Bar0tr0pic And Baroclinic Conditions

In the idealized situation discussed in the previous Section, a sea-surface slope had been set up across an ocean consisting of seawater of constant density. Consequently, the resulting horizontal pressure gradient was simply a function of the degree of slope of the surface (tan 9) - the greater the sea-surface slope, the greater the horizontal pressure gradient. In such a situation, the surfaces of equal pressure within the ocean - the isobaric surfaces - are parallel to the sea-surface, itself the topmost isobaric surface since the pressure acting all over it is that of the atmosphere.

In real situations where ocean waters are well-mixed and therefore fairly homogeneous, density nevertheless increases with depth because of compression caused by the weight of overlying water. In these circumstances, the isobaric surfaces are parallel not only to the sea-surface but also to the surfaces of constant density or isopycnic surfaces. Such conditions are described as barotropic (see Figure 3.11(a)).

BAR0TR0PIC CONDITIONS BAROCLINIC CONDITIONS

sea-surface sea-surface

BAR0TR0PIC CONDITIONS BAROCLINIC CONDITIONS

sea-surface sea-surface

Figure 3.11 The relationship between isobaric and isopycnic surfaces in (a) barotropic conditions and (b) baroclinic conditions. In barotropic conditions, the density distribution (indicated by the intensity of blue shading) does not influence the shape of isobaric surfaces, which follow the slope of the sea-surface at all depths. By contrast, in baroclinic conditions, lateral variations in density do affect the shape of isobaric surfaces, so that with increasing depth they follow the sea-surface less and less.

Figure 3.11 The relationship between isobaric and isopycnic surfaces in (a) barotropic conditions and (b) baroclinic conditions. In barotropic conditions, the density distribution (indicated by the intensity of blue shading) does not influence the shape of isobaric surfaces, which follow the slope of the sea-surface at all depths. By contrast, in baroclinic conditions, lateral variations in density do affect the shape of isobaric surfaces, so that with increasing depth they follow the sea-surface less and less.

low pressure pressure gradiert!

high pressure low pressure la) CROSS-SECTION

pressure gradiert!

force high pressure pressure gradienl torce

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