Ocean Circulation and Climate (L21-24) Flashcards
How is the atmosphere structured?
Troposphere, tropopause, stratosphere, stratopause, thermosphere
In the troposphere, what is atmospheric convection driven by?
Uneven solar heating from the angle of incoming radiation
How does latitude affect radiation received and reflected?
Received: low latitude > high latitude
Reflected: high latitude > low latitudes
How does atmospheric circulation work?
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
What is the Coriolis Force?
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
What is the Intertropical Convergence Zone (ITCZ)?
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
What is the Subtropical Jet Stream?
Coriolis causes deflection of air in the upper atmosphere
By 30° N and S, flow is zonal, W to E
What is the Subtropical high pressure zone?
Zonal flow = air accumulation at this latitude = some air sinks = high P and low precipitation
How does rotation of the Earth affect atmospheric circulation?
Without: 1 convection cell per hemisphere
With: coriolis force = 3 convection cells in each hemisphere
What are the three convection cells in each hemisphere?
0-30°N/S = Hadley cell 30-60°N/S = Ferrel cell 60-90°N/S = Polar cell
What defines the Hadley cell?
Air rises at equator Flows to 30°N Subtropical jet stream Air sinks at subtropical high P zone Some air moves back to equator
What are the Trade Winds?
Air moving along the surface deflected by the Coriolis effect
Northeast Trades = right deflection
Southeast Trades = left deflection
What are the Westerlies?
Surface air moving towards the poles from the subtropical high zone, deflected by Coriolis
What defines the Ferrel cell?
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
What effect does the Ferrel cell have?
Jet stream location
Controlling storm tracks
What effect does the polar jet stream have?
Forms meanders called Rossby waves
Rossby waves control storm tracks and long term weather in latitudes of Britain
What defines the Polar cell?
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
What is the overall action of the Polar cell?
A heat sink for the atmosphere
Define geostrophic flow
Air parcels move from areas of high P to low P and are balanced by Coriolis force
What is the tendency of global precipitation?
Condensation forms in air as it rises, cooling with the adiabatic lapse rate
What is the adiabatic lapse rate?
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
Why is the adiabatic lapse rate different from that of air in which no condensation occurs?
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
Define relative humidity
The amount of water vapour in the air compared with the amount the air can hold at that T
What initiates surface ocean circulation?
How does this happen?
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
What direction does the very surface layer of the ocean move at?
Why?
45° to the direction of prevailing winds
Deflection from Coriolis effect
What causes an Ekman spiral?
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
Surface current systems are made up of what?
A series of E-W currents and N-S ‘boundary’ currents coupled together in large, rotating surface ocean gyres centred in subtropical oceans
What do eastern boundary currents do?
Advect cold surface waters from high to low latitudes
What do western boundary currents do?
Move warm water from low to high latitudes
Why do the western boundary currents exist?
Conservation of potential vorticity
What is vorticity?
What is planetary vorticity?
What is relative vorticity?
What is absolute vorticity?
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
How do eastern and western boundary currents differ?
Eastern: slow, broad and shallow
Western: deep, narrow and fast
Where does upwelling occur?
Why?
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
Outline equatorial divergence
On both sides winds blow from E to W
Coriolis force = water moving N/S
Water upwells to make up for divergence
Why is upwelling important in biogeochemical cycling?
Brings nutrient-rich deeper waters close to the surface
What is the general T distribution in the ocean?
Surface waters are warm, deep waters are cold
Warmest restricted to surface layer and mid- or low-latitudes
What is the incoming energy disparity between the poles and equator?
4x higher at the equator than poles in energy from the sun
Where is the geothermal heat flux from Earth’s interior significant?
Only in the vicinity of hydrothermal vents at spreading ridges and stagnant locations like abyssal northern N pacific and the Black Sea
How does the high specific heat of water affect T variation timescales?
Why?
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
What is the main mechanism of heat transfer to deeper waters?
Turbulent mixing by winds and waves
Establishes a mixed surface layer
What happens to ocean T between 200 and 1000m depth?
What is this called?
What happens below this?
T declines rapidly
Permanent thermocline
Virtually no seasonal variation and T ~ 2°C
What determines the T below the permanent thermocline?
T of the cold, dense water that sinks at polar-regions and flows towards the equator
What controls the T distribution in the deep ocean?
Density-driven water movements
Where are seasonal variations in T shown in the ocean?
Above the permanent thermocline
What often happens in mid-latitudes during spring and summer?
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
What does the El Nino Southern Oscillation (ENSO) couple?
Tropical Pacific atmosphere and sea surface T, currents and height via the Walker circulation
Define the “La Nina” state
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
Define the “El Nino” state
Walker circulation weakens
Surface waters flow eastward
Convection throughout equatorial Pacific
Upwelling off Chile slows
What identifies the ITCZ?
Tropical belt of deep convective clouds or as the maximum in time-mean precipitation
How much does the ITCZ migrate?
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
Why is the mean position of the ITCZ north of the equator?
Asymmetry of Atlantic Ocean circulation transports energy N across the equator
N hemisphere warmer than S hemisphere
Define teleconnection pattern
Recurring and persistent, large-scale pattern of P and circulation anomalies spanning vast areas
Teleconnection patterns:
Frequency?
Time length?
Size?
Low-frequency
Typically several weeks to months, can be prominent for several years
Planetary-scale, many span entire ocean basins and continents
Pacific Decadal Oscillation (PDO):
Define positive phase
Define negative phase
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
How does PDO relate to ENSO?
Clear tie, but PDO is on a longer timescale and lags behind ENSO
What is the PDO a sume of variability in?
Aleutian low ENSO teleconnections on decadal timescales Stochastic atmospheric forcing Changes in N Pacific thermocline mixing Changes in oceanic gyre circulation
What are the palaeo-climate records of teleconnect changes?
Shallow marine sediment cores
Lakecores
Tree rings
Cave deposits
What are oxygen isotopes in forams in the equatorial Pacific used to reconstruct?
Sea surface temperature gradients and infer mean state of ENSO
Define salinity
Measure of the amount of dissolved substances in seawater
What is the average ocean salinity?
34.7‰ (34.7g per 1kg of seawater)
How does salinity vary in the ocean?
Very little: 75% has salinity 34-35‰
Greatest variation seen in surface waters and the halocline is the depth range where salinities change rapidly
Ocean salinity represents a balance between which effects?
Hydrologic cycle removing pure water from oceans (evaporation + ice formation) = salinity increase
Adding water to oceans (precipitation + river runoff) = salinity decrease
How does salinity vary with latitude in surface seawater?
Subtropical: evaporation dominates = very high salinity
Tropics: high evaporation but high precipitation
Highest salinity at sub-tropical central gyre regions 20°-30° N/S
Which areas have extreme salinity values?
Why?
Examples?
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
Why is salinity important?
Determines density and hence vertical flow patterns in thermohaline circulation
Records physical processes affecting a water mass when last at the surface
Conservative tracer
What does it mean for salinity to be a conservative tracer?
Why is this useful?
Not changed by processes along flow path
Determining source and mixing of water masses
What causes the down-welling of water in the N Atlantic region?
Local heating effect of the Gulf Stream
Warmer water evaporates more rapidly
Higher residual salt content
Saltier = denser = down-welling
What is the density of seawater a function of?
Which are dominant?
T, P and salinity
T and salinity
What is the average density of seawater?
Where is the significant part of this number?
1.025g/cm^3
After the third decimal
How does salinity impact density?
How does T affect density?
How does salinity affect freezing point?
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
General ocean water characteristics of:
N Atlantic
Southern Ocean
N Pacific
N Atlantic = warmest and saltiest
Southern Ocean = coldest
N Pacific = lowest average salinity
Density-driven “thermohaline” circulation:
Surface waters
Intermediate waters
Deep waters
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
What affects the interior flow of deep water circulation?
Bathymetry and Coriolis forming deep gyres in oceanic basins
Flows strongest along W boundaries
What is the purpose of deep western boundary currents?
Major transport pathway by which deep waters “ventilate” much of the interior ocean
What is North Atlantic Deep Water (NADW)?
Mixture of cold surface currents flowing out of the Arctic Ocean with saline surface waters of the N Atlantic
What happens to NADW during winter?
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
What forms Antarctic Bottom Water (AABW)?
During seasonal cooling and sea-ice formation in the Weddell Sea
What are the conditions of AABW?
T = -0.5°C S = 34.8‰
Where does AABW go?
What restricts its flow?
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
What forms Antarctic Circumpolar Water (ACW)?
As NADW upwells off Antarctica, cools further and mixes with Antarctic waters
Where does ACW flow to?
Flows E around Antarctica then N into Indian Ocean and Pacific Ocean as the bottom waters
What happens to water evaporating from the N Atlantic?
Transported by atmospheric circulation over Central America via Trade Winds to Pacific
What increases the salinity in the N Atlantic?
Saline waters flowing out of Mediterranean and Caribbean
What determines internal mixing of the ocean?
Turbulent flow
Buoyancy forces
Why is internal mixing of the ocean difficult across depths?
Mixing along surfaces of equal density (isopycnals) is easy
Mixing across isopycnals (diapycnal mixing) is difficult and requires energy
What are the three processes of diapycnal mixing in the ocean interior?
Double diffusive convection
Breaking of internal waves
Mixing of bottom current on rough bathymetry
How does downward salt fingering work?
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
Outline internal waves
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
What affects deep ocean circulation mixing on “small” timescales?
Waxing and waning of ice sheets
Hydrological cycle changes affecting surface density, deep water mass formation, and wind-forced upwelling
What affects deep ocean circulation on “long” timescales?
Tectonic changes in connectivity of ocean basins and seafloor bathymetric roughness
What does mixing and circulation of the oceans affect?
Chemical exchange between the “upper” and “lower” ocean
Horizontal distribution of elements and isotopes throughout the ocean water masses
Where is the effect of ocean mixing and circulation change recorded?
In the sedimentary and geochemical record of the ocean basins
How can the circulation path and age of deep waters be deduced?
From their radiocarbon content
Can convert it to the time length since being in contact with atmosphere (ventilation)
What are the deep water ages of the oceans?
N Atlantic: 0 to 100s of years
S Ocean and Indian Ocean: 1200-1600 years
Pacific: ~2000 years
How can mixture of NADW and AABW around Antarctica have a deep water age of the NADW?
If the water doesn’t re-equilibrate with the atmosphere, which can occur under sea ice
How does thermohaline ocean current feed into the climate system through:
heat transport?
carbon storage?
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
Outline these ocean circulation states:
Interglacial
Glacial
Off
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
