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Atmospheric and oceanic circulationsPG Lectures, Autumn 2017 Mike Byrne & Arnaud Czaja
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Aim and learning outcomesProvide new PhD students in SPAT with an overview of why and
how the atmosphere and ocean circulate and the implications for Earth’s climate
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Aim and learning outcomes
1. Describe the radiative drivers of atmospheric and oceanic circulations
2. Describe the structure of Earth’s winds (vs latitude and height) 3. Understand angular momentum conservation and implications for the
tropical Hadley cell 4. Physical origins of Ekman layers and ocean gyres 5. Baroclinic instability in atmosphere and oceans, Rossby number,
geostrophic balance 6. Atmospheric moisture transport: Influence on Earth’s water cycle and
thermohaline circulation
Provide new PhD students in SPAT with an overview of why and how the atmosphere and ocean circulate and the implications for
Earth’s climate
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Structure of the week• Monday:
• *attendance register* • Earth’s radiative budget • Observed climate: temperature and winds • Atmosphere: Hadley cell and angular momentum • Ocean: Ekman layers, gyres
• Tuesday:• Baroclinic instability in the atmosphere, ocean, and classroom (tank
experiment) • Rossby number, geostrophic balance • Impact of atmospheric circulation: Earth’s water cycle
• Friday: 3 groups present & discuss problem-set solutions
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Phenomena: Earth’s radiative balance — the circulation driver
solar energy received
NASA’s CERES satellite
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Phenomena: Earth’s radiative balance — the circulation driver
solar energy received terrestrial energy emitted
NASA’s CERES satellite
NASA’s ERBE satellite
Stefan-Boltzmann
Law:B(T ) = �T 4
e
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Phenomena: Earth’s radiative balance — the circulation driver
solar (SW) received minus terrestrial (LW) emitted
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Phenomena: Earth’s radiative balance — the circulation driver
implied atmosphere/ocean energy transport
solar (SW) received minus terrestrial (LW) emitted
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Phenomena: Poleward energy transport by atmosphere and oceans
Trenberth & Caron (2001)
total implied
ocean
atmosphere
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Phenomena: Surface temperature <-> pressure gradients -> circulations
from Schneider’s “Physics of Earth’s Climate”
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Phenomena: Surface temperature <-> pressure gradients -> circulations
from Schneider’s “Physics of Earth’s Climate”
weak gradients
strong gradients
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Phenomena: Surface temperature (annual range)
from Schneider’s “Physics of Earth’s Climate”
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Phenomena: Annual-mean surface winds — much stronger in zonal direction because of Earth’s rotation
from Schneider’s “Physics of Earth’s Climate”
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Phenomena: NH summer (JJA) surface winds, winds vary with the seasons
from Schneider’s “Physics of Earth’s Climate”
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Phenomena: NH summer (JJA) surface winds, winds vary with the seasons
from Schneider’s “Physics of Earth’s Climate”
“horse latitudes”
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Columbus knew about the surface-wind pattern
from Schneider’s “Physics of Earth’s Climate”
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Phenomena: Meridional (north-south) winds are much weaker
from Schneider’s “Physics of Earth’s Climate”
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Phenomena: Vertical structure of zonal winds
from Schneider’s “Physics of Earth’s Climate”
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Phenomena: Vertical structure of meridional mass circulation — by mass balance, Hadley and Ferrel cells must exist!
from Schneider’s “Physics of Earth’s Climate”
annual average
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Phenomena: Atmospheric meridional mass circulation — monsoons
from Schneider’s “Physics of Earth’s Climate”
Dec-Jan-Feb
southern monsoons
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Phenomena: Atmospheric meridional mass circulation — monsoons
from Schneider’s “Physics of Earth’s Climate”
Jun-Jul-Aug
northern monsoons (e.g. in India)
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Why are there strong winds in the upper atmosphere? Why are these winds increasingly westerly as you move poleward? Angular momentum and Earth’s
rotation…
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Understanding winds and the Hadley circulation using angular momentum conservation
What is the angular momentum? “velocity times distance to rotation axis”
What does it imply for upper-tropospheric winds in Hadley cell?
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Conservation of AM implies upper-level winds become stronger (more westerly) as air moves towards pole (opposite for surface winds)
umax
= ⌦asin
2 �
cos�
Latitude (degrees)0 5 10 15 20 25 30 35 40 45 50
u MAX
[m/s
]
0
50
100
150
200
250
300
~125m/s @ 30deg
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Implies infinite winds at the poles! AM conservation breaks down and Hadley circulation stops at ~30deg because of baroclinic instability and turbulence
(see tomorrow’s experiment)
umax
= ⌦asin
2 �
cos�
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In surface branch AM is not conserved because of friction. Easterly winds transfer momentum to ocean and drive oceanic circulations -> Ekman layers
Ocean
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Ekman layers
*Arnaud’s slides*
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Breakdown of Hadley cell due to Earth’s rotation
non-rotating rotating
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Macroturbulence in an Earth-like simulation
Macroturbulence in a more realistic more
Breakdown of Hadley cell due to Earth’s rotation
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Effect of Earth’s rotation on atmosphere/ocean dynamics: The Rossby number
Can begin to understand the influence of rotation on circulation by doing a scale analysis of the momentum
equation…
Du
Dt
+
1
⇢
@p
@x
� fv = friction (east� westdir.)
Dv
Dt
+
1
⇢
@p
@y
+ fu = friction (north� southdir.)
accelerationpressure-gradient
force Coriolis force
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Effect of Earth’s rotation on atmosphere/ocean dynamics: The Rossby number
Can begin to understand the influence of rotation on circulation by doing a scale analysis of the momentum
equation…
Du
Dt
+
1
⇢
@p
@x
� fv = friction (east� westdir.)
Dv
Dt
+
1
⇢
@p
@y
+ fu = friction (north� southdir.)
~V / T = V2/L
~fV
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Effect of Earth’s rotation on atmosphere/ocean dynamics: The Rossby number
Ro =
acceleration
Coriolis
⇠ V
fL
Ro = 0.1 1 10
geostrophic balance (jet
stream)
gradient balance (hurricanes)
cyclostrophic balance (tornados)
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Geostrophic balance in Earth’s atmosphere: Mid-latitude weather systems
Ro =
acceleration
Coriolis
⇠ V
fL⇡ 0.1
) f z⇥ u+
1
⇢rp = 0
Geostrophic balance: “Pressure-gradient and
Coriolis forces balance” -> flow along isobars
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Geostrophic balance in Earth’s atmosphere: Mid-latitude weather systems
Ro =
acceleration
Coriolis
⇠ V
fL⇡ 0.1
) f z⇥ u+
1
⇢rp = 0
Geostrophic balance: “Pressure-gradient and Coriolis forces balance”
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Geostrophic balance in Earth’s atmosphere: Mid-latitude weather systems
Ro =
acceleration
Coriolis
⇠ V
fL⇡ 0.1
) f z⇥ u+
1
⇢rp = 0
Geostrophic balance: “Pressure-gradient and Coriolis forces balance”
PGF
Coriolis
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Earth’s water cycle: atmospheric water vapour
ERA-40 Atlas
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Earth’s water cycle: atmospheric water vapour
ERA-40 Atlas 7%/K
�q⇤
q⇤⇡ L
RT 2�TClausius-Clapeyron:
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Earth’s water cycle: moisture transport
from Schneider’s “Physics of Earth’s Climate”
Hadley cell weather systems
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Earth’s water cycle: precipitation
Tropical Rainfall Measuring Mission (TRMM)
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Earth’s water cycle: precipitation
from Schneider’s “Physics of Earth’s Climate”
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Earth’s water cycle: evaporation
from Schneider’s “Physics of Earth’s Climate”
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Earth’s water cycle: net precipitation (P-E)
from Schneider’s “Physics of Earth’s Climate”
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Earth’s water cycle: Impact of P-E on oceans and continents
World Ocean Atlas (2005)
ocean salinity
Global Water Resource Archive
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Earth’s water cycle: P-E and the atmospheric circulation
from Schneider’s “Physics of Earth’s Climate”
) �(P � E) = �r · �F
SURFACE
P � E > 0
moisture flux ATMOS P � E = �r · F = �r · [qu]
⇡ �[qr · u]
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Earth’s water cycle: P-E and the atmospheric circulation
from Schneider’s “Physics of Earth’s Climate”
) �(P � E) = �r · �F
SURFACE
P � E > 0
moisture flux ATMOS P � E = �r · F = �r · [qu]
⇡ �[qr · u]
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Atmosphere moves moisture from dry subtropics (P-E < 0) to moist tropics & extratropics (P-E > 0)
Geophysical Fluid Dynamics Laboratory model
P � E = �r · F = �r · [qu]⇡ �[qr · u]
(from a simulation)