Thermohaline Circulation

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

1. thermohaline circulation 
2. wind-driven circulation 
3. tidal flow

• 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. 

• 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. 

S.44

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

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)

• 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.