Altklausurfragen

Pacific, Atlantic, Indian, Southern, Arctic

mean: 3734m

median: 4093m

deep maximum: 11022m (Mariana trench in western Pacific)

Seafloor divided in three distinct provinces:

1. Continental margin:

   • shelf -> gravel and sand (<150m)

   • slope -> mud (100m - several km)

   • rise -> 3000-4000m

2. Deep ocean

   • abyssal plains (mostly smooth)

   • abyssal hills

   • seamounts

   • deep sea trenches (clay/oozes (mineralic and organic)

3. Mid ocean ridges

   • shallow or steep flanks (spreading rate)

   • crest (spreading center)

• echo sounding and seismic reflection rely on sound pulses that reflect off the ocean floor and off sedimentary layers

• Altimetry: relies on satellites deducing different sea level depths due to different masses of the water column or rock/mud beneath the seasurface 

• Autonomous systems: equpped with multibeam echo sounder -> high resolution images due to proximity to seafloor

• dense bottom waters are seperated by MORs and can only leak through small gaps (fracture zones)

• trenches can steer and impact deep boundary currents

• ocean bottom roughness affects ocean mixing rates

• seamount chains refract tsunamis and change their propagation direction

• High surface tension

• high heat capacity (4187J/kg) -> heat storage -> 90% of anthropogenic global warming is absorbed in the ocean

  heat capacity of the whole atmosphere resembles heat capacity of top 3.2m of ocean

• Good solvent -> slightly negatively charges O and slightly positively charged H2 form a dipole that binds all kinds of stuff

• density anomaly at 3.98°C

Water has its highest density at 3.98°C, at lower temperatures H2O molecules form hexagonal clusters which take up more space -> ice floats

Salinity

Temperature

Pressure

It is more dense as there are more ions in tthe water

No, there is an in situ temperature (the one a parcel of water has) and a potential temperature (the one that a parcel of water has if it is moved adiabatically to the surface)

Adiabatic: thermodynamic process in which a given system is moved from one pressure tto another without exchange with its environment (e.g. a Nisken bottle)

-> the in siu T would be warmer than the potential T

• Acoustic waves are:

  • compressional waves

  • longitudinal polarized (displacement parallel to vector)

  • dispersionless (phase speed indipendent of frequency)

• Speed of sound:

  • air: 341 m/s

  • water: 1480 m/s

  • not constant in water, depends on density

    • p-change: 1.8m/s/100m

    • T-change: 5m/s/°K

    • S-change: 3m/s/g/kg

• In water masses of particularly high densities, but also high temperatures (si at surface and deep ocen fast)

• also: SOFAR-Channel

  • sound fixing and ranging channel -> sits at the minimum of the curve from vertical sound speed (mid depths) -> pressure increase to the deep (faster) counteracts temperature increase to surface (faster) -> around 500m depth

  • As a result of Snell’s law, sound waves are refracted towards regiions with lower sound speeds

    • far sound travel in SOFAR channel (esp. for low frequencies)

• Photons of longer wave lengths are absorbed before photons at shorter wave lengths and thus continuous attenuation of light other than blue, make the ocean appear blue

• Furtther: blue light is more likely to be scattered (randomly reflected back in all directions

• Depends on turbidity/clarity

• Maximum values of the euphotic zone (1% light left) for very clear waters: ~100m

Sources:

• precipitation

• river runoff

• ice melting

• ground water flow

Sinks:

• evaporation

• ice formation and brine rejection

Transport: Stuff per time

• water: m3/s

• salt: hg/s

• heat: J/s =. W

Flux: transport per unit area

• heat: W/m2 (e.g. heat loss of. sea surface)

• volume flux: m3/s*m2

Budget:

• Transport in and out of a closed system

• Average solar heating: 342 W/m2

• Albedo: fraction of incident radiation (light), that is reflected by a surface/body - High Albedos: Desert Sands < Clouds < bare ice < ice and snow (alpha = 0.9)

• There are heat fluxes

  • Ocean <-> Atmosphere

  • Atmosphere ~ Wind

  • Atmosphere ~ Condensation

• Sum of the heat fluxes are

  • flux of sunlight into the ocean(Qsw)

  • net flux of infrared radiation from the ocean (Qlw)

  • flux of heat out of the sea due to conduction (Qsen)

  • flux of heat carried by evaporated water (Qlat)

  • change of heat storage (QT)

  • divergance of heat transport (Qv)

high latitudes -> high albedos (sea ice and snow) -> net heat. deficiency

BUT: divergance of heat transport Qv!

• Areas of net gain ((sub-)tropics) and loss annihilate each other due to convection in atmosphere + THC (warm waters flow poleward, cold sink and flow equatorwards)

NADW

NEADW

LSW

AAIW

AABW

Definition of a body of water:

• temperature, salinity, oxygen, nutrients, helium, chlorofluorocarbons, SF6

• Most water masses acquire their characteristics at the sea surface

• When it sinks it carries these properties with it -> slow mixing with sourroundng waters

Polar regions - AABW - originates from Antarctic shelf - prone to:

• open ocean convection, increased density due to air-sea-ice buoyancy loss

• shelf convection (at ploynyas, caused by catabatic winds), air-sea-ice buoyancy loss, ice formation - brine rejection and buoyancy loss -> slides down continental shelf (overflow) to great depth

  • also entrain neighboring waters

Sea ice is from fresh water, brine is rejected during the formation and increases salinity of sourrounding waters

NADW:

originated from North Atlantic Ocean - deep water mixing in Greenland and Labrador sea, forms by deep convection with significant entrainment (mixing with surrounding water)

AABW:

ooriginates from coastal regions around Antarctica, forms by shelf convection with significant entrainment (sea ice important role)

• Large scale ocean circulation is forced by

  • wind stress on sea surface -> wind driven circulation

  • Buoyancy fluxes (heat and freshwater fluxes) -> Thermohaline circulation

• Forces acting on the ocean. (in all cardinal directions)

  • pressure gradient force:

    • P1 and P2 act over a distance x on different sides of water cubicle

    • Results: -(δp/δx)

    • differnet pressures because of different sea level height and different densites

  • Friction

    • wind stress τ: prportional to the square of windspeed, transmitted downward as a result of fricttion (not beyond 50-100m)

    • bottom drag: provokes frictiion boundary layer at the seafloor, dependent on the structure of seafloor

  • Coriolis - apparent force

    • dependent on:

      • lattitude

      • speed of particle

  • Gravity - towards the center of earths mass

  • Tides

Air and water not only follow pressure gradient but are deflected by Coriols

NH: deflection of objects to the right

SH: deflectiion of objects to the left

1. Zonal: UE ;  ∂u/∂t-fv=1/p*∂p/∂x+R

2. Meridional: VE ;  ∂v/∂t+fu=-1/p*∂p/∂y+R

3. Vertical: (W)??; ∂p/∂z=-ρg

Mostly EBUS or equatorial upwelling system

Major forces in the ocean:

• 1: acceleration (change of velocity due to force imbalances)

• 2a: horizontal advectioin of velocty

• 2b: vertical advection of velocity (mostly small, not discussed)

• 3: Coriolis force

• 4: Pressure gradent force

• 5: Viscous forces (turbulent viscosity, eddy friction)

• 6: Frictional forces (wind stress)

The geostrophic balance is between the coreolis force (3) and the pressure gradiient force (4)

-> forces flow along isobars (high pressure on the right)

BUT: isobars ≠ isopycnals -> baroclinic conditions:

• the deeper, thesmaller the PGF because relative pressure differences provoking pressure differences become smaller

• flow generated by PGF becomes smaller

Thermal wind equation:

• geostrophic balance (3) + hydrostatic balance / PGF (4) = thermal wind equation (6)

  • relates the vertical “shear” of the horizontal flow to the horizontal density dradientdue

  • basically: how strong is the change of flow with depth

• Ekman layer depth: 20m to maximum 150m

• calculated via

Ekman spiral:

• Balance of coreolis force (3) and frictional force (wind - 6)

• water set in motion by wind -> if several days: flow near surface described by coreolis force and vertical friction

• wind stress is transmitted downwards by internal friction

• Coriolis effect set into play creating wind driven frictional layer called called ekman layer -> propagates deep

• Coriolis force is a function of latitude and speed

  • Ekman layer depth is the e-folding deptth of decaying velocity

• Vertical integration of horizontal velocity within the Ekman layer is called Ekman Transport

Equatorial setting:

• Ekman transport due to easterly winds provoke divergence and depression of sea level height -> replenished by underlying cold waters + upwards shift of thermocline

• visible as equatorial cold tongue (Atl + Pac)

Coastal setting:

• Equatorward coast-parallel winds lead to Ekman transport off-shore

• BUT: Ekman transport tilts sea surface height provoking an addditional, opposing PGF towards coast

  YET balanced out by Coriolis force causing coast-parallel geostrophic surface flow

  THIS forces an onshore flow (see picture GF) in the bottom boundary layer that. supplies the coastal upwelling

  ADDITIONALLY wind stress curl, provoking increased water heights towards equator result in. a net laaaarge scale PGF towards the pole., which satisfied by poleward undercurrents (PF)

• Winds -> wind speed + fetch and time relevant for wave height

• Seismic disruptions (earthquakes, landslides) -> Tsunamis: off-shore, fast and long wave length, small amplitude; on shore, slows, energy conserved -> amplitude grows

• Gravitational attractoin of moon and sun

• Opposing and restoring forces:

  • Surface tensioin (creating capillary waves)

  • Gravity (creating wind waves, Tsunamis, Tides)

• Wave characzeristics:

  • Wave number: k=2π/λ (λ = Wavelength)

  • Frequency: ω= 2π/T (Period: time required for a wave crest(maximum) to reach from a  certain point to another) 

  • Phase speed: c= λ/T → c= ω/k

• earth rotates with respect to moon with a period of 24h50min -> equilibrium tidal bulges would need to travel in the opposite direction in order to maintain their positiioins relative to the moon -> two high and two low tides per lunar day

  BUT: actual tides do not behave as equilibrium tides

  • depth of ocean

  • land masses constrict preferred directiioin of flow

  • inertia causes time lag in oceans response to tractiive forces

  • Coriolis force deflects tidal flows

Similarities: Both marginal and relatively shallow Seas of North Atlantic

Salinity:

• Baltic has significant west to east salinity gradient 18-2psu, larger river catchment area + low volume

• North Sea is northwards open to ocean psu-values, coastal ares slightly lower (catchment and baltic runoff)

Exchange:

• Baltic is very enclosed wiith only one opening-> storms needed

• North Sea is open to the Atlantic (english Channel + Faire-Isle Channel and North Sea Plateau

Topography/Tides:

• Baltic has no tides + very weak circulation, exchange

• North Sea has strong seasonal and daily circulation due to winds, (anti-)clockwise circulation in basin (seasonal) + domiinant diurnal spring-neap tides superimpose everything else in the Baltic (mainly M2 and S2 constituents)+ shallow topography makes permanent displacement of water possible

Depth:

• Baltic <30m

• North Sea ~90m (including deep norwegian trench ~800m)

Temperature:

• Baltic has very high surface temperature iin summer because it is so shallow

• North Sea has mean summer temperature of 17°C

Oxygen:

• Baltiic has pronounced regions of hypoxia/anoxia

• North Sea has frequent mixing and displacement does not provoke oxygen deficiencies

• Baltic is permanently stratified water body (no pronounced autumn or winter mixing to full depths)

  • in summer: 3-layer-stratification witha thermocline at 15-20m depth and a strooong halocline at 30-70m depth -> very diffiicult vertical advection

  • in winter: still two layer stratification with strong thermocline due to immense psu differences not advected

  • Therefore, especially iin the deep basins -> water does not become advected on a winterly basis

• Deep water exchange and replenishment of oxygen is limited to singular storm events (e.g. 2014) and only weak continuous vertical advectioin in Baltic

• Baltic sea has large catchmnet ares (85 Mio. inhabitants) -> strongly eutrophied and high export productioin

• easterly winds transport warm and less dense surface water to the western end of the basin, where it is piled up

  -> tilted sea-level with negative west to east gradient (PGF develops) + warm pool at western end of basins

• replacement of water at eastern margins of basin through colder deep-water (reaching all the way to surface) -> thermocline tilted, flattens in the east

• Usually: easterly trade winds -> warm SSTs in W and colder in E -> tilted tehrmocliine, deep convection over western warm pool -> heavy rains

• El Nino conditioins: every few years trade winds weaken -> warm pool migrates eastwards -> thermocline flattens ->.less equatorial upwelling in east -> disappearance of equatorial cold tongue -> deep convectioin follows warm pool eastwards -> heavy rains over middle of pacific

• Often followed by La Nina: trade winds come back strionger than normal -> push warm water extremely westward -> steep thermocline -> center of convection extremely to the west, especially pronounced eastern pacific cold tongue

• SST and surface air pressure (Tahiti-Darwin Islands) are good proxies for La Nina/El Nino

• Indian subcontinent shows big-scale seasonal reversals

• January: from continent to ocean <—> July: from ocean

• Physical ingredients:

  • Land sea differences: differing heat capacities of land and water -> also, water stores more heat because vertical entrainment and future release

  • Planetary rotatoin: iintroduces swirl + much stronger winds

  • Moisture: Water vapor, collected over oceans throug evaporation, condenses over land + marginal seas releasing vast quantities of heat

  • Orography: Acts as elevated heat source which intensifies flow and also ducts flow

    And the in summer -> water vapor collected over ocean precipitates at west ghats and Himalayas

  • In winter, winds from north to south do not pick up any moisure siince they only drift over land