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1B ESA > Ocean Circulation and Climate (L21-24) > Flashcards

Ocean Circulation and Climate (L21-24) Flashcards

1
Q

How is the atmosphere structured?

A

Troposphere, tropopause, stratosphere, stratopause, thermosphere

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2
Q

In the troposphere, what is atmospheric convection driven by?

A

Uneven solar heating from the angle of incoming radiation

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3
Q

How does latitude affect radiation received and reflected?

A

Received: low latitude > high latitude
Reflected: high latitude > low latitudes

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4
Q

How does atmospheric circulation work?

A 
Air warmed at equator rises
Region of low P at equator
Air stops rising at top of troposphere
Flows in direction of poles
Cold air at poles descends
Region of high P at surface
Cold air flows from high P pole to low P equator
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5
Q

What is the Coriolis Force?

A

Acts on all bodies in a rotating reference frame
Acts 90° right of motion in N hemisphere
Acts 90° left of motion in S hemisphere

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6
Q

What is the Intertropical Convergence Zone (ITCZ)?

A

Warm air rises at the equator
Displacement -> zone of low P
ITCZ draws in air from subtropics
Air rising in ITCZ reaches 14km and flows towards the poles

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7
Q

What is the Subtropical Jet Stream?

A

Coriolis causes deflection of air in the upper atmosphere

By 30° N and S, flow is zonal, W to E

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8
Q

What is the Subtropical high pressure zone?

A

Zonal flow = air accumulation at this latitude = some air sinks = high P and low precipitation

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9
Q

How does rotation of the Earth affect atmospheric circulation?

A

Without: 1 convection cell per hemisphere
With: coriolis force = 3 convection cells in each hemisphere

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10
Q

What are the three convection cells in each hemisphere?

A 
0-30°N/S = Hadley cell
30-60°N/S = Ferrel cell
60-90°N/S = Polar cell
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11
Q

What defines the Hadley cell?

A 
Air rises at equator
Flows to 30°N
Subtropical jet stream
Air sinks at subtropical high P zone
Some air moves back to equator
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12
Q

What are the Trade Winds?

A

Air moving along the surface deflected by the Coriolis effect
Northeast Trades = right deflection
Southeast Trades = left deflection

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13
Q

What are the Westerlies?

A

Surface air moving towards the poles from the subtropical high zone, deflected by Coriolis

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14
Q

What defines the Ferrel cell?

A

Surface air flows N at subtropical high zone
Convergence zone at 60°N/S
Coriolis deflection = flow W to E = polar jet stream
Air rises at 60°N/S
Some flows back to 30°N/S

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15
Q

What effect does the Ferrel cell have?

A

Jet stream location

Controlling storm tracks

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16
Q

What effect does the polar jet stream have?

A

Forms meanders called Rossby waves

Rossby waves control storm tracks and long term weather in latitudes of Britain

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17
Q

What defines the Polar cell?

A 
Convergence zone at 60°N/S
Air rises at convergence zone
Air flows towards poles
Warm/moist air convects, cools and sinks at poles
Air flows towards 60°N/S
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18
Q

What is the overall action of the Polar cell?

A

A heat sink for the atmosphere

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19
Q

Define geostrophic flow

A

Air parcels move from areas of high P to low P and are balanced by Coriolis force

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20
Q

What is the tendency of global precipitation?

A

Condensation forms in air as it rises, cooling with the adiabatic lapse rate

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21
Q

What is the adiabatic lapse rate?

A

Atmospheric P decreases with altitude, volume of air expands with drop in P and T drops with expansion of volume
No condensation occurs = 10°C/km
Observed rate is 7°C/km

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22
Q

Why is the adiabatic lapse rate different from that of air in which no condensation occurs?

A

Latent heat of condensation
As rising air cools, passes through the dew point, the T a parcel of air is at saturation w.r.t. water vapour and condensation forms

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23
Q

Define relative humidity

A

The amount of water vapour in the air compared with the amount the air can hold at that T

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24
Q

What initiates surface ocean circulation?

How does this happen?

A

Winds modified by Earth’s rotation and continental barriers
Winds ‘pile up’ and ‘spread out’ surface waters causing large-scale horizontal-flowing currents in upper few 100m of oceans

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25
Q

What direction does the very surface layer of the ocean move at?
Why?

A

45° to the direction of prevailing winds

Deflection from Coriolis effect

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26
Q

What causes an Ekman spiral?

A

Ocean is stratified by density
Very surface layer imposes a force on the layers beneath it, deflected by Coriolis effect
Greater depth = even more deflection

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27
Q

Surface current systems are made up of what?

A

A series of E-W currents and N-S ‘boundary’ currents coupled together in large, rotating surface ocean gyres centred in subtropical oceans

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28
Q

What do eastern boundary currents do?

A

Advect cold surface waters from high to low latitudes

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29
Q

What do western boundary currents do?

A

Move warm water from low to high latitudes

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30
Q

Why do the western boundary currents exist?

A

Conservation of potential vorticity

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31
Q

What is vorticity?
What is planetary vorticity?
What is relative vorticity?
What is absolute vorticity?

A

Vorticity = rotation of the fluid
Planetary vorticity = everything on Earth rotates with the Earth
Relative vorticity = ocean and atmosphere don’t rotate exactly at the rate of the Earth
Absolute vorticity = sum of planetary and relative vorticity

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32
Q

How do eastern and western boundary currents differ?

A

Eastern: slow, broad and shallow
Western: deep, narrow and fast

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33
Q

Where does upwelling occur?

Why?

A

Where surface waters ‘diverge’ i.e. eastern side of ocean basins
Water pushed away by Ekman flow -> gradient in sea surface height -> water upwells to obey mass continuity

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34
Q

Outline equatorial divergence

A

On both sides winds blow from E to W
Coriolis force = water moving N/S
Water upwells to make up for divergence

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35
Q

Why is upwelling important in biogeochemical cycling?

A

Brings nutrient-rich deeper waters close to the surface

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36
Q

What is the general T distribution in the ocean?

A

Surface waters are warm, deep waters are cold

Warmest restricted to surface layer and mid- or low-latitudes

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37
Q

What is the incoming energy disparity between the poles and equator?

A

4x higher at the equator than poles in energy from the sun

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38
Q

Where is the geothermal heat flux from Earth’s interior significant?

A

Only in the vicinity of hydrothermal vents at spreading ridges and stagnant locations like abyssal northern N pacific and the Black Sea

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39
Q

How does the high specific heat of water affect T variation timescales?
Why?

A

Diurnal and seasonal T variations are relatively small compared to interannual and longer timescales
Heat conduction is extremely slow, small amount of heat is transferred downwards

40
Q

What is the main mechanism of heat transfer to deeper waters?

A

Turbulent mixing by winds and waves

Establishes a mixed surface layer

41
Q

What happens to ocean T between 200 and 1000m depth?
What is this called?
What happens below this?

A

T declines rapidly
Permanent thermocline
Virtually no seasonal variation and T ~ 2°C

42
Q

What determines the T below the permanent thermocline?

A

T of the cold, dense water that sinks at polar-regions and flows towards the equator

43
Q

What controls the T distribution in the deep ocean?

A

Density-driven water movements

44
Q

Where are seasonal variations in T shown in the ocean?

A

Above the permanent thermocline

45
Q

What often happens in mid-latitudes during spring and summer?

A

Spring: seasonal thermocline starts to develop above the permanent thermocline, as surface T rise and mixing by wind is small
Summer: seasonal thermocline reaches maximum development

46
Q

What does the El Nino Southern Oscillation (ENSO) couple?

A

Tropical Pacific atmosphere and sea surface T, currents and height via the Walker circulation

47
Q

Define the “La Nina” state

A

Strong trade winds blow warm surface waters to the west
Forms a western Pacific warm pool
Causes upwelling and cooling the eastern Pacific
Strengthens the longitudinal convectional cell - Walker circulation

48
Q

Define the “El Nino” state

A

Walker circulation weakens
Surface waters flow eastward
Convection throughout equatorial Pacific
Upwelling off Chile slows

49
Q

What identifies the ITCZ?

A

Tropical belt of deep convective clouds or as the maximum in time-mean precipitation

50
Q

How much does the ITCZ migrate?

A

Over central Atlantic and Pacific: between 9°N in July and 2°N in January
Greater migration over land from lower heat capacity and topography effects

51
Q

Why is the mean position of the ITCZ north of the equator?

A

Asymmetry of Atlantic Ocean circulation transports energy N across the equator
N hemisphere warmer than S hemisphere

52
Q

Define teleconnection pattern

A

Recurring and persistent, large-scale pattern of P and circulation anomalies spanning vast areas

53
Q

Teleconnection patterns:
Frequency?
Time length?
Size?

A

Low-frequency
Typically several weeks to months, can be prominent for several years
Planetary-scale, many span entire ocean basins and continents

54
Q

Pacific Decadal Oscillation (PDO):
Define positive phase
Define negative phase

A

Positive phase: W Pacific is cool, part of E ocean warms and deep low P in N Pacific
Negative phase: W Pacific is warm, E Pacific is cool, high P in N Pacific

55
Q

How does PDO relate to ENSO?

A

Clear tie, but PDO is on a longer timescale and lags behind ENSO

56
Q

What is the PDO a sume of variability in?

A 
Aleutian low
ENSO teleconnections on decadal timescales
Stochastic atmospheric forcing
Changes in N Pacific thermocline mixing
Changes in oceanic gyre circulation
57
Q

What are the palaeo-climate records of teleconnect changes?

A

Shallow marine sediment cores
Lakecores
Tree rings
Cave deposits

58
Q

What are oxygen isotopes in forams in the equatorial Pacific used to reconstruct?

A

Sea surface temperature gradients and infer mean state of ENSO

59
Q

Define salinity

A

Measure of the amount of dissolved substances in seawater

60
Q

What is the average ocean salinity?

A

34.7‰ (34.7g per 1kg of seawater)

61
Q

How does salinity vary in the ocean?

A

Very little: 75% has salinity 34-35‰

Greatest variation seen in surface waters and the halocline is the depth range where salinities change rapidly

62
Q

Ocean salinity represents a balance between which effects?

A

Hydrologic cycle removing pure water from oceans (evaporation + ice formation) = salinity increase
Adding water to oceans (precipitation + river runoff) = salinity decrease

63
Q

How does salinity vary with latitude in surface seawater?

A

Subtropical: evaporation dominates = very high salinity
Tropics: high evaporation but high precipitation
Highest salinity at sub-tropical central gyre regions 20°-30° N/S

64
Q

Which areas have extreme salinity values?
Why?
Examples?

A

Restricted areas that don’t mix readily with the rest of the ocean
In subtropical latitudes so lots of evaporation but little precipitation
Mediterranean Sea and Red Sea

65
Q

Why is salinity important?

A

Determines density and hence vertical flow patterns in thermohaline circulation
Records physical processes affecting a water mass when last at the surface
Conservative tracer

66
Q

What does it mean for salinity to be a conservative tracer?

Why is this useful?

A

Not changed by processes along flow path

Determining source and mixing of water masses

67
Q

What causes the down-welling of water in the N Atlantic region?

A

Local heating effect of the Gulf Stream
Warmer water evaporates more rapidly
Higher residual salt content
Saltier = denser = down-welling

68
Q

What is the density of seawater a function of?

Which are dominant?

A

T, P and salinity

T and salinity

69
Q

What is the average density of seawater?

Where is the significant part of this number?

A

1.025g/cm^3

After the third decimal

70
Q

How does salinity impact density?
How does T affect density?
How does salinity affect freezing point?

A

Salinity increase = density increase
Generally, T decrease = density increase
True for freshwater down to ~4°C, below 4°C, density decreases
Dissolved salts lower freezing point

71
Q

General ocean water characteristics of:
N Atlantic
Southern Ocean
N Pacific

A

N Atlantic = warmest and saltiest
Southern Ocean = coldest
N Pacific = lowest average salinity

72
Q

Density-driven “thermohaline” circulation:
Surface waters
Intermediate waters
Deep waters

A

Surface: evaporation as they flow poleward through sub-tropics = higher salinity at high-latitude surface, cooling here = water sinks
Intermediate: moderate density sinks at 60°N/S to 1.5-2.5km and spread laterally
Deep: further cooling at edge of sea ice + continental ice sheets, brine rejection during sea-ice formation = production of very dense deep masses at ~70°N/S

73
Q

What affects the interior flow of deep water circulation?

A

Bathymetry and Coriolis forming deep gyres in oceanic basins

Flows strongest along W boundaries

74
Q

What is the purpose of deep western boundary currents?

A

Major transport pathway by which deep waters “ventilate” much of the interior ocean

75
Q

What is North Atlantic Deep Water (NADW)?

A

Mixture of cold surface currents flowing out of the Arctic Ocean with saline surface waters of the N Atlantic

76
Q

What happens to NADW during winter?

A

Cooling, sea-ice formation and evaporation occurs
NADW sinks S of Greenland at 2-4°C and 34.9‰
Flows S over AABW in the Western Atlantic

77
Q

What forms Antarctic Bottom Water (AABW)?

A

During seasonal cooling and sea-ice formation in the Weddell Sea

78
Q

What are the conditions of AABW?

A 
T = -0.5°C
S = 34.8‰
79
Q

Where does AABW go?

What restricts its flow?

A

Flows N along the sea floor in the W Atlantic well into the N hemisphere
Restricted from the E Atlantic by the Mid-Atlantic Ridge system

80
Q

What forms Antarctic Circumpolar Water (ACW)?

A

As NADW upwells off Antarctica, cools further and mixes with Antarctic waters

81
Q

Where does ACW flow to?

A

Flows E around Antarctica then N into Indian Ocean and Pacific Ocean as the bottom waters

82
Q

What happens to water evaporating from the N Atlantic?

A

Transported by atmospheric circulation over Central America via Trade Winds to Pacific

83
Q

What increases the salinity in the N Atlantic?

A

Saline waters flowing out of Mediterranean and Caribbean

84
Q

What determines internal mixing of the ocean?

A

Turbulent flow

Buoyancy forces

85
Q

Why is internal mixing of the ocean difficult across depths?

A

Mixing along surfaces of equal density (isopycnals) is easy

Mixing across isopycnals (diapycnal mixing) is difficult and requires energy

86
Q

What are the three processes of diapycnal mixing in the ocean interior?

A

Double diffusive convection
Breaking of internal waves
Mixing of bottom current on rough bathymetry

87
Q

How does downward salt fingering work?

A

Salt makes water dense near the surface
Stratification is kept stable by T gradient
Heat diffuses more rapidly than salt
Downward moving finger of warm saline water cools by molecular diffusion and becomes more dense

88
Q

Outline internal waves

A

Can be thought of as interfacial waves
Low density water overlies high density water
Internal waves propagate along the boundary
Includes internal tides on diurnal and semidiurnal periods

89
Q

What affects deep ocean circulation mixing on “small” timescales?

A

Waxing and waning of ice sheets

Hydrological cycle changes affecting surface density, deep water mass formation, and wind-forced upwelling

90
Q

What affects deep ocean circulation on “long” timescales?

A

Tectonic changes in connectivity of ocean basins and seafloor bathymetric roughness

91
Q

What does mixing and circulation of the oceans affect?

A

Chemical exchange between the “upper” and “lower” ocean

Horizontal distribution of elements and isotopes throughout the ocean water masses

92
Q

Where is the effect of ocean mixing and circulation change recorded?

A

In the sedimentary and geochemical record of the ocean basins

93
Q

How can the circulation path and age of deep waters be deduced?

A

From their radiocarbon content

Can convert it to the time length since being in contact with atmosphere (ventilation)

94
Q

What are the deep water ages of the oceans?

A

N Atlantic: 0 to 100s of years
S Ocean and Indian Ocean: 1200-1600 years
Pacific: ~2000 years

95
Q

How can mixture of NADW and AABW around Antarctica have a deep water age of the NADW?

A

If the water doesn’t re-equilibrate with the atmosphere, which can occur under sea ice

96
Q

How does thermohaline ocean current feed into the climate system through:
heat transport?
carbon storage?

A

Heat transport: faster overturning transports more heat to the poles and vice versa, where ice sheets grow and decline
Carbon storage: Slower overturning stores more CO2 and other nutrients in the deep ocean, faster overtruning puts CO2 into the atmosphere

97
Q

Outline these ocean circulation states:
Interglacial
Glacial
Off

A

Interglacial: fast and deep overturning of NADW
Glacial: Fast, shallow overturning of Glacial N Atlantic intermediate water, overlying southern-sourced
Off: Freshwater lid on N Atlantic, little or no N Atlantic overturning, Atlantic filled with Antarctic-sourced water

1B ESA (10 decks)

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