Geoscience Reference
In-Depth Information
pressure was involved in these arguments, the resulting instability arises from the
dependenceof
β
T
ontemperature(Fig.2.9a)andbyconventioniscalled
cabbeling
.
Figure 2.11b is like Fig. 2.11a, except that here the isopyncals are drawn for
density evaluated at the pressure (depth) of the interface between the two layers,
about 9.3bar. At the higher pressure the slope of the isopycnals in
T/S
space is
less than for surface pressure, which means that for a fixed salinity, the change in
density associated with a given change in temperature is greater at depth. In this
case, the upper layer only needs a salinity increase corresponding to about 10cm
of ice growth to reach the same
in situ
density as the lower layer. Thus instabil-
ity will be triggered
before
the potential density of the upper layer reaches that of
the lower layer. The term coined by McDougall (1987) for this pressure effect is
thermobaricity
.
A method for illustrating thermobaricitypresented by Akitomo (1999)provides
additional insight into the nonlinear equation of state issues, and is easily applied
to the idealized two-layer system. Suppose that enough ice grows to increase the
salinity of the upper layer by
δ
S
=
0
.
027psu so that the
in situ
density of the two
layersisthesame attheinterface,i.e.,
ρ
(
T
u
,
S
u
,
p
93
)=
ρ
(
T
l
,
S
l
,
p
93
)
Thedifferencebetweenthedensityofthe two-layerupperoceanandanoceanwith
uniform
T
and
S
equaltotheupperlayervalues:
δρ
=
ρ
(
T
,
S
,
p
)
−
ρ
(
T
u
,
S
u
,
p
)
is plotted in Fig. 2.12. If a parcel of water from the upper layer (square marker)
is displaced downward across the interface, it will be heavier than its ambient sur-
roundingsandwillcontinuedownward.Aparceldisplacedacrosstheinterfacefrom
below (circle) will be lighter than its surroundings and will continue to rise. Con-
sequently thermobaricity is mechanism for enhanced mixing that draws from the
potentialenergyofthedestabilizingtemperaturegradient.Oncestarted,thethermo-
baricprocess is self sustaining, and is probablyan importantcomponentof mixing
in marginally stable polar oceans like much of the Weddell in late winter. As indi-
cated by Fig. 2.11, it is the curvature of the isopycnals in
T/S
space that leads to
mixing driven by nonlinearities in the equation of state. To separate cabbeling and
thermobaricityconceptuallymaybeaquestionmoreofsemanticsthanphysics,but
theimportantpointisthatwheneverthetemperatureprofileisinitselfdestabilizing,
it isimportantto considerpressureeffects.
If there are widespread regions in the Weddell where upper ocean structure is
such that only a few decimeters of ice growth could trigger deep-reachingthermo-
baric instability,whydoesan ice coverexiststhere at all? Or put anotherway, why
is the Weddell Polynya not a quasi-permanentfeature? The answer apparently lies
with what Martinson (1990) termed the “thermal barrier.” Whenever heat is mixed
up frombelow,it rapidlywarmsthe mixedlayer to the pointwhere oceanheatflux
to the ice undersurfaceexceedsconductionthroughthe ice coveror loss fromopen
water,andtheicebeginsmelting.Thisintroducespositivebuoyancythateffectively
