Thermohaline Circulation

1. Rotating sector experiments

The theory of deep circulation was first developed in the late 1950s and early 1960s. 

Stommel et al. 1958: pie-shaped experimental tool, which rotated uniformly. The circulation was observed with release of dye. 

The parabolic-shaped upper surface of the water in the rotating experiments produces a dynamical effect equivalent to the \(\beta\)-effect. 

For an ocean with uniform depth on the surface of earth, potenital vorticity \(f/h\) is low a t low latitudes; therefore, the rim of the rotating pie corresponds to the low-latitude ocean on the earth. 

From the potential vorticity argument, the system allows three types of motion only:

1. Geostrophic flow along circle of constant radius
2. Radial flow in the interior is possible only if there is a source or sink. 
3. A western boundary current going northward or southward. Similiar to the wind-driven circulation, balancing potential vorticity in the whole model basin rules out the possibility of closing the circulation by an eastern boundary current. 

An increase/sink of water level (source/sink) corresponds to an upwelling leaving the abyssal layer. Thus, a source or sink implies stretching which must be balanced by radial flow as required by the linear vorticity balance! 

The circulation driven by a point source/sink in a basin confined by two meridians is particularly interesting because the western boundary current induced by source-induced interior flow can be very non-intuitive. 

Realisitc Case: Point source at the pole: In the basin interior there is a cyclonic poleward flow, and there is an equatorward western boundary current. 

In general of a point source at the western boundary: There is an interior upwelling driven by the source, so that there is always the same cyclonic flow in the basin interior. However, the transport of the western boundary current may very depending on the exact location of the source.

Therefore, the western boundary current transport is quite non-intuitive, owing to mass balance of the model and the special nature of spherical coordinates.

Based on theoretical reasoning, Stommel (1958) postulated a framework of deep circulation in the world's oceans. Accordingly, deep circulation in the world's oceans is driven by point sources of deep water in the northern North Atlantic Ocean and the Weddell Sea. 

Stommel 1958:

1. Deep circulation in the world's oceans is driven by point sources of deep water in the northern North Atlantic Ocean and Weddell Sea. 
2. With this distribution of the deep water source, there is a southward western boundary current in the Atlantic Basin which transports deep water away from the source. 
3. On it's southward path, it gradually loses its mass to the ocean interior. 
4. At the southern edge of the world's oceans there is a simple circumpolar current around the edge of Atnartica and deep western boundary currents moving northward and deep western boundary currents moving northward in individual basins which transport deep water from the southern source to the interior of the worlds oceans. 
5. In the interior of each basin water moves uniformly upward, as assumed in the model, and this model-assumed uniform upwelling drives poleward flow, as dictated by the linear vorticity balance. 

Stommel (1958) and Stommel and Aron (1960) developed a model for the explanation of Deep Western Boundary Currents.

1. The model is driven by a distriputed upward transfer of fluid through the main thermocline. 
   1. The themrocline is remarkably sharp everywhere in mid to low latitudes. Because turbulence in the thermoclinetransport heat downwards, the thermocline should become much weaker after many years. Because this is not happening, there should be a balance between upward advection of cold water and downward diffusion of heat. 
2. This sink is supplied by interior geostrophic transport
3. The upward water transfer is balanced by a concentrated source representing downward flow of cold fluid generated in a small area at high latitude. 
4. The abyssal circulation is composed of a single layer that is steady and geostrophic everywhere in the ocean interior, except at the western boundary where a narrow, intense boundary current is permitted to depart from geostrophy. 

A good marker for LSW water is its very low potenital vorticity, through nearly homogenous water. 

the three principle spreading routes of LSW:

1. to Irminger Sea: 0.5 years
2. to the North Atlantic interior: 2-3 years
3. along the deep western boundary current: 1 years 

Along the deep western boundary current

problem: DWBC mooring arrays indicate a strong reduction of LSW at the coast of New-foundland. Is the a continous DWBC? 

Othe survey with RAFOS floats drifting at 700 and 1500m depths brings the following result: Most of the floats leave the DWBC due to interactions with meanders of the NAC. 

ausgelassen. sehr kompliziert! 

The bottom layer of the world's ocean is filled with a thick layer of very cold water with potential temperature lower than 2°C. 

The origin of this water are a few sites along the contiental margin of Antarctica, where cold water is formed during Southern Hemisphere winter:

1. very cold wind from glacial ice on Antarctica blows over the coastal ocean adjacent, driving sea ice away from the coasts and thus creating coastal polynyas (small open water areas surrounded by ice). 
2. Strong cooling over polynyas produces more sea ice, and salt rejection during sea ice formation creates cold dense water with high salinity. 
3. this dense water overflow the continental slope. 
4. The offshore transport of the newly formed bottom water is compensated by the onshore flow of water in the subsurface layer. 
5. During the descent along the slope, it entrains the water in the environment; thus, it is slightly warmed up from -2°C to -1°C. Eventually, it sinks to the bottom with a temperature of nearly 0°C. 
6. Due to virgorous entrainment, the total volume flux of the final product is greatly increased. 

Meridional circulation pattern of the Southern Oceanis dominated by the upwelling of a warm, salty water mass called Circumpolar Deep Water and its transformation into Antarctic Surface Water, which ultimately sinks to become Antarctic Intermediate Water and Antarctic Bottom Water. The circulation is driven by wind and the exchange of heat and freshwater between the ocean and the atmosphere.

sensible heat polynyas: thermdynamically driven: occur when warmer water upwells

latent heat polynyas: formed through action of wind, tides, or ocean currents

...are found in many locations around the coastline and ice shelf edge. 

Three polynya regions are most productive of AABW:

1. the southern Weddel Sea (68%)
2. the Ross Sea (8%)
3. Adelie Land (24%)

Sea Ice production is large in latent heat polynayas, so productions maps can help to indentify latent heat polynya spots!

• Warming can be followed along the path of AABW from the Weddel Sea into the Altantic. 
• Warming below 4000m contributes 16% to the total ocean heat uptake. 
• thermal expansion of water below 4000m contributes 9% to sea level rise. 

The distributionofthedepthintegral ofthefractionofAABW in theglobal ocean. NADW-derivedwatersdominatein theNorth Atlantic, whileAABW-derivedwatersdominatein theAntarcticCircumpolarCurrent(ACC) andin theIndian andPacific basins.

• The MOC is a geometric formulation, which tries to reduce a complex 3D flow field into a 2D (y-z plane) streamfunction. 
• The MOC is not the thermohaline circulation, i.e. the circulation associated with changes in temperature and salinity. 
• The MOC includes aspects of the wind-driven and the thermohaline circulation. 

The primary north-south gradient - as a result of the surface forcing - is in balance with an eastward geostrophic current which generates a secondary high and low pressure system. This, in turn, drives a northward geostrophic current, the upper branch of the MOC circulation. 

There are still competing hypothesis for the forcing of the overturning. Howeever, it is clear, tha tit is not simply pushed by meridional density gradients. 

It depends on the location of heating in a tank experiment, when circulation shall develop. 

The red parcel is expanded by heating, the blue compressed by cooling. Theleft case does not produce a significant circulation, while in the right case an overturning circulation develops. 

The vertical diffusion on the other hand warms water masses at depth in a stably stratified ocean and thus it can drive the overturning against friction. This forcing mechanism is the basic mechanism driving the Stommel-Aronsmodel discussed in lecture 10.

Another mechanism that can break the Sandströmconstraint is direct driving of a large scale circulation by windstress.

• IPCC model runs suggest future reduction of the Atlantic MOC
• Recent changes from numerical models indicate upward trend in AMOC strength between the early 1970s and mid-1990s.
• Multidecadal basin-scale changes in the Atlantic Ocean are probably related to the thermohaline circulation. 
• AMOC was nearly eliminated during the coldest deglacial interval in the North Atlantic region beginning with the catastrophic iceberg discharge Heinrich event 17500yr ago. 

1. Using mean cross-basin hydrographic sections referenced against absolute velocities derived from altimetry floats.
   1. Calculate mean temperature, salinity and density field from several sections across Labrador Sea
   2. Calculate relative geostrophic velocity
   3. Reference velocity with abolute velocities from floats
   4. Adjust absolute velocity to conserve mass.  
2. Global inverse modelling of cross-basin hydrographic sections
   1. Linear system of equation with more unknows than constraints: no single solution; it is an optimization problem. 
3. MOC temporal variability can only be observed with large observational efforts, including moored boundary current and geostrophic measurements
   1. RAPID-MOC array:
   2. An estimate of the meridional flow relating to the MOC along 26.5°N can be obtained by decomposing it into three components:
      1. transport through the Florida Straits
         1. induction messurements through a cable -> salty seawater conduct electricity. 
      2. flow induced by the interaction between wind and the ocean surface (Ekman transport)
         1. Ekman contribution to the MOC is relatively small. However, it is responsible for the largest subannual variability in the MOC. 
      3. transport related to the difference in sea water density between the American and African continents. 
4. Global assimilation models

We expect a reduction of the AMOC with ongoing climate change. Although there are some hints, measurements are not able to show a significant MOC weakening. 

1. subpolar gyre
   1. positive wind stress curl -> northward Sverdrup transport 
2. subtropical gyre
   1. negative wind stress curl -> southward Sverdrup transport 
3. tropical gyre
   1. positive windstress curl -> northward Sverdrup transport 
4. equatorial gyre
   1. negative windstress curl

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.

• 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. 

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

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.

positive values means a northward transport of freshwater. 

• The transport by the Mississipi is in comparison very weak. 

• hydrological cycle increases
• salinity differences increases (salty becomes saltier and fresh becomes fresher)

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. 

The thermosteric effect clearly dominates the halosteric effect. 

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. 

Wind-Driven Surface Currents in February and March

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 Ocean, deepening isopycnals drive strong salinity increases on pressure surfaces above 500 dbar.

subtropics and tropics becoming saltier and polar regions fresher. 

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.