Thermohaline Circulation
The thermohaline circulation of the oceans
The thermohaline circulation of the oceans
Fichier Détails
| Cartes-fiches | 89 |
|---|---|
| Langue | English |
| Catégorie | Physique |
| Niveau | Université |
| Crée / Actualisé | 21.07.2018 / 24.07.2018 |
| Lien de web |
https://card2brain.ch/box/20180721_thermohaline_circulation
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Where dominates the NADW and where the AABW?
The distributionofthedepthintegral ofthefractionofAABW in theglobal ocean. NADW-derivedwatersdominatein theNorth Atlantic, whileAABW-derivedwatersdominatein theAntarcticCircumpolarCurrent(ACC) andin theIndian andPacific basins.
What is the MOC?
- 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.
What drives the MOC?
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.
Sandström's tank experiments. What does it say?
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.
What is now the driver of the MOC?
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.
What do we know about changes of the Atlantic MOC?
- 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.
How can we estimate the MOC from observations, locally, globaly?
- Using mean cross-basin hydrographic sections referenced against absolute velocities derived from altimetry floats.
- Calculate mean temperature, salinity and density field from several sections across Labrador Sea
- Calculate relative geostrophic velocity
- Reference velocity with abolute velocities from floats
- Adjust absolute velocity to conserve mass.
- Global inverse modelling of cross-basin hydrographic sections
- Linear system of equation with more unknows than constraints: no single solution; it is an optimization problem.
- MOC temporal variability can only be observed with large observational efforts, including moored boundary current and geostrophic measurements
- RAPID-MOC array:
- An estimate of the meridional flow relating to the MOC along 26.5°N can be obtained by decomposing it into three components:
- transport through the Florida Straits
- induction messurements through a cable -> salty seawater conduct electricity.
- flow induced by the interaction between wind and the ocean surface (Ekman transport)
- Ekman contribution to the MOC is relatively small. However, it is responsible for the largest subannual variability in the MOC.
- transport related to the difference in sea water density between the American and African continents.
- transport through the Florida Straits
- Global assimilation models
What is the correct answer if someone asks me if the MOC changes?
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.
- Name the the gyres of the Atlantic, the sign of the wind stress curl, the direction of their Sverdrup transport and the main surface currents!
- subpolar gyre
- positive wind stress curl -> northward Sverdrup transport
- subtropical gyre
- negative wind stress curl -> southward Sverdrup transport
- tropical gyre
- positive windstress curl -> northward Sverdrup transport
- equatorial gyre
- negative windstress curl
