Transcript Earth`s Climate System Today

General Circulation of the Atmosphere


Tropical heating drives
Hadley cell circulation
Warm wet air rises
along the equator
 Transfers water
vapor from tropical
oceans to higher
latitudes
 Transfers heat
from low to high
latitudes
The Hadley Cell
Along equator, strong solar heating causes
air to expand upward and diverge to poles
 Creates a zone of low pressure at the
equator called
 Equatorial low
 Intertropical Convergence Zone (ITCZ)
 The upward motions that dominate the
region favor formation of heavy rainfall
 ITCZ is rainiest latitude zone on Earth
 Rains 200 days a year – aka. doldrums

General Circulation
Air parcels rise creating low pressure
 Heat and expand
 Become humid
 Transfer heat (sensible & latent) to poles
 Transfer of moisture towards poles
 In mid latitudes
 Dry air sinks creating high pressure
 Air flows away from high pressure

General Circulation
Hadley cell circulation creates trade winds
 Dry trade winds move from subtropics
to tropics and pick up moisture
 Trade winds from both hemispheres
converge in the ITCZ
 Trade winds warm and rise
 Contribute to low pressure and high
rainfall in the ITCZ

Monsoons – Circulation in ITCZ
ITCZ shifts with seasons
 Circulation driven by solar heating
 Circulation affected by seasonal heat
transfer between tropical ocean and land
 Heat capacity and thermal inertia of
land < water

Summer Monsoon

Air over land heats and rises drawing
moist air in from tropical oceans
Winter Monsoon

Air over land cools and sinks drawing dry
air in over the tropical oceans
Circulation at Mid & High Latitudes
Sinking air from Hadley circulation creates
high pressure in subtropics
 Circulation modified
 Coriolis effect
 Monsoons
 Flow of cold air from high latitudes

Coriolis Effect

Air moving from high to low pressure is deflected by
Earth’s rotation
 Clockwise rotation in the northern hemisphere
 Counterclockwise rotation in the southern hemisphere
Ocean Circulation?
Circulation in the troposphere is caused by
atmospheric pressure gradients
 Result from vertical or horizontal
temperature differences
 Temperature variations caused by
latitudinal differences in solar heating
 Ocean surfaces are heated by incoming
surface radiation
 Do the oceans circulate for the same
reason as the atmosphere?

No!



90% of solar radiation that penetrates oceans
absorbed in upper 100 m
Warm water at surface is less dense than the
colder water below
 Water column is inherently stable
 Very little vertical mixing
Water has a high heat capacity
 Lots of heat required for a small change in
temperature
 Lateral temperature and salinity differences
are small over large areas
Ocean Circulation


Ultimately driven by solar energy
 Distribution of solar energy drives global winds
 Latitudinal wind belts produce ocean currents
 Determine circulation patterns in upper ocean
 Distribution of surface ocean temperatures
strongly influence density structure
 Density structure of oceans drives deep ocean
circulation
Negative feedback
 Surface temperature gradients drive circulation
 Net effect is to move warm water to poles and
cold water towards tropics
Heat Transfer in Oceans
Heating occurs in upper ocean
 Vertical mixing is minimal
 Average mixed layer depth ~100 m
 Heat transfer from equator to pole by
ocean currents
 Oceans redistribute about half as much
heat at the atmosphere

Surface Currents



Surface circulation driven by winds
 As a result of friction, winds drag ocean
surface
Water movement confined to upper ~100 m
 Although well-developed currents ~1-2 km
 Examples, Gulf Stream, Kuroshiro Current
Coriolis effect influences ocean currents
 Water deflected to right in N. hemisphere
 Water deflected to left in S. hemisphere
Eckman Spiral
Eckman theory predicts
 1) surface currents will flow at 45° to
the surface wind path
 2) flow will be reversed at ~100 m below
the surface
 3) flow at depth will be considerably
reduced in speed
 Few observations of true Eckman Spiral
 Surface flow <45°, but still to an angle

Eckman Transport
Observations confirm net transport of
surface water is at a right angle to wind
direction
 Net movement of water referred to as
Eckman Transport

Gyre Circulation

Wind driven and large scale
 Sea level in center 2 m higher than edge
 Eckman transport producing convergence
 Circulation extends to 600-1000 m
 Volume of water moved is 100 x transport of
all Earth’s rivers
 Flow towards equator balanced by flow toward
pole on westward margin
 In Atlantic, by Gulf Stream and North
Atlantic Drift
Downwelling

In areas of convergence
 Surface water piles up in center of gyre
 Sea level in the center of gyre increases
 Surface layer of water thickens
 Accumulation of water causes it to
sink
 Process known as downwelling
Equatorial Divergence
Areas of the ocean where divergence of
surface currents occurs
 Equatorial divergence (e.g., Atlantic)
 In N. hemisphere, NW trades result in
westward flowing N. equatorial current
 Eckman transport moves water to N
 In S. hemisphere, SW trades result in
westward flowing S. equatorial current
 Eckman transport moves water to S
 Divergence occurs along the equator

Equatorial Upwelling

As surface water diverges, sea level falls,
surface layer thins and cold water “upwells”
Eckman Transport Along Coasts
Winds along a coast may result in Eckman
transport that moves water towards or
away from the coast
 Divergence from easterly winds and
southward moving currents
 SW coast of N. America
 W coast of N. Africa
 Divergence from northward moving
currents
 West coasts of S. America and S. Africa

Coastal Upwelling

Coastal divergence results in upwelling as
cold water rises to replace surface water
Geostrophic Currents
Eckman transport from wind-driven
currents piles water up in gyre center
 Gravity pulls water down slope
 Slope is opposite to Coriolis effect
 Net effect is flow 90° to slope
 Result is a geostrophic current
 Geostrophic currents push water in the
same direction as the wind-driven flow

Boundary Currents
Gyre circulation pushes water to the west
 Flow of water around gyres is asymmetric
 In the western part of gyre water is
confined to a narrow fast-moving flow
 Western boundary current
 In the eastern part of gyre flow is
diffuse, spread out and slow
 Eastern boundary current
 Eastern currents tend to be divergent
 Eckman transport away from continent

Gulf Stream

Western boundary current
in Atlantic
 Narrow, fast-moving
from Cuba to Cape
Hatteras
 Decreases speed across
N. Atlantic
 Flow broadens and slows
becoming N. Atlantic
Drift
 Movement to the south
along the Canary Current
is very slow, shallow and
broad
Deep Ocean Circulation


Driven by differences in density
Density of seawater is a function of
 Water temperature
 Salinity
 Quantity of dissolved salts
• Chlorine
• Sodium
• Magnesium
• Calcium
• Potassium
Thermohaline Circulation



Deep ocean circulation depends on temperature (thermo)
& salinity (hals)
 Controls seawater density
 Density increases as:
• Salinity increases
• Temperature decreases
Horizontal density changes small
Vertical changes not quite as small
 Water column is stable
 Densest water on bottom
 Flow of water in deep ocean is slow
 However, still important in shaping Earth’s climate
Vertical Structure of Ocean

Surface mixed layer
 Interacts with
atmosphere
 Exchanges kinetic
energy (wind,
friction) and heat
 Typically well
mixed (20-100 m)
Vertical Structure of Ocean

Pychnocline (~1 km)
 Zone of transition
between surface and
deep water
 Characterized by rapid
increase in density
 Some regions density
change due to salinity
changes – halocline
 Most regions density
change due to
temperature change –
thermocline
 Steep density gradient
stabilizes layer
Bottom Water Formation
Deep-ocean circulation begins with
production of dense (cold and/or salty)
water at high latitudes
 Ice formation in Polar oceans excludes salt
 Combination of cold water and high
salinity produces very dense water
 Dense water sinks and flows down the
slopes of the basin towards equator

Antarctic Bottom Water (AABW)
Weddell Sea major site of AABW
formation
 AABW circles Antarctica and flow
northward as deepest layer in Atlantic,
Pacific and Indian Ocean basins
 AABW flow extensive
 45°N in Atlantic
 50°N in Pacific
 10,000 km at 0.03-0.06 km h-1; 250 y

North Atlantic Deep Water (NADW)



Coastal Greenland (Labrador Sea) site of NADW
formation
NADW comprises about 50% of the deep water
to worlds oceans
NADW in the Labrador Sea sinks directly into
the western Atlantic
 NADW forms in Norwegian Basins
 Sinks and is dammed behind sills
• Between Greenland and Iceland and
Iceland and the British Isles
 NADW periodically spills over sills into the
North Atlantic
Deep Atlantic Water Masses

Deep Atlantic water comes from high
latitude N. Atlantic, Southern Ocean and
at shallower depth, the Mediterranean Sea
AABW and NADW Interact

NADW flowing south in the Atlantic joins
the Antarctic Circumpolar Current
 NADW and AABW combine
 Spin around Antarctica
 Eventually branch off into the Pacific,
Indian and Atlantic ocean basins
Ocean Circulation
Surface water at high latitudes forms
deep water
 Deep water sinks and flows at depth
throughout the major ocean basins
 Deep water upwells to replace the surface
water that sinks in polar regions
 Surface waters must flow to high latitudes
to replace water sinking in polar regions
 Idealized circulation – Thermohaline
Conveyer Belt

Thermohaline Conveyor Belt


NADW sinks, flows south to ACC and branches
into Indian and Pacific Basins
Upwelling brings cold water to surface where it
eventually returns to N. Atlantic
Ocean Circulation and Climate


Warm surface waters move from equator to
poles transferring heat pole-ward and into the
deep oceans
Oceans vast reservoir of heat
 Water heats and cools slowly
 Pools of water warmer than normal heat the
atmosphere
 Pools of water colder than normal cool the
atmosphere
 Timescale of months to years
 Time needed for heating/cooling of water
Ocean Circulation and Climate



On long timescales, average ocean temperature
affects climate
Most water is in deep ocean
 Average temperature of ocean is a function
of
 Process of bottom-water formation
 Transport of water around ocean basins
Deep water recycle times is ~1000 y
 Thermohaline circulation moderates climate
over time periods of ~ 1000 y
Ice on Earth
Important component of climate system
 Ice properties are different from water,
air and land
 Two important factors affecting climate
 High albedo
 Latent heat stored in ice

Sea Ice
Salt rejection during sea ice formation
 Important for bottom water formation
 Sea ice stops atmosphere from interacting
with surface mixed layer

Sea Ice Distribution
Most sea ice in Southern Ocean
 Enormous amount form and melt each
season
 Average thickness ~1 m
 Landmasses in Arctic prevent sea ice
movement
 Arctic sea ice persists for 4-5 years
 Reach thickness of 4 m in central Arctic
and 1 m on margins

Glacial Ice

Mountain glaciers
 Equatorial high altitude or polar lower altitude
 Few km long, 100’s m wide and 100’s m thick
Glacial Ice

Continental ice sheets
 Large ice cube
 Existing ice sheets
 Antarctica and
Greenland
• ~3% of Earth’s
surface or 11%
of land surface
• 32 million km3
(= 70 m of sea
level)