Isopycnal surfaces

Fig. 10-9 Map of winter outcrops of isopycnal surfaces in the Atlantic Ocean. (Modified with permission from W. S. Broecker and T.-H. Peng (1982). Tracers in the Sea," p. 394, Eldigio Press, Palisades, NY.)...

In Figs. 7b and 7c, we present the results of the calculations [20] of the rate of the climatic subduction of the cold Black Sea UML waters in March together with the corresponding surface temperature and salinity fields. The subduction means the transfer of cold waters from the UML to the subsurface layer owing to the action of one or several processes the decrease in the UML thickness in the course of the formation of a new seasonal pycnocline, the water downwelling, and the water transport by currents over descending isopycnal surfaces. Correspondingly, the subduction rate was estimated as the... 

The vertical distribution of dissolved oxygen in the Black Sea reflects its specific features as the density stratified basin, which has a permanent H2S zone under the pycnocline [23]. The thickness of the oxic zone varies between 70 and 100 m in areas of central cyclonic gyres with elevated isopycnal surfaces, and between 120 and 200 m in peripheral areas. 

Figure l.Z( Maps of annual mean nitrate concentrations at (a) surface, (b) on the isopycnal surface (j0 = 26.80 representing Subantarctic Mode Water in the southern hemisphere, and subpolar mode waters in the northern hemisphere, and (c) on the isopycnal surface ff2 37.00 representing North Atlantic Deep Water. Based on data from the World Ocean Atlas 2001 (Conkright et ah, 2002). 

Let us explore the thermocline distribution first. As large-scale oceanic transport occurs primarily along surfaces of equal potential density, it is instructive to inspect variations along such surfaces. Figure 1.4b shows the NO distribution along the potential density surface Gg = 26.80, which represents Sub-Polar Mode Water (SPMW) in the northern hemisphere and Sub-Antarctic Mode Water (SAMW) in the southern hemisphere (Hanawa and TaUey, 2001). Nitrate concentrations near the outcrops, i.e., where the isopycnal surface intersects the surface of the ocean are near zero, but concentrations increase rapidly as one moves away from the outcrops into the ocean s interior. 

Not all of the variations of nitrate on this isopycnal surface are due to remineraUza-tion processes, however. We need to consider also the contribution that stems from the mixing of surface source waters that already contain a significant amount ofNOj. This preformed nitrate is evident in the non-zero outcrop values noted above in Fig. i. 4a. In order to isolate the impact of remineraHzation on the interior ocean NO distribution, one therefore needs to separate the observed NO into a preformed and a remineralized component, where the latter refers to the amount of NO that has been added to a water parcel since it was last in contact with the atmosphere, thus ... 

Figure 1.12 Global plots of objectively analyzed N (pmol kg" ) on isopycnal surfaces. (A) N on the as = 26.50 surface. This surface is characteristic of subtropical mode waters, such as the 18° Water in the North Atlantic, (B) N on the as = 27.10 surface representing Subpolar Mode Waters. Solid circles denote the stations stippling denotes areas where the waters of this potential density are not present in winter time. Cross-hatched areas are regions where the estimated error in objectively analyzed JV is greater than 0.7 pmol kg". Data are from the WOCE program augmented by selected pre-WOCE cruises and were adjusted by the GLODAP project.

Takahashi, T., Broecker, W. S., and Langer, S. (1985). Redfield ratio based on chemical data from isopycnal surfaces. J. Geophys. Res. 90(C4), 6907—6924. 

Minster, J. F., and Boulahdid, M. (1987). Redfield ratios along isopycnal surfaces - A complimentary... 

Data from GEOSECS and TTO Expeditions The GEOSECS and TTO (Transient Tracers in the Oceans) programs have provided a large source of data in the Atlantic (see Broecker 1985, and Takahashi et al., 1985). The interpretation of the chemical data from isopycnal surfaces yields a Redfield ratio of P/N/C/O2 of 1 16 103 172. The ratio of P/CaCO dissolution has been estimated to be about 1 12. This indicates that the ratio of CO2 produced by the oxidation of organic carbon to that derived from the dissolution of CaC03 is about 10 1. 

A difficulty in applying a simple one-dimensional vertical explanation for the sequence of redox reactions, and for changes in the depth of the oxic-suboxic interface, is that horizontal transport appears to play a dominant role. Diffusion and mixing are much more rapid along, rather than across, equal density or isopycnal surfaces. Reactions occurring at the side boundaries of the Black Sea may have a strong influence on the distribution of properties observed in the interior (e.g., 40). In addition, the chemocline of the Black Sea appears to be subject to rapid lateral ventilation (15, 42) with waters of different histories and pathways. Both dissolved and particulate components are affected in this way (40). Unfortunately, data are not yet available for developing one- or two-dimensional horizontal and vertical interpretations. 

The vertical distributions of salinity and temperature along the longitudinal axis of the western basin, measured in September, 2007, simultaneously with the observation of the seiche discussed above, are shown in Fig. 14. We note the wave activity in the pycnocline. The characteristic wave length is about 20 km, and the vertical displacements of the isopycnal surfaces are up to 4—5 m. 

Redfield ratio based on chemical data from isopycnal surfaces. J. Geophys. Res., 90, 6907-6924. 

Figure 1 Mean vertical profiles from NATRE at 0, 5, 6, 12 and 30 months after the initial survey. The SF concentration has been averaged on isopycnal surfaces, approximately, and plotted versus height above the target isopycnal surface using the mean relation between depth and density for the 12-month survey. The profiles are normalized to have equal areas. The initial profile. ... ) is allowed to run off the graph so that the others are clear.

Chu PC, Li R (2000) South China Sea isopycnal-surface circulation. J Phys Oceanogr 30(9) 2419-2438... 

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