Transcript surface currents

Chapter 8
Circulation of the Atmosphere
Solar Radiation - initial source of energy to the
Earth. It can be absorbed, reflected and
reradiated. The redistribution of this energy
controls the structure and dynamics of the
Atmosphere and Oceans.
Gases: permanent and
variable
 Permanent =
present in
constant relative
% of total volume
 Variable =
concentration
changes with time
and location
Suspended
microscopic particles
Water droplets
Greenhouse
Gases
The Atmosphere Moves in Response to Uneven
Solar Heating and Earth’s Rotation
An estimate of the heat budget for Earth. On an average day, about half of
the solar energy arriving at the upper atmosphere is absorbed at Earth’s
surface. Light (short-wave) energy absorbed at the surface is converted into
heat. Heat leaves Earth as infrared (long-wave) radiation. Since input equals
output over long periods of time, the heat budget is balanced.
Earth’s Uneven Solar Heating Results in LargeScale Thermal Cell type of Atmospheric Circulation
A convection cell is driven by density differences
A convection current forms
in a room when air flows
from a hot radiator to a cold
window and back.
Air warms, expands,
becomes less dense, and
rises over the radiator. Air
cools, contracts, becomes
more dense, and falls near
the cold glass window.
Earth’s Uneven Solar Heating Results in LargeScale Atmospheric Circulation
What factors govern the global circulation of air?
• Uneven solar heating
• The Coriolis effect
Enters Co!
The Coriolis effect is the observed deflection of a moving object,
caused by the moving frame of reference on the spinning Earth.
How does this apply to the atmosphere?
As air warms, expands, and rises at the equator, it moves toward the
pole, but instead of traveling in a straight path, the air is deflected
eastward.
In the Northern Hemisphere air turns to the right.
In the Southern Hemisphere air turns to the left.
The Coriolis Effect Influences the Movement of
Air in Atmospheric Circulation Cells
Global air circulation as described in the six-cell circulation model. Air rises at
the equator and falls at the poles, but instead of one great circuit in each
hemisphere from equator to pole, there are three in each hemisphere. Note the
influence of the Coriolis effect on wind direction. The circulation show here is
idea – that is, a long-term average of wind flow.
Wind bands – Three convection
cells in each hemisphere
Trade winds = NE (30°N to
0°) and SE (30°S to 0°)
Westerlies = 60°N to
30°N and 60°S to 30°S
Polar easterlies = 90°N
to 60°N and 90°S to
60°S
Low pressure at 0°, 60°N,
and 60°S
Low pressure, ascending air,
clouds, increased
precipitation
High pressure at 30°N, 30°S,
90°N, and 90°S
High pressure, descending
air, clear skies, low
precipitation
The Coriolis Effect Influences the Movement of
Air in Atmospheric Circulation Cells
A large circuit of air is called an atmospheric circulation cell.
Three cells exist in each hemisphere.
Hadley cells are tropical cells found on each side of the equator.
Ferrel cells are found at the mid-latitudes.
Polar cells are found near the poles.
What are some of the wind patterns found between and within
cells?
Doldrums are calm equatorial areas where two Hadley cells
converge
Horse latitudes are areas between Hadley and Ferrel cells.
Trade winds are surface winds of Hadley cells.
Westerlies are surface winds of Ferrel cells.
Sea Breezes and Land Breezes Arise from Uneven
Surface Heating
The flow of air in coastal regions
during stable weather conditions.
(a) In the afternoon, the land is
warmer than the ocean surface,
and the warm air rising from the
land is replaced by an onshore
sea breeze.
(b) At night, as the land cools,
the air over the ocean is now
warmer than the air over the
land. The ocean air rises. Air
flows offshore to replace it,
generating an offshore flow (a
land breezes).
In the Northern Hemisphere
Air flows clockwise around high pressure systems
Air flows counterclockwise around low pressure systems
Storms Are Variations in Large-Scale Atmospheric
Circulation
Storms are regional atmospheric disturbances. Storms have high winds
and most have precipitation.
Tropical cyclones occur in tropical regions. These storms can cause
millions of dollars worth of damage and endanger life.
Extratropical cyclones occur in Ferrel cells, and are winter weather
disturbances. These storms can also cause extensive damage.
Both types of storms are cyclones, or rotating masses of low-pressure
air.
Extratropical Cyclones Form between Two Air
Masses
(a) The genesis and early
development of an
extratropical cyclone in
the Northern Hemisphere
(b) How precipitation
develops in an
extratropical cyclone.
These relationships
between two contrasting
air masses are
responsible for nearly all
the storms generated in
the polar frontal zone and
thus responsible for the
high rainfall within these
belts and the decreased
salinities of surface
waters below.
Tropical Cyclones Form in One Air Mass
The tracks of tropical cyclones. The breeding grounds of tropical cyclones are
shown as orange-shaded areas. The storms follow curving paths: First they
move westward with the trade winds. Then they either die over land or turn
eastward until they lose power over the cooler ocean of mid-latitudes. Cyclones
are not spawned over the South Atlantic or the southeast Pacific because their
waters are too chilly; nor in the still air - the doldrums - within a few degrees
of the equator.
Chapter 8 - Summary
The interaction of ocean and atmosphere moderates surface
temperatures, shapes Earth's weather and climate, and creates most of
the sea's waves and currents.
Different amounts of solar energy are absorbed at different latitudes,
and this makes the tropics warmer than the polar regions.
Uneven solar heating causes convection currents to form in the
atmosphere and leads to areas of different atmospheric pressures. The
direction of air flow in these currents is influenced by the rotation of
Earth.
To observers on the surface, Earth's rotation causes moving air (or any
moving mass) in the Northern Hemisphere to curve to the right of its
initial path, and in the Southern Hemisphere to the left. This is known as
the Coriolis effect.
Chapter 8 - Summary
The atmosphere responds to uneven solar heating by flowing in three
great circulating cells over each hemisphere. The flow of air within these
cells is influenced by Earth’s rotation (Coriolis effect). Each hemisphere
has three large atmospheric circulation cells: a Hadley cell, a Ferrel cell,
and a polar cell (less pronounced over the South Pole).
Large storms are spinning areas of unstable air that develop between or
within air masses. Extratropical cyclones originate at the boundary
between air masses.
Tropical cyclones, the most powerful of Earth's atmospheric storms,
occur within a single humid air mass.
Chapter 9
Circulation of the Ocean
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
(1,300 feet) of the ocean’s
surface, driven mainly by
wind friction.
(left) 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).
Surface Currents
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, and winds, water
often moves in a circular pattern called a gyre.
Surface Currents Are Driven by the Winds
The gyres circulate clockwise in the
Northern Hemisphere and
counterclockwise in the Southern
Hemisphere.
The North Atlantic gyre, a
series of four interconnecting
currents with different flow
characteristics and
temperatures.
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 to
the east.
Water at point B veers
right and continues to the
west.
Surface Currents Flow around the Periphery of Ocean Basins
The Ekman spiral and the mechanism by which it operates.
Surface Currents Flow around the Periphery of Ocean Basins
Consider the North Atlantic.
The surface is raised through wind
motion and Ekman transport to form a
low hill. The westward-moving water at
B ‘feels’ a balanced pull from two
forces: the one due to Coriolis effect
(which would turn the water to the
right) and the one due to the pressure
gradient, driven by gravity (which would
turn it to the left).
The hill is formed by Ekman
transport. Water turns clockwise
(inward) to form the dome, then
descends, depressing the
thermocline.
Seawater Flows in Six Great Surface Circuits
Geostrophic gyres are gyres in balance between the pressure gradient and the
Coriolis effect. Of the six great currents in the world’s ocean, five are
geostrophic gyres. Note the western boundary currents in this map.
Boundary Currents Have Different Characteristics
Western boundary currents – These are narrow, deep,
warm, fast currents found at the western boundaries of
ocean basins.
The Gulf Stream
The Japan Current
The Brazil Current
The Agulhas Current
The Eastern Australian Current
Eastern boundary currents – These currents are cold,
shallow and broad, and their boundaries are not well
defined.
The Canary Current
The Benguela Current
The California Current
The West Australian Current
The Peru Current
Boundary Currents Have Different Characteristics
Eddy formation
The western boundary of the Gulf
Stream is usually distinct, marked by
abrupt changes in water temperature,
speed, and direction.
(a) Meanders (eddies) form at this
boundary as the Gulf Stream leaves
the U.S. coast at Cape Hatteras. The
meanders can pinch off (b) and
eventually become isolated cells of
warm water between the Gulf Stream
and the coast (c). Likewise, cold cells
can pinch off and become entrained in
the Gulf Stream itself (d). (C = cold
water, W = warm water; blue = cold,
red = warm.)
Boundary Currents Have Different Characteristics
Water flow in the
Gulf Stream and
the Canary
Current, parts of
the North Atlantic
gyre.
Surface Currents Affect Weather and Climate
Wind induced vertical circulation is vertical movement induced by winddriven horizontal movement of water.
Upwelling is the upward motion of water. This motion brings cold, nutrient
rich water towards the surface.
Downwelling is downward motion of water. It supplies the deeper ocean
with dissolved gases.
Nutrient-Rich Water Rises near the Equator
Equatorial upwelling.
The South Equatorial Current,
especially in the Pacific, straddles
the geographical equator. Water
north of the equator veers to the
right (northward), and water to
the south veers to the left
(southward). Surface water
therefore diverges, causing
upwelling. Most of the upwelled
water comes from the area above
the equatorial undercurrent, at
depths of 100 meters or less.
Wind Can Induce Upwelling near Coasts
Coastal upwelling.
In the Northern Hemisphere,
coastal upwelling can be caused
by winds from the north
blowing along the west coast
of a ccontinent. Water moved
offshore by Ekman transport
is replaced by cold, deep,
nutriend-laden water. In this
diagram, temperature of the
ocean surface is shown in
degrees Celsius.
Wind Can Also Induce Upwelling Coastal Downwelling
Coastal downwelling.
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.
El Niño and La Niña Are Exceptions to Normal Wind and Current Flow
An El Niño Year
A Non-El Niño Year
In an El Niño year, when the Southern Oscillation develops, 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.
In a 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.
Thermohaline Circulation Affects All the Ocean’s Water
The movement of water due to different densities is thermohaline
circulation.
Remember that the ocean is density stratified, with the densest water at
the bottom. There are five common water masses:
•
•
•
•
•
Surface water
Central water
Intermediate water
Deep water
Bottom water
= 0-200m
= 200-thermocline
= thermocline-1500m
= 1500-4000m
= 4000-bottom
•
Thermohaline Circulation
– Vertical, density driven circulation, resulting from change in
temperature and salinity
• Continuity of flow
– Water is a relatively fixed quantity in the oceans
– Water can not accumulate in one location or be removed from
another location without movement of water between those
locations
• Vertical movement of water
• Horizontal movement of water
Chapter 9 - Summary
• Ocean water circulates in currents caused mainly by wind friction at
the surface and by differences in water mass density beneath the
surface zone.
• Water near the ocean surface moves to the right of the wind
direction in the Northern Hemisphere, and to the left in the
Southern Hemisphere.
• The Coriolis effect modifies the courses of currents, with currents
turning clockwise in the Northern Hemisphere and counterclockwise
in the Southern Hemisphere. The Coriolis effect is largely
responsible for the phenomenon of westward intensification in both
hemispheres.
• Upwelling and downwelling describe the vertical movements of water
masses. Upwelling is often due to the divergence of surface
currents; downwelling is often caused by surface current
convergence or an increase in the density of surface water.
Chapter 9 - Summary
• El Niño, an anomaly in surface circulation, occurs when the trade
winds falter, allowing warm water to build eastward across the
Pacific at the equator.
• Circulation of the 90% of ocean water beneath the surface zone is
driven by gravity, as dense water sinks and less dense water rises.
Since density is largely a function of temperature and salinity, the
movement of deep water due to density differences is called
thermohaline circulation.
• Water masses almost always form at the ocean surface. The
densest (and deepest) masses were formed by surface conditions
that caused water to become very cold and salty.
• Because they transfer huge quantities of heat, ocean currents
greatly affect world weather and climate.
Direction of wave motion
A
B
Wavelength
Height
Still water level
Crest
Frequency: Number of wave
crests passing
point A or point B
each second
Period: Time required for
wave crest at point A
to reach point B
Trough
Orbital path of
individual water
molecule at water
surface
Fig. 10-2, p. 266
Direction of wave motion
Wave-length
Crest
1/2
wavelength
depth
Trough
Still water
level
Crest
Period (& wavelength) and Wave Energy
Disturbing
force
Restoring force
Amount of energy
in ocean surface
Type of wave
Gravity
Seismic
disruption
landslides
Wind
Surface
tension
Gravity
Tide
Tsunami
Seiche
Wind wave
Capillary wave
(ripple)
2412
hr.hr.
100,000 sec 10,000 sec 1,000 sec 100 sec 10 sec
(1 1/4 days)
(3 hr)
(17 min)
1 sec
1/10 sec
1/100 sec
10
100
Period (time, in seconds for 1
two successive wave crests to Frequency (waves
pass a fixed point)
per second)
Deep- to Shallow-Water Waves
Progressive Waves
Wave Speed
L
A
H
Keep in mind: wave energy, NOT the water particles move
across the surface of the sea. Wave propagates with C,
energy moves with V
Wave Speed is C - Group Speed is V
wave speed = wavelength / period
or
C=L/T
T is determined by generating force so it remain the same
after the wave formed, but C changes. In general, the longer
the wavelength the faster the wave energy will move through
the water.
Deep Water Waves
• Period to about 20 seconds
• Wavelength to at most 600 meters
(extreme)
• Speed to about 100
kilometers/hour (70 mi/hr)
(extreme)
gL
gT
C
 1.25 L 
 1.56T
2
2
For example, for a 300 meters wave and 14 sec
period, the speed is about 22 meters per second
Deep Water Waves
* surface waves progressing in waters of D larger than 1/2 L
* as the wave moves through, water particles move in circular orbit
* diameter of orbits decrease with depth, orbits do not reach bottom,
particles do not move below a depth D = L/2
* The wave speed can be calculated from knowledge of either the
wavelength or the wave period:
C = 1.56 m/s2 T or C2 = 1.56 m/s2 L
* Group Speed (which really transport the energy) is half of the
wave speed for deep-water waves:
V = C/2
Shallow-Water Waves
C  gD  3.1 D
Seismic Sea Waves – Shallow-Water Waves
• Period to about 20 minutes
• Wavelength of about 200 kilometers
• Speed of about 750-800 km/hr (close to 500 mi/hr!!)
Shallow-Water Waves
• surface waves generated by wind and progressing in waters of D
less than (1/20) L
• wave motion: as the wave moves through, water particles move in
elliptical orbits
• diameter of orbits remains the same with depth, orbits do reach the
bottom where they ‘flatten’ to just an oscillating motion back and
forth along the bottom
* The wave speed and the wavelength are controlled by the depth D
of the waters only:
C  gD  3.1 D
* Group Speed (which transport the energy) is the same as the wave
speed for shallow-water waves:
V=C
Wind Blowing over the Ocean Generates Waves
Waves development and growth are affected by:
Wind Speed: velocity at which the wind is blowing
Fetch:
distance over which the wind is blowing
Duration:
length of time wind blows over a given area
Larger Swell
Move Faster 
waves separate
into groups
wave separation
is called
dispersion
• Storm centers and dispersion
• Winds flow around low pressure
• Variety of periods and heights are generated  grouped
into wave trains
Waves with longer period (T) and larger length move faster
- these get ahead of the ‘pack’.
Wave sorting of these free waves is dispersion
Wave Height, Wavelength & Wave Steepness
Typical ratio wave height to wavelength in open ocean = 1:7 =
wave steepness – angle of the crest = 120°
Exceed these conditions and wave will break at sea 
whitecaps
7 across
1 high
120°
Wave Height is controlled by (1) wind speed, (2) wind duration and (3)
fetch (= the distance over water that the wind blows in the same direction
and waves are generated)
Significant Wave Height - average wave height of the highest one-third
of the waves measured over a long time
1
2
a
b
Constructive
interference
(addition)
Destructive
interference
(subtraction)
Constructive
interference
(addition)
Breaker Types
Wave Refraction – slowing and bending of waves as they
approach shore at an angle
depth contours
part of wave in shallow water
slows down
crests
part of same wave still in deep
water hence faster
oblique angle between
direction of motion of
waves and depth contours
Wave refraction- propagation of waves around obstacles, for
example over a shallow ridge – energy is focused (waves get
‘interrupted’, waves generate other waves)
Wave refraction in a shallow bay – energy is spread
Wave Diffraction
narrow opening
diffraction and wave interaction
when????
Tsunamis
• Vertical sea floor displacement
• Shallow water waves
 Long wavelength
 Low period
• Wave height changes (quite dramatically!)
 At point of origin
 Close to shore where depth decreases