Thermocline circulation

The transition region between the surface and deep ocean is referred to as the thermocline. This is also a 

Concepts regarding the existence of the thermocline are changing. An older view held that the thermocline represented a vertical balance between the downward diffusion of heat and the upward advection of cold abyssal water that originated at high latitudes (see Section 9.2.4). It was presumed that these two processes are in longterm balance resulting in a steady-state. A onedimensional vertical model can be fit to most of the dat-v however, the vertical diffusivity parameters required to fit the data are unsatisfactory from several points of view (i.e. they are too large see Jenkins, 1980). 

The horizontal isopycnal thermocline model is important for determining the fate of the excess atmospheric CO2. The increase of CO2 in the atmosphere is modulated by transport of excess CO2 from the atmosphere into the interior of the ocean. The direct ventilation of the thermocline in its outcropping regions at high latitudes appears to play an important role in removing CO2 from the atmosphere (Brewer, 1978 Siegenthaler, 1983). 

Nuclear bomb-produced 02 and (as HTO) have been used to describe and model this rapid thermocline ventilation (Ostlund et ah, 1974  

Sarmiento et ah, 1982 Fine et al., 1983). For example, the distributions of tritium (Rooth and Ostlund, 1972) in the western Atlantic in 1972 (GEOSECS) and 1981 (TTO) are shown in Fig. 9-10 (Ostlund and Fine, 1979 Baes and Mulholland, 1985). In the 10 years following the atmospheric bomb tests of the early 1960s, a massive penetration of into the thermocline at all depths has occurred. Comparison of the GEOSECS and TTO data, which have a 9-year time difference, clearly shows the rapid ventilation of the North Atlantic and the value of such transient tracers. A similar distribution can be seen in the distribution of man-made chlorofluorocarbons, which have been released over a longer period (40 years) (Gammon etal., 1982). 

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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. 

Phenomena such as El Ninos or thermocline circulation in the North Atlantic lead to global instability in the processes of energy and matter exchange, which should be reflected by parameterizing non-linear feedbacks. 

Vidussi, F., Claustre, H., Manca, B.B., Luchetta, A. and Marty, J.-C. (2001) Phytoplankton pigment distribution in relation to upper thermocline circulation in the Eastern Mediterranean Sea during winter. Journal of Geophysical Research, 106, 19939-19956. 

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. 

Meier-Reimer, E., Mikolajewicz, U., Hasselmann, K. (1993) Mean circulation of the Hamburg LSG OGCM and its sensitivity to the thermocline surface forcing. J. Phys. Oceanogr., 23, 731-57. 

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. 

Jenkins W. J. (1998) Studying thermocline ventilation and circulation using tritium and He. J. Geophys. Res. 103, 15817-15831. 

Hypolimnion The portion of certain lakes below the thermocline (q.v.) which receives no heat from the sun and no aeration by circulation. 

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