5.5.1 Convection and spreading
Density of ocean water generally increases with
depth. At the surface, however, stirring by waves
and convection creates a well-mixed homogeneous
layer. Waves alone can mix the upper Ϸ50–100 m.
But convection, forced by an increase in density at
the surface via heat loss or evaporation, can
greatly increase the mixed-layer depth. During
winter, heat loss from the surface of the ocean is
high and convectively mixed surface layers are the
norm in the extratropical oceans. The deepest
(91500 m) are found in the Labrador Sea
(Fig. 5.5.1), the Greenland Sea (see Dickson et al.,
Chapter 7.3) and the Golfe du Lion in the Mediterranean Sea (see Candela, Chapter 5.7), because of
two unique features. First, they are near land
where cold air flows over the water to create the
necessary high heat loss. And second, they have
weak cyclonic circulations, which keeps the convecting water confined and relatively stationary
where the high heat loss occurs. This combination
provides the persistent heat loss from the same
body of water that is needed to force convection to
reach great depths.
An example of a layer mixed by convection is
illustrated in Figure 5.5.2 by a plot of 1.5 ( 1.5 ;
1000:potential density in kg m
93 referenced to
1500 decibars (1 decibar corresponding to about
1 m)) versus depth, on 25 February and 8 March,
in the Labrador Sea. The convective layer is the
approximately homogeneous layer next to the
surface about 750 m deep with a 1.5 of
34.652 kg m
93 on 25 February (Station 66) and
1150 m deep and 34.673 kg m
93 11 days later
(Station 118). In the potential temperature versus
salinity plots of these two stations (Fig. 5.5.3) the
homogeneous layer is identified by the concentrations of temperature and salinity values next to the
station labels. Beneath this upper layer temperature and salinity both increase well above the values observed in the mixed layer. At Station 66, for
instance, the temperature and salinity in the mixed
layer are 2.82°C and 34.806 down to Ϸ700 m but
3.02°C and 34.845 at 750 m. As deepening of the
mixed layer continues it mixes into and incorporates this warmer, saltier water, which increases
the salinity of the mixing layer. This explains why
the salinity in the mixing layer is higher at Station
118 than at Station 66 (Fig. 5.5.3). The heat added
from the warm water below also increases the
temperature of the mixing layer. In this case the
value at Station 118 is not above that at Station
66, but it is above the value it would have been
without the extra heat from below.
A common relationship studied in deep convection regions is that between heat loss from the
surface and the resulting depth of convection. A
simple way of estimating this is to integrate the
buoyancy between the existing and final conditions and convert the integrated buoyancy loss to a
heat loss. For example, in the situation shown in
Fig. 5.5.2, it is of interest to know how much heat
loss is required to increase the density of the mixed
layer to 1034.694 kg m
93 ( 1.5 :34.694) which
occurs at Ϸ2000 m. The change of slope at
this depth in the curve in Fig. 5.5.2 indicates the
greatest depth reached by convection during the
severe winters of 1993 and 1994. Starting with
5.5
Deep Convection
John Lazier, Robert Pickart and Peter Rhines
387
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
Density of ocean water generally increases with
depth. At the surface, however, stirring by waves
and convection creates a well-mixed homogeneous
layer. Waves alone can mix the upper Ϸ50–100 m.
But convection, forced by an increase in density at
the surface via heat loss or evaporation, can
greatly increase the mixed-layer depth. During
winter, heat loss from the surface of the ocean is
high and convectively mixed surface layers are the
norm in the extratropical oceans. The deepest
(91500 m) are found in the Labrador Sea
(Fig. 5.5.1), the Greenland Sea (see Dickson et al.,
Chapter 7.3) and the Golfe du Lion in the Mediterranean Sea (see Candela, Chapter 5.7), because of
two unique features. First, they are near land
where cold air flows over the water to create the
necessary high heat loss. And second, they have
weak cyclonic circulations, which keeps the convecting water confined and relatively stationary
where the high heat loss occurs. This combination
provides the persistent heat loss from the same
body of water that is needed to force convection to
reach great depths.
An example of a layer mixed by convection is
illustrated in Figure 5.5.2 by a plot of 1.5 ( 1.5 ;
1000:potential density in kg m
93 referenced to
1500 decibars (1 decibar corresponding to about
1 m)) versus depth, on 25 February and 8 March,
in the Labrador Sea. The convective layer is the
approximately homogeneous layer next to the
surface about 750 m deep with a 1.5 of
34.652 kg m
93 on 25 February (Station 66) and
1150 m deep and 34.673 kg m
93 11 days later
(Station 118). In the potential temperature versus
salinity plots of these two stations (Fig. 5.5.3) the
homogeneous layer is identified by the concentrations of temperature and salinity values next to the
station labels. Beneath this upper layer temperature and salinity both increase well above the values observed in the mixed layer. At Station 66, for
instance, the temperature and salinity in the mixed
layer are 2.82°C and 34.806 down to Ϸ700 m but
3.02°C and 34.845 at 750 m. As deepening of the
mixed layer continues it mixes into and incorporates this warmer, saltier water, which increases
the salinity of the mixing layer. This explains why
the salinity in the mixing layer is higher at Station
118 than at Station 66 (Fig. 5.5.3). The heat added
from the warm water below also increases the
temperature of the mixing layer. In this case the
value at Station 118 is not above that at Station
66, but it is above the value it would have been
without the extra heat from below.
A common relationship studied in deep convection regions is that between heat loss from the
surface and the resulting depth of convection. A
simple way of estimating this is to integrate the
buoyancy between the existing and final conditions and convert the integrated buoyancy loss to a
heat loss. For example, in the situation shown in
Fig. 5.5.2, it is of interest to know how much heat
loss is required to increase the density of the mixed
layer to 1034.694 kg m
93 ( 1.5 :34.694) which
occurs at Ϸ2000 m. The change of slope at
this depth in the curve in Fig. 5.5.2 indicates the
greatest depth reached by convection during the
severe winters of 1993 and 1994. Starting with
5.5
Deep Convection
John Lazier, Robert Pickart and Peter Rhines
387
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER