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

Pattern of Ocean Currents Composed of four main currents Equatorial currents – trade winds that blow SE in SH and NE in NH. Travel westward along the equator and form the equatorial boundary current of subtropical gyres Western Boundary Current – equatorial currents reach western land turn. Coriolis effect deflects away from equator. Northern or Southern Boundary Currents – westerlies blow from the NW in SH and SW in NH. Winds direct ocean surface water in an easterly direction Eastern Boundary Currents – currents flow back, the coriolis effect and continental barriers turn them toward equator.

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 90 o to the right of the wind in the Northern Hemisphere 90 o left in Southern Hemisphere The average for surface water is about 45 o 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)

ENSO El Niño-Southern Oscillation Irregular shift in ocean and atmosphere characteristics every 2-10 years Affects global climate Harmful and beneficial consequences

El Niño ENSO Warm Phase Most obvious in Equatorial Pacific Pacific Warm Pool moves eastward across Equatorial Pacific Changes in pattern of upwelling/downwelling in eastern Pacific Ocean Fig. 8-2

ENSO warm phase effects Warmer seawater higher sea level Warmer seawater less upwelling in eastern Pacific Lower biologic productivity Warmer seawater kills some marine life Shift in atmosphere pressure Shift in areas of precipitation

La Niña ENSO Cool Phase Stronger tradewinds Warm pool stays in western Pacific Increased upwelling in eastern Pacific Shift in precipitation patterns Shift in atmospheric pressure

El Niño

La Niña

Take Notes on the following words: El Niño What triggers it? What affects does it have in different locations. Upwell Downwell Ekman Transport Gyres Geostrophic Balance Stronger Currents Counter Current Current Measurements

Pgs. To Read Pg “Ekman Spiral and Ekman transport. Pg. 241 “Diverging Surface water” Pg. 241 “Converging Surface Water” Pg “Coastal Upwelling and Downwelling” Pg 250 “El Niño-Southern Oscillation Conditions” Pg All about El Niño, and La Niña, all the way threw “Predicting El Niño Events”.

Subtropical gyres - Ekman transport piles up “hill” of seawater at about 30 o N and S Water flows downhill under gravity and veers right (Northern hemisphere) due to Coriolis Effect Circular flow Fig. 8-7