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Planet Earth > Physics of Atmosphere and Ocean > Flashcards

Physics of Atmosphere and Ocean Flashcards

1
Q

fundamental forces to consider in atmospheric/ocean motions

A

gravitational force
pressure gradient
friction

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2
Q

Additional forces to consider in atmospheric/ocean motions

A

centrifugal
Coriolis

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3
Q

formula for gravitational force

A

F = G (m.m)/r^2

per unit mass (a) = -GM/r^2 = -g

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4
Q

quantitative formula for pressure gradient force

A

F (per kg) = -1/rho (∂P/∂x)

(in 3d ∂x, ∂y, or ∂z)

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5
Q

What can friction be caused by

A
  • viscosity
  • frictional drag at base of atm
  • windstress on ocean
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6
Q

formula for centrifugal force (per unit mass)

A

c.f. = Ω^2 . R

Ω = angular velocity = ∆theta/∆t
R = radius = 6400km

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7
Q

what direction does Coriolis force act in northern and southern hemisphere

A

N: 90º to right
S: 90º to left

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8
Q

formula for Coriolis parameter

A

f = 2Ω.sin(theta)

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9
Q

equation for Coriolis acceleration

A

a = 2Ω.v

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10
Q

Where is maximum and minimum spin on the earth

A

max = poles
min = equator

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11
Q

magnitude of Coriolis force formula

A

mag = f.v

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12
Q

value for Coriolis parameter, f, at mid latitudes

A

10^ -4 (/s)

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13
Q

force balance equation in x, y, z direction

A

∂u/∂t = -1/rho (∂P/∂x) + fv

∂v/∂t = -1/rho (∂P/∂y) - fu

∂w/∂t = -1/rho (∂P/∂z) - g

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14
Q

What forces is geostrophic balance between

A

pressure gradient + Coriolis = 0

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15
Q

what is the significance of geostrophic balance

A

on a rotating planet, winds do not flow high->low pressure, but along constant pressure contours

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16
Q

equation for velocity of geostrophic winds

A

rearrange force balance equations for velocity in x and y directions

(p.g + Coriolis = 0)

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17
Q

directions of winds around pressure systems

A

in N:

anticlockwise around low
clockwise around high

= cyclonic

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18
Q

what is the effect on geostrophic wind balance of being close to the surface

A

additional frictional force
(moving slower = Coriolis weaker, cannot balance)

=> spiral into low pressure (ascent)
=> spiral out of high pressure (descent)

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19
Q

what are the forces involved in hydrostatic balance

A

pressure gradient force + gravity = 0

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20
Q

hydrostatic balance formula

A

∂P/∂z = -rho . g

rearrange force balance equation in z direction

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21
Q

how to use hydrostatic balance formula to find pressure at given point in atm

A

integrate between that point and infinity
= weight per unit area at that point = pressure

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22
Q

equation of state for ocean (what does density depend on)

A

rho = rho(T, S, P)

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23
Q

what does the TS diagram show

A

temperature more important for determining density at usual conditions

salinity becomes more important at polar temperatures

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24
Q

definition and formula for potential temperature, θ

A

θ = temperature a fluid parcel would be if brought adiabatically to the surface

θ = T . (Ps / P) ^ 2/7

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25
Q

What is thermal wind balance in words

A

combination of geostrophic and hydrostatic balance to show how geostrophic flow varies with height

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26
Q

How to find thermal wind equations

A

partially differentiate geostrophic velocity equations with respect to height, z, and sub in hydrostatic balance equation

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27
Q

example of how thermal wind balance works in oceans and atm

A

Gulf Stream

high temp, low density off coast
= +ve wind equation
= increase velocity of currents in y with height

Jet Stream in atm

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28
Q

meaning of westerly winds and easterly winds

A

westerly = west -> east

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29
Q

meaning of eastward ocean currents

A

eastward = west -> east

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30
Q

why does wind speed up as moves north from equator (westerlies)

A

air must conserve angular momentum = mass . velocity . radius

moving polewards = decrease r, therefore increase v

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31
Q

Where are the westerlies seen

A

almost everywhere except equator, strongest at mid latitudes - subtropical jet

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32
Q

how is heat transported to the poles

A

temperature gradient gets stronger across jet stream, strengthening it, flow becomes unstable, anomalies amplify into EDDIES
= baroclinic instability

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33
Q

How are the easterly trade winds formed

A

Air moving equator-ward at the surface in the hadley cell deflected by Coriolis force

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34
Q

what do eddies do in the atmosphere

A

eddies transport hot air northwards, and cold southwards (net flux=heat N) , by a lateral motion rather than overturning

trailing tails into tropics = move westerly momentum northwards

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35
Q

describe regional climate of near equatorial regions

A
  • trade wind convergence
  • rain
    (tropical rainforests)
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36
Q

describe regional climate of subtropics

A
  • descending air from Hadley cell
  • hot and dry
    (deserts)
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37
Q

describe the regional climate of the mid latitudes

A
  • westerlies
  • eddies dominate weather
    - cyclone = wet+stormy
    - anti = calm+fine
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38
Q

what are factors of the earth to also consider when describing general atmospheric circulation

A
  • land/sea contrast (oceans warmer, mountains deflect wind)
  • seasonal variations
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39
Q

what creates low pressure systems (that dominates UK weather)

A

contrast of hot and cold air masses

40
Q

describe lifecycle of an extratropical cyclone

A
  • warm and cold front contrast
  • kink created by baroclinic instability
  • cold (W) pushes under, forces warm (E) to rise
  • low pressure at centre
  • cold front moves faster so catches up to warm
  • more catches up = wind
  • caught up = cut off from rising hot air supply = occluded front => dissipates
41
Q

conditions at warm and cold fronts

A

warm = grey skies and drizzle
cold = sudden temp drop + heavy rain

42
Q

how is flow in the upper atmosphere different to surface

A

no friction, so not spiralling in/out of high/low therefore exist in geostrophic balance

43
Q

what would happen if upper and surface highs/lows were vertically stacked

A

geostrophic balance in upper lows remains stable, and convergence into surface low will cause it to dissipate

44
Q

why do weather systems (high/low pressure systems) tilt westwards with height

A

so there is a region of divergence vertically above a low pressure, allowing it to develop
(or convergence above high)

45
Q

where are regions of divergence and convergence found in relation to jet stream ridges

A

divergence found downstream of trough
convergence upstream

46
Q

how do low pressure systems intensify/weaken

A

upper divergence > surface convergence
= low intensify

47
Q

what happens to wind as low pressure system moves over

A

‘veers’ - changes direction suddenly then returns

48
Q

what is an anticyclone

A

-high pressure system
- subsiding air = few clouds
- weak pressure grad = light winds
- hot summer, cold winter

49
Q

when does blocking by anticyclones occur

A

when a ridge in the upper atmospheric jet breaks off
=> extreme weather

50
Q

what is the NAO (North Atlantic oscillation)

A

variable strength in the jet stream leads to different conditions
- +ve = more storms = wet winter europe
- -ve = fewer storms = cold air europe

51
Q

what is ensemble weather forecasting

A

chaotic nature of atmosphere means small changes in initial conditions have large knock on impacts. Run many initial scenarios to see all possible states

52
Q

what is ocean circulation driven by

A
  • surface wind stress
  • surface heat fluxes
  • freshwater fluxes
  • tidal forces
53
Q

what is the gradient between warm and cold below water in oceans called

A

thermocline

54
Q

properties of the mixed layer

A
  • 50-100m
  • stirred by wind stress + convection
  • uniform T + S
    (below T-S thermocline rapidly -> abyss)
55
Q

Definition of transport

A

T = u . L . H
= speed . area

T = volume of fluid passing through an area per unit time

56
Q

Units of transport

A

m^3 per s
sverdrups (Sv) = 10^6

57
Q

what is the continuity equation

A

convergence in one direction must mean divergence in another

∂u/∂x + ∂v/∂y + ∂w/∂z = 0

58
Q

directions of wind driven gyres across latitudes

A

tropics = equatorial jets
subtropics = anticyclonic
subpolar = cyclonic

59
Q

What is surface wind stress and its two components

A

Force exerted on ocean by atmosphere per m^2

  • viscous shear stress
    (downward transfer of momentum)
  • form stress
    (wind accumulate one side + accelerate)
60
Q

formula for wind stress

A

Tau = C . rho . u(10)^2
(N/m^2)

C = drag coefficient =
rho = air density
u(10) = wind speed 10m above surface

61
Q

how is the ekman layer impacted by wind stress

A

wind stress acts on surface but turbulence redistributes momentum over ekman layer

62
Q

Formula for depth integrated ekman transport velocity

A

U = Tau(y) / rho . f
V = -Tau(x) / rho . f

63
Q

What does the formula for ekman transport velocities tell us

A
  • independent of ekman layer depth + structure
  • net acts at 90º to wind stress (velocity spirals down with depth)
  • force balance between wind stress and Coriolis force
64
Q

Where and how does ekman upwellings occur

A

equatorial upwelling
- easterlies = ekman R in N, L in S
- divergence => upwelling

coastal upwellings
- west coasts/ east of oceans
- equator ward wind = offshore ekman transport
(California, Antarctic circumpolar)

cold nutrient rich water brought to surface

65
Q

equation for circulation (around eddy)

A

C = (line integral) u . dl
C = 2π r. u
- for circle

66
Q

equation for vorticity

A

vorticity = measure of spin about it axis
ε = C / A
ε = ∂v/∂x - ∂u/∂y

67
Q

what is kelvins circulation theory

A

circulation is conserved following fluid parcel
(no viscosity and density change)

68
Q

what happens to vorticity when you stretch a fluid parcel parallel to axis of spin

A

increase vorticity

ε (f) / ε (i) = h(f) / h(i)
same as equation for circulation conserved

69
Q

what are the 2 components of absolute vorticity

A

planetary vorticity + relative vorticity
= f + ε

70
Q

how to know when what type of vorticity dominates

A

Rossby number = u / f . L

> 1 = relative dominate
<1 = planetary dominate

71
Q

why does an upwelling fluid column move polewards (sverdrup balance)

A
  • fluid column stretched
  • increase vorticity
  • to do so, move polewards to increase f

(reverse = squashed + equator ward)

72
Q

what do boundary currents do

A

return interior transports that flowed from subtropics towards equator back polewards, by small currents by continent boundaries where sverdrup balance breaks down (»1)
e.g. gulf stream

73
Q

what factors drive ocean gyres

A
  • meridional ekman transport (westerlies + easterlies)
  • conv/div = upwelling/downwelling
  • stretch/squash interior fluid columns -> vorticity conserved
  • western boundary currents return
74
Q

equation for heat transport from wind driven gyres

A

heat transport = V . rho . c . (Tn - Ts)

(difference between temp of water flowing N and S)

75
Q

direction of interior sverdrup transports

A

subtropical = equatorward
subpolar = poleward

76
Q

what is a water mass

A

body of water with given potential temperature and salinity.
defined by:
- formation site
- depth where settles

77
Q

what processes involved in water mass formation

A

in mixed layer:
- cooling
- evaporation
- brine rejection in sea ice formation

78
Q

where is the densest water formed

A

closed seas allowing extended evaporation e.g. med, nordic seas, weddell sea

79
Q

where does the densest water end up and why

A

intermediate
- much higher density -> rapid descent -> fast mixing -> density loss

Weddell sea (least dense of dense) -> slow descent -> less mixing -> bottom water

80
Q

characteristics of meridional overturning circulation (thermohaline circulation)

A
  • upper km N
  • large changes in T + S as moves
  • large depth + length + heat different in layers = large heat transport
81
Q

Effect of meridional overturning circulation on NW Europe (and if gone)

A

heat transport released to atmosphere over gulf stream = warmer than average for latitude

if off:
- widespread cooling in N hemisphere
- CO2 from N sequestered in cold water forming deep waters

82
Q

How do we know there must be mechanical forcing in ocean circulation

A

Overturning circulation driven by surface heat transfer is likely to be weak and shallow, however it is strong and deep

83
Q

what mechanical forcing creates deep overturning meridional circulation

A

tides
- topography on ocean floor encourages mixing

winds
- westerly winds around antarctica
= coastal upwelling

deep water formation
- N lats cold absorb more CO2 and sink

84
Q

What happened to the MOC in the Younger-Dryas event

A

Ice sheets retreating after glaciation injected freshwater into Atlantic
= sit ontop of cold saline
= weaken MOC

85
Q

How might the MOC be affected by climate change in the future

A

warmer atm -> hold more water
-> more precipitation at high latitudes
-> freshwater to Atlantic (sinks less)
-> weaken MOC

86
Q

What creates tides

A

gradient of the moons gravitational field across the diameter of the earth

87
Q

why are there tides twice a day

A

each point on earth rotates under 2 bulges with one earth rotation

88
Q

What happens at the spring and neap tides

A

spring = solar and lunar combine
neap = act at 90º

89
Q

What are ‘normal conditions’ in the context of the ENSO cycles

A
  • warm in W pacific (indonesia)
  • cold in E Pacific (S America)
  • strong convection current
  • upper level westerly flow, lower easterly
  • Equatorial thermocline shallows to east
90
Q

What is anomalously warm conditions over the Eastern pacific (S America) known as

A

El Niño
(less of a gradient than ususal)

91
Q

What is anomalously cold conditions over the Eastern pacific (S America) known as

A

La Nina
(stronger gradient across pacific than normal)

92
Q

What is the southern oscillation, and its effects on the pacific

A
  • large scale fluctuations in the atmospheric pressure over the pacific
  • -ve = high W, low E
  • +ve = low W, high E
93
Q

How is El Niño linked to the southern oscillation

A

there is a strong coupling,
they are opposites:
el nino = -ve
la nina = +ve

94
Q

What are the characteristics and effects of an El Niño event

A
  • weak temp and pressure gradient
  • weak trade winds (easterlies) -> weak upwelling -> reduced bioactivity

moves East:
- pacific warm pool (thermocline deepen)
- atm convection (thunderstorms)
- rise in sea level

95
Q

What are the characteristics of a La Niña event

A
  • stronger Walker circulation
    = stronger temp difference + winds
    -> phytoplankton bloom
  • steep thermocline
96
Q

What is ENSO

A

the coupled climate variability caused by strong coupling between tropical ocean and atmosphere

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