Atmospheric and oceanic circulations PG Lectures, Autumn 2017 Mike - - PowerPoint PPT Presentation

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Atmospheric and oceanic circulations PG Lectures, Autumn 2017 Mike - - PowerPoint PPT Presentation

Nov 03, 2022 243 likes •713 views

Atmospheric and oceanic circulations PG Lectures, Autumn 2017 Mike Byrne & Arnaud Czaja Aim and learning outcomes Provide new PhD students in SPAT with an overview of why and how the atmosphere and ocean circulate and the implications for

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Atmospheric and oceanic circulations

PG Lectures, Autumn 2017 Mike Byrne & Arnaud Czaja

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Aim and learning outcomes

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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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) = σT 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

  • cean

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 = Ωasin2 φ cos φ

Latitude (degrees)

5 10 15 20 25 30 35 40 45 50

uMAX [m/s]

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 = Ωasin2 φ cos φ

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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 ρ ∂p ∂x − fv = friction (east − westdir.) Dv Dt + 1 ρ ∂p ∂y + fu = friction (north − southdir.)

acceleration pressure-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 ρ ∂p ∂x − fv = friction (east − westdir.) Dv Dt + 1 ρ ∂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 ) fˆ z ⇥ u + 1 ρrp = 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 ) fˆ z ⇥ u + 1 ρrp = 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 ) fˆ z ⇥ u + 1 ρrp = 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

δq∗ q∗ ≈ L RT 2 δT

Clausius-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)

  • cean 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”

) δ(P E) = r · δF

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”

) δ(P E) = r · δF

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)