Early Spring Oceanic Heat Fluxes and Mixing Observed
from Drift Stations North of Svalbard
Anders Sirevaag1, 2, 3 and Ilker Fer1,2
1. Geophysical Institute, University of Bergen, Bergen, Norway
2. Bjerknes Centre for Climate Research, University of Bergen, Bergen, Norway
3. University Centre in Svalbard, Longyearbyen, Norway
Corresponding author:
Anders Sirevaag
Bjerknes Centre for Climate Research
Allegt. 70
5007 Bergen, Norway
[email protected]
1
Abstract
Observations were made in early spring of the ocean turbulent characteristics in the upper
150 m using a microstructure profiler and close to the under-ice surface using eddy
correlation instrumentation, from several drifting ice stations north of Svalbard. The data set
is used to obtain average heat fluxes at the ice-water interface, in the mixed layer, across the
main pycnocline as well over different water masses in the region. The results are contrasted
with proximity to the branches of the warm and saline Atlantic Water current, the West
Spitsbergen Current (WSC), which is the main oceanic heat and salinity source both to the
region and to the Arctic Ocean. Hydrographic properties show that the surface water mass
modification is typically due to atmospheric cooling with relatively less influence of ice
melting. Surface heat fluxes of O(100) Wm-2 are found within the branches of the WSC and
over shelf areas with elevated levels of mixing due to strong tides. Away from the shelves and
WSC, however, ocean-to-ice turbulent heat fluxes are typical of the central Arctic. Deeper in
the water column, entrainment from below together with equally important horizontal
advection and diffusion increase the heat content of the mixed layer and contribute to the heat
flux maximum in the upper layers. Our results emphasize the importance of mixing along the
boundaries, over shelves and topography for the cooling of the Atlantic Water layer in the
Arctic in general and for the regional heat budget, hence the ice cover and cooling of the
WSC north of Svalbard, in particular.
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1. Introduction
At high latitudes, air-sea interaction is affected by presence of sea ice which partly
insulates the upper ocean from solar radiation and wind stress. A significant source of oceanic
heat flux is thus the upward turbulent mixing and entrainment of heat from warmer layers
underlying the cold well-mixed layer below the ice (Aagaard et al. 1987). The West
Spitsbergen Current (WSC) is the northernmost extension of the Norwegian Atlantic Current
and carries warm and saline Atlantic Water (AW). North of Svalbard, where several drift
stations reported here are occupied, is the site where the WSC enters the Arctic. In this area,
the exchange of heat between the ocean and the ice/atmosphere plays an important role in
modifying the core of the AW and thickness of the ice cover (Untersteiner 1988).
Fram Strait, the passage between Greenland and Spitsbergen, is a site of dramatic
exchange of mass, heat and salt. On the eastern side, AW is carried northward by the WSC
and on the western side, the East Greenland Current brings colder and less saline water
southward. The WSC transports a major fraction of the heat and salt supplied to the Arctic
Ocean (Rudels et al. 2000; Saloranta and Haugan 2001). Primarily confined to the upper
continental slope along Svalbard, the WSC bifurcates into two branches around 79N where it
meets the Yermak Plateau (YP, Figure 1) (Bourke et al. 1988; Manley 1995). The Svalbard
branch continues eastward, inshore of the plateau and along the upper part of the slope
between the 400-500 m isobaths. The Yermak Branch flows along the offshore western flank
of the YP following the 1000-m isobath. While a fraction of the Yermak Branch rejoins the
Svalbard Branch and enters the Arctic Ocean, another fraction detaches north of 80N and
turns westwards contributing to the recirculating Atlantic Water (Bourke et al. 1988).
Roughly, 20% and 35% of the Atlantic Water in the region is carried by the Yermak and
Svalbard branches, respectively, whereas about 45% recirculates (Manley 1995). Furthermore
3
the Yermak Branch looses significant heat due to intense tidal mixing at the YP (Padman and
Dillon 1991; Padman et al. 1992).
The path of the WSC branches are areas of intense air-sea-ice interaction where the warm
core supplies heat to the mixed layer, leading to surface heat fluxes of O(100) W m-2
(Aagaard et al. 1987; Dewey et al. 1999). As the WSC progresses into the Arctic, it loses its
heat and surface fluxes are subsequently lower; 0 – 37 Wm-2 reported at 32˚E (Wettlaufer et
al. 1990). The objective of this study is to quantify the oceanic heat flux towards the ice west
and north of Spitsbergen at varying proximity to the warm AW core and over varying
topography. During several drifting experiments performed in early spring (within 1 April – 1
May) in the area 78.8˚N – 82˚N, 0˚E – 15.4˚E (Figure 1), the turbulent heat fluxes were either
measured directly in the mixed layer by an eddy correlation method, or inferred from
dissipation measurements from a microstructure profiler in the upper 150 m. The individual
drifts were located within, close to or away from the branches of the WSC and due to their
short durations provide only a snapshot of the early spring conditions in this area. The
measurements are from separate years, however at the same time of year, and span the period
from 2003 to 2007 (Table 1).
An overview of the measurement locations along with the instruments and the data
reduction details is presented in section 2. In section 3, a general description of each drift is
given together with salient observations. Oceanic turbulent fluxes are presented and discussed
in section 4, followed by a summary and concluding remarks in section 5.
2. Measurements and Methods
a. Drift Stations and Sampling
Geographically, the drift stations are located in the Fram Strait, over the eastern flank of
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the Yermak Plateau and on the shelf north of Svalbard, all occupied within the period 1 April
– 1 May of a given year. Upper ocean turbulence measurements were conducted using two
separate systems: i) direct eddy correlation measurements with a “Turbulence Instrument
Cluster” (TIC) and ii) dissipation measurements in the upper 150 m using a microstructure
profiler (MSS). The TIC is equipped with an acoustic 5 MHz Sontek/YSI Acoustic Doppler
Velocimeter, a SBE04 standard conductivity sensor, a SBE03 fast-response temperature
sensor and a SBE07 micro-conductivity sensor, all manufactured by SeaBird Electronics. All
TIC sensors sample at the same vertical level, providing three dimensional velocity,
temperature and salinity at 2 Hz, resolving turbulence well into the inertial sub-range
(McPhee 2002; Fer et al. 2004; McPhee 2008). MSS, on the other hand, is a loosely tethered
free fall profiler equipped with precision CTD sensors and a suite of turbulence sensors
including two shear-probes, a fast-response thermistor FP07 and a micro-conductivity sensor
(Fer 2006; Fer and Sundfjord 2007). All MSS profiles were collected from the research
vessels moored to the ice floe, whereas TIC deployments were made from hydroholes in the
ice, more than two ship lengths away from the vessels. The MSS profiler was deployed using
a manual winch equipped with 200 m cable in total, limiting the total profiling depth.
A location map of the drifts, together with a sketch of the WSC is shown in Figure 1. In
the Fram Strait, two drift stations were occupied in deep water in 2005 (FSD1 and FSD2) by
RV Lance. In total, 51 and 26 MSS profiles were collected for about 13 and 10 h duration of
FSD1 and FSD2, respectively. The Yermak Plateau Drift (YPD), was conducted in 2003
during the ARK XIX/1 cruise of RV Polarstern. The sampling included two TIC systems,
deployed 4 m apart vertically, at 1 m and 5 m below the ice, over a duration of 6 days. On the
shelf north of Svalbard, the Whaler’s Bay Drift (WBD) in 2003 (ARK XIX/1) was on the
Svalbard Branch of the WSC, whereas the two drifts on the Norwegian Bank (NBD1 in 2005
5
and NBD2 in 2007) were over shallow and colder water, both occupied by RV Lance.
Sampling consisted of TIC measurements at 1 and 5 m at WBD, at 1 m at NBD2 and MSS
profiles at NBD1. A summary of the sampling at each drift is tabulated in Table 1.
Ancillary data include atmospheric measurements and navigation data from the vessels. In
2003 and 2007, TIC measurements were supplemented by ship-board CTD profiles. As a part
of field work exercise, University Centre in Svalbard (UNIS) students conducted various
sampling during the RV Lance 2005 cruise. Here we use data from ice cores and ship-ADCP,
collected at FSD1 and FSD2 (Student report AGF-211, University Center in Svalbard, 2005)
b. Data Reduction
TIC – Time series are segmented into 15 min intervals and velocity data are rotated into a
streamline coordinate system. The chosen averaging period of 15 minutes for the turbulence
statistics preserves the covariance in the turbulent eddies (typically of several minutes in the
under-ice layer) while excluding contamination from variability at longer time scales. In order
to calculate turbulent fluxes, deviatory velocity  u , v, w  , temperature T  and salinity S  are
obtained by removing the linear trend fitted to the actual time series at each 15-minute
interval. Fluxes are obtained by zero-lag covariances assuming eddies advected past the
sensors over the averaging time are representative of the ensemble of instantaneous turbulent
fields (Taylor’s frozen turbulence hypothesis). Turbulent heat flux is calculated as
FH   CP wT  , in units of W m-2, where  is the seawater density and CP is the specific
heat of seawater. In the following, deviatory turbulent quantities are denoted by primes and
angle brackets denote averaging in time or space. The Reynolds stress per unit mass is

  u v  i vw , expressed in complex notation where i = (-1)1/2, and the local friction
 1/2
speed is u   . The term “local” indicates fluxes obtained at the measurement depth and
6
must be distinguished from the theoretically calculated ice/ocean interface fluxes which are
denoted with subscript 0.
MSS – Microstructure profile data are processed as described elsewhere (Fer 2006;
Sundfjord et al. 2007). Initial 1024 Hz sampling is averaged over 4 data points (to 256 Hz) to
reduce noise. The dissipation rate of turbulent kinetic energy per unit mass, , in units W kg-1,
is calculated using the isotropic relation   7.5 u z2 , where  is the viscosity of seawater,
uz2 is the shear variance of the horizontal small-scale velocity, and angle brackets denote
appropriate averaging. The instrument fall speed (typically  0.7 m s-1) is used to convert
from frequency domain to vertical wavenumber domain using Taylor’s hypothesis, and the
shear variance is obtained by iteratively integrating the reliably resolved portion of the shear
wavenumber spectrum of half overlapping 1-s segments. Narrowband noise peaks induced by
the probe guard cage are above the wavenumber range chosen for the analysis. Dissipation
data in the upper 8 m are unreliable owing to disturbances from the ship and the initial
adjustment to free fall. The profiles of precision CTD and ε are produced as 10-cm and 50-cm
vertical averages, respectively.
The diapycnal eddy diffusivity of mass is calculated assuming a balance between the
production of TKE, its dissipation and the buoyancy flux as K    N 2 (Osborn 1980),
using the flux coefficient (the ratio of work done against gravity to energy dissipated by
friction)  = 0.2. The choice of  is a major uncertainty, however follows the common
practice and allows for direct comparison with other studies. Both observational evidence
(Moum 1996) and direct numerical simulation results (Smyth et al. 2001) show significant
variability. According to a parameterization that accounts for the evolution over the lifetime
of an overturn (Smyth et al. 2001), to employ  = 0.2 will underestimate the median K 
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by a factor of two, whereas according to the studies by St. Laurent and Schmitt (1999) and
Arneborg (2002) it will overestimate K  by about a factor of two in shear-induced and
patchy turbulence. The buoyancy frequency, N    g /    / z   , is calculated using
1/ 2
Thorpe-ordered  profiles (Thorpe 1977), derived from the precision sensors. The density
gradient is obtained as the slope of the linear regression of depth on , over 4-m length
moving segments (i.e., typically over 40 data points). The background temperature and
salinity gradients are derived similarly from the precision temperature and salinity records as
the slope of the linear regression of depth on T and S. K  inferred from the Osborn model is
ill defined in the absence of stratification. We therefore excluded the segments where the
temperature gradient or the density gradient was not significantly different from zero
(identified as twice the uncertainty of the slope inferred from the regression described above).
An independent estimate of eddy diffusivity for heat is obtained using KT  3kT Cx
(Osborn and Cox 1972), using the temperature microstructure sampled by the pre-emphasized
2
FP07 signal. Here, Cx  (T '/ z ) 2 / T / z is the Cox number, kT = 1.410-7 m2 s-1 is the
molecular diffusivity for heat, (T '/ z ) 2
is the small scale temperature gradient variance
and T / z is the background temperature gradient. We estimate the dissipation rate of
thermal variance,   2kT 3(T '/ z ) 2
by fitting the Batchelor’s form (Dillon and Caldwell
1980), constrained by the measured , to the resolved wavenumber band of the temperature
gradient spectrum over 4 m length segments. The vertical wavenumber spectrum of the
temperature gradient T  / z , is obtained as STz (m)  WSTt  f  , where STt ( f )   2 ST ( f ) ,
  2f , W is the fall rate of the profiler, m   / W is the cyclic wavenumber, and the
temperature spectrum ST is obtained using the transfer function for the ideal
8
amplification of the circuit. The resolved wavenumber band corresponds to 1-30 Hz (1.4 – 43
cycles per meter, cpm, using the nominal fall rate of 0.7 m s-1).
CTD profiles, either from the microstructure profiler with 0.5 m vertical resolution or
from the ship-CTD system with 1.0 m vertical resolution, are used to determine the mixed
layer depth (Dml) and the pycnocline depth (Dp). Dml is determined using the split-and-merge
algorithm of Thomson and Fine (2003), where linear fits are made to individual segments of
the density profile and the range of the uppermost, vertical fit defines the extent of the mixed
layer. Dp is determined as the depth of maximum N 2 using 5-m smoothed buoyancy
frequency profiles.
c. Ice Drift Velocity and Under-ice Surface Friction Speed
Ice drift velocity is derived from the 5-min averages of navigation data collected at high
temporal sampling (10 s in RV Lance, 60 s in RV Polarstern). For each drift station latitude–
longitude pairs were converted to a polar stereographic grid centered at the mean position of
the drift, with positive y-axis aligned with north. The ice drift velocity vector is obtained as
the first difference of drift positions.
At the TIC stations, direct measurements of friction velocity, u  , in the boundary layer are
available at 1 m below the ice. However, u  can be different from the under-ice surface
(ice/ocean interface) friction velocity u0 , and comparable measurements are not available for
MSS drifts. To be consistent between all drifts, we estimate the surface friction velocity, u0
from a Rossby similarity drag law (McPhee et al. 1999),
Ui 1
 (log( Ro )  A  iB)
u*0 k
(1)
9
where Ui is the ice velocity relative to the upper ocean velocity, k = 0.4 is the Von Kármán
constant, Ro*  u0 / fz0 and A and B are similarity constants. For the TIC stations, Ui is
found from the measured velocity at 1 m, whereas for MSS stations FSD1 and FSD2 Ui is
found by subtracting the ADCP velocity at 25 m from the ice drift velocity. At NBD1, no
current measurements are available and u0 is calculated from ice drift velocity alone. For all
stations, the surface roughness is set to z0 = 2.010-3 m which is obtained from the WBD data
using the logarithmic law-of-the-wall combined with velocity and friction velocity
measurements at 5 m below the ice. The measurements from the deeper TIC are more
representative for the overall ice floe and ice in WBD, because the instruments were deployed
in an area of relatively smooth first year ice within the surrounding, more heavily ridged multi
year ice and the measurements at 1 m from the interface may only reflect the local ice
topography. The average value of z0 = 2.010-3 m is slightly smaller than that of the multiyear
floe during the SHEBA campaign (McPhee 2002).
3. General Description of Drifts
a. Hydrography and Water Masses
The sampling is limited to the upper 150 m and deep water masses are not observed. We
therefore adopt a relevant subset of the water mass categories defined in Aagaard et al.
(1985), summarized in Table 2. Representative temperature and salinity profiles and T-S
diagrams for each drift are shown in Figure 2. FSD and NBD1 profiles are time averaged
MSS profiles over the duration of the drift, whereas YPD and WBD profiles are single casts
of ship-CTD. No prominent cold halocline water is observed and the mixed layer depth varied
by one order of magnitude between very shallow, 6 m at FSD2, to about 70 m at YPD. The
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warm core of the WSC is composed of warm and saline AW. The Lower Arctic Intermediate
Water (LAIW) is of similar salinity but colder, and at this location, can be a result of cooling
AW by heat loss to the atmosphere. Arctic Surface Water (ASW), on the other hand, is both
colder and fresher and can be a result of cooling of AW by a combination of heat loss to the
atmosphere and to ice melting which accounts for the freshening (Boyd and D'Asaro 1994).
On the average, profiles from FSD1 and FSD2 show a very small fraction of LAIW
(owing to the shallow sampling) and we include LAIW in the definition of AW when
averaging properties over water masses (section 4.d).
b. Fram Strait Drifts
Fram Strait Drift I - FSD1 was established west of the WSC on the morning of 28 April
2005 (day 118). Ice thickness was about 120 cm over a water depth of about 2500 m near
79N, close to the prime meridian. The conductive heat flux in the ice is calculated as
FHi   ki  T / z i , where ki is the thermal conductivity of sea ice and
 T / z i is
the
temperature gradient in the ice. Temperature gradient measured from an ice core yielded FHi =
10 W m-2 for FSD1. Microstructure profiling started at 1254 UTC, and 54 casts were made at
a typical sampling interval of 10-15 min.
The patch of open water on the side of the vessel where the profiling was made froze
throughout the drift. Occasionally, profiling was interrupted due to ridging and rafting, and
after about 14 hours the sampling was terminated as the slack tether was unable to penetrate
the young thin ice, making it impossible to conduct free-fall profiling. A malfunction on the
automated logging of the ship meteorological and navigation data went unnoticed throughout
the drift; however, positions at the start of each cast and occasional descriptive meteorological
information were hand-logged by the authors. Air temperature was around -13C throughout
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the drift. The first couple of hours into the profiling the wind was calm with a speed of 2 m s1
, decreased practically to nil, and followed by a light breeze of about 3 m s-1 from day 118.0.
Despite the calm wind, the station drifted towards northwest at an average speed of 0.24 m s-1
gradually over warmer water (Figure 3a).
Mean profiles of hydrography, dissipation rate of TKE and diapycnal eddy diffusivity,
K  , averaged over all casts, are shown in Figure 4. On the average, the mixed layer was 21 m
deep, with temperature 0.19 K above the freezing point. Dissipation rate, in excess of 310-7
W kg-1 in the mixed layer, reduced to the noise level (210-8 W kg-1) below about 60 m depth.
Corresponding eddy diffusivity K  inferred from the Osborn model decreases from O(10-2)
m2 s-1 at the base of the mixed layer to O(10-4) m2 s-1 below the pycnocline. Time averaged
velocity profile derived from the ship ADCP showed that the speed was about 15 cm s-1 down
to 40 m, gradually decreased to about 8 cm s-1 and stayed nearly constant below 70 m (Figure
5a). The angular shear (change of current direction with depth) was significant in the upper 70
m, and despite the strong stratification, 4-m gradient Richardson number, Ri = N2/Sh2 where
Sh2 = (u/z)2 + (v/z)2 is the shear-squared, was often below unity, suggesting that the
enhanced  between 40-70 m (Figure 4c) can be due to local shear instabilities.
Fram Strait Drift II – FSD2 was established on the morning of 30 April 2005 (day 120),
on a relatively thin floe of about 30 cm. In contrast to the freezing conditions at FSD1,
repeated ice core measurements and mixed layer temperatures indicated that the ice at FSD2
was melting rapidly. Melting rate could not be determined by confidence due to the thin ice
floe and scarcity of ice cores. Conductive heat flux, averaged over two ice cores, was 15 W m2
. Air temperature gradually warmed from -12C to -3C throughout the drift when wind
averaged 6.4 m s-1 (Figure 6a-b). The mixed layer was shallow and the upper layer
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temperatures gradually increased. Time averaged velocity profile derived from ship ADCP
was nearly unidirectional at 240 in the upper 25-170 m with the speed between 35 - 40 cm s1
. Dissipation rate and eddy diffusivity were significantly larger when compared to FSD1,
however, similarly reached the noise level below about 60 m (Figure 7). Compared to FSD1,
the stratification was weaker, nevertheless, owing to the lack of shear, 4-m Ri was one order
of magnitude larger (Figure 5). Relatively enhanced mean dissipation at around 50 m was
largely as a result of the event that occurred towards the end of the drift, following the
pinching of the isotherms and isohalines (Figure 6c-d).
Using a free-drift model (McPhee 1986; see also Boyd and D'Asaro 1994) and
representative values of the air-ice (Ca) and ice-water (Cw) drag coefficients, relative ice speed
will be about 2.5% of the wind speed, U i  U a  a Ca /   wCw  . Ua is the wind velocity and ρa
and ρw are the air and water densities, respectively. The actual ice motion can be obtained by
adding the average upper ocean velocity. In FSD2, the residual between average ice velocity
inferred from the navigation (absolute ice velocity) less the 2.5% of the mean wind speed,
yields 0.35 m s-1 average upper ocean velocity which is consistent with ship-ADCP
measurements. Also, applying the same model and equating the ice-water stress and the airice stress, calculated from absolute ice and wind velocities, yields a friction velocity at the
ice/ocean interface that corresponds well in magnitude with that derived from the Rossby
similarity drag law (Eq. 1).
c. Yermak Plateau Drift
Yermak Plateau Drift - YPD, established on day 100 in 2003 had the largest duration with
6 days of sampling. Camp was established on a large floe of 2-3 m thick, multi-year ice,
however TIC instrumentation was deployed through thinner ice (~0.5 m) at the edge of the
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larger floe. The upper 70 m was well mixed in temperature and salinity (Figure 2, green
curve). The mixed layer temperature was the coldest and the temperature maximum below the
thermocline was the lowest of all stations. The floe drifted at a slow pace of 7 cm s-1,
comparable to NBD1 but about 1/4 - 1/3 the speed of FSD1 and NBD2. The location is
slightly southeast of the site where upward turbulent heat fluxes reaching 30 W m-2, due to
energetic mixing events as a result of proximity to internal wave sources, were reported by
Padman and Dillon (1991). It is also the same area where an automated buoy measured
surface heat fluxes of 22 Wm-2 over the elevated topography of Yermak Plateu, 50 days prior
to our deployment (McPhee et al. 2003)
d. Drifts on the North Spitsbergen Shelf
Whaler’s Bay Drift - WBD, north of Svalbard, was located where the continental slope
branch of warm and relatively salty AW transported by the West Spitsbergen Current enters
the Arctic. WBD was established on 1 April 2003 on an area of 1.1 m thick first year ice
within about twice as thick and more heavily ridged multiyear ice. An ice core was extracted
and ice temperatures measured at 10 cm vertical resolution yielded a temperature gradient
corresponding to an upward conductive heat flux of about 41 W m-2. The relatively high
temperature gradient reflects the low air temperature which averaged to -29.1˚C over the drift,
while the average wind was northeasterly, 9 m s-1. Water depth was about 180 m with a mixed
layer depth of 19 m with temperature 0.93 K above freezing.
Norwegian Bank Drift I – NBD1, established 1 May 2005, was closer to land and drifted
across a thermohaline front that separated the well mixed shallows of Polar Water
characteristics from deep water of Arctic Surface Water properties. Air temperature during
NBD1 was relatively warm, varying between seawater temperatures and zero. Drift started
over about 90 m deep water and crossed the thermohaline front half way into the drift. The
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shallow side of the front was characterized by unusually high dissipation rates (Figure 8),
enhanced both in the upper layer and the bottom layer suggesting strong tidal currents (section
3.e) generating turbulence in the boundary layer near the seafloor and under the ice. The T-S
profiles and stratification averaged on either side of the front are contrasted in Figure 9.
Shallows are nearly homogeneous, well-mixed. Dissipation rate is at least one order of
magnitude larger on the shallow side of the front (Figure 9c) and decreases with distance from
the boundaries towards the mid-water. Throughout NBD1, measured dissipation rates are well
above the noise level.
Norwegian Bank Drift II – NBD2, was established on 24 April 2007 in the same area as
NBD1. Compared to NBD1, the NBD2 drift was five times as fast and over a mixed layer that
was colder and twice as deep (Table 1). The hydrography from the CTD casts shows a tongue
of AW below about 60 m and a relatively strong salinity gradient at the front (Figure 10).
Average air temperature was -13˚C and an average northwesterly wind of 9 ms-1 accompanied
the drift across the shallow topography of the Norwegian Bank.
e. Tides
Tidal currents can be strong and influence the oceanic turbulent fluxes at the Yermak
Plateau (Padman and Dillon 1991; Padman et al. 1992) and over the shelves north of Svalbard
(Sundfjord et al. 2007). We estimate and contrast the tidal current ellipse parameters for each
drift using the 5 km resolution Arctic Ocean Tidal Inverse Model (AOTIM) (Padman and
Erofeeva 2004). Results predicted for the start location of each drift for the two main
constituents M2 and K1 are summarized in Table 3. Because NBD1 extends into significantly
shallow water, ellipse parameters for the end location of NBD1 are also calculated. Generally
model water depths interpolated to the drift positions agree with the actual water depth. At the
shallow end of NBD1, however, the model depth is 58 m, about double the actual depth and
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the modelled tidal currents shown in Table 3, which scale inversely by the water depth, may
therefore underestimate the actual current by a factor of 2. Except for FSD2, the tidal ellipses
have clockwise rotation at all drifts for both the diurnal and semi-diurnal constituents
(negative semi-minor axis length). Typically the semi-diurnal tide dominates, except from at
YPD where the dominant constituent is K1 and at FSD2 where both constituents are of
comparable magnitude. Tidal currents at WBD are significant whereas those towards the end
of NBD1 are the largest and will lead to significant stirring both in the under-ice boundary
layer and the bottom boundary layer near the sea bed.
4. Turbulent Fluxes
a. Heat flux at the Under-ice Surface
The under-ice surface (ice-sea water interface) heat flux can be estimated using the
parameterization
FH 0   C p cH u0 T
(2)
where c H is the turbulent exchange coefficient (Stanton number), u0 is the surface friction
velocity and T is the mixed layer temperature above freezing. The exchange coefficient has
been shown to be nearly constant, c H  0.0057 , for the under-ice boundary layer (McPhee et
al. 2003). Although the local turbulent heat flux in the under-ice boundary layer can be
directly calculated from the TIC data at 1 m below the ice (section 2.b), the actual surface flux
can be different. The surface friction velocity can be estimated using the Rossby similarity
(Eq. 1) or it can be approximated using the local u measured close to the ice undersurface
(typically 1 m) when available. To be consistent, we calculated u0 via the Rossby similarity
for all drifts. When compared to 1 m TIC measurements, u0 is 0.99, 0.30 and 1.11 times the
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local u for WBD, YPD and NBD2, respectively. The under-ice surface heat flux results are
compared for all drifts in Table 4. Also tabulated are the measured heat fluxes at 1 m below
the ice for the TIC drifts and mixed layer averages for the MSS drifts. The largest interfacial
heat flux occurred at FSD2, consistent with the observations of rapid ice melting throughout
the drift. Significantly large upward FH0 was also observed at WBD on the Svalbard branch of
the WSC. Although the drift speed during NBD2 was five times that during NBD1, u  was
only 1.4 times as large, and the mixed layer temperature above freezing in NBD2 was 0.4
times that during NBD1. Consequently, according to Eq. (1), the interfacial heat flux in
NBD2 is only 60% of that in NDB1 (Table 4). For NBD1, FH0 is calculated assuming
stagnant upper ocean, which, according to the tidal estimates in section 3.e, is probably not
the case.
For the drifts where the conductive heat flux was calculated using the measured gradient
in the ice (FSD1, FSD2 and WBD), an estimate of melting rate is made by assuming that any
discrepancy between ocean heat flux and conductive heat flux at the ice/ocean interface is
balanced by melting or freezing (e.g., Steele and Morison 1993). From this method, melting
rates of 0.3, 9 and 5 cm day-1 are estimated for FSD1, FSD2 and WBD, respectively.
Although freezing was observed in open water areas during FSD1, the ice in general was
slowly melting as the surface heat flux was larger than the conductive heat flux in the ice.
Accordingly, the salt fluxes in the mixed layer were positive (but small) which corresponds to
an input of melt water at the surface (Table 5). For FSD2, the inferred melting rate of 9 cm
day-1 is comparable to the observed rapid melting. The upward (positive) salt fluxes in the
mixed layer were significantly larger than during FSD1, which qualitatively supports
significant melting. For WBD, the melting rates are in line with the observed salt fluxes
reported in Sirevaag (2009).
17
b. Heat and Salt Flux in the Mixed Layer
The local turbulent heat flux in the under-ice boundary layer is directly calculated from
the TIC data as FH   CP wT  (section 2.b). At TIC stations, values from the cluster at 1 m
are averaged for WBD and NBD2, and those from both levels (1 and 5 m) are averaged for
YPD to obtain representative heat flux values in the mixed layer. Although WBD had another
cluster at 5 m below the ice, the mean heat flux was more than double that measured at 1 m
(620 W m-2 and 259 W m-2, respectively), with significantly large differences after day 92.1
(Figure 12d) . Large values at 5 m at WBD are likely the consequence of pressure ridge keels
and might be representative of variability in the mixed layer for irregular under-ice
topography. For MSS stations, turbulent heat and salt fluxes ( 10-3  salinity flux) are
obtained using the eddy diffusivity for heat from the Osborn-Cox model and the property
gradients (section 2.b), both evaluated at 4-m vertical segments:
FH    CP KT
FS  

1000
KS
dT
dz
dS
dz
[W m -2 ]
(3)
-1
-2
[kg s m ]
In the calculations we assume turbulent flow leading to eddy diffusivities KT  KS.
Mixed layer fluxes are obtained by averaging the values for z < Dml for each cast. MSSderived eddy diffusivity is available below 8 m depth, i.e., center of the first 4-m long
segment used in Eq. (3) is at 10 m, hence we only retain values at casts where Dml > 10 m.
TIC measurements, on the other hand, are either within the upper 5 m or 1 m below the ice.
Therefore, the values inferred from MSS / TIC are biased towards the lower / upper part of
the mixed layer.
18
Summary of the mixed layer turbulent heat flux as measured by the TIC is given in Figure
11- Figure 13 for YPD, WBD and NBD2. At YPD, the mixed layer temperature had a
pronounced diurnal signal (Figure 11a), consistent with the strong diurnal tides from
AOTIM5 (Table 3) and previous observations of strong diurnal tides in the region (Padman et
al. 1992). Friction velocity decreased from 1 m to 5 m below the ice, suggesting a mechanical
source for TKE generation at the interface. The average mixed layer heat flux of 2 W m-2, was
comparable to the typical Arctic values and compared favorably with the heat flux at the icewater interface of FH0 = 3 W m-2 obtained from the parameterization in Eq. (2). At the WBD,
the variation in the measured FH was large, both in time and between the two measurement
levels. The average value at 1 m of 259 W m-2 was larger than, but comparable, to the
interface value of 203 W m-2 (Table 4). The friction velocity was slightly larger at 5 m,
particularly in the late part of the drift, suggesting TKE contribution from distant sources. At
NBD2, TIC measured through a gradually less saline mixed layer and the front seen between
the first two CTD stations in Figure 10, was absent in the TIC data (Figure 13a). Heat flux in
the mixed layer increased from 30 W m-2 to over 80 W m-2 as the flow drifted over warmer
water, as much as 0.45 K above freezing.
The mixed layer was only occasionally deeper than 10 m at FSD2 and NBD1, hence flux
estimates in the mixed layer using Eq. (3) with profiler data from these sites are scarce, and
may not be representative of the drift average. The flux estimate from FSD1, on the other
hand, is well-resolved. Despite a mean upward heat flux of 28 W m-2, the open water at the
surface was freezing as a result of low air temperatures increasing the ocean/air temperature
difference and leading to heat loss to the atmosphere that is greater than the oceanic heat flux.
Larger fluxes of heat at the surface compared to fluxes across the pycnocline should
reduce the total amount of heat in the mixed layer. Heat content in the mixed layer,
19
H ml , is calculated from the temperature above freezing and the mixed layer depth for each
MSS profile and the rate of change of the heat content dH ml / dt is found as the slope of a line
fitted to the evolution of H ml in time. For FSD1 and FSD2, heat content in the mixed layer is
reduced at rates equivalent to heat fluxes of -387 and -496 W m-2, respectively, where the
negative fluxes mean that heat is lost from the mixed layer. As in Steele and Morison (1993),
we can estimate the contribution to changes in mixed layer heat content from horizontal
advection and diffusion relative to the drift as the residual A  d Hml dt  FH 0  FHpyc , where
FHpyc is the heat flux at the base of the mixed layer. Average values of A for FSD1 and FSD2
are -374 and -282 W m-2, respectively. Values for FSD1 are biased as the ice drifted past a
weak thermohaline front in the mixed layer and into gradually colder (and fresher) water until
day ~118.7. The contribution from horizontal advection and diffusion was below -2 kW m-2 in
this period, whereas A = 27 Wm-2 for the period after 118.7. During the first period, the
horizontal advection and diffusion is far more important than surface fluxes in determining
the mixed layer heat content. Significant contribution from horizontal advection and diffusion
was also inferred by Steele and Morison (1993). For the latter period A is of the same order as
the surface fluxes. In the case of FSD2, both the surface fluxes and the advective fluxes are
one order of magnitude larger than the last period of FSD1, due to the proximity of the WSC
branch. However, both the advective effects and vertical fluxes are of the same magnitude and
seem to be equally important in modifying the mixed layer heat content, which decreased
steadily throughout the drift.
c. Fluxes Across the Pycnocline
Time-averaged profiles of turbulent heat and salt fluxes are shown in Figure 14 for the
profiling depth at each drift. In this section, fluxes averaged between the base of the mixed
20
layer and the stratification maximum (Dml < z < DP) are presented. Average turbulent
parameters , K and KT, the buoyancy frequency and the resulting fluxes are summarized in
Table 5. The stratification varies between 3-5 cph, with the weakest at NBD1, under the
influence of strong mixing with exceptionally large eddy diffusivities.
The variability in the dissipation rate and the eddy diffusivity at FSD1 is not significant,
leading to a narrow 95% confidence interval on the parameters, however the standard
deviation on resulting fluxes are comparable to their mean owing to the variability in the 4-m
property gradients. The mean values of K and KT for FSD1, which was located a
considerable distance away from the core of the WSC, are one order of magnitude larger than
typical low latitude open-ocean pycnocline observations (Gregg 1987). This increases by one
order of magnitude at FSD2. The dissipation rate is 3-5 times larger at FSD2. The average
current profile at FSD2 lacks mean shear (Figure 5b). The pycnocline is shallower and the
boundary mixing penetrates to the pycnocline, leading to entrainment of relative warm deeper
water which also explains the slowly decreasing heat content of the mixed layer. In contrast to
FSD2, several layers of stratification maxima are observed at FSD1 (compare Figure 4 and
Figure 6). In these layers where densely spaced isotherms are separated by relatively mixed
layers, dissipation rate has to work against the fine-scale stratification.
d. Fluxes in Water Masses
Mean profiles of heat and salt fluxes at FSD1, FSD2 and NBD1 (Figure 14) show large
vertical variability with a layering structure. In all drifts the fluxes decrease with distance
from the surface in the upper ~20 m. On the shallow side of the front at NBD1, the heat flux
increases towards the bottom whereas the salt flux remains small in magnitude but changes
sign. Here, horizontal (possibly isopycnal) exchange with significantly warmer water of
comparable salinity from the deeper side of the front can lead to the observed flux
21
profiles. In order to assess the turbulent exchange between different water masses, we average
the fluxes over the extent of each water mass. All 4-m segments from the MSS casts with heat
and salt fluxes inferred from the Osborn-Cox model (section 2.b) are used to identify the
water masses (Table 2) using the segment mean temperature and salinity. Average values of
stratification, dissipation rate, eddy and inferred diffusivity and turbulent fluxes are given for
the MSS drifts (Table 6). Figure 15 shows the average heat fluxes in different water masses
for the duration of the drifts, also including those in the mixed layer for completeness. The
uppermost water mass (PW & PIW) typically extends below the mixed layer. FSD1 mixed
layer heat fluxes compare well with the Stanton number based estimates for the surface fluxes
except for the day ~118.63 when downward fluxes were observed when crossing a thermal
front in the mixed layer and towards the end of the drift when significant convection led to
enhanced upward fluxes. In FSD2, there are only four casts that resolve the mixed layer,
hence the heat flux estimates here are uncertain and are slightly smaller than the Stanton
number based estimate. An increase in heat flux towards the end of the drift is observed in all
water masses, corresponding to the mixing event at around day 120.81. During the NBD1,
heat fluxes within the PW&PIW and ASW water masses increase towards the end of the drift
as the floe approached shallower topography. Generally for all drifts, below the mixed layer,
layer averaged heat fluxes decrease with depth as a consequence of reduced mixing and
thermal diffusivity.
A summary of drift average heat fluxes and salt fluxes within the three classes of water
masses are given in Figure 16. All three drifts show a decreasing magnitude of heat and salt
fluxes with increasing depth, somewhat counterintuitive to the general understanding that the
main source of heat and salt is the layers of Atlantic Water. This is mainly because the AW
(and LAIW) has relatively weak vertical temperature gradient when compared to ASW,
22
although the eddy diffusivity for heat in these water masses are comparable (even larger KT in
AW at FSD1). Furthermore, the average value in the AW is reduced because the temperature
gradient reverses below the depth of the AW temperature maximum, and thus the direction of
the heat flux is opposite in the upper and lower parts of this water mass, reducing the average
(see e.g. Figure 14a). Heat is likely entrained from the deeper AW into PW, as observed at
FSD2, however advective heat flux is also significant (section 4b). The vertical variability of
the heat fluxes within the AW layer is significant at FSD2 (Figure 14a). At the upper side of
the AW at about 40 m where there is a subsurface temperature maximum, the heat flux is in
excess of 100 W m-2, an order of magnitude larger than the layer-averaged value. The nonzero convergence of the vertical heat flux above the AW will heat up the overlying layer.
Water mass transformations in this region can be affected by ice melting. Following
Cokelet et al. (2008) lines with constant ratio of atmospheric cooling to ice/ocean interface
melting of ice with a salinity of 5 psu, are plotted as yellow dashed lines in T-S diagram in
Figure 2. The ratio indicates if a water mass with initial properties T = 3.2˚C, S = 35.1 is
modified by melting only (R = 0), by equal contribution of atmospheric cooling and interface
melting (R = 1) or more by atmospheric cooling (R = 2). The limit R   (a vertical line on
T-S space) indicates modification by atmospheric cooling alone. Generally the upper water
mass characteristics are not produced along the melting line, but both the atmospheric cooling
and ice melting are (approximately equally) important. This is a consequence of the proximity
to the marginal ice zone and the variable ice concentration. From Figure 2, the water column
at FSD1 has a gentler slope than FSD2, WBD and NBD1. A slope with R < 1 indicates that
melting has played a more dominant role in modifying the water column. FSD1 was situated
westward of the other drifts, where it is more likely to encounter water exported from the
Arctic through the Fram Strait which has been modified by melting under a continuous ice
23
cover. The FSD2, WBD and NBD1 water column in the T-S diagram are along lines that
indicate that atmospheric cooling has made a larger contribution to modification than melting,
which is appropriate since water masses are advected along the WSC and drifts are within the
marginal ice zone where air/sea/ice interaction is intense.
5. Summary and Concluding Remarks
In total six drift stations north of Svalbard were occupied in early spring covering years
2003 to 2007. Turbulent characteristics were observed using a microstructure profiler or eddy
correlation instrumentation. The drift stations represent three different conditions; (1) within
close proximity to the West Spitsbergen Current (WSC) with shallow and warm mixed layers
and subsequent large heat fluxes (Whaler’s Bay Drift, WBD, and Fram Strait Drift 2, FSD2);
(2) away from the WSC where the mixed layer is deeper, colder and displays relatively
moderate vertical heat fluxes (Yermak Plateau Drift, YPD, and Fram Strait Drift 1, FSD1);
and (3) over the continental shelf in the Svalbard Branch where cooling and mixing of the
WSC is enhanced by the ice drift and tidal effects related to topography (Norwegian Bank
Drifts, NBD1 and NBD2).
Measurements from drifts within or close to the WSC show surface and mixed layer heat
fluxes of 200 – 300 W m-2 and estimated melting rates of 5 – 9 cm day-1. Heat fluxes are
comparable to and provide support for earlier studies which inferred vertical heat fluxes from
heat loss observed between subsequent cross sections of the WSC (e.g., Aagard et al. 1987;
Saloranta and Haugan 2004; Cokelet et al. 2008). During FSD2, turbulence in the upper layers
is one order of magnitude larger than that observed away from the WSC, and extends deep
into the pycnocline, entraining warmer water into the mixed layer. Changes of the mixed layer
heat content, however, indicate that the contribution from horizontal advection and diffusion
24
is equally important in the heat balance within the mixed layer.
Outside the WSC, turbulent fluxes in the mixed layer are small. Over the northeastern
flank of Yermak Plateau where the mixed layer is near freezing point and ice drift is slow,
average heat flux is 2 W m-2 at YPD, but varies significantly due to solar radiation and tides.
At FSD1, vertical heat fluxes and contributions from horizontal advection and diffusion
account equally for changes in mixed layer heat content. In contrast to FSD2, mixing at FSD1
is typically confined to the mixed layer with localized enhanced mixing between 40 m – 70 m
due to strong velocity shear.
On the shelf area, observed elevated levels of mixing are due to tidal and topographic
effects and lead to surface heat fluxes of O(100) W m-2. As the shallows are approached, an
increasing fraction of the water column is covered by bottom and upper boundary layers and
mixing by tides and topography gets increasingly important.
Profiles of heat flux typically decrease with depth when averaged in water mass classes.
Average fluxes in the Atlantic Water (AW) and the Lower Arctic Intermediate Water layers
are considerably lower than the surface heat fluxes, e.g. only 10% of the surface flux at FSD1.
This also shows that a considerable fraction of the mixed layer heat is entrained from deeper
layers or advected into the area. Near the WSC the vertical variability is large and heat flux on
the crucial upper side of the AW exceeds 100 W m-2, one order of magnitude larger than the
layer-averaged value.
The slope of temperature against salinity properties compared with that expected from a
given ratio of melting/atmospheric cooling shows that upper layer water masses in all our
drifts north of Svalbard are slightly more affected by atmospheric cooling than by melting of
ice. This is in line with Cokelet et al. (2008) who show that the contribution from atmospheric
25
cooling decreases along the path of the WSC as the ice cover gets more solid.
A quiescent Arctic interior with weak vertical diffusion is crucial for the maintenance of
the cold halocline layer (Fer 2009) as well as the circulation of the AW layer (Zhang and
Steele 2007). In the absence of storm events and eddies, oceanic heat fluxes in the deep basin
thus cannot contribute significantly to the ocean surface heat budget. Krishfield and Perovich
(2005), using basin-wide drifting buoy observations, estimate an annual oceanic heat flux of 3
– 4 W m-2 to the Arctic pack ice, with significantly larger values in summer. In contrast, icetethered buoy observations in the Canada Basin thermocline yield diffusive heat fluxes about
one order of magnitude less than the annual mean heat flux to the ice (Timmermans et al.
2008). Although relatively elevated turbulence is observed over deep topography (Rainville
and Winsor 2008), most of the mixing and removal of heat from the AW will occur along the
Arctic perimeters, and possibly dominate basin averaged heat flux. The warm and saline AW
layer propagates into the Arctic Ocean as a boundary current flowing along the continental
slope. Recently, Lenn et al. (2009) reported that measured vertical mixing of the boundary
current on the East Siberian continental slope could not account for the freshening and
cooling of the AW, suggesting influence of shelf waters and lateral mixing.
This study presents directly measured surface turbulent fluxes in addition to turbulent
fluxes and mixing inferred from high resolution turbulence profiling at sites where similar
data sets are scarce. The results show that mixing and heat transfer within the branches of the
WSC and over the shelf areas close to the Fram Strait are several orders of magnitude larger
than those reported by Lenn et al. (2009) farther upstream. Outside the branches of the WSC
and off the shelves, however, we observe vertical mixing and oceanic heat fluxes similar to
values found in the central Arctic. The large spatial variability and the lack of turbulent
mixing in the AW boundary current along the continental slope farther upstream suggest that
26
significant cooling of AW occurs in local regions north of Svalbard with strong currents and
tidal forcing over topography. Diapycnal mixing is important for the regional heat budget,
hence the ice cover and cooling of the WSC in the gateway to the Arctic north of Svalbard. In
contrast, mixing by processes including lateral mixing, shelf-basin exchange and intrusions of
cold shelf waters sinking down slope may be more typical of the Arctic shelves. Although our
measurements are characterized by significant lateral and temporal variability, they represent
an important contribution to the understanding of processed by which the core of the WSC
loses heat, and provide support for previous hydrography-based estimates of vertical heat
fluxes.
Acknowledgements
We thank the captains and the crews of RV Polarstern and RV Lance for their great effort
during these cruises. We are also grateful for the work of the oceanographic group during the
ARK XIX/1 cruise and of all UNIS students participating and doing the work during the RV
Lance cruises. This is publication number A232 of the Bjerknes Center for Climate Research.
27
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32
Figure Captions:
Figure 1. Map of the study area showing the locations and drift trajectories of the six
individual drifts using TIC instrumentation (squares) and MSS instrumentation (circles)
plotted over the bathymetry around Svalbard. Bathymetry is from the GEBCO dataset
(Reproduced from the GEBCO Digital Atlas published by the British Oceanographic Data
Centre on behalf of IOC and IHO, 2003) and isobaths are in meter. Wide arrows indicate the
WSC, the Svalbard Branch, the Yermak Branch and the recirculation branch. The thinner
arrow indicate recirculation across the Yermak Plateu (Bourke et al. 1988; Saloranta and
Haugan 2001).
Figure 2. Vertical profiles of a) temperature, T and b) salinity, S together with c) the
corresponding T-S diagram, observed at the indicated drifts. Profiles for FSD1 and FSD2 are
averaged over all MSS casts. The average profile at NBD1 is for the deeper side of the
thermohaline front. YPD and WBD profiles are single casts collected before and after,
respectively, the TIC measurements and their T-S curves extend to the bottom (860 m for
YPD and 178 m for WBD). The water masses shown in c) are after Aagaard et al. (1985) and
the acronyms and the T-S characteristics are summarized in Table 2. The freezing line (near 1.9 C) and density anomaly at 0.2 kg m-3 intervals are also shown. The three diagonal yellow
dashed lines indicate the trajectory along which water with (T, S) = (3.2, 35.1) is cooled and
freshened near the surface by ratios of atmospheric cooling to ice melt, R = 0, 1 and 2. The ice
is assumed to have a salinity of 5 psu.
Figure 3. Observations during the Fram Strait Drift, FSD1. Time series of a) the magnitude of
the ice drift velocity inferred from GPS fixes before each MSS cast, b) depth distribution of
dissipation rate  (color) and isotherms at 0.2 K intervals (black), and c) K (color) and
isohalines at 0.05 intervals (black). Arrow heads mark the start time of each MSS cast.
33
Evolution of the mixed layer depth is shown by the dashed line.
Figure 4. Profiles averaged over the first 51 MSS casts during the drift FDS1. a) Temperature
(black) and salinity (gray) b) potential density anomaly,  (black) and buoyancy frequency,
N (gray), c) 2-m bin averaged dissipation of TKE,  and d) eddy diffusivity using
K   0.2 N 2 . Vertical gray line in c) is the noise level for  (= 210-8 W kg-1). The gray
trace in d) is the corresponding noise level for K using the mean N profile and ignoring
segments with N < 1.5 cph.
Figure 5. Time averaged profiles of the absolute horizontal current vectors (towards north
pointing upward) in the upper 150 m, measured by the ship-ADCP during a) FSD1 and b)
FSD2. Bins are 4-m averages. 4-m first differenced shear magnitude is used together with the
4-m averaged buoyancy frequency to calculate the fine scale Richardson number (lower axis
in log-scale).
Figure 6. Observations during the Fram Strait Drift, FSD2. Time series of a) air temperature
b) wind speed (black) and ice drift velocity (red), c) depth distribution of dissipation rate 
(color) and isotherms at 0.2 K intervals (black), d) K (color) and isohalines at 0.05 intervals
(black), e) depth from the ship’s echo sounder. Arrow heads mark the start time of each MSS
cast. Evolution of the mixed layer depth is shown by the dashed line.
Figure 7. Same as Figure 4 but for FSD2 and averaged over 26 casts.
Figure 8. Observations during the Norwegian Bank Drift NBD1. Time series of a) air
temperature b) wind speed (black) and ice drift velocity (red), c) depth distribution of
dissipation rate  (color) and isotherms at 0.2 K intervals (black), d) K (color) and isohalines
at 0.05 intervals (black). Bottom topography (gray) is from the ship’s echo sounder.
34
Occasionally the profiler was run to the bottom. Arrow heads mark the start time of each MSS
cast. Evolution of the mixed layer depth is shown by the dashed line.
Figure 9. Average profiles for the Norwegian Bank Drift NBD1. Thin traces are the profiles
averaged on the deep side of the thermohaline front (casts 1 to 14) whereas the thick traces are
on the shallow (well-mixed) side of the front (casts 19 to 29). a) Temperature (black) and
salinity (gray) b) potential density anomaly,  (black) and buoyancy frequency, N (gray), c)
2-m bin averaged dissipation of TKE,  and d) eddy diffusivity using 0.2 N 2 , ignoring
segments with N < 1.5 cph.
Figure 10. Hydrographic conditions during the Norwegian Bank Drift NBD2 in 2007.
Temperature contours (color) at 0.1K intervals and salinity (black) at 0.05 intervals are
shown. Arrowheads at the top mark the SBE-CTD times and the horizontal line shows the
duration of the TIC sampling. White dashed line is the mixed layer depth. Bathymetry is from
the ship’s echo sounder.
Figure 11. TIC measurements in the under-ice boundary layer at 1 m (solid) and 5 m (dashed)
below the ice during the Yermak Plateau Drift (YPD) in 2003. Ensembles of 15-minutes are
hourly averaged and further smoothed by a 3-hour running-mean filter. a) Temperature at
both levels and elevation above freezing point at 5 m (shaded area, right axis), b) salinity, c)
local friction velocity, and d) the heat flux.
Figure 12. Same as Figure 11 but for the Whaler’s Bay Drift, on the northern shelf of
Svalbard. Because the time series is relatively short, all 15-minute segments are shown using
one hour running-mean.
Figure 13. TIC measurements at 1 m below the ice during the Norwegian Bank Drift NBD2 in
35
2007. All 15-minute segments are shown using one hour running-mean. a) Salinity (solid) and
temperature (dashed), b) elevation above freezing (shaded area, left axis) and the local friction
velocity (solid), and c) the heat flux.
Figure 14. Average profiles of turbulent heat flux, FH, (panels a and c) and salinity flux, FS,
(panels b and d) calculated using 4-m segments of MSS profiles in the Fram Strait FSD1
(black) and FSD2 (gray), and on the Norwegian Bank for the deep side (black), and shallow
side (gray) of the thermohaline front.
Figure 15. Times series of turbulent heat flux averaged over (the first row) the mixed layer
(the second row) Polar Water, (the third row) Arctic Surface Water and (the fourth row)
Atlantic Water layers. Black square markers correspond to data points derived for each MSS
cast and the gray curves are average over 10 equally spaced bins over the duration of the drift.
Values of heat flux to the ice obtained using the Stanton number parameterization (gray
squares with errorbars) are compared to the mixed layer fluxes.
Figure 16. Summary of turbulent heat fluxes for a) heat and b) salinity averaged over water
masses in FSD1, FSD2 and NBD1. The figures between the panels show the total length (km)
of data segments averaged.
36
teu
Figures
Ye
rm
ak
Pla
YPD
10
00 200
0
80
WBD
0
ch
100
NBD1
ar
Sv
alb
0
4
NBD2
00
20
ch
d
an
Br
Br
an
0
ak
60
rm
Ye
FSD2
400
0
20
FSD1
C
WS
Svalbard
Figure 1. Map of the study area showing the locations and drift trajectories of the six
individual drifts using TIC instrumentation (squares) and MSS instrumentation (circles)
plotted over the bathymetry around Svalbard. Bathymetry is from the GEBCO dataset
(Reproduced from the GEBCO Digital Atlas published by the British Oceanographic Data
Centre on behalf of IOC and IHO, 2003) and isobaths are in meter. Wide arrows indicate the
WSC, the Svalbard Branch, the Yermak Branch and the recirculation branch. The thinner
arrow indicate recirculation across the Yermak Plateu (Bourke et al. 1988; Saloranta and
37
Haugan 2001).
0
NBD1
a)
b)
D
FS
3.5
AW
2
3 27.2
2.5
2
50
D
YP
D
WB
Temperature [°C]
Depth [m]
1
D
FS
100
ASW 27.6
28
1.5
1
0
R=
0.5
0
−0.5
PW
LAIW
R=
2
−1
−1.5
−2
150
−2
0
2
Temperature [°C]
34
34.5
Salinity
c)
PIW
34.2
34.4
34.6
34.8
Salinity
35
35
Figure 2. Vertical profiles of a) temperature, T and b) salinity, S together with c) the
corresponding T-S diagram, observed at the indicated drifts. Profiles for FSD1 and FSD2 are
averaged over all MSS casts. The average profile at NBD1 is for the deeper side of the
thermohaline front. YPD and WBD profiles are single casts collected before and after,
respectively, the TIC measurements and their T-S curves extend to the bottom (860 m for
YPD and 178 m for WBD). The water masses shown in c) are after Aagaard et al. (1985) and
the acronyms and the T-S characteristics are summarized in Table 2. The freezing line (near 1.9 C) and density anomaly at 0.2 kg m-3 intervals are also shown. The three diagonal yellow
dashed lines indicate the trajectory along which water with (T, S) = (3.2, 35.1) is cooled and
freshened near the surface by ratios of atmospheric cooling to ice melt, R = 0, 1 and 2. The ice
is assumed to have a salinity of 5 psu.
38
|
u i| [m s−1]
Fram Strait Drift FSD1
0.2
0
a)
0
120
−4
−4.5
−5
−5.5
−6
−6.5
−7
−7.5
−8
−8.5
0
−2
20
−1.4
−1.6
−1.6
Depth (m)
−1.6
40
−0.4
60
100
20
Depth (m)
1.4
80
3
2.8
b)
3
3
log10 ,
θ
3
34.2
−2.5
34.2
34.25
−3
40
−3.5
34.6
60
34.85
34.7
−4.5
80
100
−4
35
c)
35.05
log10 Kρ ,
S
−5
35.1
35.1
120
−5.5
−6
118.6
118.7
118.8
118.9
Decimal Day 2005 [UTC]
119
119.1
Figure 3. Observations during the Fram Strait Drift, FSD1. Time series of a) the
magnitude of the ice drift velocity inferred from GPS fixes before each MSS cast, b) depth
distribution of dissipation rate  (color) and isotherms at 0.2 K intervals (black), and c) K
(color) and isohalines at 0.05 intervals (black). Arrow heads mark the start time of each MSS
cast. Evolution of the mixed layer depth is shown by the dashed line.
39
S
0
34.5
N [cph]
35
0
5
a)
b)
S
50
Depth [m]
T
100
σθ
N
c)
150
−1 0 1 2
T [°C]
27.6
27.8
σθ
−8
−7
−6
−1
log10 ε [W kg ]
d)
−4
−3
−2
2 −1
log10 Kρ [m s ]
Figure 4. Profiles averaged over the first 51 MSS casts during the drift FDS1. a)
Temperature (black) and salinity (gray) b) potential density anomaly,  (black) and buoyancy
frequency, N (gray), c) 2-m bin averaged dissipation of TKE,  and d) eddy diffusivity using
K   0.2 N 2 . Vertical gray line in c) is the noise level for  (= 210-8 W kg-1). The gray
trace in d) is the corresponding noise level for K using the mean N profile and ignoring
segments with N < 1.5 cph.
40
FSD1
0
FSD2
−1
−1
10 cm s
30 cm s
Depth [m]
50
100
150
0
1
10
10
Ri4m
0
10
1
10
Ri4m
2
10
Figure 5. Time averaged profiles of the absolute horizontal current vectors (towards north
pointing upward) in the upper 150 m, measured by the ship-ADCP during a) FSD1 and b)
FSD2. Bins are 4-m averages. 4-m first differenced shear magnitude is used together with the
4-m averaged buoyancy frequency to calculate the fine scale Richardson number (lower axis
in log-scale).
41
10
a) Air Temperature
b)
0.6
0.4
0.2
5
|
u i| [m s−1]
|uw| [m s−1]
Ta [◦ C]
Fram Strait Drift FSD2
0
−5
−10
0
−1.2
Depth [m]
0.8
1.2
1.2
50
3
3.6
3.2
2.6
3.8
100
3.4
−1
◦
c) log10 [W kg ], θ [ C]
3.2
0
34.7
Depth [m]
34.85
34.85
50
35.05
100
35
2 −1
d) log10 Kρ [m s ], S
35.1
35.1
35.15
Depth [m]
35.1
−4
−4.5
−5
−5.5
−6
−6.5
−7
−7.5
−8
−8.5
−2
−2.5
−3
−3.5
−4
−4.5
−5
−5.5
−6
2500
3500
e) Echo Depth
120.5
120.55
120.6 120.65 120.7 120.75
Decimal Day 2005 [UTC]
120.8
120.85
Figure 6. Observations during the Fram Strait Drift, FSD2. Time series of a) air
temperature b) wind speed (black) and ice drift velocity (red), c) depth distribution of
dissipation rate  (color) and isotherms at 0.2 K intervals (black), d) K (color) and isohalines
at 0.05 intervals (black), e) depth from the ship’s echo sounder. Arrow heads mark the start
time of each MSS cast. Evolution of the mixed layer depth is shown by the dashed line.
42
S
0
34.5
N [cph]
35
0
5
a)
b)
S
T
Depth [m]
50
100
σθ
N
c)
150
−1 0 1 2
T [°C]
27.6
27.8
σθ
−8
−7
−6
−1
log10 ε [W kg ]
d)
−4
−3
−2
2 −1
log10 Kρ [m s ]
Figure 7. Same as Figure 4 but for FSD2 and averaged over 26 casts.
43
Norwegian Bank Drift NBD1
Ta [◦ C]
1
0
a)
−1
0.2
10
5
0.1
b)
0
0
−0.8
Depth [m]
20
−0.2
−0.2
40
60
−4
−4.5
−5
−5.5
−6
−6.5
−7
−7.5
−8
−8.5
−1
−1.2
0.2
−0.4
0.4
0.6 0.4
1
−1
c)log◦10 [W kg ]
80
θ [ C]
0
34.45
Depth [m]
20
40
60
34.65
34.65 34.6
34.7
34.75
d) log10 Kρ [m2 s−1 ]
S
80
121.95
34.6
34.55
34.55
122
122.05
122.1
122.15
122.2
Decimal Day 2005 [UTC]
122.25
|
ui | [m s−1 ]
|uw| [m s−1]
−2
−2
−2.5
−3
−3.5
−4
−4.5
−5
−5.5
−6
122.3
Figure 8. Observations during the Norwegian Bank Drift NBD1. Time series of a) air
temperature b) wind speed (black) and ice drift velocity (red), c) depth distribution of
dissipation rate  (color) and isotherms at 0.2 K intervals (black), d) K (color) and isohalines
at 0.05 intervals (black). Bottom topography (gray) is from the ship’s echo sounder.
Occasionally the profiler was run to the bottom. Arrow heads mark the start time of each MSS
cast. Evolution of the mixed layer depth is shown by the dashed line.
44
S
0
34.4
N [cph]
34.8
0
2
a)
10
4
b)
c)
d)
S
20
Depth [m]
30
40
T
50
60
N
σθ
70
80
−1
0
1
27.7
T [°C]
27.8
σθ
−8 −7 −6 −5
−1
log10 ε [W kg ]
−3
−2
−1
log10 Kρ [m2 s−1]
Figure 9. Average profiles for the Norwegian Bank Drift NBD1. Thin traces are the
profiles averaged on the deep side of the thermohaline front (casts 1 to 14) whereas the thick
traces are on the shallow (well-mixed) side of the front (casts 19 to 29). a) Temperature
(black) and salinity (gray) b) potential density anomaly,  (black) and buoyancy frequency,
N (gray), c) 2-m bin averaged dissipation of TKE,  and d) eddy diffusivity using 0.2 N 2 ,
ignoring segments with N < 1.5 cph.
45
T [°C]
2.8
34.5
20
34.6
34.65
40
2
Pressure [dbar]
34.75
60
34.9
80
1.2
34.95
100
35
120
0.4
−0.4
140
−1.2
160
TIC
180
−2
114.4
114.5
114.6
Day of 2007 [UTC]
114.7
Figure 10. Hydrographic conditions during the Norwegian Bank Drift NBD2 in 2007.
Temperature contours (color) at 0.1K intervals and salinity (black) at 0.05 intervals are
shown. Arrowheads at the top mark the SBE-CTD times and the horizontal line shows the
duration of the TIC sampling. White dashed line is the mixed layer depth.
46
−1.81
0.07
−1.82
0.06
−1.83
0.05
34.17
S
a)
ΔT [K]
T [°C]
Yermak Plateau Drift, YPD
b)
34.16
FH [W m−2]
u* [m s−1]
34.15
c)
↓1m
0.02
↓5m
0.01
10
d)
0
−10
101
102
103
104
105
Day of 2003 [UTC]
106
107
Figure 11. TIC measurements in the under-ice boundary layer at 1 m (solid) and 5 m
(dashed) below the ice during the Yermak Plateau Drift (YPD) in 2003. Ensembles of 15minutes are hourly averaged and further smoothed by a 3-hour running-mean filter. a)
Temperature at both levels and elevation above freezing point at 5 m (shaded area, right axis),
b) salinity, c) local friction velocity, and d) the heat flux.
47
−0.9
1.0
−1.0
0.9
ΔT [K]
T [°C]
Whalers Bay Drift, WBD
a)
S
34.5
34.45
↓5m
↑1m
b)
u* [m s−1]
0.015
FH [W m−2]
34.4
1500
c)
0.01
d)
1000
500
0
91.6
91.7
91.8
91.9
92
Day of 2003 [UTC]
92.1
92.2
92.3
Figure 12. Same as Figure 11 but for the Whaler’s Bay Drift, on the northern shelf of
Svalbard. Because the time series is relatively short, all 15-minute segments are shown using
one hour running-mean.
48
Norwegian Bank Drift, NBD2
−1.4
T
−1.6
34.4
34.3
S
a)
−1.7
u*
b)
ΔT [K]
−1.5
0.4
7
0.3
5
u* ⋅ 10−3 [m s−1]
S
34.5
T [°C]
34.6
FH [W m−2]
0.2
80
c)
60
40
20
0
114.35
114.4
114.45
114.5
114.55
Day of 2007 [UTC]
114.6
114.65
Figure 13. TIC measurements at 1 m below the ice during the Norwegian Bank Drift
NBD2 in 2007. All 15-minute segments are shown using one hour running-mean. a) Salinity
(solid) and temperature (dashed), b) elevation above freezing (shaded area, left axis) and the
local friction velocity (solid), and c) the heat flux.
49
0
FSD1 →
Depth [m]
50
← FSD2
100
a) Fram Strait
Heat Flux
150
−50
0
50
b) Fram Strait
Salt Flux
100
0
−2
2
4
6
−1
10 × FS [kg s
FH [W m ]
6
8
−2
m ]
0
Depth [m]
20
40
60
c) Norwegian Bank
Heat Flux
80
0
200
400
−2
FH [W m ]
600
d) Norwegian Bank
Salt Flux
0
20
6
40
60
−1
10 × FS [kg s
80
−2
m ]
Figure 14. Average profiles of turbulent heat flux, FH, (panels a and c) and salinity flux,
FS, (panels b and d) calculated using 4-m segments of MSS profiles in the Fram Strait FSD1
(black) and FSD2 (gray), and on the Norwegian Bank for the deep side (black), and shallow
side (gray) of the thermohaline front.
50
Fram Strait, FSD2
1000
200
10
200
400
0
100
200
20
200
700
10
100
350
8
80
0
0
118.6 118.8
119
Day of 2005 [UTC]
PW & PIW
100
500
ASW
0
MIXED LAYER
100
Norwegian Bank, NBD1
AW & LAIW
FH [W m−2]
FH [W m−2]
FH [W m−2]
FH [W m−2]
Fram Strait, FSD1
120.5
120.7
Day of 2005 [UTC]
122
122.2
Day of 2005 [UTC]
Figure 15. Times series of turbulent heat flux averaged over (the first row) the mixed layer
(the second row) Polar Water, (the third row) Arctic Surface Water and (the fourth row)
Atlantic Water layers. Black square markers correspond to data points derived for each MSS
cast and the gray curves are average over 10 equally spaced bins over the duration of the drift.
Values of heat flux to the ice obtained using the Stanton number parameterization (gray
squares with errorbars) are compared to the mixed layer fluxes.
51
Heat Flux, FH [W m−2]
FSD1
200
FSD2
NBD1
a)
150
100
50
106×Salt Flux, FS [kg s−1 m2]
0.53
0.89
2.28
0.66
0.15
2.4
1.16
2.37
0
b)
PW+PIW
ASW
AW+LAIW
15
km
segment
10
5
0
FSD1
FSD2
NBD1
Figure 16. Summary of turbulent heat fluxes for a) heat and b) salinity averaged over
water masses in FSD1, FSD2 and NBD1. The figures between the panels show the total
length (km) of data segments averaged.
52
Tables
Table 1. Details of the ice-drift stations.
Drift
Platform
Year
Start time [doy] a
Latitude, start ˚N
Longitude, start ˚E
Drift length [km] b
Duration [h]
Number of profilesc
Drift speed [m s-1]
Dml [m]
Dp [m]
a
WBD
YPD
FSD1
FSD2
NBD1
NBD2
FS Polarstern
FS Polarstern
RV Lance
RV Lance
RV Lance
RV Lance
2003
91.51
80.429
12.817
10.05
18.0
2
0.16
19
28
2003
100.81
81.789
9.491
38.20
157.1
4
0.07
70
83
2005
118.53
78.780
0.531
11.97
14.1
54
0.24
21
51
2005
120.46
79.888
3.603
17.54
9.5
26
0.52
6
19
2005
121.95
80.244
15.402
4.53
8.7
29
0.06
5
14
2007
113.73
80.347
14.920
27.55
25.5
6
0.30
10
27
Day of year with 1 January 1200 UTC = 1.5
Cumulative length derived from navigation data.
c
Number of CTD profiles for 2003 and 2007, and number of MSS profiles for 2005.
b
53
Table 2. Water mass definitions following Aagaard et al. (1985)
Water Mass
Acronym
Temperature
Salinity
Polar Water
PW
< 0 C
< 34.4
Polar Intermediate Water
PIW
< 0 C
Arctic Surface Water
ASW
> 0 C
< 34.9
Atlantic Water
AW
> 2 C
> 34.9
LAIW
< 2 C
> 34.9
Lower Arctic Intermediate Water
34.4 < S < 34.7
54
Table 3. Tidal current ellipse parameters derived at the drift start positions using the AOTIM5
model (Padman and Erofeeva 2004). Those at the final position of the NBD1 are also shown.
M2
WBD
YPD
FSD1
FSD2
ma / mi [cm s-1] a
8.7 / -2.1
3.9 / -0.7
3.2 / -0.4
pha / inc [] b
272 / 53
240 / 51
4.8 / -1.4
48 / 51
-1 a
ma / mi [cm s ]
K1
pha / inc [] b
a
b
4.3 / 0.4
NBD1
start
8.4 / -0.1
NBD1
end
12.6 / -1.5
53 / 85
60 / 87
308 / 45
305 / 43
8.2 / -5.1
2.0 / -0.1
4.1 / 1.6
2.4 / -0.4
2.9 / -0.8
36 / 56
47 / 90
37 / 77
7 / 23
6/8
ma / mi are the semi-major / semi-minor axis lengths
pha / inc are the phase and the inclination
55
Table 4. Comparison of the average mixed layer oceanic conditions and heat flux for all drifts.
[˚C]
Tml
Sml
WBD
YPD
FSD1
FSD2
NBD1
NBD2
-0.56
-1.81
-1.68
-0.30
-1.03
-1.49
34.53
34.16
34.19
34.54
34.49
34.37
ΔT
[K]
0.93
0.05
0.19
1.58
0.86
0.37
FH
[W m-2]
259
2
28
206 a
240 a
69
u*0
10-2 [m s-1]
0.94
0.23
0.44
0.99
0.59
0.8
FH0
[W m-2]
203
3
21
303
116
69
a
Few data points
56
Table 5. Average turbulent parameters in the mixed layer and in the layer between the base of
the mixed layer and pycnocline. a
FSD1
N

Layer
[cph]
10-8 [W kg-1]
K
10-4 [m2 s-1]
KT
10-4 [m2 s-1]
FH
FS
[W m-2]
10-6 [kg s-1m-2]
a
1.6 ± 1.0
23.6
[16 34]
-
FSD2
z < Dml
3.6 ± 1.3
598
[18 19713]
-
80
[37 175]
28 ± 44
1.86 ± 5.4
61
[22 169]
206 ± 29
15.9 ± 8.2
NBD1
0.7 ± 1.0
353
276
240
30.9
FSD1
4.8 ± 0.6
5.3
[4.8 5.9]
3.9
[3 5]
3.1
[1.9 4.9]
8±8
0.44 ± 0.4
FSD2
Dml < z < DP
4.4 ± 1.4
23.5
[15.1 36.5]
26.1
[12.7 53.7]
33
[9 122]
89 ± 103
4.2 ± 5.8
NBD1
3.0 ± 1.0
917.3
[290 2900]
3302
[430 25328]
151
[60 379]
301 ± 267
33.1 ± 36.1
All values are inferred from microstructure profiling. The buoyancy frequency, heat flux
and salt flux are given as mean values ± one standard deviation. Values of the dissipation rate
and the eddy diffusivities are the maximum likelihood estimator and the 95% confidence
limits are given in brackets.
57
Table 6. Water Mass Averaged Propertiesa.
T [˚C]
S
z [m]
h [m]
N [cph]

10-8 [W kg-1]
K
10-4 [m2 s-1]
KT
10-4 [m2 s-1]
FH
[W m-2]
FS
10-6 [kgs-1m-2]
a
PW &
PIW
-1.19
34.30
10.4± 2.2
40 ± 5
4.1 ± 0.3
10.9
[8.5 14]
12.7
[7.9 20]
14
[8 24]
12 ± 15
0.77±1.8
FSD1
ASW
0.99
34.75
54.5±4.4
17.9±7.7
4.9 ± 0.9
3.0
[2.7 3.4]
1.1
[1.0 1.3]
0.45
[0.3 0.7]
6± 6
AW &
LAIW
2.51
35.03
76.4±10.8
42.2±17.4
2.0 ± 0.2
1.9
[1.9 2]
3.86
[3.5 4.2]
3.1
[2.3 4.1]
2.4 ± 3
PW &
PIW
-0.5
34.53
10.0± 0.0
9.5± 3.7
3.9 ± 1.5
85.7
[20.3 362]
265
[22 3211]
146
[10 2112]
119 ± 108
0.54±1.5
0.23±0.16
7.0±7.3
FSD2
ASW
AW &
LAIW
1.03
3.03
34.76
35.07
15.7±7.2 41.1±11
21.4±7.2 83.5±24
3.5 ± 0.6 1.5 ± 0.2
12.4
2.3
[8.8 17.4] [2.1 2.5]
8.9
7
[6.1 13]
[6.1 8]
14.7
7.2
[6.1 35] [5.5 9.4]
72 ± 78 9.5 ± 24
PW &
PIW
-0.73
34.56
10.0± 0.0
26.6±11.3
2.5 ± 0.7
458
[217 964]
1594
[450 5649]
127
[78 208]
225±157
3.5 ± 4.6
20.8±17.4
0.6±0.8
NBD1
ASW
0.42
34.69
41.0± 7.9
29.3± 9.7
2.1 ± 0.7
17.7
[9.9 31.6]
25.1
[14.1 44.7]
23.8
[11.6 49.1]
175 ± 366
4±7
Values are the drift averages for T and S, average ± one standard deviation for depth, z,
thickness, h, buoyancy frequency, N and the turbulent fluxes FH and FS, and maximum
likelihood estimator with the 95% confidence interval in brackets for the dissipation rate and
the eddy diffusivities.
58