Oceanography 100
P Anderson
Chapter 7
Ocean Circulation
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
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
Wind-driven surface currents
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
– 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
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
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
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
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
Other types of upwelling
• Equatorial upwelling
• Offshore wind
• Sea floor obstruction
• Sharp bend in coastal geometry
Antarctic surface circulation
Atlantic Ocean surface currents
North Atlantic Ocean circulation
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
Pacific Ocean surface currents
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 oceanic-atmospheric disturbance
Normal conditions in the Pacific Ocean
El Niño conditions (ENSO warm phase)
La Niña conditions (ENSO cool phase; opposite of El Niño)
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
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
Effects of severe El Niños
Indian Ocean surface currents
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
Atlantic Ocean subsurface water masses
Conveyer-belt circulation