Surface CurrentsSurface Currents

Structure of AtmosphereStructure of Atmosphere

• Almost all weather Almost all weather occurs in troposphereoccurs in troposphere– Temperature decrease Temperature decrease

with altitude in with altitude in tropospheretroposphere

• Tropopause is Tropopause is transition to transition to stratospherestratosphere– Constant temperature Constant temperature

at base, temperature at base, temperature increase in upper increase in upper portionportion

Composition of AtmosphereComposition of Atmosphere

• Lower atmosphere Lower atmosphere circulates mixes gases-circulates mixes gases-HomosphereHomosphere

• Above 80 km gases Above 80 km gases stratified-Heterospherestratified-Heterosphere– Heavier ones are lowerHeavier ones are lower

• Aerosol present in Aerosol present in AtmosphereAtmosphere– Aerosol=minute liquid and Aerosol=minute liquid and

solid particlessolid particles• Atmosphere also contains Atmosphere also contains

pollutantspollutants– Pollutant= gas or aerosol at Pollutant= gas or aerosol at

concentrations that concentrations that threaten living organismsthreaten living organisms

Origin of CurrentsOrigin of Currents

•Ocean surface currents are wind drivenOcean surface currents are wind driven

• Air movement due to Air movement due to less denseless dense air rising and air rising and more more densedense air sinking air sinking

• Horizontal air flow along Earth’s surface is Horizontal air flow along Earth’s surface is windwind• Air circulating in this manner is Air circulating in this manner is convection currentsconvection currents

Convection CurrentsConvection Currents

•Air becomes less dense when:Air becomes less dense when:

– It is warmedIt is warmed

– Atmospheric pressure Atmospheric pressure ↓↓– Water vapor (humidity) Water vapor (humidity) ↑↑

• Air becomes more dense when:Air becomes more dense when:– It is cooledIt is cooled– Atmospheric pressure Atmospheric pressure ↑↑

– Water vapor (dry air) Water vapor (dry air) ↓↓

High and Low PressureHigh and Low Pressure

•CyclonesCyclones • AnticyclonesAnticyclones– Low pressure airLow pressure air– ConvergeConverge– AscendAscend

– High pressure airHigh pressure air– DivergeDiverge– DescendDescend

In Northern Hemisphere

Wind MovementWind MovementNon-rotating EarthNon-rotating Earth

•Simple wind patternSimple wind pattern– Warm air rises at equator, flows toward polesWarm air rises at equator, flows toward poles– Air cools at poles, sinks, and flows toward equatorAir cools at poles, sinks, and flows toward equator

• Winds named by direction Winds named by direction fromfrom which they blow which they blow– North-blowing winds = southerly windsNorth-blowing winds = southerly winds– South-blowing winds = northerly windsSouth-blowing winds = northerly winds

Wind Movement Wind Movement - Rotating Earth- Rotating Earth

• At equator, warm air risesAt equator, warm air rises– Zone of low pressureZone of low pressure

– Clouds and precipitationClouds and precipitation

– Reaches troposphere Reaches troposphere and moves polewardand moves poleward

– As it spreads, it As it spreads, it coolscools• 30° N&S, cool air sinks30° N&S, cool air sinks

– Area of high pressureArea of high pressure

– Dry conditionsDry conditions

– Location of world desertsLocation of world deserts

• 60° N&S, air masses meet60° N&S, air masses meet– Form Form Polar FrontPolar Front– Air masses rise, diverge Air masses rise, diverge

and sink @ 90° and 30° and sink @ 90° and 30° N&SN&S

• Note belts of Note belts of high pressure high pressure areas at 30areas at 30oo N&S N&S

• Pressure Pressure areas linked areas linked by wind beltsby wind belts

Air rise in Intertropical Convergence Zone (ITCZ) & sinks in anticyclone

Seasonal Variation of ITCZSeasonal Variation of ITCZ•Most active weather zone, low pressure, parallels equator

•Occur where surface T is highest

•Changes position with the seasons because of tilt of earth

Sub-Saharan Sub-Saharan AfricaAfrica

• Prolonged Prolonged droughtsdroughts– Much human Much human

miserymisery• Transition from Transition from

Desert to north & Desert to north & rainforest to southrainforest to south

• Poorest nations Poorest nations on Earthon Earth

• Depend on Depend on agricultureagriculture

• Rain depends on Rain depends on ITCZITCZ

• Overgrazing also Overgrazing also compounds compounds problemproblem– Remove Remove

vegetation, less vegetation, less moisture in soilmoisture in soil

– Albedo inreases, Albedo inreases, less evaporation, less evaporation, less rainless rain

Variations in WindVariations in WindSeasonalSeasonal

•In summer:In summer:

JanuaryJuly

– Air rises over warmed land, condensesAir rises over warmed land, condenses– Sudden and large volume of precipitationSudden and large volume of precipitation

– Winds accumulate more HWinds accumulate more H22O vapor from oceanO vapor from ocean

• In winter have opposite effectIn winter have opposite effect

China, 08

Devastating monsoon floods have swept across South Asia in the past few weeks, killing at least 600 people and destroying the homes of hundreds of thousands more. REUTERS/RAFIQUR RAHMAN

South Asia Floods Severe flooding following days

of rain in Bangladesh, India, Nepal and Pakistan has affected over 30 million people. People have lost everything – their homes, livestock, possessions and

food. Infrastructure has been severely damaged. There have

been disease outbreaks as people suffer from food

shortages and survive amid stagnant waters. Large areas of land remain under water.

In Pakistan the current floods are compounding the damage resulting from severe storms in

June that left over 350,000 people homeless.www.interaction.org/sasia_floods/

High and Low Pressure High and Low Pressure AreasAreas

• Land masses modify wind bandsLand masses modify wind bands

Rotation on a GlobeRotation on a Globe

• On globe, Buffalo On globe, Buffalo rotates slowerrotates slower

• Quito rotates fasterQuito rotates faster– Circumference of Circumference of

path greaterpath greater

Speed of RotationSpeed of Rotation

Apparent DeflectionApparent Deflection

• Launch cannonball from Launch cannonball from Buffalo to QuitoBuffalo to Quito– Misses to the rightMisses to the right– Shot from slower to faster Shot from slower to faster

moving areamoving area

• Launch from Quito to Launch from Quito to BuffaloBuffalo– Miss to rightMiss to right– Shot from faster to slower Shot from faster to slower

moving areamoving area

• Opposite in Southern Opposite in Southern HemisphereHemisphere– Deflection is to the leftDeflection is to the left

Wind MovementWind MovementCoriolis EffectCoriolis Effect

•Deflected winds due to Deflected winds due to movement over spinning movement over spinning objectobject

• In Northern Hemisphere:In Northern Hemisphere:

• In Southern Hemisphere:In Southern Hemisphere:

– Winds are deflected to the rightWinds are deflected to the right

– Travel clockwise around high PTravel clockwise around high P

– Winds are deflected to the leftWinds are deflected to the left

– Travel counter-clockwise Travel counter-clockwise around high Paround high P

–Produce wind bandsProduce wind bands

Assume water-covered Earth

Variations in WindVariations in WindDailyDaily Coastal AreasCoastal Areas

•DaytimeDaytime

– Warmer land; cooler seaWarmer land; cooler sea

– Onshore flowOnshore flow

•EveningEvening– Cooler land; warmer seaCooler land; warmer sea

– Offshore flowOffshore flow

Surface Current CirculationSurface Current Circulation

Ekman TransportEkman 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 HWinds over H22O set ocean surface currents in motionO set ocean surface currents in motion• Surface HSurface H22O is deflected 45° by O is deflected 45° by Coriolis EffectCoriolis Effect

In Northern Hemisphere

• Deflection increases with depthDeflection increases with depth

Upwelling and DownwellingUpwelling and Downwelling

• West Coast of N. AmericaWest Coast of N. America

– Northerly winds Northerly winds summer summer upwellingupwelling

– Southerly winds Southerly winds winter winter downwellingdownwelling

Divergence and Divergence and ConvergenceConvergence

•ConvergenceConvergence

– Wind-driven currents collideWind-driven currents collide

– DownwellingDownwelling

• DivergenceDivergence– Currents move awayCurrents move away– UpwellingUpwelling

• Equatorial regionEquatorial region– Created by SE and NE Created by SE and NE

trade windstrade winds– DivergenceDivergence

– UpwellingUpwelling

Warm and Cold CurrentsWarm 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 Surface Currents and Coriolis EffectEffect

• 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 HWater “piles up”, gravity causes H22O to flow down slopeO to flow down slope

Geostrophic Currents GyresGeostrophic 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 gyre

gyre gyre gyre

• Boundary currents parallel to ocean marginsBoundary currents parallel to ocean margins

Pacific CurrentsPacific Currents

•Equatorial and boundary currents Equatorial and boundary currents Convergence Convergence– Warm currents in West PacificWarm currents in West Pacific

– Cold currents in East PacificCold currents in East Pacifici.e. Kuroshio, Australiai.e. Kuroshio, Australia

i.e. California, Perui.e. California, Peru

Western IntensificationWestern Intensification

•Gyres displaced to west by Earth’s rotationGyres displaced to west by Earth’s rotation

• Western IntensificationWestern Intensification– Faster, narrower currentsFaster, narrower currents

• Eastern CurrentsEastern Currents– Slower, more diffuse currentsSlower, more diffuse currents

El Niño YearEl Niño Year

•Factors producing El Niño yearFactors producing El Niño year

– Warmer HWarmer H22O moves eastO moves east

– Rainfall shifts from eastern to western PacificRainfall shifts from eastern to western Pacific• May result from atmospheric pressure changesMay result from atmospheric pressure changes

– Thermocline deepensThermocline deepens

– Trade winds weakenTrade winds weaken

– Upwelling ceases along western S. AmericaUpwelling ceases along western S. America

El Nino El Nino GenerationGeneration

• Change in atmospheric Change in atmospheric circulaion from western circulaion from western to central Pacificto central Pacific

• Trade Winds weakenTrade Winds weaken

• Warm water migrates Warm water migrates east to S.A.east to S.A.

• Thermocline (zone of Thermocline (zone of rapid temperature rapid temperature change) deepenschange) deepens

• Upwelling ceasesUpwelling ceases

El Niño El Niño ImpactImpact

•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

El Nino & Health ImpactsEl Nino & Health Impacts

El Nino Impacts - PeruEl Nino Impacts - Peru

• More diseaseMore disease– Especially malaria, wet, mosquitosEspecially malaria, wet, mosquitos

• Fishing industry collapsesFishing industry collapses– Anchovy migrate away – warm waterAnchovy migrate away – warm water

• Flooding, mudslidesFlooding, mudslides

Peru 1982 Peru 1982 El NinoEl Nino

www.columbia.edu/.../peruandelnino.html

0

5

10

15

20

25

30

35

40

45

18781880188218841886188818901892189418961898190019021904190619081910191219141916191819201922192419261928193019321934193619381940194219441946194819501952195419561958196019621964196619681970197219741976197819801982198419861988199019921994199619982000200220042006

Year

Inch

es

SEASONAL RAINFALL

5 Year Mean

Linear (SEASONAL RAINFALL)

'84

'90

'93

'41

'52'69

'78

'83

'93

'98

'14'58 '66 '73

'86

'92

'95

Data: NOAA NWS(Los Angeles/Oxnard)

'26

'02'61'99

'05

Peaks are El Nino years

Peaks below line are major La Nina events

El Nino/La Nina events impact our climate as they do developing nations

+ -Warm Phase Cool Phase

Pacific Decadal Oscillation (PDO)Pacific Decadal Oscillation (PDO)

0

5

10

15

20

25

30

35

40

45

18781880188218841886188818901892189418961898190019021904190619081910191219141916191819201922192419261928193019321934193619381940194219441946194819501952195419561958196019621964196619681970197219741976197819801982198419861988199019921994199619982000200220042006

Year

Inch

es

SEASONAL RAINFALL

5 Year Mean

Linear (SEASONAL RAINFALL)

'84

'90

'93

'41

'52'69

'78

'83

'93

'98

'14'58 '66 '73

'86

'92

'95

Data: NOAA NWS(Los Angeles/Oxnard)

'26

'02'61'99

'05

Warm and Cold FrontsWarm and Cold Fronts

• Caused by eastward moving air massesCaused by eastward moving air masses– Warm air rises over cold, dense air – condenses and precipitatesWarm air rises over cold, dense air – condenses and precipitates

– Steeper cold front pushes warm air up – cools, condenses Steeper cold front pushes warm air up – cools, condenses and precipitatesand precipitates

Wind MovementWind Movement

• Air that sinks does not flow back in a straight north-south Air that sinks does not flow back in a straight north-south path – it curves (Coriolis Effect)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

• Lagos averages 68.5" of rain annually. The Lagos averages 68.5" of rain annually. The four seasons are:four seasons are:

– Rainy season March to July, with peak in June.Rainy season March to July, with peak in June.– Short dry season in August lasts for 3-4 Short dry season in August lasts for 3-4

weeks. This ITCZ moves north of region.weeks. This ITCZ moves north of region.– Bbrief wet period in August to early Bbrief wet period in August to early

September to mid-October, ITCZ moves south.September to mid-October, ITCZ moves south.– Long dry season late October and lasts to Long dry season late October and lasts to

early March with peak between early early March with peak between early December and late February. Vegetation December and late February. Vegetation growth hampered, grasses dry and leaves fall growth hampered, grasses dry and leaves fall from deciduous trees due to reduced from deciduous trees due to reduced moisture.moisture.

• Kano averages 32.5" (825 mm) of rain Kano averages 32.5" (825 mm) of rain annually. There are only two season since annually. There are only two season since the ITCZ only moves into the region once a the ITCZ only moves into the region once a year before returning south :year before returning south :

– The long dry season October to mid-May. ITCZ The long dry season October to mid-May. ITCZ in the Southern Hemisphere, North-east winds in the Southern Hemisphere, North-east winds over the Sahara prevail. over the Sahara prevail.

– Short rainy season from June to September. Short rainy season from June to September. The rains are generally heavy and short in The rains are generally heavy and short in duration, and often characterized by frequent duration, and often characterized by frequent storms. This results in flash floods storms. This results in flash floods

Images & text: http://www.srh.weather.gov/srh/jetstream/tropics/itcz.htm