Ocean Currents• Are masses of ocean water that flow from one place

to another. • Water masses in motion

• Surface Currents - wind driven currents move water horizontally and occur primarily in the ocean’s surface waters• Deep Currents – a density driven circulation that is

initiated at the ocean surface by temperature and salinity conditions that produce a high- density water mass, which sinks and spreads slowly beneath surface waters

Measuring ocean currents

•Direct measurement• Floating objects/instruments• Fixed instruments

• Indirect measurement• Distribution of density• Satellite data• Doppler flow meter• Chemical tracers• Distinctive water masses

Surface Currents

• Surface Currents – within and above the pycnocline ( layer of rapidly changing density) to a depth of about 1 km (0.6 mile) • Affects only 10% of the

world’s ocean water.

Origins Surface Currents

• Friction between the ocean and the wind that blows across its surface • 2% of wind’s energy is

transferred to the ocean surface• Driven by wind belts and

continents• Other factors

• Gravity• Friction• Coriolis effect

Pattern of ocean currents

• Subtropical gyres – large circular loops of water driven by major wind belts

Gyres

• Equatorial countercurrents - water on the western margins then flows downhill under the influence of gravity that flow to the east counter to and between the adjoining equatorial currents

• Subpolar gyres – driven in a westerly direction by the polar easterlies that rotate opposite the adjacent subtropical gyres.

Factors affecting ocean surface circulation• Ekman spiral• Geostrophic currents • Western intensification of subtropical gyres

Why does surface water move in a different direction than the wind?

Ekman Spiral

• Affects the ocean surface circulation• Circulation model developed by Swedish physicist V.

Walfrid Ekman• Describes the speed and direction of flow of

surface waters at various depths.

Ekman spiral and Ekman transport

• Ekman transport moves shallow seawater about 90o to the right of the wind in the Northern Hemisphere• 90o left in Southern

Hemisphere• The average for surface

water is about 45o

Fig. 8-6a

What causes upwelling and downwelling• Upwelling – vertical

movement of cold, deep, nutrient-rich water to the surface

• Downwelling – vertical movement of surface water to deeper parts of the ocean.

Upwelling Occurs when:

• Ekman transport moves seawater offshore• Ekman transport

moves seawater away from another water mass• Water moves up

Fig. 8-11a

• Cooler, nutrient-rich water rises vertically toward sea surface

• High biologic productivity

• Downwelling opposite

Western intensification

• “Hill” of seawater is steeper on western side• Western currents are fast, narrow, deep

Antarctic circulation

• Antarctic Circumpolar Current (or West Wind Drift)• Greatest volume• Connects main

oceans• East Wind Drift• Antarctic Divergence

(upwelling)