Surface Currents

Origin of Currents Ocean surface currents are wind driven Ocean surface currents are wind driven Air movement due to less dense air rising and more dense air sinkingAir movement due to less dense air rising and more dense air sinking Horizontal air flow along Earth’s surface is windHorizontal air flow along Earth’s surface is wind Air circulating in this manner is convection currentsAir circulating in this manner is convection currents

Convection Currents Air becomes less dense when: Air becomes less dense when: – It is warmed – Atmospheric pressure ↓ – Water vapor (humidity) ↑ Air becomes more dense when:Air becomes more dense when: – It is cooled – Atmospheric pressure ↑ – Water vapor (dry air) ↓

Wind Movement Non-rotating Earth Simple wind pattern Simple wind pattern – Warm air rises at equator, flows toward poles – Air cools at poles, sinks, and flows toward equator Winds named by direction from which they blowWinds named by direction from which they blow – North-blowing winds = southerly winds – South-blowing winds = northerly winds

Wind Movement Rotating Earth At equator, warm air risesAt equator, warm air rises – Zone of low pressure – Clouds and precipitation – Reaches troposphere and moves poleward – As it spreads, it cools 30° N&S, cool air sinks30° N&S, cool air sinks – Area of high pressure – Dry conditions – Location of world deserts 60° N&S, air masses meet60° N&S, air masses meet – Form Polar Front – Air masses rise, diverge and 90° and 30° N&S

Rotation on a Globe

Speed of Rotation

Apparent Deflection

Coriolis on a Cylinder

Coriolis impact on fast moving object minimal Coriolis impact on fast moving object minimal Impact on slow moving wind/ocean current greater Impact on slow moving wind/ocean current greater Current flowing 0.5 lat, travels 1800 m/hr, deflected 300 m from original path Current flowing 0.5 lat, travels 1800 m/hr, deflected 300 m from original path

Wind Movement Coriolis Effect Deflected winds due to movement over spinning object Deflected winds due to movement over spinning object In Northern Hemisphere:In Northern Hemisphere: In Southern Hemisphere:In Southern Hemisphere: – Winds are deflected to the right – Travel clockwise around high P – Winds are deflected to the left – Travel counter-clockwise around high P – Produce wind bands Assume water-covered Earth

Surface Currents Uneven solar heating produces ocean temperature patternUneven solar heating produces ocean temperature pattern Clockwise rotation in Northern HemisphereClockwise rotation in Northern Hemisphere Ocean currents redistribute heat and influence climateOcean currents redistribute heat and influence climate

Surface Currents and Coriolis Effect Moving water deflected by Coriolis EffectMoving water deflected by Coriolis Effect Wind deflection creates ocean circulation gyresWind deflection creates ocean circulation gyres Ocean currents are driven by windOcean currents are driven by wind Water “piles up”, gravity causes H 2 O to flow down slopeWater “piles up”, gravity causes H 2 O to flow down slope

Ekman Transport Net water movement 90° to right of wind direction in N. Hemi.Net water movement 90° to right of wind direction in N. Hemi. Winds over H 2 O set ocean surface currents in motionWinds over H 2 O set ocean surface currents in motion Surface H 2 O is deflected 45° by Coriolis EffectSurface H 2 O is deflected 45° by Coriolis Effect In Northern Hemisphere Deflection increases with depthDeflection increases with depth

Upwelling and Downwelling West Coast of N. AmericaWest Coast of N. America – Northerly winds  summer upwelling – Southerly winds  winter downwelling

Divergence and Convergence ConvergenceConvergence – Wind-driven currents collide – Downwelling DivergenceDivergence – Currents move away – Upwelling Equatorial regionEquatorial region – Created by SE and NE trade winds – Divergence – Upwelling

Note Equatorial Upwelling

Peru Upwelling

California Upwelling Spring & Summer High Productivity Diatomaceous sediments accumulate, preserved in basins

Geostrophic Currents Gyres Convergence thickens surface layer – builds a domeConvergence thickens surface layer – builds a dome Circular current systems in major ocean basins: GyresCircular current systems in major ocean basins: Gyres gyre Boundary currents parallel to ocean marginsBoundary currents parallel to ocean margins

Pacific Currents Equatorial and boundary currents  ConvergenceEquatorial and boundary currents  Convergence – Warm currents in West Pacific – Cold currents in East Pacific i.e. Kuroshio, Australia i.e. California, Peru

California Current Most studied-separates from east flowing North Pacific Current Flow from shelf break to 1000km offshore Movement down to 500m, speeds to.5m\sec Interacts with sea floor Eddy produces counter current

California Current Colder water transported southward Changes in current strenght,position impacts sediment accumulation Changes associated with El Nino/La Nina events, glacial/interglacial period

Santa Barbara Basin Core-Monterey Rocks Laminated diatomaceous sediment Laminated diatomaceous sediment Changes in forams distinct at 1925 and later. More tropical-subtropical forms Changes in forams distinct at 1925 and later. More tropical-subtropical forms Low oxygen level preserve layers Low oxygen level preserve layers (c) 2005 David Field

Western Intensification Gyres displaced to west by Earth’s rotationGyres displaced to west by Earth’s rotation Western IntensificationWestern Intensification – Faster, narrower currents Eastern CurrentsEastern Currents – Slower, more diffuse currents

El Niño Year Factors producing El Niño yearFactors producing El Niño year – Warmer H 2 O moves east – Rainfall shifts from eastern to western Pacific May result from atmospheric pressure changesMay result from atmospheric pressure changes – Thermocline deepens – Trade winds weaken – Upwelling ceases along western S. America

El Niño Impact Leads to death of cold-water organismsLeads to death of cold-water organisms Warm-water organisms migrate north and southWarm-water organisms migrate north and south

Temperature Variations britannica