—greater than 2 km.
4. Most deep and bottom water originated at the surface where cooling
and increased salinity raised their density until they sank.
5. Ocean basins interconnect and exchange water with each other and with
the surface. Inter-ocean basin circulation and exchange between surface
and deep water appears largely driven by waters of the North Atlantic.
J.
The major thermohaline currents appear to flow mainly equatorward, but this
is because they originate in the polar regions and their outward flow is
confined between the continents.
1. Warmer water (>10oC) is confined between 45o north and south
latitude.
2. Poleward of 45o, density of water increases because of declining
temperature and increased salinity because of evaporation or ice
formation.
3. The water sinks to a density-appropriate level and then slowly flows
outward in all directions across the basin until they are blocked by a
continent.
4. Deep water gradually mixes with other water masses and eventually
rises to the surface.
5. The Atlantic Ocean has the most complex ocean stratification
containing the following layers:
a. Antarctic Bottom Water forms in the Weddell Sea mainly in
the winter when water is very cold and ice formation increases
salinity.
- It is the densest water in the ocean and flows northward into
the N. Atlantic.
b. Antarctic Deep Water forms a less saline and slightly warmer
water mass off the coast of Antarctica where conditions are not as
less severe.
- It flows northward at the surface to the Antarctic
convergence, where it sinks and then continues northward
between the Antarctic Bottom Water and the North Atlantic
Deep Water.
c. North Atlantic Deep Water forms a cold, salty water mass off
the coast of Greenland that sinks and flows southward along the
bottom until it encounters and rides over the denser Antarctic
Bottom Water.
d. Arctic Intermediate Water forms in the Subarctic region and
flows southward between the surface water of the Sargasso Sea
and the North Atlantic Deep water.
e. Mediterranean Intermediate Water flows out of the
Mediterranean Sea as a warm, highly saline water mass that sinks
to about 1 km and then spreads westward across the Atlantic
Ocean
6. The Pacific Ocean has a less complex stratification, is weakly layered,
displays sluggish circulation and is remarkably uniform below 2000m.
a. Common Water is main water mass and is formed by a blending
of the Antarctic Bottom Water and North Atlantic Deep Water in
the Antarctic Circumpolar Current.
- It is injected into the Pacific basin and flows northward to
10oS where it meets Pacific Subarctic Water.
b. Above 2000m, the North Pacific Intermediate Water and
Antarctic Intermediate Water dominates.
7. The Indian Ocean has the simplest stratification consisting of:
a. Common Water which occupies most of the basin.
b. Antarctic Intermediate Water.
c. Red Sea Intermediate Water.
- It is a warm and highly saline (40 o/oo) water from the Red
Sea that flows into the Indian Ocean and sinks to about
3000 m.
8. Climate change affects thermohaline circulation.
a. Increased freshwater influx to the North Atlantic will reduce
salinity and density of surface water, and could shut down
NADW downwelling, changing weather patterns in Europe and
North America.
b. There is evidence that North Atlantic circulation shut down in
the Holocene, reversing the warming trend at the end of an ice
age.
6-4. Water Flow in Semi-enclosed Seaways
K. Most seas are indentations into continents, partially isolated from the ocean
and strongly influenced by continental climate and river drainage.
1. As Atlantic Ocean water flows through the Straits of Gibraltar into the
Mediterranean Sea at the surface, warm, highly saline Mediterranean
Sea water flows out through the Straits at the bottom.
a. As the surface water flows eastward, salinity increases because of
excess evaporation.
b. Sea surface slopes eastward and surface water flows down the
resulting pressure gradient.
c. At the eastern edge of the basin the warm, but highly saline (38
o/oo), water sinks, flows westward and eventually out into the
Atlantic Ocean as Mediterranean Intermediate Water where it
sinks to 1000 m.
e. Mediterranean Sea is nutrient poor because there is no upwelling
of deep water to resupply nutrients.
3. In the Black Sea the surface water is brackish because of excess
precipitation and river inflow.
a. The water column is highly stratified with a strong halocline and
thermocline preventing mixing.
b. Water below the pycnocline is anoxic and rich in dissolved
hydrogen sulfide gas (H2S) because of the decay of organic
material sinking from the very productive surface water.
c. Cooling of surface water combined with strong storms results in
vertical mixing of the hydrogen sulfide into the surface waters,
causing massive fish kills.