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Global and Planetary Cha

Impact of flaw polynyas on the hydrography of the Laptev Sea

Igor A. Dmitrenkoa,T, Konstantin N. Tyshkob, Sergey A. Kirillovb, Hajo Eickenc,

Jens A. Holemannd, Heidemarie Kassense

aInternational Arctic Research Center, University of Alaska Fairbanks, 930 Koyukuk Drive, P.O. Box 757335, Fairbanks, AK 99775-7335, USAbThe State Research Centre of the Russian Federation, the Arctic and Antarctic Research Institute, St. Petersburg, Russia

cGeophysical Institute, University of Alaska Fairbanks, USAdInstitute for Polar and Marine Research, Bremerhaven, GermanyeCentre for Marine Geosciences, Kiel University, Kiel, Germany

Received 6 January 2004; received in revised form 22 April 2004; accepted 9 December 2004

Abstract

Based on hydrological data from 1979 to 1999 the average long-term salinity of the flaw polynya in the Eastern Laptev Sea

is estimated. A new method to evaluate ice production based on hydrological rather than sea-ice observations is proposed.

Average annual ice production in the polynya ranges between 3 and 4 m. The probability of convective mixing penetrating

down to the seafloor is highest in the regions of the flaw polynya, but does not exceed 20% in the Eastern and 70% in the

Western Laptev Sea. Conductivity–temperature–depth (CTD) measurements and observations of currents carried out in April–

May 1999 allowed us to investigate the surface circulation along the margins of the Laptev Sea flaw polynya. The convective

nature of the surface currents, with velocities measured as high as 62 cm/s, is discussed. Currents are most likely part of

circulation cells, which arise as a result of brine rejection due to intensive ice formation in the polynya. It is shown that the

spatial alignment of sea ice crystals in the marginal part of the polynya is most likely a consequence of the quasi-stationary

cellular circulation.

D 2005 Elsevier B.V. All rights reserved.

Keywords: hydrography; polynya; ice production; salinity variance; convection

1. Introduction

Large, persistent areas of open water and young

ice off the land-fast ice are referred to as flaw

polynyas (World Meteorological Organization

0921-8181/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.gloplacha.2004.12.016

T Corresponding author.

E-mail address: igordm@iarc.uaf.edu (I.A. Dmitrenko).

(WMO), 1970; Zakharov, 1997). The system of

flaw polynyas on the Russian Arctic shelf, known

as the Great Siberian Polynya, is an important

component of the Arctic climate system. Extensive

stretches of open water up to 200 km wide

combined with extremely low air temperatures

induce intensive ice formation and local increases

in salinity of the water column during winter and

early spring. The Great Siberian Polynya is a

nge 48 (2005) 9–27

2

3

4

5

6

Ko

telnyy

Khata

nga

Olenek

YanaLena

Laptev Sea

Pack Ice

Fast Ice

Fast Ice

80°N

80°N120°E

75°N

150°E

120°E

75°N

70°N

Ta

imyr

1

Fig. 1. Location of flaw polynyas in the Laptev Sea according to

Zakharov (1997). 1—Eastern Severnaya Zemlya, 2—Northeastern

Taimyr, 3—Taimyr, 4—Anabar Lena, 5—Western New Siberian,

6—New Siberian. Dashed lines show the working area of the

TRANSDRIFT VI expedition in April–May 1999.

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2710

possible source of both saline shelf waters for the

Arctic Ocean (Martin and Cavalieri, 1989; Cavalieri

and Martin, 1994; Winsor and Bjork, 2000) and

Transpolar Drift ice (Eicken et al., 1997; Dethleff et

al., 1998; Alexandrov et al., 2000). The total ice

thickness formed in the Great Siberian Polynya

during winter is estimated to range from 3.6 m

[(Zakharov, 1966), central Laptev Sea] to 21 m

[(Martin and Cavalieri, 1989), eastward from the

Severnaya Zemlya Islands]. Among western scien-

tists, Martin and Cavalieri (1989) were the first to

point out the importance of the Siberian coastal

polynya in the Arctic climate system. Although flaw

polynyas in the Russian Arctic have long been of

interest to researchers, relevant studies are under-

represented in the literature; for example, in their

Table 1

Recurrence and average width of the flaw polynyas in the Laptev Sea (Z

Polynyas Recurrence, %

Feb. Mar. Apr. Ma

Eastern Severnaya Zemlya 89 68 63 3

Northeastern Taimyr 100 80 90 5

Taimyr 83 63 40 3

Anabar Lena 96 86 66 9

Western New Siberian 67 84 88 10

New Siberian 85 82 80 7

comprehensive overview paper on physical pro-

cesses and the environment of polynyas, Smith et

al. (1990) include only one short paragraph on the

Siberian Shelf Polynya.

Previous studies have described the Great Siberian

Polynya as a latent heat polynya; offshore components

of surface wind forcing may create open water and

induce new ice formation in the absence of sensible

heat input from below (Zakharov, 1966, 1997;

Dethleff et al., 1998). However, based on studies

performed from 1993 to 1998, Dmitrenko et al.

(1999a,b) have suggested that among other factors,

sensible heat transport impacts the location of the fast-

ice edge.

In the Siberian Arctic seas, flaw polynyas are most

distinct in the Laptev Sea (Fig. 1; Table 1). In

comparison with the polynyas of the Barents, Kara

and East Siberian Seas, they are the largest in size and

occur most frequently (Zakharov, 1997). Zakharov

(1966) is the most recent Russian publication describ-

ing the impact of the flaw polynyas on the hydrography

of the Laptev Sea shelf. Since then, extensive data sets

on Laptev Sea hydrography and ice conditions have

been collected during a number of international and

Russian expeditions in the area. Our understanding of

the physics underlying the relevant oceanographic

processes has been substantially improved, not least

due to advances in measurement techniques, including

the application of satellite remote sensing. Newly

available data sets now provide an opportunity to

evaluate the impact of the flaw polynyas on the

hydrography of the Laptev Sea in more detail and to

a higher degree of accuracy and, furthermore, allow us

to expand on and possible revise earlier, mostly

qualitative assessments and hypotheses.

In the first part of this contribution, we will discuss

the impact of the flaw polynyas on the hydrography of

akharov, 1996)

Width, km

y Jun. Feb. Mar. Apr.–May Jun.

1 18 45 20 30 5

5 80 140 80 65 60

0 24 40 45 20 25

0 97 70 80 55 95

0 100 70 65 75 75

3 83 55 55 55 65

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–27 11

the Laptev Sea, taking into account historical oceano-

graphic data collected by the Arctic and Antarctic

Research Institute, St. Petersburg, Russia (AARI). The

average winter salinity distribution and its variability

is estimated for the period of 1979–1999. The impact

of the coastal polynya on surface salinity, along with

variable river runoff and atmospheric forcing, is

discussed and ice production rates are estimated based

on the polynya hydrography. Ice production can be

obtained from the salinity distribution, which reflects

the amount of brine rejection in the polynya. The rate

of salinity adjustment in response to ice formation is

evaluated statistically from time series of winter

salinity observations. The probability of winter con-

vective mixing penetrating down to the seafloor is

evaluated statistically from the annual time series of

surface and bottom salinity. It will be shown that both

the vertical density stratification, mainly controlled by

river runoff, and the position of the main polynyas

determine the degree of convection penetrating down

to the seafloor.

Results of the joint Russian–German expeditions

carried out in the Eastern Laptev Sea in 1996 and

1999 are discussed in the second half of this paper.

The rate of surface salinity increase from land-fast

ice toward the polynya was determined through

direct conductivity–temperature–depth (CTD) salin-

ity measurements in spring of 1999. Preferred

alignment of ice crystals in the ice cover along

the fast-ice margin of the Laptev Sea coastal

polynya was also observed and will be discussed

1980 1985 1990 1995 20000

40

1980 1985 1990 1995 20000

40

80

120 1

No.

of s

tatio

ns

YearSummer

I

III

Fig. 2. Distribution of oceanographic stations over the Laptev Sea. The p

Fig. 3a.

in the context of circulation patterns forced by

intensive brine rejection during ice formation.

2. Hydrography of the flaw polynya in the Laptev

Sea: analysis of historical data

2.1. Data sets

We have used temperature and salinity records

obtained in the Laptev Sea during the AARI bSeverQexpeditions (1979–1990, 1992, and 1993), together

with CTD-soundings performed during the Russian–

German expeditions TRANSDRIFT IV (1996) and

TRANSDRIFT VI (1999). The observation time

varied from March until May. A total of 1685

hydrological stations was analyzed. The distribution

of stations in the four principal sub-regions of the

Laptev Sea is shown in Fig. 2.

Ten-day mean air temperature records from 1979

till 1993, for the Tiksi, Dunay and Kotelnyy mete-

orological stations were used to calculate the total

amount of freezing degree-days. Station positions are

shown in Fig. 3a.

The location of the fast-ice edge for February

11–20 each year from 1979 until 1999 (Fig. 3b) was

obtained from AARI ice charts based on satellite

images and airborne ice observations. In February

the location of the fast-ice edge becomes nearly

stationary and hardly changes at all till the end of

May.

1980 1985 1990 1995 20000

40

1980 1985 1990 1995 20000

40

80

20

YearWinter

IV

II

anels (I)–(IV) represent the sub-regions marked by red squares in

21

1

2

5

6

4

3

2

Lena DeltaKha

tanga

Anabar

Olenek

Yana

21

105oE 110o 115o 120o 125o 130o 135o 140o 145oE

71o

N

72o

73o

74o

75o

76o

77o

78o

N

105oE 110o 115o 120o 125o 130o 135o 140o 145oE

72o

73o

74o

75o

76o

77o

71o

N

78o

N

b

Kot

elnyyaimyrT

24

32

26

2830

2230 28

1618

20

Yana

78o

N

10

10 10

10

10

1010

10

10

10

10

10

10

10

10

10

10

10

10

1010

10

1010 10

10

10

20

20

20

20

20

20

20

2020

20

20

20

2020

20

20

2020

20

20

30

30

30

30

30

30 30

30

30

30

30

40

40

4040

40

4040

40

40

40

50

50

50

50

50

50

50

50

100

32

105oE 110o 115o 120o 125o 130o 135o 140o 145oE

71o

N

72o

73o

74o

75o

76o

77o

78No

105oE 110o 115o 120o 125o 130o 135o 140o 145oE

72o

73o

74o

75o

76o

77o

71o

N

40

50

100

30

10

Kot

elnyy40

Khatan

ga

Olenek

Anabar

aimyrT

Lena Delta

Dunay

Kot

elnyyKotelnyy

3

45

4

Tiksi

I II

III

IVa

Fig. 3. Long-term average (1979–1999) salinity S (a, bold solid line) and (b, dashed line) its variance Ds2 (psu2) at the surface of the Laptev Sea

in winter. Arrows on panel (a) are the directions of the summer surface circulation from (Dobrovolskiy and Zalogin, 1982). Thin solid line on

panel (a) represents bottom topography. Dashed red line on panel (b) shows the average location of the fast-ice edge in mid-February. Grey solid

lines correspond to locations of the fast-ice edge in mid-February from 1979 to 1999.

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2712

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–27 13

2.2. Salinity increase in the flaw polynya and ice

production estimates

2.2.1. Approach

Intensive ice formation in the flaw polynya,

accompanied by the rejection of brine, leads to an

increase in the salinity of the surface water layer down

to the depth of mixed layer density interface. The

location of the flaw polynyas varies from year to year.

The flaw polynyas themselves are not stationary, as

they change their configuration and size depending on

weather conditions resulting in a spatial variability of

surface salinity during any given winter. Over longer

interannual time scales, this spatial variability affects

the variance of salinity. Local maxima in salinity

variance are hence postulated to coincide with the

longer-term mean location of the flaw polynya. This

assumes that all other factors, such as interannual

variability of thermohaline and wind-forced circula-

tion, do not significantly contribute to interannual

salinity variations.

Statistically the average surface salinity S from a

random-variable time series is determined as

S =SFDs, where Ds is the confidence interval.

For 208 of freedom at a 70% significance level Ds

corresponds to the root-mean-square salinity devia-

tion (RMSD). Therefore, the average range of

salinization, corresponding to the doubled RMSD

value (2Ds), has been further considered to be the

range of average salinization caused by ice for-

mation in the polynya.

To reveal the impact of flaw polynyas on Laptev

Sea hydrography, we studied the average winter

surface salinity (S) and its variance (Ds2) for the

period from 1979 till 1999. The historical hydro-

graphical data from 1685 stations were mainly

obtained during the AARI airborne expeditions from

March to May, i.e. conditions still representative of

the winter regime. The average long-term salinity

(Fig. 3a) was estimated by averaging the annual

measurements, linearly interpolated to a regular grid

with intervals of 0.58 of latitude and 28 of longitude.A 10 km search radius was employed in the

interpolation procedure. The salinity variance Ds2,

presented in Fig. 3b, was calculated for each grid

point using the interpolated value of yearly measured

surface salinity. Given a rather uniform data coverage

over most of the Laptev Sea, we assume that the

obtained salinity field is representative of the actual

conditions.

Due to intensive summer river runoff and ice

melting, together with limited input of river water

through the eastern Lena River channels during

winter, the salinity of the southeastern Laptev Sea

remains relatively low in winter. It increases north-

ward from 12 to 14 practical salinity units (psu) near

the Lena River delta to 28–30 psu offshore in the

northeastern part of the sea. In the Western Laptev Sea

salinity ranges between 30 and 32 psu (Fig. 3a). The

variability of surface salinity is relatively low

throughout the Laptev Sea, with the variance typically

not exceeding 4–5 psu2 (Fig. 3b). The highest Ds2 was

detected in the estuarine areas of the Yana, Olenek,

and Buor-Haya Bays, and are attributed to interannual

variations in winter river runoff.

The eastern part of the sea also exhibits high

salinity variance, with the Ds2 maximum coinciding

with the average position of the fast-ice edge (Fig.

3b). This is also where according to Zakharov (1997)

the Western New Siberian Polynya develops in March

or later in the year (see Fig. 1 and Table 1). The

northern boundary of the zone of increased Ds2 (Ds

2N3)

coincides approximately with the long-term average

northern limit of the Western New Siberian Polynya

(with a width of 75 km, Table 1). The southern

boundary lies within the limits of interannual variation

of the fast-ice edge (Fig. 3b).

River runoff is considerably reduced during winter-

time. Hence, interannual variability of winter river

discharge most likely only affects the area adjoining

the river mouths, as evident in Fig. 3b. It is unlikely

that the local zone of high Ds2 values situated 300–400

km away from the river mouth is the result of river

discharge interannual variability. Interanual variations

in the surface circulation regime may be one cause of

the surface salinity variance increase, especially in the

vicinity of the land-fast-ice edge.

The large-scale salinity patterns are governed by

the comparatively stable Lena discharge signal and do

not vary as much from year to year. The wind-forced

circulation is also subject to interannual variability,

with the balance between the high sea-level pressure

(SLP) center in the Western Arctic (the Siberian High)

and the Icelandic Low mainly controlling atmospheric

circulation over the Siberian Arctic (Johnson and

Polyakov, 2001). During anticyclonic circulation

1940 1950 1960 1970 1980 1990 2000

1

0.5

0

-0.5

Years

Fig. 4. The time series of winter mean (November–April) vorticity

index (mb) accounted according to Walsh et al. (1996). Bold curve

corresponds to the polynomial smoothed data.

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2714

phases (Johnson and Polyakov, 2001), the Siberian

High is well developed and the Icelandic Low is

suppressed, with winds over the Laptev Sea tending

towards the East Siberian Sea. During the cyclonic

phases, the SLP high in the Western Arctic is weaker

and the Icelandic Low is stronger, extending farther

into the Barents and Kara seas. As a result, the

prevailing wind over the Laptev Sea turns toward the

Eurasian Basin of the Arctic Ocean (Johnson and

Polyakov, 2001).

In order to evaluate the impact of atmospheric

circulation on surface salinity, the National Center for

Environmental Prediction (NCEP)/National Center for

Atmospheric Research (NCAR) Reanalysis data on

30

20

50

100

2000 1000500

10

20

2.4

1.6

Fig. 5. Correlation between interannual winter surface salinity variations a

Black bold curve corresponds to surface salinity RMSD maxima from (P

SLP were analyzed. The winter (November–April)

averaged vorticity index derived from monthly

averaged SLP data has been applied to describe the

atmospheric circulation over the Laptev Sea. The

vorticity index, first employed to describe the

circulation over the Arctic Ocean by Walsh et al.

(1996), is the finite-differenced numerator of the

Laplacian of SLP for the area within 550 km of

848N and 1258E. The winter averaged vorticity index

shows the intensity of wind vorticity in the Eurasian

Low during winter (Fig. 4).

Although larger areas of the Eastern Laptev Sea are

covered during winter by land-fast ice, the Lena River

freshwater plume is nevertheless redistributed due to

wind-forced sea level changes in the vicinity of the

fast-ice edge (Dmitrenko et al., 2002). To examine the

role of this factor, the correlation between interannual

winter salinity variations and the winter averaged

vorticity index was calculated for the same regular

grid extended to the western East Siberian Sea. The

calculation period covers all available winter surveys

in the 1950s to 1990s for the Laptev and East Siberian

seas.

Our results (Fig. 5) are in very good agreement

with the basic conclusions of Nikiforov and Shpaiyher

(1980), who postulated wind-driven redistribution of

Siberian river runoff. Under the cyclonic regime

(positive vorticity) the eastward diversion of Lena

River water results in a negative salinity anomaly to

20

40100

10

-0.6 -0.2 0.2 0.6

nd winter averaged vorticity index for the period of 1950s to 1990s.

olyakov et al., 2003). Thin solid line represents bottom topography.

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–27 15

the east of the Lena Delta and farther to the East

Siberian Sea, and a positive anomaly to the north of

the Lena Delta. Anticyclonic (negative) vorticity

results in negative salinity anomalies northward from

the Lena Delta due to freshwater advection toward the

north, and a corresponding salinity increase eastward.

The relatively high correlation values (dark red

color in Fig. 5) appear to delineate the areas in which

interannual salinity variations are mainly wind-driven.

In the Eastern Laptev Sea, this area is situated

southward of the land-fast-ice edge in a sector

associated with recurring polynyas and leads. Poly-

akov et al. (2003) published data on winter surface

salinity variations for the same period of the 1950s to

1990s. A comparison of this data with the correlation

field in Fig. 5 indicates that the area with high salinity

variance is situated within the contour where the

impact of atmospheric circulation on salinity varia-

tions is negligible (correlation coefficient ranges

within F 0.3). Therefore, we surmise that variability

in the surface wind field is only of marginal

importance for the generation of the maximum in

salinity variance northward of the land-fast-ice edge.

A period of relatively uniform vorticity index

values in the 1980s to 1990s (with mostly stable

negative values from 1980 to 1995 as compared to

earlier time periods, Fig. 4) also supports the

assumption that salinity variations are driven primar-

ily by ice production rates. In addition, the correlation

of salinity and vorticity index suggests that the impact

of winds on salinity variations is only of relevance

inside the landfast-ice edge.

Hence, the root-mean-square salinity deviation

range 2Ds=3.5–4.2 psu is representative of long-term

salinization, which is a consequence of ice formation

in the Western New Siberian Polynya (Fig. 3b). This

value exceeds the 2–2.5 psu suggested by Zakharov

(1966) but is close to the measured values presented

in the second part of this article. Estimates of 5.95–

6.34 psu (Winsor and Bjork, 2000) exceed our values

probably due to the different approach of the latter

authors. We did not consider advection of saline

waters from the polynya and, hence, obtained a lower

estimate of the possible salinity increase.

2.2.2. Method

The salinity increase in the polynya can be directly

linked to the intensity of ice formation. Based on the

Ds values, the long-term average cumulative temper-

ature below freezing (freezing degree-days, h), and on

the average long-term temperature and salinity pro-

files in the region of the flaw polynya [S,T=S,T(H),

where H is the depth], one can estimate the amount of

ice produced in the polynya. In contrast with the

method suggested here, which is based on the impact

of ice formation on the surface salinity, most other

estimates of ice formation are based on either direct

field observations of ice growth in the polynyas

(Kupetskiy, 1959; Zakharov, 1966), calculations of ice

production rates as a function of meteorological

conditions, supported by satellite observations (Martin

and Cavalieri, 1989; Cavalieri and Martin, 1994;

Dethleff et al., 1998; Winsor and Bjork, 2000) or a

combination of both (Drucker and Martin, 2003).

The basic goal is to estimate the rate of ice

production in the polynya, which corresponds to the

surface salinity variation of 2Ds. Zubov (1963)

assumed convective adjustment to the input of salt

by ice formation. His simplest convective model,

obtained with the assumption that the salinity of ice is

zero, related the ice growth I with the salinity S of the

convectively mixed layer:

Ik ¼Xki¼1

1:1Hi�1ðSi � Si�1ÞSi�1

ð1Þ

Sk ¼ f qk ; TÞð ð2Þ

where the index i indicates the depth level in the

vertical profile (i =1 is at the sea surface); Hi and Siare the depth and salinity, respectively, at level i; Hk,

Sk, and qk are the depth, salinity, and density,

respectively, of the mixed layer; Ik is the ice thickness

formed when mixing penetrates down to the level k;

and function f is the equation of state for seawater

solved numerically for salinity. The coefficient 1.1 is

the density ratio of ice and water.

To increase the accuracy of the estimates, both the

winter and summer average long-term temperature

and salinity profiles were used. The profiles were

calculated from data obtained during summer (mainly

August–September) and winter (mainly March–May)

hydrographic surveys in the Laptev Sea between 1979

and 1999 (Fig. 2) by averaging the annual measure-

ments, interpolated onto a regular grid. An example of

the average long-term vertical temperature and sal-

SkSo

-2 -1 32

40

30

20

10

016 2018 30 32

0 2 8

-2 2-1 3

30

20

10

0

16 20 30 32

0 2 8

341834

2∆S

I∆

Ho

Hk

Io Ik 6

28So Sk22

4

22 24 26 28

2∆S

Hk

4 6I, ice thickness (m) I, ice thickness (m)

T, temperature (oC) T, temperature (oC)

S, salinity (psu)

H, d

epth

(m

)

S, salinity (psu)

40

30

20

10

020 22 30 32

40

30

20

10

0

20 22 30 32

H, d

epth

(m

)

S, salinity (psu)3418 3418

-2 -1 1 2

0 2 6 8

-2 -1 1 2

0 2 6 8

T, temperature (oC)

I, ice thickness (m)Io Ik

So Sk

2∆S

I∆

Ho

Hk

2∆S

SkSo

Hk

24 28

4IkI, ice thickness (m)

T, temperature (oC)S, salinity (psu)

ba

dc

Io

I∆

Io Ik

Fig. 6. The long-term average vertical temperature and salinity profiles in the Western New Siberian Polynya and estimates of the ice thickness

Ik required to explain mixing of the upper water layer down to depth Hk; a, b—768N, 132830VE; c, d—758N, 1308E; a, c—summer profiles; b,

d—winter profiles. Green and red curves show the vertical average multi-annual salinity S(H) and temperature T(H) distributions respectively.

Blue curve is the ice thickness derived from Eqs. (1) (2) using S(H) and T(H). Estimates of Ik and Hk are also represented in Table 2.

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2716

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–27 17

inity profile in the Western New Siberian Polynya is

shown in Fig. 6.

An empirical relationship exists between the sea

ice thickness in the Arctic and the freezing degree-

days h (Zubov, 1963):

I ¼ � 25þ ð25þ IiniÞ2 � 8hh i1=2

ð3Þ

Here, Iini is the initial ice thickness at the start of the

calculation (t =0); the freezing degree-days h are

given by the cumulative temperature below freezing:

h ¼Z t

o

Tf � TaÞdtð ð4Þ

where Tf is the freezing point of water; Ta is the air

temperature; and t is time. The long-term average for

the freezing degree-days h has been derived from the

data of meteorological observations at Tiksi, Dunai

Island, and Kotelnyi Island stations from 1979 to

1994. For further convenience h is approximated by

polynomials:

h ¼ � 52:931650þ 2:269147d � 0:384070d2

þ 0:001622d3 � 0:000002d4 ð5Þ

h ¼ � 56:047127� 0:065949d � 0:347992d2

þ 0:001505d3 � 0:000002d4 ð6Þ

h ¼ � 48:037763� 0:649094d � 0:358000d2

þ 0:001572d3 � 0:000002d4 ð7Þ

Here d is the ordinal number of the day starting from

the average fall freeze-up date (October 1). The Eqs.

(5)–(7) are for the meteorological stations Tiksi,

Dunai Island, and Kotelnyi Island, respectively. The

initial ice thickness Iini on October 1 was set to zero.

Table 2

Estimates of vertical convection elements from Fig. 6 in the Western New

Position Season So (psu) Sk (psu) Io (m)

768N 132830VE summer 24.49 28.43 1.74

768N 132830VE winter 26.66 30.60 0

758N 1308E summer 21.06 25.28 1.72

758N 1308E winter 23.41 27.63 0

The second 10-day period in April was considered as

the mid-point of the winter hydrographic observa-

tions. The ice thickness Io for this time was estimated

from Eqs. (3)–(7).

Convective cooling without ice formation precedes

the freezing phase, accompanied by increase in

salinity due to brine rejection. Therefore, the average

summer profiles S ,T =S ,T(H) were recalculated

according to Eq. (2) in order to adjust the temperature

of the upper layer (up to the seasonal pycnocline) to

the freezing temperature (T=Tf). Furthermore, we

have estimated the salinity So and depth Ho of the

upper convective mixed layer at ice thickness Io from

adjusted summer long-term average temperature and

salinity profiles (Fig. 6a and c; Table 2) based on Eqs.

(1) (2). The salinity increase from So up to Sk caused

by ice formation in the polynya was determined as

2Ds. The total ice production in the polynya Ik was

derived from Eqs. (1) (2) according to:

Ik ¼ I So þ 2DsÞð ð8Þ

Finally the polynya ice production ID is described by:

ID ¼ Ik � Io: ð9Þ

Formally, ID is the ice thickness needed to

increase salinity So of the upper convective mixed

layer with thickness Ho by 2Ds. As this takes place,

the thickness of the convective mixed layer grows

up to the value Hk.

Fig. 6a and c, and Table 2 display the So, Sk, Io, Ik,

Ho, Hk, ID values calculated from the average summer

multi-annual temperature and salinity profiles for the

Western New Siberian Polynya.

Average winter temperature and salinity profiles

have mostly been obtained between April 11 and 20,

and have hence already adjusted to the ice formation

that had occurred prior to this date (Io=0). Eq. (8) is

applicable for winter profiles only, with So specified

Siberian Polynya

Ik (m) Ho (m) Hk (m) ID (m) Panel in

Fig. 6

3.95 11.8 17.3 2.21 a

2.11 0 20.5 2.11 b

3.91 9.8 13.1 2.19 c

1.32 0 12.6 1.32 d

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2718

as the average winter salinity at the sea surface. 2Ds is

taken to be the salinity increase due to ice formation in

the polynya. It hence corresponds to an increase of the

convective mixing depth up to the value of Hk (Fig.

6b and d; Table 2).

2.2.3. Results

Our calculations, performed separately for sum-

mer and winter data sets, indicate that the total ice

production Ik in the Western New Siberian Polynya

varies from 3 to 4 m depending on the data set,

and the geographical location of the individual

points (Table 2). In the northwestern part of the

Western New Siberian Polynya the ice production

estimates derived from summer and winter profiles

coincide quite well, deviating by only a few

centimeters (Fig. 6a and b; Table 2). In the

southeastern part of the polynya the differences

are greater and can reach up to 0.9 m (Fig. 6c and

d; Table 2). The substantial impact of winter river

discharge in the southeastern sector of the polynya

may partly explain these discrepancies. The hydrog-

raphy-based estimates of total ice production in the

polynya of 3 to 4 m are close to Zakharov’s

(1966) estimate of 3.6 m based on in-situ ice

observations and Zubov’s freezing degree-day

model (Eq. (3)). Winsor and Bjork (2000) have

105°E 110° 115° 120° 125°

105°E 110° 115° 120° 125°

71°N

72°

73°

74°

75°

76°

77°

78°N

30

7040

60403020

10

0

50

70

Lena D

Olenek

AnabarKha

tanga

aimyrT

Fig. 7. Probability (%) of convective mixing penetrating down to the seaflo

the fast-ice edge in mid-February.

estimated Western New Siberian Polynya ice produc-

tion as 4 km3. At 3.4 km mean polynya width and a

length of 83 km (Winsor and Bjork, 2000) this

corresponds to 14.2 m of ice production, which

appears to be too high. The Winsor and Bjork

(2000) estimate is also contradicted by the hydro-

graphic observations. If such amounts of ice were

formed, convective mixing would penetrate down to

the seafloor, which is not what has been observed in

the field (see Section 2.3; Figs. 6b,d and 7).

The magnitude of 2Ds calculated from the long-

term average data is smaller than the actual total

salinization due to advection of saline water from the

polynya, salt exchange with the underlying layers, and

ice salinity exceeding the assumption of zero psu.

With actual new ice salinities ranging around 4 psu,

total ice production based on the hydrographic data

should be considered a lower bound. Following

Zubov (1963), the difference between 0 and 4 psu

ice salinities were assessed for the ice production

estimates shown in Table 2, indicating that actual ice

production may be higher by as much as 15–17%,

with a maximum estimate of around 5 m. The

observations discussed in the second part of this

article furthermore indicate that vertical exchange

processes in the flaw polynya also have to be taken

into account.

130° 135° 140° 145°E

130° 135° 140° 145°E

71°N

72°

73°

74°

75°

76°

77°

78°N

00

10

10

20

10

20

0

0

0

010

P=0

P=0

Kot

elnyy

Yana

elta

or in the Laptev Sea. Dashed black line shows the average location of

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–27 19

2.3. Convection penetration down to the seafloor

The question of whether convective mixing in

the polynyas can penetrate to the seafloor is an

important, unsettled issue that is of prime impor-

tance for the analysis of dense water formation and

seafloor sediment dynamics, in particular the poten-

tial resuspension of sediments due to convective

action. The winter sea surface and the bottom

salinity fields (for the time period from 1979 to

1999) were used to determine the probability of

convective mixing penetrating down to the seafloor.

In the calculations, a statistical normal distribution was

assumed for the salinity at each location in all years.

Penetration of convective mixing down to the sea floor

results in an equalization of the surface and bottom

salinity. Such a mixing event has been assigned

probability P. Mathematically, P can be expressed as

the integral of the surface and bottom salinity

probability density functions fsurface and fbottombetween the point of intersection S0 ( fsurface(S0)=

fbottom(S0)) and the maximum and minimum salinities,

respectively:

P ¼Z Smax

So

fsurfacedSsurface þZ So

Smin

fbottomdSbottom ð10Þ

The probability P calculated for the Laptev Sea is

shown in Fig. 7. In general, it reflects the vertical

stratification characteristics of the water column.

Thus, low values of P (0 to 10%) in the Eastern

Laptev Sea and in the vicinity of the river mouths

reflect strong stratification due to river runoff. In the

east, only the Western New Siberian Polynya

exhibits values of P as high as 20%, with the

southern boundary of this zone located within the

boundaries of the interannually varying fast-ice edge

(Fig. 3b). Due to weak vertical density stratification

in the Western Laptev Sea, the probability of

convection penetrating to the seafloor is 2–4 times

higher and may exceed 70%. The zone of the

highest convection probability is located in the area

of the Anabar Lena Polynya. Winter surveys of the

bSeverQ expedition have repeatedly recorded the

presence of cold, saline bottom waters formed in

this Anabar Lena sector of the Laptev Sea coastal

polynya (Churun and Timokhov, 1995).

3. Circulation patterns in the Laptev Sea flaw

polynya: analysis of the data from the 1999 winter

expedition

3.1. Measurements and data sets

3.1.1. CTD, current and meteorological observations

Oceanographic investigations in the region of the

Anabar Lena and Western New Siberian polynyas in

the Laptev Sea were carried out in April–May 1999

with the help of helicopters within the framework of a

joint Russian–German project bLaptev Sea System

2000Q. The study area of the TRANSDRIFT VI

helicopter expedition is shown in Fig. 1. In order to

study oceanographic processes in the flaw polynya,

oceanographic stations TI9908, 7/22, 18/19, 14, 02/

06/23, and 24 were occupied directly at or close to the

fast-ice edge (Fig. 8a). At stations TI9902/23, 7, and

8, hydrographic measurements were conducted along

transects of 500–600 m length, perpendicular to the

fast-ice edge and extending over the fast ice, open

water, and young ice. Oceanographic data were

obtained with a ME (Meerestechnik Electronik)

OTS-3 CTD sonde (manufactured by Meerestechnik

Electronik GmBH, Germany) under the fast ice and

with a Seacat SBE 19 CTD (manufactured by Sea-

Bird Electronics, Inc., USA) deployed from a rubber

boat in open water, or directly under thin new ice.

Currents were measured with an acoustic Doppler

current meter (3D-ACM, FSI, USA).

Routine current observations were only carried out

from stationary ice, with the instrument lowered

through a hole 22 cm in diameter. Starting at the ice

bottom, measurements were taken for 3–4 min each at

2–3 m vertical spacing down to the seafloor. At each

depth level, data were averaged from 6 to 8 sets of 30-

s mean measurements. Current observations were

computed at 6 stations (TI9918, 19, 20, 22, 23, and

24, Fig. 8a). Stations TI9918 and 19 were 2.5 km

apart from each other, the first on 108 cm thick fast

ice, the other on grey young ice 24 cm thick at a

distance of 5 m from open water. TI9922 was located

100 m and TI9924 6 km away from the fast-ice edge.

Station TI9920 was located on the fast ice at a

distance of about 120 km from the flaw polynya.

The mean daily air temperature at the Tiksi and

Kotelnyy meteorological stations from October 1,

1998 to May 10, 1999 was used to estimate the

P

YN

O

AY

25.04.99

8

1017

7/22

16

18/1913

5

9 24

2/6/2314

3

121/20

2115

11

120°E 125° 130° 135°E 120°E 125° 130° 135°E

4

Fast Ice

Pack Ice

Lena Delta

Fast Ice

72°N

73°

74°

75°

76°N

72°N

73°

74°

75°

76°N

72°N

73°

74°

75°

76°N

72°N

73°

74°

75°

76°N

120°E 125° 130° 135°E 120°E 125° 130° 135°E

L

10

10 1020

2020

20

40

40

5025.04.99

20 400

Lena Delta

-1cm c

a b

ADCP

Fig. 8. TRANSDRIFT VI Expedition (April–May 1999): location of stations (a) and surface currents and ice crystal C-axis orientations (b).

Thin solid line on panel b represents bottom topography. O—Oceanographic stations (TI99..); c—vectors of surface currents; —C-axes

orientation.

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2720

freezing degree-days based on Zubov’s (1963) empir-

ical relationship (Eqs. (3) and (4)).

3.1.2. Ice crystal analysis

At stations TI19901, 2, 3, 4, 7, 8, 9, 11, 16, 17,

18, and 24 (Fig. 8a), ice cores were obtained close

to the hydrographic measurement site for an analysis

of crystal microstructure and orientation. Azimu-

thally oriented cores were cut into segments of

about 20 to 30 cm length. From each segment, one

or two vertical and one horizontal thin section (less

than 1 mm thick, 9�12 cm wide and high) were

prepared. From an inspection of thin sections

between crossed polarizing filters, we derived the

texture, size, and the azimuthal orientation of the

crystal basal planes. While the horizontal sections

indicated whether crystals were azimuthally aligned,

the vertical sections, obtained between two adjacent

horizontal sections helped identify the depth at

which alignment set in.

3.2. Evolution of the flaw polynya in the Eastern

Laptev Sea during Late winter 1998–1999

As indicated by satellite imagery (Radarsat

Scanning Synthetic Aperture Radar [ScanSAR]),

Advanced Very High Resolution Radiometer data

in the infra-red range and passive microwave data)

the system of flaw polynyas along the Laptev Sea

fast-ice edge was not very well developed or was

absent prior to the mid-March during the winter of

1998–1999. Open water occurred only in a chain of

separate, narrow flaw leads until the second half of

March, as exemplified by the Radarsat ScanSAR

image of March 21 (Fig. 9a), in which a flaw

polynya is completely absent. Kotelnyy Island

meteorological data indicate that southeasterly winds

increased up to 16 m/s after March 25, continuing

up until March 29 (Fig. 10a). Later, winds

decreased to 5–7 m/s, but their direction remained

unchanged until April 7. The opening of a polynya

(reaching up to 10 km in width) and new ice

formation (over a zone 20–23 km wide) are clearly

evident in the satellite scenes of April 5 (Fig. 9b).

On April 12 southeasterly winds of up to 12–16 m/s

picked up again, continuing until April 17 (Fig.

10a). This induced further widening of the polynya

area, and by April 25 its width had increased to 90

km in some locations (Fig. 9c). With new ice

thickening in this region, another opening event

occurred from May 4 to 7, with southerly winds

blowing at up to 16 m/s, creating an area of open

8

10

22

3

2/6/23

20

4

11

22

3

2/6/23

24

20

4

8

10

11

22

3

2/6/23

24

20

4

8

10

11

22

3

2/6/23

24

20

4

8

10

11

0 64 128 km

Lena DeltaLena Delta

Lena DeltaLena Delta

a b

c d

N

24

Fig. 9. Evolution of the polynya in the Laptev Sea between March and May 1999, as seen in Radarsat ScanSAR satellite images (a—March 21,

1999; b—April 5, 1999; c—April 25, 1999; d—May 5, 1999).

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–27 21

water and new ice up to 35 km wide (Fig. 9d). It

was these repeated, strong southeasterly winds that

fostered the opening of the polynya and widespread

formation of young ice between late March and

early May, during a period of overall low air

temperatures and subsequently high ice formation

rates (Fig. 10b).

3.3. Hydrography of the flaw polynya and the

adjacent fast ice

The impact of the flaw polynya on the salinity

distribution in the Eastern Laptev Sea has been

discussed in Section 2.2. According to Zakharov

(1966), the surface salinity is higher by about 2 to 2.5

10 15 20 25 31 5 10 15 20 25 30 5-40

-20

March

-30

-10

April May

Tem

pera

ture

(°C

)

10 20 m/s0b

a

N

Fig. 10. Wind (a) and air temperature (b) in March–May 1999, average daily data for Kotelnyy Island meteorological station.

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2722

psu (up to a maximum differential of 6 psu) in the

polynya region. Our analysis has shown that the long-

term mean increase of salinity in polynyas of the

Eastern Laptev Sea varies between 3.5 and 4.2 psu. A

system of meso-scale currents should develop in

association with the polynya due to such local salinity

variations. Information about currents in the Laptev

Sea in winter is restricted to a few, episodic

observations that are inadequate to support conclu-

sions about the specifics of the winter surface water

circulation, in particular in the narrow polynya zone.

The descriptions provided in a number of summary

articles are based more on general geographical

knowledge than on systematic observations (Dobro-

volskiy and Zalogin, 1982; Pavlov et al., 1996). Thus,

the alongshore southward current from the northwest

has been described as turning farther to the northeast,

contributing to the cyclonic surface circulation (Fig.

3a). The main features of the thermohaline and wind-

driven surface circulation can be obtained from

hydrodynamic models [for example, Pavlov and

Pavlov (1999)]. These models indicate that the

circulation is mostly controlled by the large-scale

surface salinity distribution (Fig. 3a), with winter

current velocities generally below 5–8 cm/s. The

geostrophic currents in the area northward of the Lena

River Delta are directed to the north, dominating the

offshore winter circulation regime in this area. The

local salinity increase in the polynyas and its impact

on surface flow has not been taken into account in any

of the studies referred to above, however.

Salinity increases due to ice formation in the

polynya measured along short transects across the

fast-ice edge varied between 1 psu (stations TI9902,

Fig. 11a, and TI9908) and 5 psu (station TI9923, Fig.

11b). At the stations closest to the fast-ice edge (100 m

to 2 km distance) sub-ice currents were recorded in the

surface water layer down to the pycnocline, flowing

towards the open water of the flaw polynya (stations

TI9918, TI9923a, and TI9924, Figs. 8b and 11b). The

velocity of these currents varied from 10 to 24 cm/s. In

the underlying layer, current direction (except at station

TI9918) changed by 90–1808. A maximum current

velocity of 62 cm/s, directed to the northeast (268), wasmeasured in the sub-ice layer directly at the fast-ice

edge at station TI9923. At a depth of 4.6 m, directly

underneath the pycnocline, the current was setting in

almost the opposite direction (2398) at a velocity of 26cm/s (Fig. 11b). The direction of 268 was normally

oriented to the local position of the fast-ice edge.

During measurements at station TI9923 the weather

was relatively calm, with winds below 5 m/s from the

northeast. A similar situation was observed at the fast-

ice edge at St. TI9922, where a current of 5.9 cm/s was

flowing northwestward (3208) towards open water at adepth of 3.8 m. In the pycnocline at 8.5 m depth, a near-

opposite current (1158) with a velocity of 5.8 cm/s was

registered. Below the thin new ice, the sub-ice currents

were homogeneous from the surface to the pycnocline

depth, and opposite to the sub-ice currents below the

fast-ice edge (stations TI9919 and TI9923d, Fig. 11b).

Thus, at the fast-ice edge, two jets constitute the current

system in the polynya, with water in the sub-ice layer

directed towards the polynya and flow beneath the

pycnocline in the opposite direction.

A bottom-moored upward-looking Acoustic Dop-

pler Current Profiler (ADCP) has been deployed in

September 1998 for a 1-year period in the eastern part

TI9923TI9923bTI9923a TI9923c TI9923dnew ice new iceice edgefast icefast ice

TI9902bTI9902TI9902a TI9902c TI9902dopen water open waterice edgefast icefast ice

Dep

th (

m)

Dep

th (

m)

Distance (m)

b

a

N

0 50 100 150 200 250 300

0 20 40cm/s

5

10

15

20

5

10

15

20

Fig. 11. Distribution of salinity and currents along the short transects across the fast-ice edge at stations (a) TI9902 (April 19, 1999) and (b)

TI9923 (May 5, 1999).

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–27 23

of the Laptev Sea (Fig. 8a). During April–May 1999

the mooring was situated in the vicinity of the fast-ice

edge near station TI2499 (Fig. 8a). The ADCP

observations have shown that the semidiurnal fre-

quency band of currents appears to be dominated by

tides (Dmitrenko et al., 2002). During the ice-covered

period, a strong baroclinic internal tide with maximum

current of 20 cm/s at the density interface was recorded.

In general, however, the maximum tidal current (lunar

semidiurnal tide M2) did not exceed 5–6 cm/s

(Dmitrenko et al., 2002) during the time interval of

the polynya’s existence. The tidal energy was rather

uniformly distributed with depth. Therefore the dis-

tribution of currents observed at station TI9923 could

not be explained by the baroclinicity of the internal tide.

3.4. Ice texture, crystal orientation and circulation

patterns in the flaw polynya and the adjacent fast ice

It has been shown in several studies that consistent

sub-ice currents determine the azimuthal crystal

alignment in a sea-ice cover (Weeks and Gow, 1978,

1980; Stander and Michel, 1989; Strakhov, 1989).

The dependence of crystal alignment on the current

regime, hypothesized based on field measurements,

has been verified in laboratory experiments (Cher-

epanov and Strakvov, 1989). Hence, the data on

crystal alignment in fast-ice cores collected as part of

this study can provide insight into the under-ice

currents during the period of ice growth.

Thin-section analysis revealed an azimuthal align-

ment of ice crystals at stations TI9901, 2, 3, 4, 8, 16,

18, and 23 (Figs. 8b and 12). At stations TI9901, 2, 4,

8, 16, and 18, crystal C-axes were generally trending

in a southeasterly to northwesterly direction, whereas

at station TI9924 the alignment was south to north.

Based on the stratigraphic analysis of horizontal and

vertical thin sections, we were able to determine the

approximate level of the onset of crystal alignment (as

exemplified in Fig. 12 for station TI9902). To convert

the depth of crystal alignment onset to an actual date,

we computed the change in ice thickness with time

based on the empirical relationship expressed in Eqs.

(3) and (4), forced by daily temperature data from the

meteorological stations on Dunay Island (Eq. (11))

and Kotelnyy Island (Eq. (12)) during the winter of

Fig. 12. Thin-section photographs of sea ice microstructure at the station TI9902 recorded between crossed polarizers.

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2724

1998–1999. The age of ice (d) at depth level was then

derived based on the freezing degree-days h according

to the polynomial approximations:

h ¼ � 21:028120� 5:433169d � 0:220287d2

þ 0:000554d3 � 0:0000002d4 ð11Þ

h ¼ � 20:366962� 7:805000d � 0:164232d2

þ 0:000223d3 þ 0:0000008d4 ð12Þ

The dates of crystal alignment onset that have been

determined based on this method are listed in Table

3. Note that a more detailed analysis with a multi-

layer ice-growth model taking into account radiative

and other forcing (including measured snow depth)

yields only slightly different dates (Eicken et al.,

this volume).

Table 3

Approximate onset dates of ice crystal spatial alignment in the

southern Laptev Sea during the winter of 1998–1999

Station

number

(TI99..)

Date

(dd.mm.yy)

Ice

thickness

(m)

Ice thickness

at onset of

alignment

(m)

Approximate

date of onset

of alignment

(dd.mm.yy)

1 17.04.99 2.08 1.00 16.11.98

2 19.04.99 0.92 0.70 26.03.99

3 21.04.99 1.68 1.35 25.02.99

4 23.04.99 1.60 0.80 13.01.99

8 26.04.99 0.68 0.30 26.03.99

16 30.04.99 2.08 1.70 13.02.99

18 01.05.99 1.08 0.80 24.03.99

24 06.05.99 1.39 0.90 27.02.99

At all stations except TI9903, the crystal alignment

in the ice cores confirms the general geostrophic

surface circulation regime. Both the direction of

alignment and the surface current measurements at

stations TI9918, 22, and 23 correlate well with the

surface circulation obtained from hydrodynamic

modeling by Pavlov and Pavlov (1999). However,

as is evident in Table 3, crystal alignment at the fast-

ice margin stations TI9902 (Fig. 12), 8, and 18 only

sets in at the end of March. At the stations close to the

flaw polynya (TI9903 and 24) the onset of alignment

occurred in mid- or late February. At those locations

separated by a greater distance from the flaw polynya

(TI9901 in fast ice and TI9904 in drifting ice),

alignment commenced much earlier in the year.

Hence, crystal alignment in the lower part of relatively

thick fast ice (TI9903, 09, 10, and 11) is controlled by

different physical forces than alignment in young ice

formed adjacent to the flaw polynya. The onset of

alignment along the fast-ice margin (Table 3, March

24–26) coincides with the intensification of south-

easterly winds on March 25. The latter induced

expansion of the flaw polynya (Figs. 9a and b, 10a).

This suggests that crystal alignment near the fast-ice

edge is due to the establishment of persistent sub-ice

currents oriented towards open water in the flaw

polynya. In the remaining cases studied here, align-

ment is most likely controlled by the geostrophic or

tidal circulation patterns.

Persistent sub-ice surface currents that are oriented

towards open water adjacent to an ice cover have been

observed repeatedly in leads in the Arctic ice cover

during winter (Smith et al., 1990; Morison et al.,

1992). Smith (1973) was the first to explain that these

pack ice fast iceflaw polynya

brine

pycnoclinepycnocline

pycnocline

rejection

Fig. 13. Schematic depiction of the cellular circulation in the flaw

polynya (isohalines indicated by solid lines).

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–27 25

currents are the result of circulation cells induced by

an increase in surface layer salinity due to ice

formation. Thus, the surface circulation is driven by

salt rejection from the growing ice sheet, resulting in

off-ice flow of the upper water layers and a counter-

current developing below the quasi-homogeneous

sub-ice layer that sets towards the ice cover (Fig.

13). Velocities of such currents have been observed to

vary between 2 and 12 cm/s (Morison et al., 1992).

Smith and Morison (1993) derived the circulation

pattern under leads and their impact on the hydrog-

raphy of the Beaufort Sea. For the case of free

convection the resulting current velocity was esti-

mated at 8 cm/s, with convective cells approximately

1000 m in lateral extent.

The general character of the circulation and the

density structure measured along the short transects at

the fast-ice margin corresponds closely to those

calculated by Smith and Morison (1993) for the case

of free convection forced by salt rejection during ice

growth. Hence, the surface currents are most likely

due to salinization of the surface water caused by ice

formation in the flaw polynya, resulting in the

circulation patterns and crystal alignment discussed

above (see also Fig. 13).

4. Conclusions

1. The persistence of a quasi-stationary system of

flaw polynyas is one of the main factors respon-

sible for the variability of the surface hydrography

in the Eastern Laptev Sea. The salinity increase in

the upper water layer caused by ice production in

the Western New Siberian Polynya by mid-April

corresponded closely to the double root-mean-

square deviation from the average long-term sur-

face salinity (2Ds) in the Laptev Sea in winter,

amounting to 3.5–4.2 psu. These estimates were

confirmed by salinity measurements in the polynya

in April 1999.

2. A new method to evaluate the long-term average

annual ice production in the flaw polynya has been

described, based on hydrographic observations

rather than sea-ice data. The extent and depth of

convective mixing during winter has been calcu-

lated based on long-term average temperature and

salinity profiles (winter and summer). The total ice

growth required to increase the long-term average

surface salinity by 2Ds has been taken to corre-

spond to the ice production in the flaw polynya.

The value of 3–4 m total ice production by mid-

April agrees well with estimates based on direct ice

observations but is much smaller than results of

model calculations based on satellite data.

3. The probability of winter convective mixing

penetrating down to the seafloor has been linked

to the distribution of river runoff in the Eastern

Laptev Sea and the location of the main flaw

polynyas. In the Eastern Laptev Sea penetration of

convection down to the seafloor is most probable

in the Western New Siberian Polynya (20%),

whereas in the Western Laptev Sea the probability

is highest in the Anabar Lena Polynya (70%).

4. Preferred alignment of ice crystals in the ice cover

along the fast-ice margin of the Anabar Lena and

Western New Siberian Polynya was identified

during ice studies in the spring of 1999. This

alignment is most likely due to the development of

quasi-stationary circulation cells resulting from

convergent flow in the polynya, forced by the

salinization of surface waters as a result of ice

growth. The surface currents are directed towards

the open water of the polynya and can reach

velocities as high as 62 cm/s. In the underlying

water layer, a countercurrent is established, thereby

promoting the turbulent exchange of heat and mass

with the underlying ocean.

Acknowledgements

This research was supported by the German

Ministry for Science and Education and National

I.A. Dmitrenko et al. / Global and Planetary Change 48 (2005) 9–2726

Science Foundation (OPP-9876843). Our colleagues

M. Antonov, V. Churun, S. Kovalev, and M. Schmidt

have rendered invaluable assistance in performing

measurements in the Laptev Sea in spring 1999. The

authors are very grateful to V. Gribanov for processing

the historical oceanographic data. Four anonymous

reviewers provided helpful comments and criticism.

The English manuscript was kindly improved by Dr.

Candace S. O’Connor.

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