Chapter 7 Ocean Circulation

Essentials of Oceanography

7th Edition

Ocean currents

Surface currentsAffect surface water within and above the pycnocline (10% of ocean water)Driven by major wind belts of the world

Deep currentsAffect deep water below pycnocline (90% of ocean water)Driven by density differencesLarger and slower than surface currents

Measuring surface currents

Direct methodsFloat meters

Intentional

Inadvertent

Propeller meters

Indirect methodsPressure 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 waterSubtropical gyres

Five main gyres (one in each ocean basin):North PacificSouth PacificNorth AtlanticSouth 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 depthsFactors:

WindCoriolis 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:

FastNarrowDeep

Western boundary currents are also warmEastern 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 downMoves 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 Figure 7-9

Equatorial upwelling

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 currentMeanders as it moves into the North AtlanticCreates 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 ChristmastimeSouthern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El NiñoENSO describes a combined oceanic-atmospheric 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 yearsPacific 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

Figure 7-23

Northeast monsoon Southwest monsoon

Deep currents

Deep currents:Form in subpolar regions at the surfaceAre created when high density surface water sinksFactors 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