Ž .Journal of Marine Systems 21 1999 199–205

Mixing and vertical heat flux estimates in theArctic Eurasian Basin

Richard Dewey a,), Robin Muench b, John Gunn b

a UniÕersity of Victoria, Victoria BC, Canada V8W 2Y2b Earth and Space Research, 1910 FairÕiew East, Suite 102, Seattle, WA 98102, USA

Received 14 November 1997; accepted 8 December 1998

Abstract

This paper presents results of an analysis into the magnitude and distribution of vertical heat flux from the warm AtlanticWater core into the upper ocean over the southern Eurasian Basin of the Arctic Ocean as determined by Acoustic Doppler

Ž . Ž .Current Profiles ADCP and Conductivity, Temperature, and Depth CTD casts made in 1993 and 1995. Verticaldiffusivity parameterizations based on shear and buoyancy frequency squared are utilized, including one that assumes thatturbulent dissipation rates associated with internal wave variance dissipation can be estimated from vertical shear and

wbuoyancy frequency profiles Gregg, M.C., 1989. Scaling turbulent diffusion in the thermocline. J. Geophys. Res., 94, pp.Ž9686–9698; D’Asaro, E.A., Morison, J.H., 1992. Internal waves and mixing in the Arctic Ocean. Deep-Sea Res., 39 Suppl.

. x w2 , pp. S459–S484 , and another based on an inverse Richardson number parameterization Pacanowsky, R.C., Philander,S.G.H., 1981. Parameterization of vertical mixing in numerical models of tropical oceans. J. Phys. Oceanogr., 11, pp.

x1443–1451 . Due to the scarcity of measurements in the Arctic, these data provide a preliminary indication as to themagnitude and spatial distribution of vertical heat flux from the warm core, below the halocline, into the upper ocean. Thedata suggest that vertical heat flux peaks over the continental slope region of the Eurasian Basin. It is possible that thisdiffusion is associated with elevated tidal forcing over the shelf break and slope regions, although our ship board ADCPrecords are too short to resolve tidal currents directly. q 1999 Published by Elsevier Science B.V.

Keywords: Arctic Ocean; mixing; vertical heat flux

1. Introduction

The distributions of heat and salt in the upperArctic Ocean reflect a balance among lateral advec-tion via Fram and Bering straits, the Barents Sea,and peripheral shelf seas, and lateral and verticalmixing. Heat is input primarily through the warmAtlantic Water core by way of Fram Strait. Heat is

) Corresponding author. Fax: q1-250-472-4030; e-mail:rdewey@uvic.ca.

distributed from this deep core throughout the ArcticOcean by the peripheral currents and through lateraland vertical mixing. Vertical profiles of temperatureand salinity, and time series profiles of horizontalcurrents, were measured at locations from the Karaand Laptev shelf breaks well out into the centraldeep basin of the Arctic Ocean during the summers

Ž .of 1993 and 1995 Fig. 1 . The shelf regions are sitesof relatively strong tidal currents, while the sloperegions are overlain by the highest subsurface coretemperatures in the Arctic. These two factors predis-pose the region to potentially large heat fluxes out of

0924-7963r99r$ - see front matter q 1999 Published by Elsevier Science B.V.Ž .PII: S0924-7963 99 00014-7

( )R. Dewey et al.rJournal of Marine Systems 21 1999 199–205200

ŽU .Fig. 1. The Laptev Sea, slope region, and Nansen Basin, where CTD and ADCP stations were occupied in 1993 during ARK-IX andŽ .1995 during ARK-XI I .

the warm core through vertical diffusion, making itpotentially one of the more important regions of theArctic for warm core modification and erosion of theice cover through melting from below.

The data presented here were collected during twoŽcruises by the RrV Polarstern in 1993 ARK IX

. Žfrom August 6 to October 5 and 1995 ARK XI.from July 7 to September 20 to the Laptev and Kara

Ž .Seas Fig. 1 . Cold, relatively fresh water flows fromthese shelf regions into the Nansen Basin from twosources: as buoyancy driven plumes of dense winterwater, and as a permanent inflow of the Barents Sea

Ž .branch of the Atlantic Water Schauer et al., 1997 .Both interact with the warm core of the AtlanticWater, which enters the Arctic Ocean through FramStrait. As the shelf waters spill over into the NansenBasin they displace, interleave, and mix with theAtlantic Water, modifying the core properties. Therole and distribution of mixing processes is impor-tant in understanding the water mass formation andmodification processes in the Arctic, and in deter-

mining the circulation and transport of water, saltand heat.

Generally, tidal currents are weak in the ArcticŽOcean Kowalik and Proshutinsky, 1994; Polyakov,

.1997 , with basin speeds of order a few centimetresper second. Over the broad, shallow shelf seas, tidalcurrents are elevated, with peak M2 velocities of

y1 Ž .nearly 0.1 ms at the shelf breaks Polyakov, 1997 .The interaction of the tides with the shelf dissipateskinetic energy, either locally through direct turbulentmixing and boundary layer dissipation, or throughthe generation of internal tides and waves. Ulti-mately, this internal wave energy must dissipate also,through either wave–wave or wave–shear interac-tions. Elevated turbulent mixing due to tidal currentsand isopycnal displacements has been observed near

Žbathymetric features further east Padman and Dil-.lon, 1991 . Although the ship board observations

made during ARK IX and XI are limited to shortduration hydrographic station profiling, an estimateof the turbulent diffusivity can be made from esti-

( )R. Dewey et al.rJournal of Marine Systems 21 1999 199–205 201

Žmates of the vertical stratification and shear D’Asaro.and Morison, 1992 .

2. Data processing

ŽThe data consist of hydrographic Conductivity,.Temperature, and Depth CTD profiles and short

Ž .Acoustic Doppler Current Profiles ADCP time se-ries collected during periods while the RrV Po-larstern remained on-station. These time series rangefrom a few hours to 12 h in duration, while variousother sampling programs were executed. Surface tobottom CTD profiles were made using a Neil BrownMark III profiler in 1993 and a SeaBird 911 profilerin 1995. A hull mounted 150 kHz ADCP was set tosample 1 min ensembles with 8 m bins. ADCP datawere only collected while the ship was stationaryand not steaming through open water or breakingthrough ice.

CTD profiles were used to provide estimates ofthe density anomaly s , and subsequently the buoy-t

ancy frequency squared,

g Est2N sy 1Ž .r E zo

where g is the acceleration due to gravity, r is theo

background density, and ErEz represents a differen-tiation over vertical distances of approximately 10 m.A minimum threshold for N 2 exists that is associ-ated with the smallest density difference resolvableby the CTD profiler. In many of the deeper CTDcasts collected, density differences become very small

2 Ž y6 y2 .and N was set to a minimal value e.g. 10 s .N 2 is a measure of the stratification, and N repre-sents the upper limit to the frequency of oscillationof internal wave fluctuations. In the upper 300 m ofthe water column, density variations are dominatedby variations in the salinity. Shown in Fig. 2 arerepresentative hydrographic profiles showing tem-perature, salinity, and s for a station near the shelft

Ž .break ARK-XI-032, Fig. 1 and deep within theŽ .Nansen Basin ARK-XI-047, Fig. 1 .

The ADCP data were first corrected for shipmotion using the available navigation records. All ofthe ADCP records were too short to remove tidalvariations. Consequently, only two relevant quanti-

ties were calculated: vertical shear and rotary spec-tra. The vertical shear is one measure of the dynamicstability of the flow, and can be used to parameterizethe available energy for small scale mixing andturbulent dissipation. In addition, a spectrum of thevertical shear can indicate the wavenumber distribu-tion of the shear variance, an indicator of the energyand vertical scales of the internal wave field. Therotary spectra, calculated by the complex fast Fourier

Ž .transform of uq iÕ , where u and Õ are the hori-zontal components of the velocity field, provides ameasure of the energy in both the clockwise andcounter-clockwise rotating components of the veloc-

Žity field. Rotary components either clockwise or.counter-clockwise with upward phase propagation

have group velocities and energy propagation di-rected downwards, suggesting a source of internalwave generation near the ocean surface. Alternately,downward propagating phase is associated with up-ward propagating internal wave energy, suggestingnear-bottom generation. A composite spectrum of

Ž .upward downward energy propagation can be con-structed by summing frequency bands in both theclockwise and counter-clockwise rotary spectra with

Ž .downward upward propagating phase. An exampleŽ .of the depth averaged 25–250 m rotary spectra

calculated from ADCP time series are shown in Fig.3, for representative stations ARK-XI-032 over theslope and ARK-XI-47 in the central Nansen BasinŽ .Fig. 1 . The spectral energy over the resolved inter-nal wave band from continental slope regions aregenerally 3 to 10 times larger than found in deeperwater, away from topography.

To investigate the vertical mixing that might beassociated with the elevated shears in these regions,two parameterizations were explored. First was the

Ž .parameterization of Gregg 1989 for the dissipationŽrate and effective diffusivity D’Asaro and Morison,

.1992 associated with enhanced shears from internalwaves. The vertical diffusivity associated with inter-nal waves may be parameterized by,

S4y6K s5=10 2Ž .iw 4¦ ;N

4 w 2where S is calculated from the vertical shear S sŽ .2 Ž .2 xEurEz q EvrEz between 8 m bins, which is

Žcorrected for loss due to first differencing Gregg

( )R. Dewey et al.rJournal of Marine Systems 21 1999 199–205202

Ž . Ž . Ž .Fig. 2. Representative CTD and estimated diffusivity profiles, showing Temperature solid , Salinity dashed and s dotted and Kt iwŽ . Ž . Ž . Ž .solid and K dashed for stations a ARK-XI-032, over the western Laptev slope, and b ARK-XI-047, deep within the Nansen Basin.PP

. ²:and Sanford, 1988 , denotes a sufficient timeaverage, and N is the buoyancy frequency. The

Ž y6 .coefficient 5=10 assumes that the internal waveŽ .variance is consistent with a Garrett–Munk GM

Žspectrum Garrett and Munk, 1979, D’Asaro and.Morison, 1992 . Although measurements from mid-

latitudes suggest that the ocean typically has aninternal wave variance consistent with a GM spec-

Ž . Ž .Fig. 3. Upward and downward energy from depth averaged rotary spectra of ADCP time series at a ARK-XI-032 and b ARK-XI-047,wŽ y1 .2 xover the western Laptev slope and the central Nansen Basin, respectively. Also shown are the total variances in each direction cm s .

( )R. Dewey et al.rJournal of Marine Systems 21 1999 199–205 203

Žtrum, the limited data from the Arctic Levine et al.,.1985 suggests that the internal wave field is spa-

tially and temporally more variable, with wave en-ergy under the pack ice being lower than found inopen ocean regions and higher near topography wheretidal currents are elevated.

As a check against the magnitude and spatialvariation of K , the bulk diffusivity of Pacanowskyiw

Ž .and Philander 1981 was calculated from the meanshear and N profiles,

2y3 y45=10 q10 1q5RiŽ .y5K s q10 3Ž .PP 31q5RiŽ .

where RisN 2rS 2 is the gradient Richardson num-Ž .ber. This estimate of the vertical diffusivity K isPP

not necessarily associated with internal wave pro-cesses, but represents a bulk parameterization whereregions of weak stratification and strong shear aremore likely to correspond with regions of high verti-cal diffusivity. This diffusivity is more commonlyused in baroclinic circulation models, where themean shear and stratification are the only knownparameters.

Both K and K are shown in Fig. 2 foriw PP

hydrographic stations ARK-XI-032 and ARK-XI-047. It is interesting to note both the similarities anddifferences between the structure of the two diffusiv-

Ž .ities. The parameterization of Gregg 1989 is typi-cally lower in the upper 100 m of the water column,but becomes larger and highly variable towards thebottom of the profiles, where N 2 is very small. Theparameterization of Pacanowsky and PhilanderŽ .1981 reveals a similar trend of larger diffusivitieswith decreasing N 2; however, the magnitude of K PP

Ž . 2is constrained by 3 for very low N . At mid-depthsŽ .e.g., 150 m , the two parameterizations often agree.Geographically, there is an order of magnitude larger

Ždepth averaged diffusivity for both parameteriza-.tions over the slope regions than in the deep central

basin, consistent with the elevated spectral energiesŽ .Fig. 3 .

3. Vertical heat flux

Using estimates of the vertical diffusivity and theobserved temperature gradients, estimates of the ver-

tical heat fluxes between 100 and 200 m depth wereŽ .made at each station Fig. 4 ,

E TH srC K 4Ž .f p E z

where C is the heat capacity of seawater and K thep

vertical diffusivity. In order to calculate the averagevertical heat flux between 100 and 200 m, the ADCPrecords need to be of sufficient length and quality to

Žallow resolution of the internal wave band several.hours , and be uncontaminated by navigation errors

and back-scatter interference, e.g., from apparatusbeing lowered on the hydrographic wire. Addition-ally, the station water depth must exceed 200 m. Dueto the increasingly noisy K values near the loweriw

portion of the diffusivity profiles, heat fluxes calcu-lated using K were often extremely large, andiw

sensitive to the choice of the averaging depth anddata processing. Due to the similarities between K iw

Ž 2and K over most of the water column where NPP2 .is greater than the expected noise level in N , heat

fluxes calculated using the more stable K diffusiv-PP

ities are plotted. Heat flux estimates were made for53 of the 78 stations occupied in 1995, and onlyeight of the 74 stations occupied in 1993.

The warm core of the Atlantic Water typically liesbelow 200 m, with the strongest temperature gradi-ents between 100 and 200 m depth. In Fig. 4, wehave plotted the average vertical heat flux over thisdepth interval for each hydrographic station for whichthe above quantities could be readily calculated. Wehave used the diffusivity K for the heat fluxPP

estimates since it is more uniform and where possi-ble, the rotary spectra suggest that the internal waveenergy is below the GM level. The use of K as thePP

diffusivity may underestimate the heat flux whencompared to that predicted by Gregg’s internal waveparameterization. The spatial distribution of heat fluxclearly reveals enhanced vertical heat flux over theslope regions between the Kara and Laptev Seas andthe Central Arctic Ocean. We believe that this is dueto elevated tidal and internal wave shears. Awayfrom the shelf and slope regions, the vertical heatflux estimates are typically of the order 15 to 30 Wmy2 . Over the slope regions west of the Laptev SeaŽ .Fig. 4 , vertical heat fluxes above the warm AtlanticWater core rise to values in the 150 to 160 W my2

range. Consequently, the slope regions of the Eastern

( )R. Dewey et al.rJournal of Marine Systems 21 1999 199–205204

Fig. 4. Estimates of the average vertical heat flux between 100 and 200 m depth using the vertical diffusivity K and the observedPP

temperature profile. Larger circles indicate higher heat flux, and circles with a ‘q’ are data collected in 1993, while those with an ‘x’ arefrom data collected in 1995.

Arctic Ocean and the tidal forcing of the internalwave and shear fields may play a significant role inthe redistribution of heat and the modification ofwater masses in the Arctic.

4. Conclusions

CTD and ADCP data collected during multiplecruises to the Arctic Ocean have been used to esti-mate the variation of internal wave energy, andconsequently, the potential vertical diffusivity andheat flux, over the continental slope of the LaptevSea and within the Nansen Basin. Internal waveenergy is significantly elevated over the shelf andslope regions as compared to the deeper regions ofthe Arctic basins. The most likely source for theenergy is stronger baroclinic tides near the shelf and

Ž .slope regions of the Arctic Ocean Polyakov, 1997 .The dissipation of tidal energy both locally throughturbulent dissipation and remotely through the gener-ation of internal tides and waves may have a domi-

nant role in determining the modification of thevarious water masses that meet and inter-leave inthis region.

Acknowledgements

This work has been supported in part by the U.S.Office of Naval Research through grants to Earth

Ž .and Space Research RDM, RKD, and JTG and theŽ .University of Victoria RKD . The data were col-

lected from the icebreaking research vessel Po-larstern, and the support in this endeavour of theAlfred-Wegener-Institute of Bremerhaven, Germany,is gratefully acknowledged.

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