Pw

Fig. 5.156 Equilibrium 2: the conveyor belt: a Atlantic and b Pacific meridional streamfunction in Sv; c Atlantic and d Pacific zonally averaged salinity; e northward heat flux in PW for Atlantic (dotted), Pacific (dashed), and the two basins combined (solid) (Marotzke and Willebrand, 1991).

Accordingly, many of the state-of-art climate models predict that meridional overturning circulation in the Atlantic Basin will be substantially reduced over the next hundred years. • Due to global climate change, within the next 30-50 years the Arctic Ocean may be ice-free in summer-time. Without the ice, the large amount of relatively fresh water in the Arctic Ocean may not be held within the Arctic Basin. If some water from this freshwater pool floods the North Atlantic Ocean, it may cause a halocline catastrophe, similar to that simulated by F. Bryan and others.

Note that the results obtained from these models were based on the assumption that diapycnal mixing coefficients are invariant under different climate conditions. From the

Fig. 5.157 Left column: a sea surface temperature from EXP I; b sea surface temperature from EXP II; c the difference between the two experiments (a minus b). Right column: a sea surface salinity from EXP I; b sea surface salinity from EXP II; c the difference between EXP I and EXP II (a minus b) (Manabe and Stouffer, 1988).

Fig. 5.157 Left column: a sea surface temperature from EXP I; b sea surface temperature from EXP II; c the difference between the two experiments (a minus b). Right column: a sea surface salinity from EXP I; b sea surface salinity from EXP II; c the difference between EXP I and EXP II (a minus b) (Manabe and Stouffer, 1988).

point of view of the new energy theory, wind stress - and, to a lesser degree, tidal dissipation - can be quite different under different climate conditions. Thus, the results from such model experiments remain questionable. It is hoped that, with the rapid progress being made toward unraveling the mystery of mixing and oceanic circulation, we will be able to simulate and predict the oceanic circulation and climate more accurately in the near future.

5.4.7 Thermohaline oscillations

Thermohaline circulation encompasses a wide spectrum of different phenomena. Although each of them has its own characteristics, most of these phenomena are closely linked to the salinity balance in the system, including the connection with the hydrological cycle in the climate system or the turbulent diffusion of salty (or fresh) water in the oceanic interior. In this section, we deal with a hierarchy of saline/thermohaline oscillations.

A heat-salt oscillator

First of all, let us introduce a prototype of a thermohaline oscillator which can be illustrated using a simple box model (Welander, 1982). The model consists of a well-mixed layer, forced from above by a heat flux kt (Ta - T) and salt flux ks (Sa - S) (Fig. 5.158).

The turbulent flux between the mixed layer and the water below is parameterized in terms of a transient mixing coefficient, k = k(p) (where p is the density of the well-mixed layer), depending on the density difference (Fig. 5.159). Thus, if we set T0 = So = po = 0, the basic equations are

Fig. 5.158 Welander's box model. The values Ta and Sa are the effective temperature and salinity external forcing; To and So are the temperature and salinity of the deep reservoir; kt, ks, and k are the coefficients in assumed Newtonian-type transfer laws.

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