Transcript Atmosphere-Oceans - Cal State LA

Surface Currents
Structure of Atmosphere
• Almost all weather occurs
in troposphere
– Temperature decrease with
altitude in troposphere
• Tropopause is transition
to stratosphere
– Constant temperature at
base, temperature increase
in upper portion
Composition of Atmosphere
Origin of Currents
• Ocean surface currents are wind driven
• Air movement due to less dense air rising and more dense
air sinking
• Horizontal air flow along Earth’s surface is wind
• Air circulating in this manner is convection currents
Convection Currents
• Air becomes less dense when:
– It is warmed
– Atmospheric pressure ↓
– Water vapor (humidity) ↑
• Air becomes more dense when:
– It is cooled
– Atmospheric pressure ↑
– Water vapor (dry air) ↓
Wind Movement
Non-rotating Earth
• 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 blow
– North-blowing winds = southerly winds
– South-blowing winds = northerly winds
Wind Movement Rotating Earth
• At 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 sinks
– Area of high pressure
– Dry conditions
– Location of world deserts
• 60° N&S, air masses meet
– Form Polar Front
– Air masses rise, diverge and
sink @ 90° and 30° N&S
• Note belts of high
pressure areas at
30o N&S
• 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
• Prolonged droughts
– Much human
misery
• Transition from
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Desert to north &
rainforest to south
Poorest nations on
Earth
Depend on
agriculture
Rain depends on
ITCZ
Overgrazing also
compounds problem
– Remove
vegetation, less
moisture in soil
– Albedo inreases,
less evaporation,
less rain
Sub-Saharan Africa
Variations in Wind
Seasonal
July
January
• In summer:
– Air rises over warmed land, condenses
– Sudden and large volume of precipitation
– Winds accumulate more H2O vapor from ocean
• In winter have opposite effect
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/
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
China, 08
High and Low Pressure Areas
• Land masses modify wind bands
Rotation on a Globe
• On globe, Buffalo
•
rotates slower
Quito rotates faster
– Circumference of path
greater
Speed of Rotation
Apparent Deflection
• Launch cannonball from
Buffalo to Quito
– Misses to the right
– Shot from slower to faster
moving area
• Launch from Quito to Buffalo
– Miss to right
– Shot from faster to slower
moving area
• Opposite in Southern
Hemisphere
– Deflection is to the left
Wind Movement
Coriolis Effect
• Deflected winds due to
movement over spinning
object
– Produce wind bands
• In Northern Hemisphere:
– Winds are deflected to the right
– Travel clockwise around high P
• In Southern Hemisphere:
– Winds are deflected to the left
– Travel counter-clockwise around
high P
Assume water-covered Earth
Surface Current Circulation
Ekman Transport
In Northern Hemisphere
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Winds over H2O set ocean surface currents in motion
Surface H2O is deflected 45° by Coriolis Effect
Deflection increases with depth
Net water movement 90° to right of wind direction in N. Hemi.
Upwelling and Downwelling
• West Coast of N. America
– Northerly winds  summer upwelling
– Southerly winds  winter downwelling
Warm and Cold Currents
• Uneven solar heating produces ocean temperature pattern
• Ocean currents redistribute heat and influence climate
• Clockwise rotation in Northern Hemisphere
Surface Currents and Coriolis Effect
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•
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Ocean currents are driven by wind
Moving water deflected by Coriolis Effect
Wind deflection creates ocean circulation gyres
Water “piles up”, gravity causes H2O to flow down slope
Geostrophic Currents Gyres
gyre
gyre
gyre
gyre
gyre
• Convergence thickens surface layer – builds a dome
• Circular current systems in major ocean basins: Gyres
• Boundary currents parallel to ocean margins
Pacific Currents
• Equatorial and boundary currents  Convergence
– Warm currents in West Pacific
i.e. Kuroshio, Australia
– Cold currents in East Pacific
i.e. California, Peru
El Niño Year
• Factors producing El Niño year
– Trade winds weaken
– Warmer H2O moves east
– Upwelling ceases along western S. America
– Thermocline deepens
– Rainfall shifts from eastern to western Pacific
• May result from atmospheric pressure changes
El Nino
Generation
• Change in atmospheric
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circulaion from western to
central Pacific
Trade Winds weaken
Warm water migrates
east to S.A.
Thermocline (zone of
rapid temperature
change) deepens
Upwelling ceases
El Niño
Impact
• Leads to death of cold-water organisms
• Warm-water organisms migrate north and south
El Nino & Health Impacts
El Nino Impacts - Peru
• More disease
– Especially malaria, wet, mosquitos
• Fishing industry collapses
– Anchovy migrate away – warm water
• Flooding, mudslides
Peru 1982 El
Nino
www.columbia.edu/.../peruande
lnino.html
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
SEASONAL RAINFALL
45
40
5 Year Mean
'84
'05
Linear (SEASONAL RAINFALL)
'90
35
'78
'41
'83
30
'93
'95
'14
25
Inches
'93
'69
'52
'98
'58
'26
20
'66
'92
'73
'86
15
10
5
'99
'61
0
'02
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1878
Year
Data: NOAA NWS(Los Angeles/Oxnard)
How Does Climate Change
Impact El Nino
• Two Extreme events in this century
• Increased frequency of events in past 50
years
• If climate change responsible could mean
disruption
• But data not clear
Pacific Decadal Oscillation (PDO)
+
Warm Phase
Cool Phase
Linear (SEASONAL RAINFALL)
'84
'95
'14
25
30
'52
'90
'86
20
0
2006
2004
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1998
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Data: NOAA NWS(Los Angeles/Oxnard)
Year
'02
'61
'99
'92
'73
'66
'93
'69
'93
'78
'98
'83
'41
35
'05
5 Year Mean
40
'58
'26
Inches
45
SEASONAL RAINFALL
15
10
5
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Lagos averages 68.5" of rain annually. The four
seasons are:
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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 seasons since the ITCZ only
moves into the region once a year before
returning south :
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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: http://www.srh.weather.gov/srh/jetstream/tropics/itcz.htm