Geoscience Reference
In-Depth Information
AIDJEX Station Blue Fox,
δ
T = T
5
-
T
f
(S
5
)
0.2
0.1
0
150
200
250
300
350
Day of 1975
Fig. 6.1
Elevation of near surface (5m from ice) temperature in the water column above its
freezing temperature (at surface pressure) at Station Blue Fox during the 1975-1976 AIDJEX
experiment
stations,theSHEBAdatain1998,andseveralunmannedbuoydriftscoveringmost
of the Arctic Ocean. In this chapter we will explore how this storage comes about
andwhatitimpliesaboutsurvivabilityofsea ice.
6.1 Enthalpy and Salt Balance at the Interface
Thethermalbalanceattheinterfacemayberepresentedintermsofacontrolvolume
following the interface (Fig. 6.2). The interface velocity,
w
, comprises two terms:
w
0
h
istheisostaticallyadjustedbottommeltratewhere
h
isicegrowthrate;
and
w
p
is a “percolation velocity,” expressing the idea that even if there were no
bottom melting, a hydrostatic head at the surface might force meltwater through a
porousicecover,resultinginabasalvelocity.Ignoringsensibleheatassociatedwith
the percolation velocity,
1
the idealized control volume reflects primarily a balance
among (i) conduction in the ice column; (ii) turbulent heat flux from the ocean;
and (iii) latent heat release or absorptionfrom melting or freezing. Consequently a
simpleexpressionforthe“kinematic”interfaceheatbalanceis
=
−
ρ
ice
ρ
w
+
w
T
0
=
−
q
w
0
Q
L
(6.1)
where“kinematic”refersto the actualheatequationdividedbythe productof den-
sity andspecific heat
2
, with unitsKm s
−
1
. Notethatif thesum of termson
thelefthandsideisnegative(iceconductionexceedsoceanheatflux),
w
0
isnegative
indicatingfreezing.
(
ρ
w
c
p
)
1
Inasituationwhere
w
p
was important, thetemperature gradient near theinterface would bevery
small.
2
At surface pressure the specific heat of seawater is a weak function of temperature and salinity
(see, e.g., Appendix A3.4 of Gill 1982). For typical Arctic mixed layer properties, it is very close
to4
000J kg
−
1
K
−
1
.
,
