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Sketch the heat fluxes through the sea surface in latitudinal dependency and explain!

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definitions: downward flux >0; upward flux <0;

shortwave:

  • solar radiation (nearly transparent atmosphere for solar radiation)
  • main: increases with decreasing latitude; second importance: cloud climatology
  • maxima can be found at equatorial cold tongues (east side of the ocean basins), where low cloudiness can be found

latent:

  • major outgoing flux of the ocean surface
  • evaporation 
  • global maxima can be found at western boundary outflow regimes 
    • Kuroshio Stream, Gulf Stream
    • here warm SST's meet cold and dry air from the continents
  • strong latent heat loss exists at extratropics/subtropics
  • relatively low latent heat flux at the cold tongues of surface waters in the eastern equatorial Pacific and Atlantic Oceans. 
  • small latent heat loss at high latitudes due to low SST's

longwave radiation:

  • longwave radiation is controlled by the difference of ingoing and outgoing longwave radiation
    • at low latitudes, the outgoing radiation is rather low because SST don't exceed atmospheric temperatures
    • in subtropical regions and the western boundary, the outgoing longwave radiation is the highest because of heat transport northwards. 

sensible:

  • controlled by the difference between ocean surface temperature and the atmosphere's temperature 
    • is maximal at the western boundary outflow regimes
    • positive at high latitudes due to stronger atmospheric heat transport
    • positive at upwelling regions (east-side of the ocean basins)

net flux:

  • in first-order increases with decreasing latitude
    • poleward heat transport! 
    • the strongest gain at the Pacific and Atlantic cold tongues
    • the biggest loss at the western boundary outflow regimes 
    • major loss at Atlantics high latitudes due to strong meridional overturning circulation.

 

Rui Xing Huang et al. 2010: S. 6

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In which three components the large-scale flow in the ocean can be divided? 

  1. thermohaline circulation 
  2. wind-driven circulation 
  3. tidal flow
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What is the wind-driven circulation?

  • Wind-driven circulation generally refers to the circulation in the upper ocean (~1km) which is primarily driven by wind stress. 
  • Wind stress can drive horizontal circulation in a homogeneous ocean; thus, the existence of a wind-driven circulation is independent of the surface thermohaline forcing. 
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What is the thermohaline circulation? 

  • generally refers to the circulation associated with differences in temperature and salinity in the ocean, although the exact definition remains debatable.

the definition:

  1. Thermohaline circulation is a circulation driven by mechanical stirring, which transports mass, heat, freshwater and other properties in the meridional/zonal direction. Mechanical stirring is supported by external sources of mechanical energy from wind stress and tidal dissipation. 
  2. Surface heat and freshwater fluxes are necessary for setting up the circulation. 

 

  • variability: decadal to millenial and centennial timescales. 
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Explain the rough picture of the unified thermohaline and wind-driven circulation! 

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S.44

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How is the precipitation distributed?

  1. Maxima in the Intertropical Convergence Zone and minima over subtropical gyres associated with Hadley circulation 
  2. strong east-west difference in the Pacific Ocean associated with Walker Circulation 
  3. Maxima over the Western Boundary Currents 
  4. Maxima over monsoon regions (West India, Bay of Bengal, Southeast Asia)
  5. Minima over upwelling regions
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surface temperature distribution in the worlds ocean

  1. sea surface temperature is reduced in poleward direction 
  2. sea surface temperature is the highest in the Warm Pool in the western equatorial Pacific and eastern equatorial Indian Oceans.
  3. strong zonal temperature gradient along the equatorial band of the Pacific Ocean with the Cold Tongue at the eastern basin. (but also in other Oceans)
  4. In the subtropical basin, temperature is generally high at the western basin and low at the eastern basin due to anticyclonic subtropical gyres. In subpolar basin, the zonal temperature gradient flips sign. 
  5. in the Southern Ocean, there is a very strong thermal front around the latitudal band of 40°-50° S. This cold front is due to the strong upwelling driven by the Southern Westerlies. 
  6. the surface density is primarily controlled by SST except for the high latitudes. 

 

Abbildung S. 18 (Huang)

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  • surface salinity distribution of the worlds oceans.
  • strong linkage between the surface salinity and net freshwater flux (evaporation-precipitation)(nearly linear relationship)
  • "halocline catastrophe": linkage between salinity and freshwaterflux is unidirectional and anomalies can persist (instead of SST for example). The halocline catastrophe is the persistence of freshwater anomalies at high latitudes. 
  • surface density is primarily controlled by surface salinity at high latitudes 
  • surface salinity is the lowest in the Gulf of Bengal due to high precipitation and huge amount of river run-off
  • surface salinity is very high in the Arabian Sea, due to strong evaporation.

 

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surface density distribution of the worlds ocean

  • is dynamically linked to the currents
  • is primarily controlled by the temperature distribution, except the high latitudes, where it is primarily controlled by surface salinity.
  • the lowest sea surface density can be found at the Gulf of Bengal and the Warm Pool due to strong freshwater input. 
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meridional distribution of surface density, temperature, and salinity.

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Surface water in the Southern Hemisphere is colder and saltier than that in the Northern Hemisphere. 

The combination of the Antartic continent and the adjacent circumpolar water chanel creates the coldest and densest surface water in the worlds ocean's, which sinks to the bottom of the world's oceans and dominates the abyssal circulation. 

 

Abb. 1.18, S.22

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zonal distribution of sea surface temperature, salinity and density. 

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  1. closely related to the circulation by wind stress (upwelling and downwelling regions. 
  2. the second major factor are the wind-driven gyres
    1. Cyclonic gyres in the subpolar basins bring relatively warm water to the eastern part of the subpolar basin and relatively cold water to the western part of the subpolar basin.
    2. Anticyclonic subtropical gyres bring relatively cold water to the eastern part of the subtropical basin and relatively warm water to the western part of the subpolar basin. 
  3. Strong cold air from the Eurasien and American continent must contribute to the low surface temperatures at the western basin of the Pacific and Atlantic.

 

Additionally, there is a major contrast between Atlantic and Pacific.

  • SST in the eastern and middle parts of the northern North Atlantic Ocean is about 5°C warmer than the zonal mean temperature. In comparison, sea surface temperature along the eastern boundary of the northern Pacific Ocean is only slightly warmer than the zonal mean temperature. 
    • The dramatic difference arises due the strong MOC in the Atlantic, which is missing in the Pacific. 
  • Sea surface salinity is also much higher in the Atlantic than in the Pacific (especially north Atlantic). 

 

Although higher temperatures compensating this partly, the SS density is much higher in the North Atlantic, than in the North Pacific. This is the most crucial dynamic factor regulating the deepwater formation in the Norther Hemisphere. 

reasons for the strong salinity differences may be found in: differences in the hydrological cycle, land-sea distribution and the fact, that there is a strong freshwater transport from the Atlantic to the Pacific over narrow middle america due to the westerlies.

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Sketch the meridional oceanic and atmospheric freshwater transport. 

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positive values means a northward transport of freshwater. 

  • The transport by the Mississipi is in comparison very weak. 
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What is the main change in the hydrological cycle and salinity distribution due to global warming?

  • hydrological cycle increases
  • salinity differences increases (salty becomes saltier and fresh becomes fresher)
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What factors drives the temporal evolution of the sea level?

  1. freshwater flux 
    1. precitpitation 
    2. evaporation 
    3. river run-off
  2. divergence
  3. thermosteric effect (by temperature only)
  4. halosteric effect (by salinity only)

 

the steric effect is defined as the difference between the hight of a reference fluid and a fluid with specific salinity and temperature (also steric height).

the steric effect causes the steric height of a fluid with a specific salinity and temperature. 

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Sea level change due to global warming. Is the thermosteric or the halosteric effect stronger?

The thermosteric effect clearly dominates the halosteric effect. 

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Explain Gulf Stream Precipitation!

In the marine boundary layer, atmospheric pressure adjustment to sharp sea surface temperature gradients lead to a surface wind convergence, which anchors a narrow band of precipitation along the Gulf Stream. 

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Sketch the Wind driven Ocean circulation! 

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Wind-Driven Surface Currents in February and March

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Sketch the Large-Scale Buoyancy Driven Ocean Circulation. 

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salinity trends through global warming (noch eher unklar) 

  1. enhancement of the hydrological cycle. -> salty gets saltier and fresh gets fresher
  2. Adiabatic vertical shifts (or heave) of density surfaces driven by wind changes will cause salinity change on a pressure surface without any real change to the ocean freshwater inventory or water masses.

above the reasons and beneath the main results :

  1. Subsurface salinity changes on pressure surfaces are attributable to both isopycnal heave and real water mass modification of the temperature- salinity relationship. 
  2. Broad-scale surface warming and the associated poleward migration of isopycnal outcrops drive a clear and repeating pattern of subsurface isopycnal salinity change in each independent ocean basin. 
  3. Qualitatively, the observed global multidecadal salinity changes are thus consonant with both broad-scale surface warming and the amplification of the global hydrological cycle. 

In the strongly warming tropical and subtropical Atlantic Oceandeepening isopycnals drive strong salinity increases on pressure surfaces above 500 dbar.

subtropics and tropics becoming saltier and polar regions fresher. 

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How large is the groundwater flow? 

here are three different results:

  1. Surface flow balances E-P over the ocean and P-E over land: no additional groundwater flow required
  2. The total flux of submarine groundwater discharge to the Atlantic Ocean is similar in volume to the riverine flux
  3. groundwater discharge 3 to 4 times greater than river discharge, but largest amount is salty water showing strong interaction between land and ocean. 
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Meridional heat transport

three components:

  1. sensible heat flux in the Oceans
  2. sensible heat flux in the atmosphere 
  3. latent heat flux in the atmosphere-ocean coupled system 

 

  • In the subtropics, oceanic sensible heat flux is the dominating contributor
  • mid latitudes, latent heat flux is the dominating contributor 
  • at high latitudes, atmospheric sensible heat flux is the dominating contributor

 

S.590

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Annual range of sea surface temperature and how well it penetrates into deeper layers.

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  • The annual variations in temperature decrease with depth and are rarely perceptible below 100 to 300 m. 
  • The maxima/minima at the sea surface occurs at the end of the warming/cooling season.
    • Below the sea surface, the times of occurrence of the maxima and minima are delayed by as much as 2 months relative to the times at the surface. 
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Sketch the main features of the Evaporation-Precipitation and sea surface salinity map. What does it say? 

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As sea water evaporates the salt remains behind, only the freshwater is transferred from the ocean to the atmosphere. A region of excess evaporation, such as the subtropics tend to become salty, while the areas of excess rainfall become fresher. Salinity reflects the workings of the hydrological cycle: the movement of freshwater through the earth/ocean/atmosphere system.

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Main stratification of the world oceans. 

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  • Water warmer than 10°C dominate the sea surface but do not extend much below 500m in the ocean. 
  • The thermocline is the sharp drop off in temperature with depth and marks the boarder between the surface zone (mixed layer) and the deep zone. 
  • Deeper cold waters derive their properties at the sea surface during winter at high latitude. 
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Sketch the meridional vertical cross-section of the potential temperature distribution of the Atlantic Ocean!

  1. bowl-shaped isothermals in the subtropical Ocean around 30° off the equator and a few 100m below the sea surface. 
  2. subsurface maximum of the vertical temperature gradient -> main thermocline 
  3. strong front from the sea surface to the deep Ocean in the Southern Ocean.
    1. strong circumpolar current seperates cold water from the south and relative warm water from the north
  4. cold water formed near the edge of Antarctica spreads northwards as bottom water 
  5. northern source of cold water formation
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Sketch the meridional vertical cross-section of the potential temperature distribution of the Pacific Ocean!

  1. bowl-shaped isothermals in the subtropical Ocean around 30° off the equator and a few 100m below the sea surface. 
  2. subsurface maximum of the vertical temperature gradient -> main thermocline 
  3. strong front from the sea surface to the deep Ocean in the Southern Ocean.
    1. strong circumpolar current seperates cold water from the south and relative warm water from the north
  4. cold water formed near the edge of Antarctica spreads northwards as bottom water 

 

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Sketch the meridional vertical cross-section of the potential temperature distribution of the Indian Ocean!

  1. bowl-shaped isothermals in the subtropical Ocean around 30° off the equator and a few 100m below the sea surface. 
  2. subsurface maximum of the vertical temperature gradient -> main thermocline 
  3. strong front from the sea surface to the deep Ocean in the Southern Ocean.
    1. strong circumpolar current seperates cold water from the south and relative warm water from the north
  4. cold water formed near the edge of Antarctica spreads northwards as bottom water 

 

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Sketch the meridional vertical cross-section of the salinity distribution of the Atlantic Ocean!

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  • In the North Atlantic Ocean, there is a tongue-like feature of high salinity, starting from the upper ocean and pentrating vertically to the depth of 2km. 
  • At the 2km level, the core of this high-salinity tongue extends southward and across the equator.
    • This salinity tongue is mostly the signature of the high-salinity Mediterranean Water. 
    • Since it is a subsurface maximum it is not directly connected to the surface salinity along this section. 
    • In fact, it is produced by a westward lateral transport of high salinity at a depth of roughly 1km. 
  • In the Southern Hemisphere, an outstanding tongue of low salinity, originating from the sea surface at 50° S, extends to a depth of of 1km. 
    • This is the Antarctic Intermediate Water (AAIW)
  • The relatively low salinity below the level of 4km is associated with Antartic Bottom Water (AABW) formed near the edge of the Antartic continent. 

 

The tongue-like features cannot be interpreted as flow directions since many other processes are involved!

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Sketch the meridional vertical cross-section of the salinity distribution of the Pacific Ocean!

  • relatively high salinity in the upper 400m in the subtropics
  • between 500m and 1,5km the salinity distribution is dominated by tongues of relatively low salinity from high latitudes. 
    • In comparison with the Atlantic section, the low salinity tongue from the south (AAIW) is less prominent.
  • In contrast to the Atlantic section, here the AABW appears as slightly saltier water spreading northward in the deep ocean below 4km. 
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Sketch the meridional vertical cross-section of the salinity distribution of the Indian Ocean!

  • The low-salinity tongue associated with AAIW is the dominant feature in the Souther Hemisphere, similar to the Atlantic section. 
  • The relatively high-salinity intrusion from the north dominates the depth between 2 and 4 km in the Souther Hemisphere. 
  • In the North Indian Ocean, the influence of high-salinity water from the Red Sea can be seen clearly at the depth of 1km.