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) ↓

Warm and Cold Fronts Caused by eastward moving air massesCaused by eastward moving air masses – Warm air rises over cold, dense air – condenses and precipitates – Steeper cold front pushes warm air up – cools, condenses and precipitates

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

Wind Movement Air that sinks does not flow back in a straight north-south path – it curves (Coriolis Effect)Air that sinks does not flow back in a straight north-south path – it curves (Coriolis Effect) At equator, warm air rises, condenses and precipitatesAt equator, warm air rises, condenses and precipitates At 30° and 90°, cool air sinksAt 30° and 90°, cool air sinks

Rotation on a Globe Buffalo and Quito located on same line of longitude (79ºW)Buffalo and Quito located on same line of longitude (79ºW) Both cities circles the globe in one day (360º/24 hours = 15º/1 hour)Both cities circles the globe in one day (360º/24 hours = 15º/1 hour) Quito has larger circumference; thus, travels fartherQuito has larger circumference; thus, travels farther Quito needs to travel faster than BuffaloQuito needs to travel faster than Buffalo

Apparent Deflection Hypothetical war gameHypothetical war game If a cannonball is shot north from QuitoIf a cannonball is shot north from Quito It will travel a straight pathIt will travel a straight path But, because Earth is rotating east to westBut, because Earth is rotating east to west The cannonball appears to veer to the right in Northern HemisphereThe cannonball appears to veer to the right in Northern Hemisphere This is the Coriolis EffectThis is the Coriolis Effect

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

High and Low Pressure CyclonesCyclones AnticyclonesAnticyclones – Low pressure air – Converge – Ascend – High pressure air – Diverge – Descend In Northern Hemisphere

High and Low Pressure Areas Land masses modify wind bandsLand masses modify wind bands

Variations in Wind Daily Coastal Areas DaytimeDaytime – Warmer land; cooler sea – Onshore flow EveningEvening – Cooler land; warmer sea – Offshore flow

Variations in Wind Seasonal In summer:In summer: January July – Air rises over warmed land, condenses – Sudden and large volume of precipitation – Winds accumulate more H 2 O vapor from ocean In winter have opposite effectIn winter have opposite effect

Surface Current Circulation

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

Warm and Cold 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

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

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