Thermocline layers

The dynamics behind the Kelvin wave front changes completely. The divergence of the Ekman offshore transport is balanced by the divergence of the coastal jet in the surface layer and the divergence of the Ekman compensation current in the sub-thermocline layer is balanced by the longshore divergence of a coastal undercurrent flowing in opposite direction to the upper layer coastal jet. 

It is not to be expected that the straight-line Equation 21 would apply to the thermocline layers of other lakes. Both N and K were calculated from temperature profiles the shape of which depends in a complex manner on the climate of the area, thermal regime, depth, and volume of the lake. It seems, however, that by arguments presented earlier in this section, an inverse relationship between the stability frequency and eddy diffusion coefficient would, in general, hold in the pycnocline layers of lakes. If such a relationship is established, it would be possible to obtain estimates of K from the values of the stability frequency N, which are much easier to compute. 

Ocean. Despite the fact that no changes in the chemical composition of the ocean water have been established in recent time, it is instructive to consider the transient behavior of reacting chemical species in the oceanic water column. The case of the nuclear fallout products transported through the surface and thermocline layer of the ocean is the best known, although not yet completely understood, case of a transient chemical event on a world-wide scale (34, 35,36,37). 

Thermocline layer, from 100 to abont 1500 m, in which the temperature decreases sharply and... 

The oceans are subdivided into surface (100—1000-m) ocean and deep ocean. The zone separating the warmer, surface water from the lower, cooler layer (oceanic thermocline) is characterized by a density gradient that prevents mixing. 

Figure 7. Map of 6 Zn (%o) in the surface layer of FeMn-nodules. High-latitude samples have isotopically heavier Zn than low-latitude samples. This feature was interpreted by Marechal et al. (2000) as reflecting the presence of a Zn-depleted seasonal thermocline at high latitude. Map drawn using the GMT software package (Wessel and Smith 1991).

The depth of the mixed layer is important for two reasons. First, phytoplankton can be carried out of the photic zone and, hence, halt net primary production if the mixed layer is deeper than the photic zone. Second, the bottom of the mixed layer marks the upper limit to which density stratification in the thermocline inhibits upward vertical transport of nutrients. If the photic zone extends into the thermocline, phytoplankton... 

In Illustrative Example 19.1, we calculated the vertical exchange of water across the thermocline in a lake by assuming that transport from the epilimnion into the hypolimnion is controlled by a bottleneck layer with thickness 5 = 4m. From experimental data the vertical diffusivity was estimated to lie between 0.01 and 0.04 cm2s 1. Closer inspection of the temperature profiles (see figure in Illustrative Example 19.1) suggests that it would be more adequate to subdivide the bottleneck boundary in two or more sublayers, each with its own diffusivity. 

Until May 1985, the PCE content of Greifensee was relatively small (between about 40 and 80 moles, see Fig. 23.7). On June 3, 1985, a PCE content of more than 200 moles was found in the lake. The PCE was mainly detected in the surface mixed layer (about 4 m deep) and in the thermocline. On July 1, 1985, the PCE content was still around 200 moles, but the concentration maximum had moved from the surface to the thermocline. This concentration peak remained visible throughout the summer and fall until the PCE content had returned to its normal level. 

The mean residence time of carbon in the mixed layer of the sea before transfer into the deep sea is of considerable interest, for as has already been pointed out, the rate of this transfer will eventually govern the levels of excess 14C in the atmosphere. There have been several estimates of this residence time. Craig (29) concluded that it was most probably not more than 10 years, and in one of his calculations he deduced a value of 4 years. Broecker et al. (14) concluded it was 5 years in the Atlantic Ocean and 8 years in the Pacific Ocean. Nydal (45) found that for the North Atlantic it was around 3 years or less. The profiles of Figure 6, and a few others which are not shown, all show a significant penetration of excess 14C below the mixed surface layer, pointing to a short residence time, of the order of 2 years, in the mixed layer of the sea before transfer below the thermocline into the deep sea. Considering the size of the oceans these data are very meager, and no firm conclusions can be drawn from them. However, continued measurements of 14C in the sea should help to establish a firmer estimate of this quantity. 

The turbulent fluxes of carbon on the thermocline-deep ocean border is considered to be proportional to the coefficient kT of the difference in carbon concentrations in the bordering layers ... 

The shallow-water character of the sea provides rapid propagation of wind and convective mixing down to the bottom, which leads to equalizing the vertical temperature distribution in most cases, the temperature difference is less than 1 °C. Meanwhile, during summertime calm periods, the thermocline is formed which prevents the near-bottom layer from water exchange. 

Fig. 3 Vertical profiles of the water potential temperature (T , degrees Celsius), water salinity (S, practical salinity unit), and water specific potential density (or , kgm-3) a in upper layer of the Black Sea central area in August 1995 and b in deep layer (mean values based on high vertical resolution CTD measurements). 1 Upper mixed layer, 2 seasonal pycnocline (thermocline), 3 cool intermediate layer, 4 main pycnocline (halocline), 5 deep pycnocline, 6 bottom mixed layer...

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