Geoscience Reference
In-Depth Information
In general, this approach to interpreting fluxes of momentum (and additionally
scalar variables) in terms of mean quantityprofilesin the surface layer hasbecome
knownasMonin-Obukhovsimilarity.
Aslongasrotationisunimportant,(4.4)providesaformulaformixinglengthin
thesurfacelayer
z
φ
m
whichcombinedwith anempiricalformulalike (4.5)showshowtheverticalextent
of the energy containing eddies depends on buoyancy. If
λ
sl
=
κ
ζ
is large and positive
(i.e.,
L
0
>
z
), mixing length is severely reduced from its neutral values.
Conversely,it increasesdramaticallyif
0;
L
0
<
is largeandnegative(statically unstable).
This observationintroducesan importantprinciple for understandingthe impactof
variousscalesonturbulence:
it is the smaller scale that governs
.
ζ
4.2 The Outer Layer
As distance from the boundary increases, rotation in the IOBL can no longer be
ignored. Figure 4.1, showing horizontal currents at several levels averaged for 5h
duringastormattheAIDJEXPilotStudystationin1972(McPheeandSmith1976),
is representative of the IOBL velocity structure often observed under sea ice. The
dashedcurveconnectsvelocitymeasurementsatvariouslevelsfrom2to32mfrom
the ice. Below 32m, angular shear was small and velocity at that level measured
relative to the drifting ice was close to the apparent bottom velocity measured by
an acoustic bottom referencesystem. Thus absolute currentat 32m was small, and
vectors drawn from that depth represent the actual velocity in the IOBL. In this
representation,thex-axisisalongthedirectionofthe2-mcurrent,whichistakento
be the direction of stress acting on the interface (opposite the surface stress acting
ontheIOBL).Thedashedvectorindicateswherethenearsurfacecurrentisaligned
45
◦
cum sole
fromthesurfacestress,takentobeapproximatelytheupperlimitofthe
Ekmanlayer.Figure4.1bpresentsthesameinformationintermsofverticalprofiles
of the velocity. Despite large current shear in the upper few meters, the integral
of velocity (i.e., volume transport) is nearly all in the positive
y
direction, at right
anglestothesurfacestress.
In the short term, velocity measurements at a particular site with (relative)
currentsemanatingfromaparticularupstreamdirectionareofteninfluencedbyun-
derice morphology (e.g., note the irregularities between 4 and 16m in Fig. 4.1).
This is especially evident in multiyear pack ice including different ice types and
thicknesses.Withalongenoughrecord,however,wecanexamineflowsfrommany
directionswith differentstress conditions.Duringthe ISPOL experimentwe moni-
toredcurrentsintheupperoceanmoreorlesscontinuouslywithanacousticDoppler
profiler(ADP)formostofthemonthofDecember.Weconsideredevery3-haverage
data set, sampled the profiles at 2-m intervals from 10 to 30m, and nondimension-
alized the complex (2-d) velocity vectors by dividing by the complex current at
30m.Results,showninFig.4.2,showareasonablywelldevelopedEkmanspiralin
