Geoscience Reference
In-Depth Information
Warm
and
salty
Cool and fresh
Figure 8.5 A region of warm salty water in a tank of
cooler, fresh water.
(a)
(b)
T 1 , S 1 , p 1
T 1 , S 1 , p 1
Warm and salty
T 2 , S 2 , p 2
Cool and fresh
T 2 , S 2 , p 2
Figure 8.6 (a) A two-layer ocean system with warm salty water
overlying cold freshwater. (b) A perturbation of the two-layer
ocean system.
stable. Note that temperature dominates in the determination of density in
this idealized problem, but this is the case in most of the ocean (high latitudes
excepted).
Next, test the stability of the two-layer system by moving a parcel of water
from the upper layer to the lower layer ( Fig. 8.6b) . The system is stable, and if
we make the adiabatic assumption, the less dense, displaced parcel will float
back to the surface. More realistically, however, the parcel will tend to lose
heat quickly through diffusion, especially if the displacement is slow or the
rebound is delayed. Salt will also diffuse out of the displaced parcel, but at a
much slower rate. Therefore, the density of the displaced parcel will increase
as it cools (due to heat diffusion). If the temperature of the parcel equilibrates
with that of the bottom layer but the parcel retains its higher salinity, it will be
denser than the fluid in the bottom layer and continue to sink. Thus, double-
diffusive convection makes this apparently stable two-layer system unstable.
Salt fingering occurs in the ocean where salty, warm waters overlie fresher,
cool waters. Subtropical regions, such as the Mediterranean Sea, are favorable
for this process since the excess of evaporation over precipitation increases the
salinity of surface waters. Parts of the Atlantic basin have warm, salty surface
waters over cooler freshwater due to the outflow of warm, salty Mediterranean
water.
Caballing is a vertical mixing process that occurs because the equation of
state for ocean water is nonlinear. Counterintuitively, mixing together two par-
cels of saltwater with the same density can result in a parcel with a density
greater than that of the original parcels.
Consider the example of a layer of water beneath an ice shelf. The water is
at 0ºC and has a salinity of 34 psu. It overlies warmer but saltier water at 10ºC
 
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