13.2: Importance of the Deep Circulation

The deep circulation carries heat, salinity, oxygen, CO₂, and other properties from high latitudes in winter to lower latitudes throughout the world. This has very important consequences.

1. The contrast between the cold deep water and the warm surface waters determines the stratification of the ocean, which strongly influences ocean dynamics.
2. The volume of deep water is far larger than the volume of surface water. Although currents in the deep ocean are relatively weak, they have transports comparable to the surface transports.
3. The fluxes of heat and other variables carried by the deep circulation influences earth’s heat budget and climate. The fluxes vary from decades to centuries to millennia, and this variability modulates 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.

Two aspects of the deep circulation are especially important for understanding Earth’s climate and its possible response to increased carbon dioxide, or CO₂, in the atmosphere: i) the ability of cold water to store CO₂ and heat absorbed from the atmosphere, and ii) the ability of deep currents to modulate the heat transported from the tropics to high latitudes.

The Ocean as a Reservoir of Carbon Dioxide

The ocean are the primary reservoir of readily available CO₂, an important greenhouse gas. The ocean contain \(40,000 \ \text{GtC}\) of dissolved, particulate, and living forms of carbon. The land contains \(2,200 \ \text{GtC}\), and the atmosphere contains only \(750 \ \text{GtC}\). Thus the ocean hold 50 times more carbon than the air. Furthermore, the amount of new carbon put into the atmosphere since the industrial revolution, \(150 \ \text{GtC}\), is less than the amount of carbon cycled through the marine ecosystem in five years. \((1 \ \text{GtC} = 1 \ \text{gigaton of carbon} = 10^{12} \ \text{kilograms of carbon})\). Carbonate rocks such as limestone, the shells of marine animals, and coral are other, much larger, reservoirs. But this carbon is locked up. It cannot be easily exchanged with carbon in other reservoirs.

More CO₂ dissolves in cold water than in warm water. Just imagine shaking and opening a hot can of Coke^TM. The CO₂ from a hot can will spew out far faster than from a cold can. Thus the cold deep water in the ocean is the major reservoir of dissolved CO₂ in the ocean.

New CO₂ is released into the atmosphere when fossil fuels and trees are burned. Very quickly, 48% of the CO₂ released into the atmosphere dissolves into the ocean (Sabine et al, 2004), much of which ends up deep in the ocean.

Forecasts of future climate change depend strongly on how much CO₂ is stored in the ocean and for how long. If little is stored, or if it is stored and later released into the atmosphere, the concentration in the atmosphere will change, modulating Earth’s long-wave radiation balance. How much and how long CO₂ is stored in the ocean depends on the transport of CO₂ by the deep circulation and on the net flux of carbon deposited on the sea floor. The amount that dissolves depends on the temperature of the deep water, the storage time in the deep ocean depends on the rate at which deep water is replenished, and the deposition depends on whether the dead plants and animals that drop to the sea floor are oxidized. Increased ventilation of deep layers and warming of the deep layers could release large quantities of the gas to the atmosphere.

The storage of carbon in the ocean also depends on the dynamics of marine ecosystems, upwelling, and the amount of dead plants and animals stored in sediments. But I won’t consider these processes.

Oceanic Transport of Heat

The ocean carries about half the heat out of the tropics needed to maintain Earth’s temperature. Heat carried by the Gulf Stream and the North Atlantic drift keeps the Far North Atlantic ice-free, and it helps to warm Europe. Norway, at 60\(^{\circ}\)N is far warmer than southern Greenland or northern Labrador at the same latitude. Palm trees grow on the west coast of Ireland, but not in Newfoundland, which is further south.

Wally Broecker (1987), working at Lamont-Doherty Geophysical Observatory of Columbia University, calls the oceanic component of the heat-transport system the Global Conveyor Belt. The basic idea is that surface currents carry heat to the far north Atlantic (figure \(\PageIndex{1}\)). There the surface water releases heat and water to the atmosphere, and it becomes sufficiently cold, salty, and dense that it sinks to the bottom in the Norwegian and Greenland Seas. It then flows southward in cold, bottom currents. Some of the water remains on the surface and returns to the south in cool surface currents such as the Labrador Current and Portugal Current (see figure \(11.4.2\)). Richardson (2008) has written a very useful paper surveying our understanding of the global conveyor belt.

Figure \(\PageIndex{1}\): The surface (narrow dashes) and deep (wide dashes) currents in the North Atlantic. The North Atlantic Current brings warm water northward where it cools. Some sinks and returns southward as a cold, deep, western-boundary current. Some returns southward at the surface. From Woods Hole Oceanographic Institution.

The deep bottom water from the North Atlantic is mixed upward in other regions of the ocean, and eventually it makes its way back to the Gulf Stream and the North Atlantic. Thus most of the water that sinks in the North Atlantic must be replaced by water from the Far South Atlantic. As this surface water moves northward across the equator and eventually into the Gulf Stream, it carries heat out of the South Atlantic.

So much heat is pulled northward by the formation of north Atlantic bottom water in winter that heat transport in the Atlantic is entirely northward, even in the southern hemisphere (figure \(5.7.1\)). Much of the solar heat absorbed by the tropical Atlantic is shipped north to warm Europe and the Northern Hemisphere. Imagine, then, what might happen if the supply of heat is shut off. I will get back to that topic in the next section.

We can make a crude estimate of the importance of the North Atlantic surface and deep circulation from a calculation based on what we know about waters in the Atlantic as compiled by Bill Schmitz (1996) in his wonderful summary of his life’s work. The Gulf Stream carries \(40 \ \text{Sv}\) of \(18^{\circ}\text{C}\) water northward. Of this, \(14 \ \text{Sv}\) return southward in the deep western boundary current at a temperature of \(2^{\circ}\text{C}\). The water must therefore lose \(0.9 \ \text{PW}\) \((1 \ \text{petawatt} = 10^{15} \ \text{watt})\) in the north Atlantic north of 24\(^{\circ}\)N. Although the calculation is very crude, it is remarkably close to the value of \(1.2 \pm 0.2 \ \text{PW}\) estimated much more carefully by Rintoul and Wunsch (1991).

Note that if the water remained on the surface and returned as an eastern boundary current, it would be far warmer than the deep current when it returned southward. Hence, the heat transport would be much reduced and it would probably not keep the Far North Atlantic ice-free.

The production of bottom water is influenced by the surface salinity and winds in the North Atlantic (Toggweiler and Russell, 2008). It is also influenced by the rate of upwelling due to mixing in other oceanic areas. First, let’s look at the influence of salinity.

Saltier surface waters form denser water in winter than less salty water. At first you may think that temperature is also important, but at high latitudes water in all ocean basins gets cold enough to freeze, so all oceans produce \(-2^{\circ}\text{C}\) water at the surface. Of this, only the most salty will sink, and the saltiest water is in the Atlantic and under the ice on the continental shelves around Antarctica.

The production of bottom water is remarkably sensitive to small changes in salinity. Rahmstorf (1995), using a numerical model of the meridional overturning circulation, showed that a \(\pm 0.1 \ \text{Sv}\) variation of the flow of fresh water into the North Atlantic can switch on or off the deep circulation of \(14 \ \text{Sv}\). If the deep-water production is shut off during times of low salinity, the \(1 \ \text{petawatt}\) of heat may also be shut off. Weaver and Hillaire-Marcel (2004) point out that the shutdown of the production of bottom water is unlikely, and if it did happen, it would lead to a colder Europe, not a new ice age, because of the higher concentrations of CO₂ now in the atmosphere.

I write may be shut off because the ocean is a very complex system. We don’t know if other processes will increase heat transport if the deep circulation is disturbed. For example, the circulation at intermediate depths may increase when deep circulation is reduced.

The production of bottom water is also remarkably sensitive to small changes in mixing in the deep ocean. Munk and Wunsch (1998) calculate that \(2.1 \ \text{TW}\) \((\text{terawatts} = 10^{12} \ \text{watts})\) are required to drive the deep circulation, and that this small source of mechanical mixing drives a poleward heat flux of \(2000 \ \text{TW}\). Some of the energy for mixing comes from winds which can produce turbulent mixing throughout the ocean. Some energy comes from the dissipation of tidal currents, which depend on the distribution of the continents. Some of the energy comes from the flow of deep water past the mid-ocean ridge system. Thus during the last ice age, when sea level was much lower, tides, tidal currents, tidal dissipation, winds, and deep circulation all differed from present values.

Role of the Ocean in Ice-Age Climate Fluctuations

What might happen if the production of deep water in the Atlantic is shut off? Information contained in Greenland and Antarctic ice sheets, in North Atlantic sediments, and in lake sediments provide important clues. Several ice cores through the Greenland and Antarctic ice sheets provide a continuous record of atmospheric conditions over Greenland and Antarctica extending back more than 700,000 years before the present in some cores. Annual layers in the core are counted to get age. Deeper in the core, where annual layers are hard to see, age is calculated from depth and from dust layers from well-dated volcanic eruptions. Oxygen-isotope ratios of the ice give air temperature at the glacier surface when the ice was formed. Deuterium concentrations give ocean-surface temperature at the moisture source region. 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. And isotopes of some elements give solar and cosmic ray activity (Alley, 2000).

Cores through deep-sea sediments in the North Atlantic made by the Ocean Drilling Program give information about i) surface and deep temperatures and salinity at the location above the core, ii) the production of North Atlantic deep water, iii) ice volume in glaciers, and iv) production of icebergs. Ice-sheet and deep-sea cores have allowed reconstructions of climate for the past few hundred thousand years.

1. The oxygen-isotope and deuterium records in the ice cores show abrupt climate variability many times over the past 700,000 years. 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). For example, \(\sim\) 11,500 years ago, temperatures over Greenland warmed by \(\sim 8^{\circ}\text{C}\) in 40 years in three steps, each spanning 5 years (Alley, 2000). Such abrupt warming is called a Dansgaard/Oeschger event. Other studies have shown that much of the Northern Hemisphere warmed and cooled in phase with temperatures calculated from the ice core.
2. The climate of the past 8,000 years was constant with very little variability. Our perception of climate change is thus based on highly unusual circumstances. All of recorded history has been during a period of warm and stable climate.
3. Hartmut Heinrich and colleagues (Bond et al. 1992), studying the sediments in the North Atlantic, found periods when coarse material was deposited on the bottom in mid-ocean. Only icebergs can carry such material out to sea, and the find indicated times when large numbers of icebergs were released into the North Atlantic. These are now called Heinrich events.
4. The correlation of Greenland temperature with iceberg production is related to the deep circulation. When icebergs melted, the surge of fresh water increased the stability of the water column, shutting off the production of North Atlantic deep water. The shut-off of deep-water formation greatly reduced the northward transport of warm water into the North Atlantic, producing a very cold Northern Hemisphere climate (figure \(\PageIndex{2}\)). The melting of the ice pushed the polar front, the boundary between cold and warm water in the North Atlantic further south than its present position. The location of the front, and the time it was at different positions, can be determined from analysis of bottom sediments.

   

   Figure \(\PageIndex{2}\): Periodic surges of icebergs during the last ice age appear to have modulated temperatures of the Northern Hemisphere by lowering the salinity of the Far North Atlantic and reducing the meridional overturning circulation. Data from cores through the Greenland ice sheet (1), deep-sea sediments (2,3), and alpine-lake sediments (4) indicate that: Left: During recent times the circulation has been stable, and the polar front which separates warm and cold water masses has allowed warm water to penetrate beyond Norway. Center: During the last ice age, periodic surges of icebergs reduced salinity and reduced the meridional overturning circulation, causing the polar front to move southward and keeping warm water south of Spain. Right: Similar fluctuations during the last interglacial appear to have caused rapid, large changes in climate. The Bottom plot is a rough indication of temperature in the region, but the scales are not the same. After Zahn (1994).

5. 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. This results in a climate ’sea-saw’ between the Northern and Southern Hemispheres.
6. The switching on and off of the deep circulation has large hysteresis (figure \(\PageIndex{3}\)). The circulation has two stable states. The first is the present circulation. In the second, 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. Once the circulation is shut off, the system switches to the second stable state. 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).

   

   Figure \(\PageIndex{3}\): The meridional-overturning circulation in the North Atlantic may be stable near 2 and 4. But, the switching from a warm, salty regime to a cold, fresh regime and back has hysteresis. This means that as the warm salty ocean in an initial state 1 freshens, and becomes more fresh than 2 it quickly switches to a cold, fresh state 3. When the area again becomes salty, it must move past state 4 before it can switch back to 1.

7. Heinrich events seem to precede the largest Dansgaard/Oeschger events (Stocker and Marchal, 2000). Here’s what seems to happen. The Heinrich event shuts off the Atlantic deep circulation, which leads to a very cold north Atlantic (Martrat et al, 2007). This is followed about 1000 years later by a Dansgaard/Oeschger event with rapid warming.
8. Dansgaard/Oeschger–Heinrich tandem events have global influence, and they are related to warming events seen in Antarctic ice cores. Temperatures changes in the two hemispheres are out of phase. When Greenland warms, Antarctica cools. Recent data from the European Project for Ice Coring in Antarctica (EPICA) shows that in the period between 20,000 and 90,000 years ago, 40% of the variance in the Greenland temperature data can be explained by Antarctic temperature data (Steig, 2006).
9. A weakened version of this process with a period of about 1000 years may be modulating present-day climate in the North Atlantic, and it may have been responsible for the Little Ice Age from 1100 to 1800.

The relationship between variations in salinity, air temperature, deep-water formation, and the atmospheric circulation is not yet understood. For example, we don’t know if changes in the atmospheric circulation trigger changes in the meridional overturning circulation, or if changes in the meridional overturning circulation trigger changes in the atmospheric circulation (Brauer et al, 2008). Furthermore, surges may result from warmer temperatures caused by increased water vapor from the tropics (a greenhouse gas) or from an internal instability of a large ice sheet. We do know, however, that climate can change very abruptly, and that circulation in the northern hemisphere has a very sensitive threshold, that when crossed, causes large changes in the circulation pattern.

For example, Steffensen (2008) found that 11,704, 12,896, and 14,694 years before 2000 AD, the temperature of the source water for Greenland precipitation warmed \(2–4^{\circ}\text{C}\) in 1–3 years. This indicates a very rapid reorganization of the atmospheric circulation at high latitudes in the Northern Hemisphere and a shift in the location of the source region. During the earliest event, air temperature over Greenland warmed by \(\approx 10^{\circ}\text{C}\) in 3 years. At the later events, air temperature over Greenland changed more slowly, over 60 to 200 years. Brauer et al (2008) found an abrupt change in storminess over Germany at almost exactly the same time, 12,679 years ago.