exam questions

Pacific, Atlantic, Indian, Arctic and Southern Ocean (Basin) – last one connects P, A &I

On average the ocean is 3734 m (mean - average) and 4093 m (median) deep,

the deepest point is the Mariana trench and its deepest point is in a depth of around 11.022 m (western pacific - ~11°N, 142° E)

Sea floor subdivided in 3 distinct provinces:

(1)     Continental margin: shelf (gravel & sand - < 150 m depth), slope (mud – 100m to several km) & rise (3000 – 4000 m)

(2)     Deep ocean basin: abyssal plains (mostly smooth), abyssal hills, seamounts and deep-sea trenches (clay, oozes (mineral, organic)

(3)     Mid-ocean ridges: midocean ridge flanks, midocean ridge crest (sea floor spreading)

Altimetry: echo sounding and seismic reflection rely on sound pulses that reflect off the ocean floor and off sedimentary layers (time the sound pulse travels) – altimetry uses satellites to determine bathymetry

Autonomous systems: newer method (autonomous underwater vehicles (AUV)), AUV equipped with multibeam echosounder -> very high solution

Bottom waters (dense) are separated by ocean ridges and can leak across ridges only through narrow gaps (fracture zones); trenches can steer and impact deep boundary currents; ocean bottom roughness affects ocean mixing rates; seamount chains can e.g. refract (brechen, ablenken) tsunamis – impact in which direction they propagate

-          dense bottom water is separated by ocean ridges and can leak across ridges only through narrow gaps (fracture zones)

-          trenches can steer and impact deep boundary currents

-          ocean bottom roughness affects ocean mixing rates

-          seamount chains can refract tsunamis and change their propagation direction

(1)     Hydrogen bonds (molecules form clusters (tetrahedron – densest possible packing), HB are steadily renewed)

(2)     dipole structure

(3)     due to 1 + 2: high surface tension, very high heat capacity (4187 J/kg/K), high latent heats of vaporization and fusion (good solvent, density in solid less than fluid phase)

Temperature, salinity, pressure

Saline water is heavier and that makes it denser by constant temperature (density increases by increasing mass and/or decreasing volume)

Because ice (solid water) has a lower density than fluid water (water expands during the phase change from liquid to solid)

• hexagonal clusters

No, because temperature depends on pressure, an increase in pressure causes a water parcel to slightly compress leads to an increase in temperature) – in situ temp. & potential temp. (temp that water would have at the surface)

o   In-situ-T: temperature of water parcel at particular depth + pressure (no correction for compressibility effect)

o   Potential-T θ: Temperature a water parcel would have if moved adiabatically to surface (takes compressibility into account)

§  Adiabatic: thermodynamic process in which a given system is moved from one point (pressure p1) to another (pressure p2) without exchanging heat with its surrounding (imagine water sample in a CTD-rosette)

o   If we measure in-situ-T, it will be warmer, than it´s potential-T (hypothetical T, this water parcel would have at surface-pressure) à if p↑ then T↑, now when sampling: p↓, then T↓ à to compare, need to convert

Faster; (sound speed c (water): 1480 m/s; c(air): 341 m/s

-          Acoustic waves are:

o   Compressional waves

o   longitudinal polarized (displacement parallel to vector)

o   Dispersionless (phase speed ~ independent of frequency)

-          Speed of Sound:

o   Air: 341 m/s

o   Water: 1480 m/s

o   à not constant in water, depends on density

§  p-change: 1.8m/s*100m

§  T-change: 5m/s*°C

§  PSU: 3m/s*1g/kg

Sound speed not constant, depends primarily on temperature (increases with increasing temp.) and pressure (increases with increasing pressure) with only small corrections for salinity; sound speed minimum at mid-depth (~500 – 1000 m, SOFAR channel), sound speed highest near the surface (and then again higher in greater depth

-          Photons of longer wavelengths are absorbed before blue light and thus continuous attenuation of lights other than blue, are making the ocean to appear blue

-          Further: blue light is more likely to be scattered (randomly reflected back in all directions)

Appears deep blue in tropical and equatorial seas & green blue in higher latitudes; the blue light reaches the deepest, colors like red and yellow are absorbed

Depends on clarity of water; ~1m (45% remains of energy, read absorbed, all colors mainly green, blue), ~10m only 16%    (violet, green, blue), ~100m (1%, end of euphotic zone, blue)

(1)     Sources: melting sea ice + icebergs, river runoff, precipitation (moving of water), ground water flow

(2)     Sinks: formation of ice, evaporation (moving of water, ground water)

(1) transport (stuff per time): - volume (m³/sec) (106 m³/sec = Sv); salt (kg/sec); heat (J/sec = W)

(2) flux (transport per unit area = stuff per time per area (e.g. surface): - volume flux (m³/sec/m²); heat flux (W/m²)

-          Budget:

o   Transport in and out of a closed volume

o   We are assuming e.g.:

§  Conservation of Volume:

·       Vi + 𝑅iver + 𝑃recipitation = 𝑉0 + 𝐸vaporation

·       𝐹 ≡ 𝑉0 − Vi = 𝑅 + (𝑃 − 𝐸)

§  Conservation of salt over time periods of decades

·       Salt in: Vi * (ρi) *Si = Salt out: V0 * (ρ0) * S0

§  à Knudsen-Relation:

·       Vi = F* (S0/(Si – So)) and V0 = F * (Si/(Si – S0))

·       Calculate freshwater gain or loss for specific regions

§  F positive: P + R > E ↔ F negative: P + R < E

Surface covered with (sea) ice and snow (α=0.9)

-          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 + Snow (alpha = 0.9)

-          There are heat fluxes

o   Ocean↔Atmosphere

o   Atmosphere ~ Wind

o   Atmosphere ~ Condensation

-          Sum of heat fluxes: Qsum = QSW – QLW – Qsen - Qlat =QT + Qv

o   QSW: flux of sunlight into the sea (shortwave radiation)

o   QLW: net heat flux of infrared radiation from the ocean

o   Qsen: flux of heat out of the sea due to conduction

o   Qlat: flux of heat carried by evaporated water

o   QT: change of heat storage

o   Qv: divergence of heat transport à …

In the annual average, ocean gains heat in the tropics and looses heat in higher latitudes, to maintain a balance, the atmosphere and ocean redistribute heat from low to high latitudes (warm waters flowing poleward and cold waters returning)

-          high lats à high albedos (sea-ice + snow) à net heat deficit

-          But: Divergence of heat transport Qv!

o   Areas of net gain ((sub-)tropics) and loss annihilate each other due to convection in atm. + THC (warm waters flow polewards, cold sink and flow equatorwards)

(1)     Ocean basins: Atlantic, Pacific, Indian, Antarctic, Arctic

(2)     References to the hemisphere/climate zone: Equatorial, Subtropical, Tropical, Polar, Subpolar

(3)     Depth range: Surface, Central & Mode, Intermediate, Deep, Bottom Water

-          NADW, NEADW, LSW, AAIW, AABW

Temperature, salinity, Oxygen, Nutrients (Nitrate, Phosphate, Silicate), Helium, Chlorofluorocarbons (CF6), SF6 …

o   Most water masses acquire their characteristics at the sea surface

o   When it sinks it carries these properties with it à slow mixing with surrounding waters

 (AABW – Antarctic Bottom Water) – originates from coastal regions around Antarctica

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

o   open ocean convection, increases density due to air-sea-ice buoyancy loss

o   shelf convection (at polynyas, caused by catabatic winds), air-sea-ice buoyancy loss, ice formation – brine rejection and buoyancy loss à slides down continental shelf (overflow) to great depth

§  also entrain neighboring waters

When sea ice is formed the salinity increases due to the loss of freshwater, the salt in the sea ice remains in the surrounding waters

-          NADW: originates from the North Atlantic Ocean – deep water mixing in Greenland and Labrador Sea, forms by deep convection with significant entrainment (mixing with surrounding water)

-          AABW: originates from coastal regions around Antarctica, forms by shelf convection with significant entrainment (sea ice important role)

Due to currents, (1) wind stress acting on sea surface – forces wind-driven circulation

(2) buoyancy fluxes (heat and freshwater fluxes) between ocean and atmosphere – forces thermohaline circulation

(1)     Pressure Gradient Force (PGF)

(2)     Coriolis Force (CF)

(3)     Friction (Surface Wind Stress, Bottom Drag (between ocean & solid earth))

(4)     Gravity

(5)     Others (Tides)

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

§  Results: -(δp/δx)

§  Different pressures because of different sea-level height and different densities

§  wind stress τ: proportional to the square of windspeed, transmitted downward as a result of friction (not beyond 50-100m)

§  bottom drag: provokes friction boundary layer at the seafloor, dependent on the structure of seafloor

§  dependent on:

·        latitude

·       speed of particle

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

Yes, if the water is in motion it will be deflected to the right (NH) or to the left (SH) due to the Coriolis force on a large-scale

-        zonal, so x-direction: u                         

-        meridional, so y-direction: v                 

-        horizontal, so z-direction: w        

-      total current notation: : U(mit strich) = (u,v,w)

The Coriolis force deflects objects in motion (e.g. water currents) to the right in the northern hemisphere and to the left in the southern hemisphere (on large-scale basis)

-          mostly at eastern bound upwelling or equatorial upwelling systems

-          The geostrophic balance is between the Coriolis force (3) and the pressure gradient force (4):

o   forces flow along isobars (high pressure on the right):

o   but: isobars ≠ isopycnals à baroclinic conditions:

§  the deeper, the smaller the PGF because relative pressure differences provoking pressure differences become smaller

§  à flow generated by PGF becomes smaller

Dominant balance of large-scale flows of the atmosphere and the ocean; this is the balance between the pressure gradient force and the Coriolis force; the pressure gradient force sets the water in motion (from high to low pressure) if the water is in motion it will be deflected by the Coriolis force. The two forces then balance each other until the water flows along lines of constant pressure (with high pressure to the right (NH))

Ekman layer depth;  depth at which Ekman transport = 0, function of windspeed

velocity down to a depth of about 20 – 150 m (Ekman depth) – direct influence of wind

Wind parallel to the coastline lead to offshore (NH – winds blowing towards the equator) Ekman transports of water in the surface layer that causes upwelling along the coast (surface water is transported away from the coast and needs to be replaced), drives vertical advection of nutrient-rich cold waters from the deeper layers to the near surface due to continuity (SH – winds blowing in the opposite direction) – not only at the coasts

Mostly wind generated, and gravitational force (moon and sun), (by surface tension (only very short waves))

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

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

-          Gravitational attraction of moon and sun

-          earth rotates with respect to moon with a period of 24h50min à equilibrium tidal bulges would need to travel in the opposite direction in order to maintain their positions relative to the moon

o   à Two high and two low tides per lunar day

-          But: actual tides do not behave as equilibrium tides

o   depth of ocean

o   land masses constrict preferred direction of flow

o   inertia causes time lag in oceans response to tractive forces

o   Coriolis force deflects tidal flows

Semidiurnal tide; most common one, the other areas are (maybe) separated due to land masses

(1): similarities: Both marginal seas of the North Atlantic, surrounded by land (Baltic more), shallow sea,

(2): differences: NS widely open, BS isolated (only connected with NS), BS low oxygen areas, more brackish water BS (freshwater dominates)

-          Several reasons:

o   First of all: Baltic à permanently stratified water body (no pronounced autumn or winter mixing to full depths)

§  In summer: 3-layer-stratification with a thermocline at 15-20m depth and a strooong halocline at 30-70m depth à very difficult vertical advection

§  In winter: still 2-layer-stratification with strong thermocline due to immense PSU differences not advected

§  Therefore, esp in the deep basins à water does not become advected on a winterly basis

o   Second: deep water exchange and replenishment of oxygen is limited to singular storm events (e.g. 2014) and only weak continuous vertical advection in Baltic

o   Third: Baltic sea has large catchment area (85Mio. Inhabitants) à strongly eutrophied and high export production

due to strong stratification in summer oxygen-rich surface waters can not reach the bottom (strong thermo- and halocline), wind-driven mixing in winter can also not reach the bottom due to the halocline

The easterly winds transport the warm surface waters to the west where the tilted up (higher sea level on this site), the water has to be replaced, colder water from below replaces the water from above and reaches the surface (therefore cooler sea surface), so now there is a pressure gradient from the west to the east (from high to low SSH/pressure) and a colder tongue in the east, the thermocline flattens in the east

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

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

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

-          Sea Surface Temperature and surface air pressure (Tahiti-Darwin Islands) are good proxies for La Nina / El Nino

El Nino is the warm water phase of the monsoon (also called northeast monsoon), winds blowing from the land onto the ocean – dry season, every few years the trade winds weaken, the western pacific warm pool as well as the deep atmospheric convection migrates eastward, the thermocline flattens out and the equatorial upwelling is reduced in the east - often followed by the cold phase (La Nina).

a.     Due to monsoon = which is a circulation (seasonal wind) that reverses direction in seasonal time scales

b.     Cause: strong thermal contrast btw land and sea à sharp seasonal contrasts In precipitation

c.       Heavy summer rains (southwestern monsoon)

d.      Very dry winters (northeastern monsoon)

-          Ekman-spiral:

o   Balance of Coriolis force (3) and frictional force (wind) (6)

o   Water set in motion by wind à if several days: flow near surface described by Coriolis force and vertical friction

o   à wind stress is transmitted downwards by internal friction

o   Coriolis effect set into play creating wind driven frictional layer called Ekman layer à propagates deep

o   Coriolis force is a function of latitude and speed

§  Ekman layer depth is the e-folding depth of decaying velocity

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

a.       Providing reasonable numbers and units to all axis

b.       Filling the empty boxes with appropriate names

-          y-axis: in [m] à 100m, 500m, 900m, 1000m

-          Left: Thermocline: in [°C] from -1.8°C to ~ 28°C

-          Middle: Halocline: in [%o] or PSU from 33 – 37%o

-          Right: Pycnocline: in [kg/m3] from 1022 – 1028 kg/m3

-          All average numbers, of course there are extremes

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

o   2a: horizontal advection of velocity

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

o   3: Coriolis force

o   4: Pressure Gradient force

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

o   6: Frictional forces (wind stress)

o   3 + 4 = geostrophic currents

o   3 + 6 = Ekman spiral

o   5 + 4 = down gradient flow (rivers)

o   1 + 4 = gravity waves (tsunami)

o   1 + 3 = inertial circulation (inertial waves)

-          Surface wind stress and induced surface current are at an angle of 45° to each other

-          This is provoked by the deflection of the Coriolis force

-          However: it is dependent on the latitude à ß-effect also Rossby parameter

o   Coriolis parameter f changes sing at equator and differs with latitude: in N – positive; in S – negative

o   f increases with latitude and leads to asymmetric circulation (see western intensification)

Constant mid latitude westerlies and low latitude trade winds blow in opposite directions. Friction by wind stress sets water into motion which is deflected by the Coriolis force. Since the subtropical Atlantic gyre is in the northern hemisphere, deflection is to the right. This causes Ekman transport convergence, resulting in a mound of water. The difference in sea level height results in pressure difference from the middle (high p) to the outer bounds (low p) of the water mound. This results in a pressure gradient force, setting the water in motion down the gradient. Again, moving water is deflected to the right by Coriolis force, whereby it reaches a geostrophic balance. An equilibrium between Coriolis force and the pressure gradient force is called geostrophic balance, at which water flows exactly perpendicular to both forces, thus creating a circular gyre around the water mound. The water flows along isobars. This gyre is subject to western intensification, a result of conservation of vorticity but changing Coriolis force with latitudes and changing friction along continental shelves (North America). Further, speed within this gyre decreases with depth as the formation of the mound forces lighter waters to the right. Thereby, baroclines and pycnoclines are tilted, resulting in a decrease of the pressure gradient force with depth.

-          Canary current along the East African shore

-          North Equatorial Current breaching over Atlantic at ~ 15°N

The graph shows temperature in [°C] (see values at minor-thermoclines) and reveals a strong shift of the thermocline (deepening in west). The prominent thermocline is at about 200m depth in the west and around 100m depth in the east, until completely at surface (upwelling of cold waters in EBUS). The constant winds blowing towards west result in a piling up of warm surface water at the western end of the basin. This also results in an increase of sea level height at the west.

-          Equatorial Counter Current, specifically named Cornwell Current in the Pacific

-          Induced by global surface winds:

o   Easterly winds directly on Equator à Coriolis = 0 à  pushes warm surface water towards west of basin à meets land à piles up at western boundary à zonal pressure gradient force directed towards east

-          Additionally:

o   Mostly warm water gets piled up at western boundary à deeper thermocline à remember PGF and inclined pycnoclines (higher densities at thermocline level)

-          In the mixed surface layer wind forcing and PGF forcing balance each other out, but in deep à only PGF acting eastwards resulting in Counter Current à this current feeds the upwelling in the east

-          Somali current is heavily influence by the monsoon. Differences in heat capacity of land and ocean provoke a shift in the prominent wind pattern between summer and winter. In summer, hot high-pressure air over the continent is lifted up and is constantly replenished by air from the low-pressure system over the Indian Ocean. This causes a prominent wind from south-west and over the western Indina ocean. In winter the situation and prominent wind direction is reversed. The wind blows from north east to south west. During the southwest monsoon, the wind stress, which is parallel to the coast, causes the Ekman transport to the east, so that on the west coast of Somalia and Oman there is significant upwelling, which is very good for fishery. During the northeast monsoon, the resulting Ekman transport from wind stress is directed towards the east. It causes downwelling at the Somalian coast and a deflection of the Somali-current.

-          Follow fluid particles along their trajectories

-          Shows position of a particle through time and space and gives path line

-          Get velocity of particle at any time and point in space t

-          Methods:

o   Drifters: measure position, SST, SSS, Wind, Currents

o   Floats: measure at greater depths, programmed to sink and rise in a cyclic fashion

o   Tracers: inert substances injected in ocean and follow spread over time and space

-          Follow over time, how particles change velocity and direction at a given point in space

o   Get a trend over time for a current

-          Methods:

o   Fixed station points

o   Moorings

§  Rotor current meters, acoustic current meters and ADCPs

§  e.g. Labrador Sea array

-          readily mixing along isopycnals (horizontal) by mesoscale eddies → diffusivity ~ 102 – 104 m2/s

-          Slow (against gravity gradient!) mixing across isopycnals (diapycnal mixing) → diffusivity ~ 10-5 m2/s

-          Deduced by tracer experiments:

o   intentional injection of inert substance into column at defined density

o   Follow spread of the tracer along different isopycnals

o   e.g.: Guinea exp: injection + 3yrs → 1000km horizontal + 200m vertical spread

-          Isopycnal mixing is largely done by currents ↔ diapycnal mixing either by diffusion or processes like upwelling, downwelling, which in the first place are not there anyway, that’s why there are diapycnal boundaries!

Constant mid latitude westerlies and low latitude trade winds blow in opposite directions. Friction by wind stress sets water into motion which is deflected by the Coriolis force. Since the subtropical Pacific gyre is in the northern hemisphere, deflection is to the right. This causes Ekman transport convergence, resulting in a mound of water. The difference in sea level height results in pressure difference from the middle (high p) to the outer bounds (low p) of the water mound. This results in a pressure gradient force, setting the water in motion down the gradient. Again, moving water is deflected to the right by Coriolis force, whereby it reaches a geostrophic balance. An equilibrium between Coriolis force and the pressure gradient force is called geostrophic balance, at which water flows exactly perpendicular to both forces, thus creating a circular gyre around the water mound. The water flows along isobars. This gyre is subject to western intensification, a result of conservation of vorticity but changing Coriolis force with latitudes and changing friction along continental shelves (North Asia). Further, speed within this gyre decreases with depth as the formation of the mound forces lighter waters to the right. Thereby, baroclines and pycnoclines are tilted, resulting in a decrease of the pressure gradient force with depth.

-          California current along the western coast of North America

-          North Equatorial Current at ~ 15°N

San Francisco: mixed, dominant semi-diurnal type

-          Two low and two high tides per day, but different amplitude à at some days of the month amplitudes of similar height though

Manila: mixed, dominant full diurnal

-          Mixed type, two low and two high per day, different amplitudes, some days of the month only one high and one low à dominant diurnal type

Do San: full diurnal type

-          Diurnal type has a period of 24h50min à one high and one low per day

Earth and moon rotate around a common center of mass which lies within the earths diameter. In the center of the earth gravity force and centripetal force are equal. On the moon face side however gravity is stronger, pulling the water towards it, creating a bulge. As a result of centripetal force this bulge, although less pronounced, can also be found on the opposite side of the earth.

• -1.8 °C

kg/m3

a) Pressure gradient and Coriolis force