The direct forcing of the oceanic circulation by wind discussed in the last few chapters is limited mostly to the upper kilometer of the water column. Below a kilometer lies the vast water masses of the ocean extending to depths of 4 km – 5 km. The water is everywhere cold, with a potential temperature less than 4°C. The water mass is formed when cold, dense water sinks from the surface to great depths at high latitudes. It spreads out from these regions to fill the ocean basins. Deep mixing eventually pulls the water up through the thermocline over large areas of the ocean. It is this upwelling that drives the deep circulation. The vast deep ocean is usually referred to as the abyss, and the circulation as the abyssal circulation.
The densest water at the sea surface, water that is dense enough to sink to the bottom, is formed when frigid air blows across the ocean at high latitudes in winter in the Atlantic between Norway and Greenland and near Antarctica. The wind cools and evaporates water. If the wind is cold enough, sea ice forms, further increasing the salinity of the water because ice is fresher than sea water. Bottom water is produced only in these two regions. In other polar regions, cold, dense water is formed, but it is not quite salty enough to sink to the bottom.
At mid and low latitudes, the density, even in winter, is sufficiently low that the water cannot sink more than a few hundred meters into the ocean. The only exception are some seas, such as the Mediterranean Sea, where evaporation is so great that the salinity of the water is sufficiently great for the water to sink to intermediate depths in the seas. If these seas are can exchange water with the open ocean, the waters formed in winter in the seas spreads out to intermediate depths in the ocean.
Defining the Deep Circulation
The term thermohaline circulation was once widely used, but it has disappeared almost entirely from the oceanographic literature (Toggweiler and Russell, 2008). It is no longer used because it is now clear that the flow is not density driven, and because the concept has not been clearly defined (Wunsch, 2002b).
The meridional overturning circulation is better defined. It is the zonal average of the flow plotted as a function of depth and latitude. Plots of the circulation show where vertical flow is important, but they show no information about how circulation in the gyres influences the flow.
Following Wunsch (2002b), I define the deep circulation as the circulation of mass. Of course, the mass circulation also carries heat, salt, oxygen, and other properties. But the circulation of the other properties is not the same as the mass transport. For example, Wunsch points out that the North Atlantic imports heat but exports oxygen.
The deep circulation is mostly wind driven, but tidal mixing is also important. The wind enters several ways. It cools the surface and evaporates water, which determines where deep convection occurs. And, it produces turbulence in the deep ocean which mixes cold water upward.
13.1 Importance of the Deep Circulation
The deep circulation which carries cold water from high latitudes in winter to lower latitudes throughout the world has very important consequences.
Two aspects of the deep circulation are especially important for understanding Earth's climate and its possible response to increased carbon dioxide CO2 in the atmosphere:
The Oceans as a Reservoir of Carbon Dioxide
More CO2 dissolves in cold water than in warm water. Just imagine shaking and opening a hot can of CokeTM. 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 in the cold waters of the ocean, 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 deep circulation and the net flux of carbon deposited on the seafloor. 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 we won't consider these processes.
Oceanic Transport of Heat
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 the Gulf Stream carries heat to the far north Atlantic (Figure 13.1). There the surface water releases heat and water to the atmosphere. Some of the ocean water becomes sufficiently cold, salty, and dense that it sinks to the bottom in the Norwegian and Greenland Seas. It then flows saothward in very cold, bottom currents along western boundaries as a western boundary current. Some of the water remains at the surfae and returns to the south in cool surface currents such as the Labrador Current and the Portugal Current (see Figure 11.8).
The deep bottom water from the north Atlantic is mixed upward in other regions and 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 5.11). 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. We will get back to that topic in the next section.
We can make a crude estimate of the importance of the conveyor-belt circulation from a simple calculation based on what we know about waters in the Atlantic compiled by Bill Schmitz (1996) in his wonderful summary of his life's work. The Gulf Stream carries 40 Sv of 18°C water northward. Of this, 14 Sv return southward in the deep western boundary current at a temperature of 2°C. The flow carried by the conveyor belt must therefore lose 0.9 petawatts (1 petawatt = 1015 watt) in the north Atlantic north of 24°N. Although the calculation is very crude, it is remarkably close to the value of 1.2 ± 0.2 petawatts 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. Furthermore, the warm water would not reach high latitudes in the North Atlantic, and they would freeze over in winter, leading to further cooling of the region.
The production of bottom water is influenced by the salinity of surface waters in the north Atlantic. 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 ocean produce -2° 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 ±0.1 Sv variation of the flow of fresh water into the north Atlantic can switch on or off the deep circulation of 14 Sv. If the deep-water production is shut off during times of low salinity, the 1 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 TW (terawatts = 1012 watts) are required to drive the deep circulation, and that this small source of mechanical mixing drives a poleward heat flux of 2000 TW. Most 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. 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
Several ice core through the Greenland ice sheet and three through the Antarctic sheet provide a continuous record of atmospheric conditions over Greenland and Antarctica extending back more than 400,000 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. 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; 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 sea-surface temperature and salinity above the core, the production of north Atlantic deep water, ice volume in glaciers, and production of icebergs.
The relationship between variations in salinity, air temperature, and deep-water formation is not yet well understood. For example, we don't know what causes the ice sheets to surge. 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. Nor do we know exactly 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.
The oceans play a key role in the development of the ice ages. Every 100,000 years for the past million years, ice sheets have advanced over the continents. Shackleton (2000) finds that the 100,000-year period of Earth's orbital eccentricity, deep-sea temperature, and atmospheric carbon-dioxide concentration are well correlated over the 100,000-year cycle. He also finds that ice-sheet volume lagged behind CO2 changes in the atmosphere, implying that ice sheets changed as a result of CO2 changes, not the other way around.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, email@example.com
All contents copyright © 2005 Robert H. Stewart,
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Updated on October 24, 2008