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
major outgoing flux of the ocean surface
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 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.
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)
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
In which three components the large-scale flow in the ocean can be divided?
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.
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.
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.
Surface heat and freshwater fluxes are necessary for setting up the circulation.
variability: decadal to millenial and centennial timescales.
Explain the rough picture of the unified thermohaline and wind-driven circulation!
Maxima in the Intertropical Convergence Zone and minima over subtropical gyres associated with Hadley circulation
strong east-west difference in the Pacific Ocean associated with Walker Circulation
Maxima over the Western Boundary Currents
Maxima over monsoon regions (West India, Bay of Bengal, Southeast Asia)
Minima over upwelling regions
surface temperature distribution in the worlds ocean
sea surface temperature is reduced in poleward direction
sea surface temperature is the highest in the Warm Pool in the western equatorial Pacific and eastern equatorial Indian Oceans.
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)
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.
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.
the surface density is primarily controlled by SST except for the high latitudes.
Abbildung S. 18 (Huang)
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.
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.
meridional distribution of surface density, temperature, and salinity.
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
zonal distribution of sea surface temperature, salinity and density.
closely related to the circulation by wind stress (upwelling and downwelling regions.
the second major factor are the wind-driven gyres
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.
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.
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.
Sketch the meridional oceanic and atmospheric freshwater transport.
positive values means a northward transport of freshwater.
The transport by the Mississipi is in comparison very weak.
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)
What factors drives the temporal evolution of the sea level?
thermosteric effect (by temperature only)
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.
Sea level change due to global warming. Is the thermosteric or the halosteric effect stronger?
The thermosteric effect clearly dominates the halosteric effect.
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.
salinity trends through global warming (noch eher unklar)
enhancement of the hydrological cycle. -> salty gets saltier and fresh gets fresher
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 :
Subsurface salinity changes on pressure surfaces are attributable to both isopycnal heave and real water mass modification of the temperature- salinity relationship.
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.
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 Ocean, deepening isopycnals drive strong salinity increases on pressure surfaces above 500 dbar.
subtropics and tropics becoming saltier and polar regions fresher.
How large is the groundwater flow?
here are three different results:
Surface flow balances E-P over the ocean and P-E over land: no additional groundwater flow required
The total flux of submarine groundwater discharge to the Atlantic Ocean is similar in volume to the riverine flux
groundwater discharge 3 to 4 times greater than river discharge, but largest amount is salty water showing strong interaction between land and ocean.
Meridional heat transport
sensible heat flux in the Oceans
sensible heat flux in the atmosphere
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
Annual range of sea surface temperature and how well it penetrates into deeper layers.
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.