Surface Currents

Structure of Atmosphere Almost all weather occurs in troposphere Almost all weather occurs in troposphere –Temperature decrease with altitude in troposphere Tropopause is transition to stratosphere Tropopause is transition to stratosphere –Constant temperature at base, temperature increase in upper portion

Composition of Atmosphere Lower atmosphere circulates mixes gases-Homosphere Lower atmosphere circulates mixes gases-Homosphere Above 80 km gases stratified- Heterosphere Above 80 km gases stratified- Heterosphere –Heavier ones are lower Aerosol present in Atmosphere Aerosol present in Atmosphere –Aerosol=minute liquid and solid particles Atmosphere also contains pollutants Atmosphere also contains pollutants –Pollutant= gas or aerosol at concentrations that threaten living organisms

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) ↓

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

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

Note belts of high pressure areas at 30 o N&S Note belts of high pressure areas at 30 o N&S

Pressure areas linked by wind belts Pressure areas linked by wind belts

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

Seasonal 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 Africa Prolonged droughts Prolonged droughts –Much human misery Transition from Desert to north & rainforest to south Transition from Desert to north & rainforest to south Poorest nations on Earth Poorest nations on Earth Depend on agriculture Depend on agriculture Rain depends on ITCZ Rain depends on ITCZ Overgrazing also compounds problem Overgrazing also compounds problem –Remove vegetation, less moisture in soil –Albedo inreases, less evaporation, less rain

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

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.

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

Rotation on a Globe On globe, Buffalo rotates slower On globe, Buffalo rotates slower Quito rotates faster Quito rotates faster –Circumference of path greater

Speed of Rotation

Apparent Deflection Launch cannonball from Buffalo to Quito Launch cannonball from Buffalo to Quito –Misses to the right –Shot from slower to faster moving area Launch from Quito to Buffalo Launch from Quito to Buffalo –Miss to right –Shot from faster to slower moving area Opposite in Southern Hemisphere Opposite in Southern Hemisphere –Deflection is to the left

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

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

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 Nino Generation Change in atmospheric circulaion from western to central Pacific Change in atmospheric circulaion from western to central Pacific Trade Winds weaken Trade Winds weaken Warm water migrates east to S.A. Warm water migrates east to S.A. Thermocline (zone of rapid temperature change) deepens Thermocline (zone of rapid temperature change) deepens Upwelling ceases Upwelling ceases

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

El Nino & Health Impacts

El Nino Impacts - Peru More disease More disease –Especially malaria, wet, mosquitos Fishing industry collapses Fishing industry collapses –Anchovy migrate away – warm water Flooding, mudslides Flooding, mudslides

Peru 1982 El Nino lnino.html

Year Inches 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 PhaseCool Phase Pacific Decadal Oscillation (PDO)

Year Inches 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 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 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

Lagos averages 68.5" of rain annually. The four seasons are: Lagos averages 68.5" of rain annually. The four seasons are: –Rainy season March to July, with peak in June. –Short dry season in August lasts for 3-4 weeks. This ITCZ moves north of region. –Bbrief wet period in August to early September to mid-October, ITCZ moves south. –Long dry season late October and lasts to early March with peak between early December and late February. Vegetation growth hampered, grasses dry and leaves fall from deciduous trees due to reduced moisture. Kano averages 32.5" (825 mm) of rain annually. There are only two season since the ITCZ only moves into the region once a year before returning south : Kano averages 32.5" (825 mm) of rain annually. There are only two season since the ITCZ only moves into the region once a year before returning south : –The long dry season October to mid-May. ITCZ in the Southern Hemisphere, North-east winds over the Sahara prevail. –Short rainy season from June to September. The rains are generally heavy and short in duration, and often characterized by frequent storms. This results in flash floods Images & text: