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thicknessesthereisapotentiallyinterestingwrinkle.Ifasignificantfractionoftotal
ice production is in the form of frazil crystals, growth of categories near the low
end of the thickness distributionwill be slower than otherwise, while thick ice will
accrete faster. If the growth of thin ice is retarded, its steep temperature gradient
(responsible for most of the total heat transfer) will persist for a longer time, with
the possibility of more overall heat transfer out of the ocean. Holland et al. (1997)
examinedthis in a modeling study couplingan upper ocean model to an ice model
with eight thickness categories. Using the “three-equation” parameterization sug-
gested by McPhee et al. (1987),they foundthat the equilibriumannualaverage ice
thicknessincreased by about10cm comparedwith an identicalmodelrun thatwas
thesameexceptthattheexchangecoefficientsremainedequal.Thereweresubstan-
tialdifferencesinmodeledbasalaccretion.
In the multiyear ice pack of the Arctic, observationsindicate that neither super-
coolingnorfrazilproductionisextensiveduringwinter.Byexaminingthinsections
in sea ice, it is relatively straightforward to distinguish between columnar ice ac-
cretedbycongelationwithhorizontal c -axisorientationversusthatfromfrazil,with
more random orientation. Weeks and Ackley (1986) report that frazil accounts for
only about 5% of total ice volume in Arctic pack ice and in fast sea ice from both
hemispheres.Itis foundmainlynearthesurface,producedduringinitialice forma-
tion. In the Antarctic, frazil-dominated structure is much more common, probably
as a resultof intenseair-seainteractionin the vast marginalice zonesof theSouth-
ern Ocean. Over most of the Weddell Gyre, for example, the seasonal ice remains
quitethin,oftenwitha bi-modalthicknessdistributionfromraftingbywaves.Such
conditionsareconduciveto frazilproduction.
While notcommon,supercooledwater hasbeenobservedbeneaththeArctic ice
pack.UntersteinerandSommerfeld(1964)reportedsupercoolingofapproximately
4mK (i.e.,water temperatureabout0.004K below its freezingtemperature,depen-
dentonsalinityandpressure)neariceislandARLIS2(adriftingtabularberg)from
measurements in water under the adjacent pack ice. They used a differential tem-
peraturemeasurementtechniquethatdidnotrequireaccuratesalinitydetermination,
an important consideration at the time. In that case, the supercooling was possibly
attributable to the “ice-pump” effect described by Foldvik and Kvinge (1974) and
Lewis and Perkin (1983). In typical pack ice, water in the well mixed IOBL will
contactice at varyingpressures, e.g., ridge keels at pressures up to 10dbar and be-
yond.Waterthatisatitfreezingtemperatureattheleveloftheundeformedice(say,
2dbar)wouldbeabout6mKabovefreezingat10dbar. 3 Theicepumpoccurswhen
this water melts ice at depth, thus attaining a freezing temperature associated with
thepressurewheremeltingoccurred.Asthiswaterrisesfollowingtheicemorphol-
ogy, it will be supercooled relative to its in situ pressure and will deposit ice as it
encounters nucleation sites at the ice/water interface. In this way, ice can be trans-
ported through the thickness distribution from thicker to thinner categories. The
icepumpisespeciallyeffectiveunderfloatingiceshelveswherelargebasalmelting
nearthegroundinglineis“redeposited”asseaiceathigherlevelsneartheterminus.
3 We use the UNESCOformula for the freezing point of seawater from Millero(1978) as reported
by Gill(1982), who points out that theformula fitsmeasurements toan accuracy of
±
4mK.
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