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The Atlantic Meridional Overturning Circulation (AMOC) - Driving Processes

The Atlantic Meridional Overturning Circulation has been the subject of much research over many years as it is of major relevance for the climate of the world, though the energy that powers it, the mechanisms that ultimately drive it by providing the necessary energy has remained controversial. In this paper the authors1 review the observational data and models connected with the 2 main proposed mechanisms - vertical processes in the interior of the oceans and Ekman Upwelling induced by the wind over the Southern Ocean. The authors1 discuss the roles of surface heat and fluxes of freshwater, influences on the volume transported by the meridional overturning circulation and shape its pattern of circulation, though not actually supplying energy to the overturning itself in steady state. The authors1 conclude that upwelling driven by wind and vertical mixing are likely to contribute to the driving of the circulation that is observed. They suggest research in the future needs to address some of the open questions which they outline.

There are 4 main branches of the deep Atlantic Meridional Overturning Circulation (AMOC) - upwelling transports volume from the depths to near the surface of the ocean; relatively light water is transported towards high latitudes by surface currents; in deep water formation regions waters become denser and sink to form deep currents that close the loop. The entire Atlantic Ocean, including the Northern and Southern Atlantic Ocean, is spanned by these 4 branches forming a circulation system comprising 2 overturning cells, a deep one with North Atlantic Deep Water (NADW) and an abyssal one with Antarctic Bottom Water (AABW). The stratification and distribution of water masses, the amount of heat the ocean transports and the storage and cycling of species of chemical, such as CO2 in the deep ocean, are all strongly controlled by the AMOC, therefore the AMOC has a key role in the climate of the Earth. The maximum northwards heat transport in the North Atlantic is about 1 PW (1015 W) (Hall & Bryden, 1982; Ganachaud & Wunsch, 2000; Trenberth & Caron, 2001), which contributes to the mild climate of northwestern Europe. The authors1 suggest any reduction in the AMOC would likely have strong implications for the El Niņo-Southern Oscillation (ENSO) (Timmermann et al., 2005); the location of the Intertropical Convergence Zone (Vellinga & Wood, 2002), and the Atlantic marine ecosystems (Schmittner, 2005). Reorganisations of the AMOC in the past has been suggested by evidence to be involved in changes in climatic temperature of several degrees in as little as a few decades (see also reviews by Clark et al., 2002). The authors suggest that in the future there is a risk that global warming could result in substantial circulation changes (Manabe & Stouffer, 1994; Rahmstorf & Ganopolski, 1999; Wood et al., 1999; Schaeffer et al., 2002; Zickfeld et al., 2007).

The Sun and the Moon are the ultimate causes of circulation in the oceans and the atmosphere. The ocean waters are set in motion, either directly or indirectly, by intermediate processes such as waves, by surface fluxes of heat, freshwater and momentum, as well as gravity and tides. The main aim of this paper is to discuss the physical mechanisms responsible for driving the AMOC, in the sense that they provide an input of energy into the ocean that is capable of sustaining a steady state deep overturning circulation.

There are 2 distinct mechanisms considered for the driving of the meridional overturning circulation (MOC). One is the traditional thermohaline mechanism that has been proposed (Sandstrom, 1916; Jeffreys, 1925). According to this proposal the driver is the transport of heat from the surface to the deep water masses, down across surfaces of equal density (diapycnal mixing), that has been described in detail (Munk & Wunsch, 1998). Internal waves in the oceans are generated by the action of winds and tides, these waves dissipating into small-scale motion causing turbulent mixing. The water masses in the deep ocean are lightened by the mixing of heat which causes them to rise in low latitudes. The resulting surface and intermediate waters being advected poleward into North Atlantic where atmospheric cooling and the rejection of salt during sea ice formation transforms them into dense waters, which then sink to the depths where they spread, setting up the deep water mass of the ocean, establishing a density gradient between high and low latitudes.

The second proposal is upwelling that is driven by the wind (Toggweiler & Samuels, 1993b, 1995, 1998), they concluded, based on observational radiocarbon constraints, that diapycnal mixing caused an insufficient amount of upwelling of abyssal water to sustain the estimated overturning of about 15 Sv (1 Sv = 1 Sverdrup = 106 m3/sec) in the Atlantic Ocean. They proposed that most of the upwelling is wind driven and is located in the Southern Ocean. Vigorous water transport of water northward, Ekman transport, near the surface of the ocean is driven by the strong westerly circumpolar winds. Upwelling from depth is induced by the so-called Drake Passage Effect, as there is a horizontal divergence of the Ekman transport. According to this view the strengths of the winds over the Southern Ocean, and not the oceanic diapycnal mixing, govern the AMOC. The winds induce large-motion of water masses in the Southern Ocean which flow into the Atlantic Ocean then north to the sites where deep water is formed. Wind-driven mixing is small-scale turbulent motion induced by surface wind stress that is a component of the mixing process, though not considered to be a direct upwelling that is wind-driven.

There is great interest in determining which of the 2 processes is the main mechanism driving the MOC as different sensitivities to variations of external forcings (Schmittner & Weaver, 2001; Prange et al., 2003), and therefore a different direction of evolution of the MOC under conditions of continued climate change. In this paper the authors1 review work on theory, modelling and observations that suggest either or both as the main driving mechanisms.

The authors1 say they wish to emphasise that the spatial extent and strength of the AMOC is not fully determined by the driving process. A variety of processes, such as horizontal gyre circulation, atmospheric cooling, precipitation, evaporation and ice melting, control the amount of water that actually sinks in the North Atlantic at the sites of deep water formation. The spatial pattern of the AMOC can be changed drastically by these processes which can temporarily increase or decrease the amount of deep water that is formed, which has a strong impact on climate. According to the authors1 their aim in this paper is on the AMOC as a large-scale coherent circulation system and on longer timescales, on which mechanism provides the energy to the ocean that is required to sustain a steady state deep overturning circulation.

Though the terms "meridional overturning circulation" and "thermohaline circulation" (THC) have at times been used like synonyms they actually have different meanings. "MOC" is a descriptive geographic term, being a circulation in a meridional-vertical plane, as depicted by an overturning stream function, and doesn't refer to a particular driving mechanism.

The term "THC" defines flow by the driving mechanism, with 3 physical driving mechanisms, that are qualitatively different, for driving ocean flows - direct transfer of momentum by surface winds, acceleration of water by tidal forces, and thermohaline forcing, this classification being in textbooks on oceanography since the early 20th century (e.g. Defant, 1929; Neumann & Pierson, 1966). A simple archetypal example of THC, though in this case "thermal circulation", suggested by the authors1 is at hotspots of geothermal heating on the ocean floor in the vicinity of mid-ocean ridges (Joyce & Speer, 1987; Thompson & Johnson, 1996). Strong surface cooling of a water body that was previously warmer, as occurs when a polynya opens in sea ice (Buffoni et al., 2002) is another example. Thermohaline fluxes at ocean boundaries, ocean surface or bottom, cause density changes that result in flow by setting up pressure gradients.

When considering the steady state large-scale thermohaline circulation a complication arises, as surface buoyancy fluxes are not sufficient alone to sustain this steady state. A mechanical input of energy is necessary to sustain the turbulence required to mix down heat so the pressure gradients can be maintained in addition to the surface fluxes. The large-scale thermohaline circulation has been defined as "currents driven by heat and fresh water across the sea surface and subsequent mixing of heat and salt" (Rhamstorf, 2002, p. 208, 2003) to account for this fact. The same fact has also been defined as [The THC is]1 "an overturning in the ocean driven by mechanical stirring/mixing, which transports mass, heat and fresh water and other properties. In addition, the surface heat and freshwater fluxes are necessary for setting up the flow." (Huang, 2004, p. 497). Mechanical mixing is not necessary for the examples of transient flow or regional thermohaline flows that were mentioned above, being necessary only for the steady state large-scale MOC. The authors1 state they use the term "driver" in the rest of this paper as meaning "the physical process that provides the necessary energy input to sustain a steady state deep MOC." See Link 1

 

Sources & Further reading

  1. Kuhlbrodt, T., A. Griesel, M. Montoya, A. Levermann, M. Hofmann, and S. Rahmstorf (2007), On the driving processes of
    the Atlantic meridional overturning circulation, Rev. Geophys., 45, RG2001, doi:10.1029/2004RG000166.

Links

  1. On the Driving Processes of the Atlantic Meridional Overturning Circulation
  2. Atlantic Meridional Overturning Circulation (AMOC) of CMIP5 models: RCP and Historical Simulations

 

Author: M. H. Monroe
Email: admin@austhrutime.com
Last updated: 07/04/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading