Thermocline depth

Similar to large, deep lakes, much of the ocean is thermally stratified, particularly in warmer equatorial areas. Thermocline depths are of the order of a thousand meters. As in the case of hypolimnetic waters in a lake, deep waters in such stratified ocean areas are cut off from atmospheric exchange of gases such as O2 and CO2. Unlike the seasonal formation and dissipation of stratification in temperate-zone lakes, however, this oceanic stratification is persistent, and deep waters can be isolated from atmospheric exchange for decades. 

Several key factors modify the timing, extent, and stability of thermal stratification, notably climate together with lake-specific morphological features. Regionally, air temperature and solar radiation are important factors. Other parameters of importance are the water temperatures, the thermocline depth (which contfols the volume of the epilimnion) and the duration of stratification. Direct measurement of these parameters is preferable but correlations also exist to estimate these quantities as reported in the literature (e.g., Cahill et al., 2005 Putter, 2003 Gao and Stefan, 1999). 

Schnoor (1987) has compiled these coefficients for a number of lakes. In most cases, the thermocline depth is 10 5m. The vertical dispersion coefficients across the thermocline are approximately 0.02 cm /s plus or minus a factor of 5, that is, 0.004-0.10 cm /s whereas whole lake coefficients are considerably larger and in the range 0.10-4 cm /s. 

Lake area Lake depth Lake volume Thermocline depth Water outflow Reaction rate constant Volatilization MTC Net MTC to sediment Thermocline diffusivity Path length Thermocline MTC Chemical input... 

South America to about the date line (180 ). These trade winds also drive near-equatorial surface flow westward as the South Equatorial Current (SEC). This piles up warm surface water in the western Pacific to create a deep warm pool and results in depression of the depth of the thermocline from east to west. The westward flow in the surface SEC is partly compensated by a return flow to the east in the thermocline ( 150m) called the Equatorial Undercurrent (EUC). 

Nuclear bomb produced " 002 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, changes in the distributions of tritium (Rooth and Ostlund, 1972) in the western Atlantic between 1972 (GEOSECS) and 1981 (TTO) are shown in Fig. 10-10 (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 F1 (tritium) into the thermocline has occurred at all depths. 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 transient effect can be seen in the penetrative distribution of manmade chlorofluorocarbons, which have been released over a longer period (40 years) (Gammon et al., 1982). 

Fig. 3 The annual development of the seasonal thermocline (expressed as temperature gradients) (a) for a selected set of years when hypolimnetic withdrawal resulted in a deep thermocline and an extensive metalimnion, and (b) for a selected set of years when epilimnetic water flowed through the intermediate outlet inducing the development of a shallower thermocline. The solid black line shows the daily development of the withdrawal depth and the gray areas the bottom of the reservoir. Modified from Moreno-Ostos et al. [37]...

Pit lake at the former El Mine is a meromictic lake with a thermocline and a chemocline coinciding at the depth 20 m, where former open pit was connected to the shaft. 

Technically the thermocline is the depth zone over which the vertical temperature gradient reaches maximal values. This depth zone varies with season, latitude and longitude, and local environmental conditions. The top of the thermocline is defined by... 

The nutrient profiles are characterized by much higher concentrations in the deep-waters than in the surface. In some locations, such as shown in Figure 9.1, mid-water concentration maxima are present. The depth region over which concentrations exhibit the largest vertical gradients is usually defined by the thermocline. All biolimiting elements have similar depth profiles, having surfece-water depletions and deep-water enrichments. 

The increase in 2CO2 within the thermocline is accompanied by a decrease in pH caused by the hydrolysis of CO2 produced by the remineralization of POC. This supply is so large that significant amoimts of CO2 and H2CO3 are present at these depths. The [COj ] reaches minimum values in the depths over which POC remineralization is most... 

Depth profiles from the eastern tropical North Pacific (Figure 24.8) show the effects of nitrogen metabolism under 02-deficient conditions. The thermocline is characterized by a sharp decline in O2 concentrations that coincides with increasing nitrate and phosphate concentrations. The oxycline is produced by the respiration of sinking POM under vertically stagnant conditions. Below the oxycline, in depths where O2 concentrations are suboxic, phosphate concentrations continue to increase, but at a slower rate. In contrast, nitrate concentrations decline and reach a mid-water minimum that coincides with a nitrite maximum. The latter is referred to as the secondary nitrite maximum. (At this site the primary nitrite maximum is located at 50 m.)... 

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... 

Pycnocline The vertical region which density increases rapidly with depth. This usually coincides with the thermocline. 

Thermocline The depth range over which temperature decreases rapidly with depth. At mid latitudes this zone usually spans depths ranging from 300 to 1000 m. 

However, salinity values are easily obtained with a salinometer (which measures electrical conductivity and is appropriately calibrated with standard solutions and adjusted to account for T effects). The salinity of seawater increases if the loss of H2O (evaporation, formation of ice) exceeds the atmospheric input (rain plus rivers), and diminishes near deltas and lagoons. Salinity and temperature concur antithetically to define the density of seawater. The surface temperature of the sea reflects primarily the latitude and season of sampling. The vertical thermal profile defines three zones surface (10-100 m), where T is practically constant thermoclinal (100-1000 m), where T diminishes regularly with depth and abyssal... 

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