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Chapter 13 Deep Circulation in the Ocean Physical oceanography Instructor: Dr. Cheng-Chien Liu Department of Earth Sciences National Cheng Kung University Last updated: 4January 2004
Introduction • Surface circulation (upper km) • Abyss circulation (1 km ~ 4, 5 km) • Thermohaline circulation • Meridional Overturning Circulation • The vertical movements of ocean water masses caused by density differences that are due to variations in temperature and salinity • Carries cold water from high latitudes in winter to lower latitudes throughout the world
Importance of the Thermohaline Circulation • Three important consequences • Stratification • layers according to density; applies to fluids. Stable stratification occurs when density decreases continuously (but not necessarily uniformly) with distance from the Earth's center • Volume • Weak but comparable to the volumes of surface transports • Earth's heat budget and climate • The deep circulation varies from decades to centuries to millennia • Modulate climate over such time intervals?!! • The ocean may be the primary cause of variability over times ranging from years to decades, and it may have helped modulate ice-age climate
Importance of the Thermohaline Circulation (cont.) • The Oceans as a Reservoir of CO2 • CO2 greenhouse gas • Amount of carbon • Ocean: 40,000 GtC • Land: 2,200 GtC • Atmosphere: 750 GtC • GTC: gigaton of carbon = 1012 kilograms of carbon • New carbon since Industrial Revolution: 150 GTC • Carbon cycled through the marine ecosystem in five years > 150 GTC
Importance of the Thermohaline Circulation (cont.) • The Oceans as a Reservoir of CO2 (cont.) • Major exchangeable reservoir • Not those lock-up reservoir • Rocks, shell, coral, … • The cold deep water • Image the cold cola • New CO2 • Burning of fossil fuels and trees • Where do they go?
Importance of the Thermohaline Circulation (cont.) • The Oceans as a Reservoir of CO2 (cont.) • Forecast of the future climate How much CO2 is stored in the ocean and for how long • The amount T of the deep water • The storage time the rate at which deep water is replenished • The deposition whether the dead plants and animals that drop to the sea floor are oxidized.
Oceanic Transport of Heat • Global Conveyor Belt • Wally Broecker • Fig 13.1 • crude estimate • 40 Sv of 180C water northward • 14 Sv of 20C water return southward • Lose 0.9 petawatts (1 petawatt = 1015 watt) in the north Atlantic north of 24°N • Another estimation: 1.2 ± 0.2 petawatts
Oceanic Transport of Heat (cont.) • The production of bottom water • Remarkably sensitive to small changes in S • Influence of S • More saline surface waters form denser water in winter than less saline water • The saltiest water is in the Atlantic and under the ice on the continental shelves around Antarctica. • Remarkably sensitive to small changes in mixing
Oceanic Transport of Heat (cont.) • Role of the Ocean in Ice-Age Climate Fluctuations • Ice core • Two from Greenland ice sheet and three through the Antarctic sheet • A continuous record of atmospheric conditions (400,000 years) • Annual layers counted to get age • Deeper in the core, where annual layers are hard to see, depth age • Occasional world-wide dustings of volcanic ash provide common markers in cores • Oxygen-isotope ratios in the ice give temperatures over parts of the northern hemisphere • Bubbles in the ice give atmospheric CO2 and methane concentration • Pollen, chemical composition, and particles give information about volcanic eruptions, wind speed, and direction • Thickness of annual layers gives snow accumulation rates • Isotopes of some elements give solar and cosmic ray activity
Oceanic Transport of Heat (cont.) • Role of the Ocean in Ice-Age Climate Fluctuations (cont.) • Deep-sea sediments core • Ocean Drilling Program • Sea-surface temperature • Salinity above the core • Production of north Atlantic deep water • Ice volume in glaciers • Production of icebergs
Oceanic Transport of Heat (cont.) • Role in Ice-Age Climate Fluctuations (cont.) • Abrupt T variability over the past 100,000 years • The oxygen-isotope record in the ice cores • Many times during the last ice age, temperatures near Greenland warmed rapidly over periods of 1-100 years, followed by gradual cooling over longer periods (Dansgaard et al., 1993) • Dansgaard/Oeschger event • 11,500 years ago, temperatures over Greenland warmed by 80C in 40 years in three steps, each spanning 5 years (Alley, 2000) • Other studies have shown that much of the northern hemisphere warmed and cooled in phase with temperatures calculated from the ice core • The climate of the past 8,000 years was constant • Our perception of climate change • Based on highly unusual circumstances • All of recorded history has been during a period of warm and stable climate
Oceanic Transport of Heat (cont.) • Role in Ice-Age Climate Fluctuations (cont.) • Heinrich events (iceberg production time ticeberg) • Hartmut Heinrich and colleagues (Bond et al., 1992) • Studying the sediments in the north Atlantic • Periods: coarse material was deposited on the bottom in mid-ocean • Mechanism: only icebergs can carry such material out to sea • Indication: times when large numbers of icebergs were released into the north Atlantic • T variability + ticeberg the meridional overturning circulation • Icebergs melted the surge of fresh water the stability of the water column shut off the production of North Atlantic Deep Water the transport of warm water in the north Atlantic producing very cold northern hemisphere climate (Figure 13.2) pushed the polar front further south • The location of the front, and the time it was at different positions can be determined from analysis of bottom sediments
Oceanic Transport of Heat (cont.) • Role in Ice-Age Climate Fluctuations (cont.) • Antarctic warming • When the meridional overturning circulation shuts down, heat normally carried from the south Atlantic to the north Atlantic becomes available to warm the southern hemisphere • Hysteresis (Figure 13.3) • Four states (two of which are stable) • 1: The present circulation stable • 2: Deep water is produced mostly near Antarctica, and upwelling occurs in the far north Pacific (as it does today) and in the far north Atlantic • 3: The circulation is shut off stable • 4: The return to normal salinity does not cause the circulation to turn on. Surface waters must become saltier than average for the first state to return (Rahmstorf, 1995)
Oceanic Transport of Heat (cont.) • Role in Ice-Age Climate Fluctuations (cont.) • Dansgaard/Oeschger-Heinrich tandem events • Heinrich events seem to precede the largest Dansgaard/Oeschger events • Heinrich event shuts off the Atlantic thermohaline circulation very cold North Atlantic 1000 years later Dansgaard/Oeschger event with rapid warming • Have global influence • Related to warming events seen in Antarctic ice cores • Temperatures changes in the two hemispheres are out of phase • When Greenland warms, Antarctica cools • Little Ice Age • A weakened version of this process with a period of about 1000 years • May be modulating present-day climate in the north Atlantic • May have been responsible for the Little Ice Age from 1100 to 1800
Oceanic Transport of Heat (cont.) • Summary • Not yet understand well • The variability in S, Tair and deep-water formation • What causes the ice sheets to surge? • Surges T water vapor from the tropics (a greenhouse gas) • Surges T an internal instability of a large ice sheet • How the oceanic circulation responds to changes in the deep circulation or surface moisture fluxes? • Recent work by Wang, Stone and Marotzke (1999), who used a numerical model to simulate the climate system, shows that the meridional overturning circulation is modulated by moisture fluxes in the southern hemisphere
Oceanic Transport of Heat (cont.) • Summary (cont.) • 100,000-year cycle • Every 100,000 years for the past million years, ice sheets have advanced over the continents • Correlation: Earth's orbital eccentricity, deep-sea temperature, and atmospheric carbon-dioxide concentration • Ice-sheet volume lagged behind CO2 changes in the atmosphere • CO2 changes ice sheets change, not the other way around • The oceans play a key role in the development of the ice ages
Theory for the Thermohaline Circulation • Stommel, Arons and Faller theory • Three papers from 1958 to 1960 • Three fundamental ideas • Supplied by deep convection • Cold, deep water is supplied by deep convection at a few high-latitude locations in the Atlantic, notably in the Irminger and Greenland Seas in the north and the Weddell Sea in the south • Mixing • Brings the cold, deep water back to the surface. • Geostrophic flow • The abyssal circulation is strictly geostrophic in the interior of the ocean, and therefore potential vorticity is conserved.
Theory for the Thermohaline Circulation (cont.) • Velocity • Sverdrup equation • Integration • Velocity • Direction: everywhere toward the poles • Fig 13.4 • The abyssal flow in the interior of the ocean
Theory for the Thermohaline Circulation (cont.) • Western boundary current Tw • Deep western boundary (Stommel and Aron) • Simplified ocean (equator + 2 meridians + flat bottom) • Source at the pole S0 • Tw = -2S0 at j = 900 • Tw gradually diminishes to zero at j = 00 • If S0 exceeds the volume of water upwelled in the basin the western boundary current carries water across the Equator • Fig 13.4: the western boundary current sketched in the north Atlantic • No source • Tw gradually diminishes to zero at j = 300 • Fig 13.4: the western boundary current sketched in the north Pacific • Fig 13.5: ridges control the flow of deep circulation • Time scale of deep circulation: hundreds to thousands years
Theory for the Thermohaline Circulation (cont.) • Some comments • Convection Mixing • Convection • Reduce the potential energy of the water column • Self powered • Mixing in a stratified fluid • Increases the potential energy • Driven by an external process. • The meridional over-turning circulation is very sensitive to Az • Numerical models of the deep circulation show that the meridional over-turning circulation is very sensitive to the assumed value of vertical eddy diffusivity in the thermocline (Gargett and Holloway, 1992). • Tw is very sensitive to Az • Numerical calculations by Marotzke and Scott (1999) indicate that the transport is not limited by the rate of deep convection, but it is sensitive to the assumed value of vertical eddy diffusivity, especially near side boundaries.
Theory for the Thermohaline Circulation (cont.) • Some comments (cont.) • Mixing takes place at • Seamounts, mid-ocean ridges, and along strong currents such as the Gulf Stream • Numerical models large errors • The deep circulation calculated from numerical models probably has large errors • Heat transport may be not that sensitive to S • Because the meridional overturning circulation is pulled by mixing and not pushed by deep convection, the transport of heat into the north Atlantic may not be as sensitive to surface salinity as described above
Observations of the Deep Circulation • Difficulties in observing the abyssal circulation • Direct measurement is not available until recently • The measurements do not produce a stable mean value for the deep currents • Mean = 1 mm/s but variability = 100 mm/s • Solution • Indirect observation • Inferred from measured distribution of temperature, salinity, oxygen, silicate, tritium, fluorocarbons and other tracers • These measurements are much more stable than direct current measurements • Observations made decades apart can be used to trace the circulation.
Observations of the Deep Circulation (cont.) • Water mass • Originate from meteorology • Cold front warm • Atmosphere: large contrast in both r and T strong wind • Sea : small contrast in both r and T weak currents • Definition • Common formation history • Physical entities with a measurable volume • Delineation: T-S plot • The properties formed at the surface on in the ML • Conserved properties • Only change little by mixing as the water mass sinks • (T, S) unique water mass • Fig 13.6 • Left: d-T, d-S plots • Right: S-T plot
Observations of the Deep Circulation (cont.) • Fig 13.7: mixing of water masses • Mixing two water types leads to a straight line on a T-S diagram • Fig 13.8: densification • Because the lines of constant density on a T-S plot are curved, mixing increases the density of the water. This is called densification
Observations of the Deep Circulation (cont.) • Fig 13.9: • A T-S plot calculated from hydrographic data collected in the south Atlantic • The mixing among three water masses shows the characteristic rounded apexes • Table 13.1: • Three important water masses listed in order of decreasing depth • Antarctic Bottom Water AAB • North Atlantic Deep Water NADW • Antarctic Intermediate Water AIW
Observations of the Deep Circulation (cont.) • Core method • Core • A layer of water with extreme value (in the mathematical sense) of salinity or other property as a function of depth • An extreme value is a local maximum or minimum of the quantity as a function of depth • Assumptions • The flow is along the core • Water in the core mixes with the water masses above and below the core and it gradually loses its identity • The flow tends to be along surfaces of constant potential density • Fig 13.10 (South Atlantic Ocean) works very well • AIW • NADW • ABW
Observations of the Deep Circulation (cont.) • Problems with the core method • The flow is probably not along the core • Lateral boundary (seamounts, mid-ocean ridges) weak vertical mixing • Flow in a plan the core (along the western boundary ) horizontal mixing • Fig 13.11: • T-S plots of water in the various ocean basins
Observations of the Deep Circulation (cont.) • Other tracers • S • Conserved • Influences s much less than T • O2 • Partly conserved • Its concentration is reduced by the respiration by marine plants and animals and by oxidation of organic carbon • Silicates • Used by some marine organisms • They are conserved at depths below the sunlit zone • Phosphates • Used by all organisms • Provide additional information
Observations of the Deep Circulation (cont.) • Other tracers (cont.) • 3He • Conserved • There are few sources, mostly at deep-sea volcanic areas and hot springs • 3H (tritium) • Produced by atomic bomb tests in the atmosphere in the 1950s • Enters the ocean through the mixed layer • Useful for tracing the formation of deep water • Fluorocarbons (Freon used in air conditioning) • Injected into atmosphere • Can be measured with very great sensitivity • They are being used for tracing the sources of deep water • Sulphur hexafluoride SF6 • Can be injected into sea water • Can be measured with great sensitivity for many months
Antarctic Circumpolar Current • Significance of the Antarctic Circumpolar Current (ACC) • Transport waters among three oceans • Contribute the deep circulation in all basins • Fig 13.12: Density plot in the Drake Passage • Three fronts • the Subantarctic front • the Polar front • the Southern ACC front • Distribution (Fig 13.13) • Density slopes at all depths the current extend to the bottom
Antarctic Circumpolar Current (cont.) • Transpose of ACC • Slow (10 cm/s – 50 cm/s) but deep and wide transpose more water than the western boundary water • 125 11 Sv • Range: 95 – 158 Sv • Fig 13.14: Variability of transpose in the ACC • Max late winter and early spring
Antarctic Circumpolar Current (cont.) • Influence of topography steering • Circumpolar Deep Water • A mixture of deep water from all oceans • The upper branch of the current contains oxygen-poor water from all oceans • The lower (deeper) branch contains a core of high-salinity water from the Atlantic • A giant mix-master • The coldest and saltiest water on the continental shelf around Antarctica in winter • Too dense to cross the Drake Passage • Seeps into other basins • It is not the circumpolar water
Important concepts • The deep circulation of the ocean is very important because it determines the vertical stratification of the oceans and because it modulates climate. • The cold, deep water in the ocean absorbs CO2 from the atmosphere, therefore temporarily reducing atmospheric CO2. • Eventually, however, most of the CO2 must be released back to the ocean. (Some is used by plants, some is used to make sea shells). • The production of deep bottom waters in the north Atlantic causes a transport of one petawatt of heat into the northern hemisphere which warms Europe. • Variability of deep water formation in the north Atlantic has been tied to large fluctuations of northern hemisphere temperature during the last ice ages
Important concepts (cont.) • The theory for the deep circulation was worked out by Stommel and Arons in a series of papers published from 1958 to 1960. They showed that vertical velocities are needed nearly everywhere in the ocean to maintain the thermocline, and the vertical velocity drives the deep circulation. • The deep circulation is driven by vertical mixing, which is largest above mid-ocean ridges, near seamounts, and in strong boundary currents. • The deep circulation is too weak to measure directly. It is inferred from observations of water masses de ned by their temperature and salinity and from observation of tracers. • The Antarctic Circumpolar Current mixes deep water from all oceans and redistributes it back to each ocean. The current is deep and slow with a transport of 125 Sv