Circulation, oceanic thermocline

In this section we briefly review what controls the density of seawater and the vertical density stratification of the ocean. Surface currents, abyssal circulation, and thermocline circulation are considered individually. 

The density of seawater varies from a maximum of c = 29, observed in deep antarctic waters, to a minimum of c = 25 in subtropical oceanic thermoclinal waters. The high density of polar waters causes them to sink beneath subtropical waters and constitutes the driving force of deep oceanic circulation. 

The distribution of CO2 and the associated carbonic acid system species in the upper ocean (here loosely defined as waters above the thermocline and generally only a few hundred meters in depth) is primarily controlled by the exchange of CO2 across the air-sea interface, biological activity, and circulation of the ocean, mainly through vertical mixing processes. Other factors, such as the temperature and salinity of the water, can also contribute to variations by influencing the solubility of CO2 in seawater and the equilibrium constants of the carbonic acid system. 

As a consequence of the general deep circulation of the ocean (Box 3.2), a permanent thermocline is present in temperate and tropical oceans (at depths of c.300 and 100m, respectively).The permanent thermocline persists throughout the year at middle and low latitudes but is absent at latitudes above c.60° because the cooler climate and reduced insolation do not cause sufficient... 

Table 1.3 shows which of the four environmental compartments store significant amounts of the essential elements. Table 1.4 shows the estimated quantities of the elements that circulate rapidly through the environment—C, fixed N, P, S, and water. The amounts shown for the oceans are those above the thermocline, a temperature inversion at about 50 m depth, which separates the surface water from the deeper water. The soil is the largest reservoir of almost all the essential elements and is the only reservoir for most of the essential elements. 

Intermediate between the mixed layer and the deep sea, at depths between 100 and 1000 m, lies the main thermocline, a region where mixing is imperfect and exchange with the atmosphere is slow but nevertheless faster than in the denser and cooler waters of the deep ocean. At high latitudes, a mixed layer often cannot be discerned and the surface waters mix directly with the deeper strata. Circulation in the main body of the ocean is accomplished by the downward motion of cold surface waters near the poles and a slow updrift at low latitudes. Estimated rates of downflow are 30 x 106 m3/s as given by Munk (1966) and Gordon (1975) for the Antarctic, and (10-30) x 106 m3/s as derived by Broecker (1979) for the North Atlantic. The resulting turnover time of deep ocean waters is 700-1000 yr. 

In the subtropics, where wind-stress is convergent, water tends to be forced downward from the surface ocean into the thermocline. This downwelling is an important process for ventilation of the thermocline, and for driving the shallow gyre circulation. Figure 8 shows the measured tritium-helium age as measured... 

It is unusual to think of any type of atmospheric contamination - especially by a radioactive species -as beneficial however, bomb-produced radiocarbon (and tritium) has proven to be extremely valuable to oceanographers. The majority of the atmospheric testing, in terms of number of tests and production, occurred over a short time interval, between 1958 and 1963, relative to many ocean circulation processes. This time history, coupled with the level of contamination and the fact that becomes intimately involved in the oceanic carbon cycle, allows bomb-produced radiocarbon to be valuable as a tracer for several ocean processes including biological activity, air-sea gas exchange, thermocline ventilation, upper ocean circulation, and upwelling. 

Radiocarbon has been used to study thermocline ventilation using tools ranging from simple 3-box models to full 3D ocean circulation models. Many of the ID and 2D models are based on work by W. Jenkins using tritium in the North Atlantic. In a recent example, R. Sonnerup and co-workers at the University of Washington used chlorofluorocarbon data to calibrate a ID (meridional) along-isopycnal advection-diffusion model in the North Pacific with WOCE data. [4] is the basic equation for the... 

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