How does thermohaline circulation take place

This warmer, fresher water from the Pacific flows up through the South Atlantic to Greenland, where it cools off and undergoes evaporative cooling and sinks to the ocean floor, providing a continuous thermohaline circulation. Hence, a recent and popular name for the thermohaline circulation, emphasizing the vertical nature and pole-to-pole character of this kind of ocean circulation, is the meridional overturning circulation.

The deep water masses that participate in the MOC have chemical, temperature and isotopic ratio signatures and can be traced, their flow rate calculated, and their age determined. The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates at the tip of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean.

The process of western intensification causes the Gulf Stream to be a northward accelerating current off the east coast of North America. Although there has been recent debate, there is consensus that the climate of Western Europe and Northern Europe is warmer than it would otherwise be due to the North Atlantic drift, [10] [11] one of the branches from the tail of the Gulf Stream. It is part of the North Atlantic Gyre. Its presence has led to the development of strong cyclones of all types, both within the atmosphere and within the ocean.

The Gulf Stream is also a significant potential source of renewable power generation. All these dense water masses sinking into the ocean basins displace the older deep water masses which were made less dense by ocean mixing. To maintain a balance water must be rising elsewhere. However, because this thermohaline upwelling is so widespread and diffuse, its speeds are very slow even compared to the movement of the bottom water masses.

It is therefore difficult to measure where upwelling occurs using current speeds, given all the other wind-driven processes going on in the surface ocean.

Deep waters have their own chemical signature, formed from the breakdown of particulate matter falling into them over the course of their long journey at depth.

A number of scientists have tried to use these tracers to infer where the upwelling occurs. Wallace Broecker, using box models, has asserted that the bulk of deep upwelling occurs in the North Pacific, using as evidence the high values of silicon found in these waters.

Other investigators have not found such clear evidence. Computer models of ocean circulation increasingly place most of the deep upwelling in the Southern Ocean, [14] associated with the strong winds in the open latitudes between South America and Antarctica. While this picture is consistent with the global observational synthesis of William Schmitz at Woods Hole and with low observed values of diffusion, not all observational syntheses agree.

Recent papers by Lynne Talley at the Scripps Institution of Oceanography and Bernadette Sloyan and Stephen Rintoul in Australia suggest that a significant amount of dense deep water must be transformed to light water somewhere north of the Southern Ocean.

The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions, although poleward heat transport outside the tropics is considerably larger in the atmosphere than in the ocean. Insofar as the thermohaline circulation governs the rate at which deep waters are exposed to the surface, it may also play an important role in determining the concentration of carbon dioxide in the atmosphere.

While it is often stated that the thermohaline circulation is the primary reason that Western Europe is so temperate, it has been suggested that this is largely incorrect, and that Europe is warm mostly because it lies downwind of an ocean basin, and because of the effect of atmospheric waves bringing warm air north from the subtropics.

Large influxes of low-density meltwater from Lake Agassiz and deglaciation in North America are thought to have led to a disruption of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas. Coincidentally, scientists at Woods Hole had been measuring the freshening of the North Atlantic as Earth becomes warmer.

Their findings suggested that precipitation increases in the high northern latitudes, and polar ice melts as a consequence. The engine moving these deep currents is thermohaline circulation. So cold, salty water has a high density and sinks to the ocean floor. Gravity and geography or geomorphology then come into play. Imagine a water tank with a sloped bottom, and I place a drop of mercury on the surface of the water. The mercury sinks to the bottom of the tank since it is much denser than the water.

But once it reaches the floor of the container, it continues to move down the sloped bottom to the lowest point in the tank. Gravity and the tank shape have worked together, creating this outcome.

If we take this simple example and apply it to the oceans, we find the same principles at work with thermohaline circulation. Cold saline water sinks to the ocean bottom.

Gravity then takes over, and the water moves downslope, always seeking the lowest elevation. If the supply of dense water is continuous, then the gravity-driven flow of water becomes a deep ocean current. Thus, the discovery of cold water in the deep ocean at low latitudes was a great surprise. Upon the passage, I made several trials with the bucket sea-gage, in the latitude 25' 13'' north; longitude 25'12'' west.

I charged it and let it down to different depths, from feet to feet; when I discovered, by a small thermometer of Fahrenheit, made by Mr. Bird, which went down in it, that the cold increased regularly, in proportion to the depths, till it descended to feet: from whence the mercury in the thermometer came up at 53 degrees; and tho' I afterwards sunk it to the depth of feet, that is a mile and 66 feet, it came up no lower.

The warmth of the water on the surface, and that of the air, was at that time by the thermometer 84 degrees. I doubt not but that the water was a degree or two colder, when it enter'd the bucket, at the greatest depth, but in coming up had acquired some warmth Warren, Bottom water properties in the world's oceans Deep water at low latitudes is much colder than the lowest temperature at the sea surface in winter-time; thus, such cold water cannot be formed locally, and the source of such water mass must be traced back to higher latitudes where cold winter conditions make the formation of water mass with such low temperature possible.

The reasoning that cold water is formed at high latitudes and transported to low latitudes eventually led to the theories of thermohaline circulation in the world's oceans.

Subsequent observations through scientific expeditions established the fact that the bottom of the world's oceans is covered by cold water originating from a very few narrow sites at high latitudes, where severe winter conditions produce the coldest water in the.

Over the past century, extensive observation data over the world's oceans have been accumulated. Potential temperature distribution on the sea floor of the world's oceans, based on clima-tological data, is shown in Figure 5.

As discussed in Chapter 2, potential temperature is a better tracer to use in the description of the oceanic environments because the effect of compression varying with depth is eliminated.

From Figure 5. From these source regions, bottom water is carried northward and eastward by currents and eddies. The cold water mass formed around the edge of the Antarctic continent that sinks to the bottom of the world's oceans is called Antarctic Bottom Water AABW. Bottom water temperature in the South Atlantic Ocean is the coldest among all basins. The eastern basin, the Angola Basin, does not receive AABW from the south; it is a basin closed to cold bottom water from the south.

The ocean, including its abyss, is warming at a rate of 0. Although historical observations and paleoclimatic data reveal significant climate variability on decadal to millennial time scales, this ocean warming during the last several decades is linked to global climate change.

Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and land surface properties have altered the energy balance of the climate system. Consequently, the change of the global energy balance has seen a decrease in sea ice Fig. This excess of high latitudinal freshwater influx can substantially modify the deep ocean circulation.

The sinking that drives the thermohaline circulation depends critically on the water being sufficiently cold and salty. Therefore, any factor that changes the state of the conditions for circulation, can result in a slow-down of the thermohaline circulation, and thereby dramatically influence the climatic state and driving further climate change.

This raises concerns about potential abrupt climate changes in the future. Is a complete shutdown of the thermohaline possible? In light of the deficit of the scientific understanding of the thermohaline circulation and the feedback potentials between the two deepwater sources, it is difficult to predict the influence of global climate change on the dynamics of the thermohaline.

Even within the scientific realm there is disagreement on the possibility of a complete shutdown of the thermohaline circulation. Given this discrepancy, a simulation experiment was conducted Seidov, D. In this simulation, the authors set out to demonstrate the degree to which the thermohaline circulation is driven by both the NADW and the AABW, by means of a designed series of simplified freshwater influx events, in which all ocean model parameters are held constant except salinity.

The results of the experiments were described for: a North Atlantic freshwater influx, a Southern Ocean freshwater influx, and a combined influx for the North Atlantic and Southern Ocean.

The results of the experiment simulating a low-salinity impact -2 psu than present on the North-Atlantic conform to what is already known from previous work, namely that the conveyor is weaker and shallower. Temperature differences between the current conditions and this low-salinity scenario indicate cooling in high latitudes of the Atlantic Ocean. This occurs because the reduced NADW production led to a shallow conveyor and cooler and fresher water than today in these latitudes characterises the deep ocean water.

In addition to this, the surface ocean has more time to lose heat to the atmosphere because the overturning slowed. The reduced NADW outflow has an evident imprint in the oceanic heat transport.

Northward cross-equatorial heat transport is dramatically reduced in the scenario with a strong freshwater impact, which indicates the possibility of a cold episode following a freshwater influx event. If the present-day global warming was potent enough to induce a low-salinity episode in the North Atlantic, caused by iceberg and Arctic sea ice melting, the result could be a tendency towards colder temperature conditions in the Northern Hemisphere. It is important though to note that even with an excessive northern low-salinity signal of -2 psu during the simulation, no complete termination of the conveyor occurred.

In contrast to the predictable results of the northern low-salinity impact, the results of the Southern Ocean surface freshening are less intuitive. Two aspects are noteworthy: first the circulation changes driven by the low-salinity signal were much stronger, and second, they led to a very strong warming of the deep ocean.

Warming takes place over the entire deep ocean and its maximum shifts to the southern edges. In the North Atlantic scenario, the freshwater influx impact on the conveyor caused thermal effects only in the deep Atlantic Ocean, whereas in the Southern Ocean, the freshwater scenarios impact is global. The increased NADW outflow in the deep layers leads to increased compensating northward surface water flow. This flow carries more warm and salty subtropical water to convection sites, which might further increase NADW production until the atmosphere warms up to reduce the cooling of the sea surface and subsequently reduce the deep convection.

The positive feedback of NADW production and northward heat transport can be viewed as a first link toward high-latitudinal warming in the Northern Hemisphere caused by freshwater influx events in the Southern Ocean. The results of a combined low-salinity event of the North Atlantic and the Southern Ocean, display a deepwater regime that is qualitatively similar to a Southern-Ocean-only experiment. Less Deep Ocean warming is observed, however the impact still remains global and substantial.

Results of the runs with perturbations to two sources, demonstrates a more powerful response to a freshwater influx event in the Southern Ocean than for those in the North Atlantic. However, much of this power stems from increased NADW production. Sea level rise can be caused by either melting of major ice sheets Fig. As the deep ocean warms up, the sea elevation will change as a result of the thermal expansion of sea water.

Historic hydrographic data suggest that thermal expansion of the ocean can contribute tens of centimeters to the observed sea level rise over the last century Godfrey, J. Some simulations Church, J. In the simulation experiment conducted, Seidov, D. In many sensitive coastal areas the sea-level rise could be over 1 meter.

It is important to note that this sea-level rise could occur without significant melting of the ice sheets, including the Western Antarctic Ice Sheet WAIS , which is considered the most vulnerable to climate change Seidov, D.