Chapter 7
Ocean Circulation
Essentials of Oceanography
7th Edition
Ocean currents
Surface currents
Affect surface water within and above the
pycnocline (10% of ocean water)
Driven by major 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
Measuring surface currents
Direct methods
Float meters
Intentional
Inadvertent
Propeller meters
Indirect methods
Pressure gradients
Satellites
Doppler flow meters
Figure 7B
Surface currents closely follow
global wind belt pattern
Trade winds at 0-30º
blow surface currents
to the east
Prevailing westerlies
at 30-60º blow
currents to the west
Figure 7-3
Wind-driven surface currents
Figure 7-4
Current gyres
Gyres are large circular-moving loops of water
Subtropical gyres
Five main gyres (one in each ocean basin):





North Pacific
South Pacific
North Atlantic
South Atlantic
Indian
Generally 4 currents in each gyre
Centered at about 30º north or south latitude
Current gyres
Gyres (continued)
Subpolar gyres
Smaller and fewer than subtropical gyres
Generally 2 currents in each gyre
Centered at about 60º north or south latitude
Rotate in the opposite direction of adjoining
subtropical gyres
Ekman spiral
Ekman spiral
describes the speed
and direction of flow
of surface waters at
various depths
Factors:
Wind
Coriolis effect
Figure 7-6
Ekman transport
Ekman transport is the
overall water movement
due to Ekman spiral
Ideal transport is 90º
from the wind
Transport direction
depends on the
hemisphere
Internet visualization
Figure 7-6
Geostrophic flow and western
intensification
Geostrophic flow
causes a hill to form in
subtropical gyres
The center of the gyre
is shifted to the west
because of Earth’s
rotation
Western boundary
currents are intensified
Figure 7-7
Western intensification of
subtropical gyres
The western boundary currents of all
subtropical gyres are:
Fast
Narrow
Deep
Western boundary currents are also warm
Eastern boundary currents of subtropical
gyres have opposite characteristics
Currents and climate
Warm current 
warms air  high
water vapor  humid
coastal climate
Cool current  cools
air  low water vapor
 dry coastal climate
Figure 7-8a
Upwelling and downwelling
Vertical movement of water ()
Upwelling = movement of deep water to surface
Hoists cold, nutrient-rich water to surface
Produces high productivities and abundant marine life
Downwelling = movement of surface water down
Moves warm, nutrient-depleted surface water down
Not associated with high productivities or abundant
marine life
Coastal upwelling and
downwelling
Ekman transport moves surface water away
from shore, producing upwelling
Ekman transport moves surface water
towards shore, producing downwelling
Figure 7-11
Other types of upwelling
Equatorial
upwelling
Offshore wind
Sea floor
obstruction
Sharp bend in
coastal geometry
Equatorial upwelling
Figure 7-9
Antarctic surface circulation
Figure 7-13
Atlantic Ocean surface currents
Figure 7-14
North Atlantic Ocean
circulation
Figure 7-15
The Gulf Stream and sea
surface temperatures
The Gulf Stream is
a warm, western
intensified current
Meanders as it
moves into the
North Atlantic
Creates warm and
cold core rings
Figure 7-16
Pacific Ocean surface currents
Figure 7-17
El Niño-Southern Oscillation
(ENSO)
El Niño = warm surface current in
equatorial eastern Pacific that occurs
periodically around Christmastime
Southern Oscillation = change in
atmospheric pressure over Pacific Ocean
accompanying El Niño
ENSO describes a combined oceanicatmospheric disturbance
Normal conditions in the Pacific
Ocean
Figure 7-18a
El Niño conditions (ENSO
warm phase)
Figure 7-18b
La Niña conditions (ENSO cool
phase; opposite of El Niño)
Figure 7-18c
The 1997-98 El Niño
Sea surface
temperature anomaly
map shows warming
during severe 1997-98
El Niño
Internet site for El
Niño visualizations
Current state of the
tropical Pacific
Figure 7-19a
El Niño recurrence interval
Typical recurrence interval for El Niños =
2-12 years
Pacific has alternated between El Niño and
La Niña events since 1950
Figure 7-20
Effects of severe El Niños
Figure 7-21
Indian Ocean surface currents
Northeast monsoon
Southwest monsoon
Figure 7-23
Deep currents
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 also known as thermohaline
circulation
Deep ocean characteristics
Conditions of the deep ocean:
Cold
Still
Dark
Essentially no productivity
Sparse life
Extremely high pressure
Identification of deep currents
Deep currents
are identified by
measuring
temperature (T)
and salinity (S),
from which
density can be
determined
Figure 7-24
Atlantic Ocean subsurface water
masses
Figure 7-25
Conveyer-belt circulation
Figure 7-27
End of Chapter 7
Essentials of Oceanography
7th Edition