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The near surface hydrography beneath the Odden ice tongue
Deep-Sea Research II 46 (1999) 1301}1318
The near surface hydrography beneath the Odden ice tongue M.A. Brandon*, P. Wadhams Scott Polar Research Institute, University of Cambridge, Lensxeld Road, Cambridge CB2 1ER, UK Received 26 February 1998; received in revised form 4 November 1998; accepted 10 November 1998
Abstract As part of the European Subpolar Ocean Programme (ESOP), the German research icebreaker Polarstern worked in the Greenland Sea in the late winter of 1993. Whilst on passage, the ship encountered a severe winter storm with winds consistently above 20 m s\ coupled to air temperatures of below !103C. The underway sensors revealed heat #uxes of greater than 700 W m\ across most of the Nordic Basin, peaking at greater than 1200 W m\ when the ship crossed the cold, fresh water of the Jan Mayen Current. This large heat #ux coupled to the unique hydrographic conditions present in the Jan Mayen Current allowed sea-ice generation in the form of frazil ice at a rate of 28 cm d\. This frazil ice then developed into pancake ice. Measurements also were made in the late winter beneath this pancake ice in two remnants of the Odden. In the Jan Mayen Current, hydrographic conditions are such that the ice can exist for a long period of time before eventually decaying due to short-wave radiation at the surface. Towards the centre of the Greenland Sea, hydrographic measurements reveal that the ice is more transient and decays four times more rapidly than ice in the Jan Mayen Current. We discuss the development of the Odden ice tongue in light of these results and add evidence to the argument that the eventual fate of the water stored in the ice is important and could be a relevant factor in the formation of Greenland Sea Deep Water. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The Nordic Basin is the general term for the enclosed basin that contains the Icelandic Sea, the Norwegian Sea and the Greenland Sea. It lies between the Arctic
* Corresponding author. Now at British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK. Tel.: 0044-1223-221400; fax: 0044-1223-362616. E-mail address: [email protected] (M.A. Brandon) 0967-0645/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 0 2 4 - 7
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Basin and the North Atlantic Ocean and has been recognized as a region of deepwater formation for almost 100 years (Nansen, 1902). Although the precise mechanism is still not well understood, the region is thought to drive much of the ventilation of the world's oceans that occurs in the northern hemisphere (Aagaard and Carmack, 1994; Killworth, 1983; Schmitz, 1995). Within the basin the sea ice area varies seasonally by up to 6;10 km (Gloersen et al., 1992), and although on a global scale this is relatively small, it represents a local variation within the basin of almost 200%. This area of ice is made up partially from the export of multiyear ice through Fram Strait (Vinje and Finneka sa, 1986) and partially through the development of a winter ice feature that extends from the East Greenland Current (EGC) across the southern part of the Greenland Sea. This ice feature is known as the Odden (Wadhams, 1981). The Odden ice tongue develops in winter over the cold, fresh Jan Mayen Current (JMC), which forms the southern limb of the Greenland Sea gyre (Bourke et al., 1992). There has been much debate about whether the sea ice in the Odden is locally grown or advected into the central Greenland Sea from the EGC, with some authors neglecting this very important point. If the ice in the Odden is locally grown there will be salt rejection and resulting convection, but as the Odden melts the net in#ux of fresh water is zero. If the ice is advected from the EGC it will represent a net contribution of fresh water to the Greenland Gyre and therefore increase the vertical stability of the water column, inhibiting convective overturning. The Odden ice tongue has been shown to vary greatly in size from year to year (Vinje, 1977) and on smaller time scales down to 2}4 d (Gloersen, 1990). This rapid size variation strongly suggests local ice growth that would have a signi"cant e!ect on the upper ocean. Satellite evidence of such ice growth and historical hydrographic data led Pawlowicz (1995) to suggest that changes in the extent of the ice cover in the Odden could be used as a proxy for characterising the onset and depth of convection in the Greenland Sea. However, Pawlowicz (1995) had only a limited hydrographic data set when ice was actually present. In this paper we present hydrographic measurements from late winter 1993 taken either in the presence of sea ice or beneath a sea ice cover. Initially, using data from a transect of the Nordic Basin we show how a pancake ice cover develops in the Greenland Sea, and then calculate the sea}air heat #uxes and the resulting rate of ice formation. Visits later in the winter enabled the calculation of ice melt rates. This enables us to show that, like in the Arctic Ocean, the short-wave radiation #ux is the most important factor in the decay of the pancake ice cover. Finally, we discuss the relevance of the Odden to the formation of Greenland Sea Deep Water in the light of our measurements.
2. The transect across the Nordic Basin Measurements were taken on board the German ship RV Polarstern on its "rst winter deployment in the Nordic Basin as part of ESOP in the winter of 1993. Polarstern sailed from Bremerhaven to Fram Strait, making a transect of the Nordic Basin from 2}5 March. The portion of the transect between 703 and 803N (Fig. 1) was
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Fig. 1. The station positions in the Nordic Basin in the winter of 1993 along with the cruise track of F.S. Polarstern through the Nordic Basin 2}5 March.
designed to cross a region of the Odden that had been visited on 12}14 February 1993 by M.V. Northern Horizon. On this previous visit the Odden (Fig. 1) consisted entirely of pancake ice embedded in frazil ice (Tadross and Brandon, 1993). However, as Polarstern neared the region of the previous Odden a severe winter storm prevented any scienti"c operations other than logging the ship's underway sampling system. 2.1. The surface data Fig. 2 shows the surface temperature and the salinity of the shallowest hydrographic bottle of the CTD measurements of ARKTIS IX/1, the CTD stations being taken between 25 March and 18 April 1993 (Meincke et al., 1994). These late winter data show that the cruise track on 3}5 March 1993 crossed the JMC, and that the Odden ice tongue was over the JMC. From 703N to almost 733N the ship was sailing through warm, saline Atlantic-derived waters in the Norwegian Atlantic Current (Figs. 2 and 3). Just south of 733N the ship crossed the Arctic Front, and both the sea surface temperature and salinity fell rapidly. At this late stage in the winter the gradients were greater than those encountered by van Aken et al. (1995) when the Meteor crossed the Arctic Front in November 1988. Water temperatures began to rise north of 74.33N as the ship left the JMC. The salinity was slightly di!erent, with a peak in the signal within the cold JMC water. After a malfunction in the package,
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Fig. 2. The potential temperature and salinity at the shallowest hydrographic bottle from the Polarstern ARKTIS IX deep CTD stations, 23 March to 18 April 1993.
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Fig. 3. Data from the Polarstern underway sampling system on the transect from 2}5 March 1993. (A) the sea surface temperature and sea surface salinity; (B) the wind speed and air temperature. The latitudes in which ice was observed are bracketed by the two vertical chained lines.
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from 74.9}76.33N the temperature and salinity structure became more variable, and close to Fram Strait the changes were rapid and large. The ice edge was reached at 79.53N and the rapidly varying observations towards Fram Strait suggest that Polarstern crossed small eddies, common close to the ice edge (Johannessen et al., 1987). During the crossing of the Arctic Front the wind was consistently strong, peaking at 23.1 m s\ (Fig. 3b). This strong wind, coupled with low atmospheric temperatures (averaging } 12.63C between 72.4 and 74.53N), created conditions under which ice could form in the open ocean. Despite large areas of pancake and frazil ice in February, ice was encountered only on 3 March between the latitudes of 73.43N and 73.83N, as frazil ice slicks and brash ice (Vieho! et al., 1993). The salinity pulse noted within the chained lines in Fig. 3 suggests salt rejection from the formation of the frazil ice. Photographs of the ice show many whitecaps visible from breaking waves and a slick of frazil ice with small pieces of brash ice embedded (Fig. 4a), as well as a more developed frazil slick with small pancakes #oating in it (Fig. 4b). 2.2. Atmospheric heat yuxes We can use the underway sensor data (Fig. 3) to derive the sea}air heat #uxes that drive the formation of the ice in Fig. 4. The heat exchange in ice-covered waters has been covered previously in great detail (e.g. Maykut, 1986; Parkinson and Washington, 1979), and here we consider the atmospheric terms of the heat budget. The atmospheric heat budget consists of four terms; the short wave radiation, Q , the long wave radiation, Q , the sensible heat, Q , and the latent heat of evaporation, Q . The net atmospheric heat #ux, Q , is then Q "(1!a)Q #e(Q !p¹ )#Q #Q .
(1)
In (1) a is the surface albedo, estimated as 0.5 for the ice/water mix, e is the emissivity of water and taken to be 0.97, and p is the Stefan}Boltzmann constant, which is 5.67;10 W m\ C\. Fig. 5 shows Q against latitude along with the four indi vidual components for the transect (a positive #ux being from the ocean to the atmosphere). The net short-wave radiation is negligible along with the net long-wave radiation north of 723N. Thus the greatest variability within the net atmospheric heat #ux is within the turbulent terms of sensible and latent heat. South of 70.73N, Q is negative and the surface of the ocean is being heated. As Polarstern heads north, the sign of Q changes and cooling becomes strong as the turbulent terms increase in magnitude, reaching an intense maximum of almost 1250 W m\ just south of the Arctic Front. At this maximum the ocean is warm (almost 3.53C, Fig. 3) and no ice can form. This is always the case in this region as heat is constantly supplied in the Norwegian Atlantic Current. In contrast, above the JMC, Q ranges between 850 and 1050 W m\. This heat #ux associated with the cool, fresh surface layer (Figs. 2 and 3) drives the surface layers below the freezing point, forming ice in the open ocean.
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Fig. 4. (a) A small slick of frazil ice and brash ice developing on 3 March 1993. (b) A more developed frazil ice slick on 3 March 1993.
2.3. Open-ocean ice formation For the conditions present over the JMC, the problem of seeding the ocean to allow ice formation is solved by the ambient weather conditions and the existence of the brash ice debris (Fig. 4). The strong cold winds and confused sea with surface waves up to 7 m threw up much spray and spume that froze rapidly, causing severe superstructure icing problems on Polarstern. Each of these frozen droplets, when returning to the ocean, can start secondary nucleation when the surface is at the freezing point, creating the thin frazil ice slicks visible in Fig. 4a. After initial formation the frazil
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Fig. 5. The net atmospheric heat #ux from the ocean to the atmosphere for the transect across the Nordic Basin 2}5 March 1993 (positive represents a heat #ux from the ocean to the atmosphere).
slicks grow through secondary nucleation, creating larger and thicker slicks even after the storm has passed. Fig. 4b shows a close up of a dense frazil ice slick, its thickness is estimated as between 5 and 10 cm. The surface of the ocean now acts as if it had a mass loading, with the ice/water mix behaving as a slurry. Inside the slick (the mass-loaded region) the higher frequency waves are damped out; thus only the lower frequency swell can propagate. In contrast, Fig. 4b shows that outside the slick breaking waves are still visible. This selective damping of higher frequency waves allows frazil ice to be detected using Synthetic Aperture Radar (SAR) imagery, the frazil ice appearing dark in images (Wadhams and Holt, 1991). Within the slick, the lower frequency wave energy allows the individual crystals to bond, creating the small pancakes that then grow. In the absence of damping of the lower frequency waves, the pancakes will not freeze together, as has been observed in the Weddell Sea (Lange et al., 1989), and they remain a loose ice cover. As the frazil ice forms and subsequently grows, the rejected salt will increase the density of the surface layers of the ocean. It is di$cult to infer much about the hydrographic structure because of lack of data; however, some features are apparent. The intense winds and rough seas deepen the mixed layer; also salt rejection from ice formation and subsequent growth increases its density. As the mixed layer deepens,
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warmer Lower Arctic Intermediate Water (LAIW) is entrained from depth to mix with and replace the cooler dense saline layer developing due to the ice growth. This rising warm water acts against continued ice growth, supplying heat that may melt the pancakes. Beneath the upper highly turbulent surface layer the convection is then thought to become ordered into plumes (Rudels, 1990), which would be similar to those observed in laboratory and numerical experiments (Jones and Marshall, 1993; Maxworthy and Narimousa, 1994). Carsey and Roach (1994) have suggested that from these ordered convection cells, the rising patches of warm water would melt the frazil ice slicks in Fig. 4 into patterns that could be observable in SAR imagery. This issue is discussed further by Backhaus and KaK mpf (1999) and KaK mpf and Backhaus (1999). We use bulk formulations to derive parameters relevant to the convection process. Over the JMC, which is close to freezing and being seeded by the freezing spume, the rate of production of ice is given by Cavalieri and Martin (1994) as dh Q " , dt o¸
(2)
where o is the density of the frazil ice, taken as 900 kg m\, and ¸ is the latent heat of fusion of ice which is 3.34;10 J kg\. Typically, where ice is forming, the rate of production is high at 3.25;10\ m s\ (approximately 28 cm d\). We cannot determine the east}west extent of the 30 km of the transect in Fig. 1 that had growing ice because of the absence of suitable SAR imagery; we therefore consider an area of 30 km;30 km. The volume of ice formed in one day is then Q ;86400;(9;10) Vol" o¸
(3)
and the salt rejection from this area, *s is given by *s "Vol(s!s )10\,
(4)
where s is the salinity of the ice given by s "0.31 s (Martin and Kau!man, 1981). Using (3), the volume of ice formed is 2.457;10 m and from (4) the resulting salt #ux is then 5.282;10 kg. Using a mixed layer of typical thickness in the Greenland Sea of 100 m (Clarke et al., 1990), the salinity would increase by 0.065 PSU d\. The temperature of the surface layer was at the freezing point (Fig. 3), and the resulting density increase would be 0.053 kg m\ d\.
3. The late winter measurements The next two visits to the region of the Odden were on 3 and 10 April 1993 as part of the full-depth CTD programme of the ARKTIS-IX/1 campaign. By this stage in the winter of 1993 the Odden had separated into two discrete concentrations of ice (Wadhams and Wilkinson, 1999). The southern concentration of ice, named the South
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Island and centred around 733N 1.53W (Fig. 1), was visited on 3 April 1993. The northern concentration, called the North Island, was centred around 753N 03E and was visited on 10 April 1993. Each island had very di!erent ice and oceanographic conditions. On visits to each island, samples of individual pancakes and frazil ice were taken along with shallow CTD stations with a Seacat SBE-19 pro"ler and a deep CTD station with a Seabird SBE-9. 3.1. The South Island of the Odden The ice cover in the South Island was di!erent to that observed on 12 February from the Northern Horizon and consisted almost exclusively of very large pancakes with the addition of the occasional large multiyear ice #oe, the latter showing heavy ablation. The pancakes and multiyear ice #oes were embedded in a dense frazil ice layer that was measured as being approximately 10 cm thick (Brandon, 1995). Wadhams and Wilkinson (1999), on the basis of structural analysis, suggest that the pancakes at the South Island could be up to 35 d old. This would imply that once the pancakes were formed and after the initial haline- and wind-driven convection, conditions would have been stable within the Odden. Near-surface potential temperature}salinity data from the mean casts of three 1-h deployments of the shallow CTD and a simultaneous deep CTD station from Polarstern (station 40) (Fig. 6) show that the mixed layer was approximately 30 dbar thick. The most obvious feature was a `kneea in the pro"les at 65 dbar, potential temperature !1.8523C and salinity 34.665 PSU (Fig. 6). This knee is not a direct product of the mixing of any local water masses present but rather implies that the surface waters had advected into the region. Lacking nutrient tracer data, the origin of the surface water is indeterminate; however, the proximity of this station to warmer, more saline water to the northwest (Fig. 2) would suggest exchange with these waters. The CTD data in Fig. 6 show no impact from melt water from the pancakes. This will be discussed below. 3.2. The North Island of the Odden Wadhams and Wilkinson (1999) show that the only ice types at this station, pancake and frazil ice, were very similar to the ice observed on 14 February 1993 from the Northern Horizon. Wadhams and Wilkinson (1999), again using structural analysis of the ice, estimate the maximum age of the pancakes as approximately 5 d. Fig. 7 shows the mean of pro"le for almost 1 h of shallow CTD measurements made beneath the pancake ice cover. All the measurements were within the 100 dbar-thick mixed layer that was revealed by a CTD station (Fig. 8). Both the temperature and salinity of the surface waters decreased towards the surface (Fig. 7). At this station the ratio of the coe$cient of haline expansion to the thermal coe$cient of expansion, b/a, was greater than 30.0 and thus the small salinity gradient determined the stable density structure shown in Fig. 7c. Moore and Wallace (1988) have shown that the addition of ice melt water to a water mass means that the water will follow a line in potential temperature}salinity space
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Fig. 6. The potential temperature-salinity plot of the near-surface data from three 1-h Seacat SBE-19 deployments and deep CTD station 40 on 3 April 1993.
given by d¹ ¸ " ds c (s!s ) N
(5)
and ¸ is the latent heat of ice salinity s given by
s ¸ "¸ 1! , s
(6)
where ¸ is the latent heat of pure ice (3.34;105 J kg\). A line de"ned by Eq. (5) is shown in Fig. 7d, a potential temperature against salinity plot, as a dotted line that clearly matches the potential temperature}salinity data. This indicates that the surface waters are principally derived at this stage in the winter by the addition of melt water from the decaying pancake ice. The increase in the gradient of the potential temperature in the surface of 4 dbar is because of the addition of the pancake melt water is mixed over this shallow depth by surface wave energy.
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Fig. 7. The mean pro"le from one hour of shallow data recorded at the North Island of the Odden The mean is the solid line and the dotted line is one standard deviation from the mean. 7(d) is a potential temperature}salinity pro"le for the mean cast along with a `melt linea as calculated from the formula of Moore and Wallace (1988).
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Fig. 8. The potential density referenced to the surface at the shallowest hydrographic bottle from the Polarstern ARKTIS IX deep CTD stations, 23 March to 18 April 1993.
3.3. Ice melt rates We can calculate the rate of addition of melt water to the surface waters demonstrated by Fig. 7d and use the empirical model by McPhee (1992), which is based on results from the MIZEX and CEAREX experiments. We can then estimate the possible further lifetime of the pancakes. McPhee (1992) demonstrated that instead of solving the full boundary-layer formulation for the #uxes at the ice}ocean interface, a simple drag law can be used to parameterize the ice}ocean temperature #ux. The ice}ocean temperature #ux, F , (3C m s\) is then given by F "c u (¹!¹ ), O
(7)
where c is a transfer coe$cient estimated from CEAREX data as 0.0055, ¹ is the ocean temperature, ¹ is the melting point of the sea ice, and u is the ice}ocean skin O friction given by u ,q, where q is the surface wind stress. Steele and Morison (1993) O showed that under similar conditions, u varies between 0.01 and 0.02 m s\, and for O these calculations a representative value of 0.01 is used. Eq. (7) is now multiplied by the speci"c heat of sea water, C , and the density of the water, o , to give Q , the heat N #ux from the ocean to the ice (W m\) Q "C o F . N
(8)
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Table 1 The heat #ux and melt rate at the ice}ocean interface Place and date
F (3C m s\)
Q (W m\)
dh /dt (cm d\) G
South Island: 3 April 1993 North Island: 10 April 1993
1.5;10\ 3.3;10\
6.3 13.7
0.2 0.8
Finally, to calculate an ice melt rate from this heat #ux we divide Eq. (8) by the latent heat, ¸ , and density, o , of the sea ice to give dh Q " . (9) dt ¸o The calculated melt rate from Eq. (9) will be an underestimate as the model assumes melting only from the underside of the pancakes, and excludes the frazil ice. In reality the #oes also melt from the sides and, as the #oe diameter decreases, the signi"cance of lateral melt increases (Steele, 1992). The rate of #oe melting is also a function of wave energy, with higher wave energy increasing the speed of melting (Wadhams et al., 1979). The results of Eqs. (7), (9) with local values of the input parameters are presented in Table 1. For both islands of the Odden the melt rate from the underside of the pancakes is low. Calculated melt rates suggest that the pancakes at the South Island could survive for a further 100 d, but only 20 d at the North Island, assuming similar atmospheric and oceanic conditions. This would be an extremely long lifetime for the pancakes at the South Island, and is clearly physically unrealistic. The contradiction is solved by calculating the full heat budget equation (1) with the addition of Q , which reveals that at this stage in the winter the controlling factor in the heat budget is the incident short-wave radiation. Brandon (1995) and Wadhams and Wilkinson (1999) have noted the existence of melt pools on the surface of the pancakes at the South Island. These pools reduce the surface albedo of the ice and would have a signi"cant impact on increasing the speed of the pancake decay. The calculation, however, does show the stable nature of the pancakes at the South Island once formed, the ice at the North Island being more transient.
4. Discussion To discuss our results on the hydrographic structure beneath the Odden ice tongue we "rst must put the ice tongue into context in the role of formation of the important water mass, Greenland Sea Deep Water (GSDW). The Odden ice tongue has been implicated in the formation of GDSW (e.g., Clarke et al., 1990; Rudels, 1990), and here we encounter a conundrum. The density of the shallowest hydrographic bottle at the surface of the Greenland Sea (calculated from the data in Fig. 2) reveals that the densest surface water encountered in the Greenland Sea was in a pool of open water to the northwest of the Odden. This region is frequently ice free in winter, and is called Nordbukta.
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Wadhams (1981) described the development of the Odden as being initially a broad swath of ice across the Greenland Sea before the ice retreats and Nordbukta develops. This scenario has been con"rmed by passive microwave data (e.g., Visbeck et al., 1995), although in some years, like 1993, the Odden only forms above the JMC in the southern part of the Greenland Gyre. When the Odden initially develops in the open ocean, conditions mean that ice generation will follow the scheme detailed in Section 2.3 and develop into pancakes, the multiyear ice exported from Fram Strait tending to remain in the EGC (Vinje and Finneka sa, 1986). To facilitate this ice formation across the sea (and ultimately limit its thickness) there must be a su$cient salinity strati"cation at the surface to limit the cooling required for ice formation. This strati"cation can arise in two ways; the fresh water could be a remnant from the previous winter or it could arise from a greater than average ice export from the Arctic Ocean. Small anomalies of the multiyear ice export through Fram Strait are common (N. Davis, personal communication, 1995), and once in the Nordic Basin it would be the following winter before the resulting fresh-water anomaly would have advected around the Greenland Gyre to the region of the Nordbukta. When combined with the conditions demonstrated in Figs. 3 and 4 this strati"cation would allow ice to grow over the entire Greenland Sea, inducing haline-driven convection. There are now two possibilities to remove the pancake ice from the region of Nordbukta. The "rst is that the haline-driven convection brings warmer LAIW towards the surface, the stored heat melting the ice layer initially grown and representing no net input of salt. Another mechanism is through prevailing northerly winter winds exporting ice to the south. Using a one-dimensional ice}ocean model, Visbeck et al. (1995) have demonstrated that ice export in this way is possibly a prerequisite for deep convection. In reality the opening of Nordbukta most likely would result from a mixture of these two processes. Further convection in the now-open Nordbukta would deepen the mixed layer. Schott et al. (1993) have shown that once the ice cover is removed from Nordbukta the mixed layer deepens rapidly. Simultaneous acoustic tomography showed that this deepening to be coupled to cooling extending from the surface to the intermediate layers of the Greenland Sea (Worcester et al., 1993). Once the intermediate layers are cooled, for deeper convection and the "nal stage of GSDW formation, the dependence of compressibility on temperature may become signi"cant (BudeH us et al., 1993). The haline strati"cation in the JMC is greater than that in the central Greenland Sea (Brandon, 1995); thus more ice can grow without releasing heat from the deeper water. Schott et al. (1993) showed, with data from a mooring that in the JMC close to the South Island, that conditions throughout the winter were similar to the ARKTIS IX/1 data, i.e. reasonably stable. Here the mixed layer would deepen slowly throughout the winter after the initial pancake-ice formation.
5. Summary Conditions such as those presented above cool the surface of the ocean su$ciently to enable ice formation. The ice forms as frazil slicks, which then develop into
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pancakes. Depending on the severity of a storm or the degree of haline strati"cation, the Odden can grow across the entire Greenland Sea. This ice formation can be self-destructive, as may be the case in the Nordbukta region of the Odden. In the region of the JMC, such as the South Island, the greater strati"cation means that the pancakes can survive. After initial pancake formation, the salt #ux into the surface layer will decrease as the ice develops slowly over a winter. The decay of the pancakes in late winter in the more stable region is driven at the surface by short wave radiation and not from the ocean}ice heat #ux. As previous authors have suggested (Rudels, 1990; Carsey and Roach, 1994) sea ice is an important component in the Greenland Sea system. However, the fate of the ice is the relevant factor. In the Nordbukta region, depending on suitable conditions, this may lead to eventual deep-water formation; in the southern Greenland Sea Gyre the ice is stable and melt water returned locally.
Acknowledgements The authors thank J. Meincke and G. BudeH us for the CTD bottle data and Steven Wells, Eleanor Prussen and David Crane for their extensive help during the ARKTIS IX/1 expedition. This work was funded by the Commission of the European Communities under contract MAS2-CT93-0057 of the MAST-II programme.
References Aagaard, K., Carmack, E.C., 1994. The arctic ocean and climate: a perspective. In: Johannessen, O.M., Muench, R.D., Overland, J.E. (Eds.), The Polar Oceans and their Role in Shaping the Global Environment. American Geophysical Union, Washington, DC, pp. 5}20. Backhaus, J.O., KaK mpf, J., 1999. Simulations of sub-mesoscale oceanic convection and ice}ocean interactions in the Greenland Sea. Deep-Sea Research II 46, 1427}1455. Bourke, R.H., Paquette, R.G., Blythe, R.F., 1992. The Jan Mayen Current of the Greenland Sea. Journal of Geophysical Research 97 (C5), 7241}7250. Brandon, M.A., 1995. Winter surface water mass modi"cation in the Greenland Sea. Ph.D. Thesis. University of Cambridge, Cambridge, 246 pp. BudeH us, G., Maul, A.-A., Krause, G., 1993. Variability in the Greenland Sea as revealed by a repeated high spatial resolution conductivity-temperature-depth survey. Journal of Geophysical Research 98 (C6), 9985}10000. Carsey, F.D., Roach, A.T., 1994. Oceanic convection in the Greenland Sea Odden region as interpreted by satellite data. In: Johannessen, O.M., Muench, R.D., Overland, J.E. (Eds.), The Polar Oceans and their Role in Shaping the Global Environment. American Geophysical Union, Washington, DC, pp. 211}222. Cavalieri, D.J., Martin, S., 1994. The contribution of Alaskan, Siberian, and Canadian coastal polynyas to the cold halocline layer of the Arctic Ocean. Journal of Geophysical Research 99 (C9), 18343}18362. Clarke, R.A., Swift, J.H., Reid, J.L., Koltermann, K.P., 1990. The formation of Greenland Sea Deep Water: double di!usion or deep convection? Deep-Sea Research 37 (9), 1385}1424. Gloersen, P., 1990. A time lapse motion study of the `Oddena during 1978}1987 as observed with the Nimbus 7 scanning multichannel microwave radiometer. CRREL Monograph 90 (1), 177}179. Gloersen, P., Campbell, W.J., Cavalieri, D.J., Comiso, J.C., Parkinson, C.L., Zwally, H.J., 1992. Arctic and Antarctic Sea ice, 1978}1987: satellite passive-microwave observations and analysis. National Aeronautics and Space Administration, Report of NASA SP-511, 290 pp.
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Johannessen, J.A., Johannessen, O.M., Svendsen, E., Shuchman, R., Manley, T., Campbell, W.J., Josberger, E.G., Sandven, S., Gascard, J.C., Olaussen, T., Davidson, K., Van Leer, J., 1987. Mesoscale eddies in the Fram Strait marginal ice zone during the 1983 and 1984 marginal ice zone experiments. Journal of Geophysical Research 92 (C7), 6754}6772. Jones, H., Marshall, J., 1993. Convection with rotation in a neutral ocean: a study of deep-ocean convection. Journal of Physical Oceanography 23, 1009}1039. KaK mpf, J., Backhaus, J.O., 1999. Ice}ocean interactions during shallow convection under conditions of steady winds: three-dimensional numerical studies. Deep-Sea Research II 46, 1335}1355. Killworth, P.D., 1983. Deep convection in the world ocean. Reviews of Geophysics and Space Physics 21 (1), 1}26. Lange, M.A., Ackley, S.F., Wadhams, P., Dieckmann, G.S., Eicken, H., 1989. Development of sea ice in the Weddell Sea. Journal of Glaciology 12, 92}96. Martin, S., Kau!man, P., 1981. A "eld and laboratory study of wave damping by grease ice. Journal of Glaciology 27, 283}314. Maxworthy, T., Narimousa, S., 1994. Unsteady turbulent convection into a homogenous rotating #uid with oceanographic applications. Journal of Physical Oceanography 24 (5), 865}887. Maykut, G.A., 1986. Surface heat and mass balance. In: Untersteiner, N. (Ed.), The Geophysics of Sea Ice. Plenum Press, New York, pp. 395}463. McPhee, M.G., 1992. Turbulent heat #ux in the upper ocean under sea ice. Journal of Geophysical Research 97 (C4), 5365}5379. Meincke, J., Verch, N., Purschel, W., Baering, S., Dengler, M., Senet, C., BudeH us, G., Darnell, C., Johnson, J., Gascard, J.C., Rouault, C., 1994. Physical oceanography during ARKIX/1. Berichte zur Polarforschung 134, 53}57. Moore, R.M., Wallace, D.W.R., 1988. A relationship between heat transfer to sea ice and temperaturesalinity properties of Arctic Ocean waters. Journal of Geophysical Research 93 (C1), 565}571. Nansen, F., 1902. The oceanography of the north polar basin. The Norwegian North Polar Expedition, 1893}1896, 3(9), 427 pp. Parkinson, C.L., Washington, W.M., 1979. A large scale numerical model of sea ice. Journal of Geophysical Research 84 (C1), 311}337. Pawlowicz, R., 1995. A note on the seasonal cycles of temperature and salinity in the upper waters of the Greenland Sea Gyre from historical data. Journal of Geophysical Research 100 (C3), 4715}4726. Rudels, B., 1990. Haline convection in the Greenland Sea. Deep-Sea Research 37 (9), 1491}1511. Schmitz Jr, W.J., 1995. On the interbasin-scale thermohaline circulation. Reviews of Geophysics 33 (2), 151}173. Schott, F., Visbeck, M., Fischer, J., 1993. Observations of vertical currents and convection in the Central Greenland Sea during the winter of 1988}1989. Journal of Geophysical Research 98 (C8), 14401}14421. Steele, M., 1992. Sea ice melting and #oe geometry in a simple ice}ocean model. Journal of Geophysical Research 97 (C11), 17729}17738. Steele, M., Morison, J.H., 1993. Hydrography and vertical #uxes of heat and salt northeast of Svalbard in autumn. Journal of Geophysical Research 98 (C6), 10013}10024. Tadross, M., Brandon, M.A., 1993. A report on the sea ice conditions encountered by M.V. Northern Horizon 27.1.93 } 19.2.93. Scott Polar Research Institute, Sea Ice Group Technical Report, 20 pp. van Aken, H.M., BudeH us, G., HaK hnel, M., 1995. The anatomy of the Arctic Frontal Zone in the Greenland Sea. Journal of Geophysical Research 100 (C8), 15999}16014. Vieho!, T., Eicken, H., Ramseier, R., Wadhams, P., 1993. Sea-ice conditions during ARKIX/1a, b with RV `Polarsterna: shipboard observations and satellite imagery. Alfred Wegener Institut-FuK r Polar-undMeeresforschung, Berichte aus dem Fachbereich Physik, 43. Vinje, T.E., 1977. Sea ice conditions in the European sector of the marginal seas of the Arctic, 1966}1975, As rbok Norsk Polarinstitutt, 1975, pp. 163}174. Vinje, T.E., Finneka sa, "., 1986. The ice transport through the Fram Strait. Skrifter NR 186, Norsk Polarinstitutt, Oslo. Visbeck, M., Fischer, J., Schott, F., 1995. Preconditioning the Greenland Sea for deep convection: ice formation and ice drift. Journal of Geophysical Research 100 (C9), 18489}18502.
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Wadhams, P., 1981. The ice cover in the Greenland and Norwegian Seas. Reviews of Geophysics and Space Physics 19 (3), 345}393. Wadhams, P., Gill, A.E., Linden, P.F., 1979. Transects by submarine of the East Greenland Polar Front. Deep-Sea Research 26, 1311}1327. Wadhams, P., Holt, B., 1991. Waves in frazil ice and pancake ice and their detection in Seasat Synthetic Aperture Radar imagery. Journal of Geophysical Research 91 (C5), 8835}8852. Wadhams, P., Wilkinson, J.P., 1999. The physical properties of sea ice in the Odden ice tongue. Deep-Sea Research II 46, 1275}1300. Worcester, P.F., Lynch, J.F., Morawitz, W.M.L., Pawlowicz, R., Sutton, P.J., Cornuelle, B.D., Johannessen, O.M., Munk, W.H., Owens, W.B., Schumann, R., Spindel, R.C., 1993. Evolution of the large scale temperature "eld in the Greenland Sea during 1988}89 from tomographic measurements. Geophysical Research Letters 20 (20), 2211}2214.
The near surface hydrography beneath the Odden ice tongue M.A. Brandon*, P. Wadhams Scott Polar Research Institute, University of Cambridge, Lensxeld Road, Cambridge CB2 1ER, UK Received 26 February 1998; received in revised form 4 November 1998; accepted 10 November 1998
Abstract As part of the European Subpolar Ocean Programme (ESOP), the German research icebreaker Polarstern worked in the Greenland Sea in the late winter of 1993. Whilst on passage, the ship encountered a severe winter storm with winds consistently above 20 m s\ coupled to air temperatures of below !103C. The underway sensors revealed heat #uxes of greater than 700 W m\ across most of the Nordic Basin, peaking at greater than 1200 W m\ when the ship crossed the cold, fresh water of the Jan Mayen Current. This large heat #ux coupled to the unique hydrographic conditions present in the Jan Mayen Current allowed sea-ice generation in the form of frazil ice at a rate of 28 cm d\. This frazil ice then developed into pancake ice. Measurements also were made in the late winter beneath this pancake ice in two remnants of the Odden. In the Jan Mayen Current, hydrographic conditions are such that the ice can exist for a long period of time before eventually decaying due to short-wave radiation at the surface. Towards the centre of the Greenland Sea, hydrographic measurements reveal that the ice is more transient and decays four times more rapidly than ice in the Jan Mayen Current. We discuss the development of the Odden ice tongue in light of these results and add evidence to the argument that the eventual fate of the water stored in the ice is important and could be a relevant factor in the formation of Greenland Sea Deep Water. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The Nordic Basin is the general term for the enclosed basin that contains the Icelandic Sea, the Norwegian Sea and the Greenland Sea. It lies between the Arctic
* Corresponding author. Now at British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK. Tel.: 0044-1223-221400; fax: 0044-1223-362616. E-mail address: [email protected] (M.A. Brandon) 0967-0645/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 0 2 4 - 7
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Basin and the North Atlantic Ocean and has been recognized as a region of deepwater formation for almost 100 years (Nansen, 1902). Although the precise mechanism is still not well understood, the region is thought to drive much of the ventilation of the world's oceans that occurs in the northern hemisphere (Aagaard and Carmack, 1994; Killworth, 1983; Schmitz, 1995). Within the basin the sea ice area varies seasonally by up to 6;10 km (Gloersen et al., 1992), and although on a global scale this is relatively small, it represents a local variation within the basin of almost 200%. This area of ice is made up partially from the export of multiyear ice through Fram Strait (Vinje and Finneka sa, 1986) and partially through the development of a winter ice feature that extends from the East Greenland Current (EGC) across the southern part of the Greenland Sea. This ice feature is known as the Odden (Wadhams, 1981). The Odden ice tongue develops in winter over the cold, fresh Jan Mayen Current (JMC), which forms the southern limb of the Greenland Sea gyre (Bourke et al., 1992). There has been much debate about whether the sea ice in the Odden is locally grown or advected into the central Greenland Sea from the EGC, with some authors neglecting this very important point. If the ice in the Odden is locally grown there will be salt rejection and resulting convection, but as the Odden melts the net in#ux of fresh water is zero. If the ice is advected from the EGC it will represent a net contribution of fresh water to the Greenland Gyre and therefore increase the vertical stability of the water column, inhibiting convective overturning. The Odden ice tongue has been shown to vary greatly in size from year to year (Vinje, 1977) and on smaller time scales down to 2}4 d (Gloersen, 1990). This rapid size variation strongly suggests local ice growth that would have a signi"cant e!ect on the upper ocean. Satellite evidence of such ice growth and historical hydrographic data led Pawlowicz (1995) to suggest that changes in the extent of the ice cover in the Odden could be used as a proxy for characterising the onset and depth of convection in the Greenland Sea. However, Pawlowicz (1995) had only a limited hydrographic data set when ice was actually present. In this paper we present hydrographic measurements from late winter 1993 taken either in the presence of sea ice or beneath a sea ice cover. Initially, using data from a transect of the Nordic Basin we show how a pancake ice cover develops in the Greenland Sea, and then calculate the sea}air heat #uxes and the resulting rate of ice formation. Visits later in the winter enabled the calculation of ice melt rates. This enables us to show that, like in the Arctic Ocean, the short-wave radiation #ux is the most important factor in the decay of the pancake ice cover. Finally, we discuss the relevance of the Odden to the formation of Greenland Sea Deep Water in the light of our measurements.
2. The transect across the Nordic Basin Measurements were taken on board the German ship RV Polarstern on its "rst winter deployment in the Nordic Basin as part of ESOP in the winter of 1993. Polarstern sailed from Bremerhaven to Fram Strait, making a transect of the Nordic Basin from 2}5 March. The portion of the transect between 703 and 803N (Fig. 1) was
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Fig. 1. The station positions in the Nordic Basin in the winter of 1993 along with the cruise track of F.S. Polarstern through the Nordic Basin 2}5 March.
designed to cross a region of the Odden that had been visited on 12}14 February 1993 by M.V. Northern Horizon. On this previous visit the Odden (Fig. 1) consisted entirely of pancake ice embedded in frazil ice (Tadross and Brandon, 1993). However, as Polarstern neared the region of the previous Odden a severe winter storm prevented any scienti"c operations other than logging the ship's underway sampling system. 2.1. The surface data Fig. 2 shows the surface temperature and the salinity of the shallowest hydrographic bottle of the CTD measurements of ARKTIS IX/1, the CTD stations being taken between 25 March and 18 April 1993 (Meincke et al., 1994). These late winter data show that the cruise track on 3}5 March 1993 crossed the JMC, and that the Odden ice tongue was over the JMC. From 703N to almost 733N the ship was sailing through warm, saline Atlantic-derived waters in the Norwegian Atlantic Current (Figs. 2 and 3). Just south of 733N the ship crossed the Arctic Front, and both the sea surface temperature and salinity fell rapidly. At this late stage in the winter the gradients were greater than those encountered by van Aken et al. (1995) when the Meteor crossed the Arctic Front in November 1988. Water temperatures began to rise north of 74.33N as the ship left the JMC. The salinity was slightly di!erent, with a peak in the signal within the cold JMC water. After a malfunction in the package,
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Fig. 2. The potential temperature and salinity at the shallowest hydrographic bottle from the Polarstern ARKTIS IX deep CTD stations, 23 March to 18 April 1993.
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Fig. 3. Data from the Polarstern underway sampling system on the transect from 2}5 March 1993. (A) the sea surface temperature and sea surface salinity; (B) the wind speed and air temperature. The latitudes in which ice was observed are bracketed by the two vertical chained lines.
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from 74.9}76.33N the temperature and salinity structure became more variable, and close to Fram Strait the changes were rapid and large. The ice edge was reached at 79.53N and the rapidly varying observations towards Fram Strait suggest that Polarstern crossed small eddies, common close to the ice edge (Johannessen et al., 1987). During the crossing of the Arctic Front the wind was consistently strong, peaking at 23.1 m s\ (Fig. 3b). This strong wind, coupled with low atmospheric temperatures (averaging } 12.63C between 72.4 and 74.53N), created conditions under which ice could form in the open ocean. Despite large areas of pancake and frazil ice in February, ice was encountered only on 3 March between the latitudes of 73.43N and 73.83N, as frazil ice slicks and brash ice (Vieho! et al., 1993). The salinity pulse noted within the chained lines in Fig. 3 suggests salt rejection from the formation of the frazil ice. Photographs of the ice show many whitecaps visible from breaking waves and a slick of frazil ice with small pieces of brash ice embedded (Fig. 4a), as well as a more developed frazil slick with small pancakes #oating in it (Fig. 4b). 2.2. Atmospheric heat yuxes We can use the underway sensor data (Fig. 3) to derive the sea}air heat #uxes that drive the formation of the ice in Fig. 4. The heat exchange in ice-covered waters has been covered previously in great detail (e.g. Maykut, 1986; Parkinson and Washington, 1979), and here we consider the atmospheric terms of the heat budget. The atmospheric heat budget consists of four terms; the short wave radiation, Q , the long wave radiation, Q , the sensible heat, Q , and the latent heat of evaporation, Q . The net atmospheric heat #ux, Q , is then Q "(1!a)Q #e(Q !p¹ )#Q #Q .
(1)
In (1) a is the surface albedo, estimated as 0.5 for the ice/water mix, e is the emissivity of water and taken to be 0.97, and p is the Stefan}Boltzmann constant, which is 5.67;10 W m\ C\. Fig. 5 shows Q against latitude along with the four indi vidual components for the transect (a positive #ux being from the ocean to the atmosphere). The net short-wave radiation is negligible along with the net long-wave radiation north of 723N. Thus the greatest variability within the net atmospheric heat #ux is within the turbulent terms of sensible and latent heat. South of 70.73N, Q is negative and the surface of the ocean is being heated. As Polarstern heads north, the sign of Q changes and cooling becomes strong as the turbulent terms increase in magnitude, reaching an intense maximum of almost 1250 W m\ just south of the Arctic Front. At this maximum the ocean is warm (almost 3.53C, Fig. 3) and no ice can form. This is always the case in this region as heat is constantly supplied in the Norwegian Atlantic Current. In contrast, above the JMC, Q ranges between 850 and 1050 W m\. This heat #ux associated with the cool, fresh surface layer (Figs. 2 and 3) drives the surface layers below the freezing point, forming ice in the open ocean.
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Fig. 4. (a) A small slick of frazil ice and brash ice developing on 3 March 1993. (b) A more developed frazil ice slick on 3 March 1993.
2.3. Open-ocean ice formation For the conditions present over the JMC, the problem of seeding the ocean to allow ice formation is solved by the ambient weather conditions and the existence of the brash ice debris (Fig. 4). The strong cold winds and confused sea with surface waves up to 7 m threw up much spray and spume that froze rapidly, causing severe superstructure icing problems on Polarstern. Each of these frozen droplets, when returning to the ocean, can start secondary nucleation when the surface is at the freezing point, creating the thin frazil ice slicks visible in Fig. 4a. After initial formation the frazil
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Fig. 5. The net atmospheric heat #ux from the ocean to the atmosphere for the transect across the Nordic Basin 2}5 March 1993 (positive represents a heat #ux from the ocean to the atmosphere).
slicks grow through secondary nucleation, creating larger and thicker slicks even after the storm has passed. Fig. 4b shows a close up of a dense frazil ice slick, its thickness is estimated as between 5 and 10 cm. The surface of the ocean now acts as if it had a mass loading, with the ice/water mix behaving as a slurry. Inside the slick (the mass-loaded region) the higher frequency waves are damped out; thus only the lower frequency swell can propagate. In contrast, Fig. 4b shows that outside the slick breaking waves are still visible. This selective damping of higher frequency waves allows frazil ice to be detected using Synthetic Aperture Radar (SAR) imagery, the frazil ice appearing dark in images (Wadhams and Holt, 1991). Within the slick, the lower frequency wave energy allows the individual crystals to bond, creating the small pancakes that then grow. In the absence of damping of the lower frequency waves, the pancakes will not freeze together, as has been observed in the Weddell Sea (Lange et al., 1989), and they remain a loose ice cover. As the frazil ice forms and subsequently grows, the rejected salt will increase the density of the surface layers of the ocean. It is di$cult to infer much about the hydrographic structure because of lack of data; however, some features are apparent. The intense winds and rough seas deepen the mixed layer; also salt rejection from ice formation and subsequent growth increases its density. As the mixed layer deepens,
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warmer Lower Arctic Intermediate Water (LAIW) is entrained from depth to mix with and replace the cooler dense saline layer developing due to the ice growth. This rising warm water acts against continued ice growth, supplying heat that may melt the pancakes. Beneath the upper highly turbulent surface layer the convection is then thought to become ordered into plumes (Rudels, 1990), which would be similar to those observed in laboratory and numerical experiments (Jones and Marshall, 1993; Maxworthy and Narimousa, 1994). Carsey and Roach (1994) have suggested that from these ordered convection cells, the rising patches of warm water would melt the frazil ice slicks in Fig. 4 into patterns that could be observable in SAR imagery. This issue is discussed further by Backhaus and KaK mpf (1999) and KaK mpf and Backhaus (1999). We use bulk formulations to derive parameters relevant to the convection process. Over the JMC, which is close to freezing and being seeded by the freezing spume, the rate of production of ice is given by Cavalieri and Martin (1994) as dh Q " , dt o¸
(2)
where o is the density of the frazil ice, taken as 900 kg m\, and ¸ is the latent heat of fusion of ice which is 3.34;10 J kg\. Typically, where ice is forming, the rate of production is high at 3.25;10\ m s\ (approximately 28 cm d\). We cannot determine the east}west extent of the 30 km of the transect in Fig. 1 that had growing ice because of the absence of suitable SAR imagery; we therefore consider an area of 30 km;30 km. The volume of ice formed in one day is then Q ;86400;(9;10) Vol" o¸
(3)
and the salt rejection from this area, *s is given by *s "Vol(s!s )10\,
(4)
where s is the salinity of the ice given by s "0.31 s (Martin and Kau!man, 1981). Using (3), the volume of ice formed is 2.457;10 m and from (4) the resulting salt #ux is then 5.282;10 kg. Using a mixed layer of typical thickness in the Greenland Sea of 100 m (Clarke et al., 1990), the salinity would increase by 0.065 PSU d\. The temperature of the surface layer was at the freezing point (Fig. 3), and the resulting density increase would be 0.053 kg m\ d\.
3. The late winter measurements The next two visits to the region of the Odden were on 3 and 10 April 1993 as part of the full-depth CTD programme of the ARKTIS-IX/1 campaign. By this stage in the winter of 1993 the Odden had separated into two discrete concentrations of ice (Wadhams and Wilkinson, 1999). The southern concentration of ice, named the South
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Island and centred around 733N 1.53W (Fig. 1), was visited on 3 April 1993. The northern concentration, called the North Island, was centred around 753N 03E and was visited on 10 April 1993. Each island had very di!erent ice and oceanographic conditions. On visits to each island, samples of individual pancakes and frazil ice were taken along with shallow CTD stations with a Seacat SBE-19 pro"ler and a deep CTD station with a Seabird SBE-9. 3.1. The South Island of the Odden The ice cover in the South Island was di!erent to that observed on 12 February from the Northern Horizon and consisted almost exclusively of very large pancakes with the addition of the occasional large multiyear ice #oe, the latter showing heavy ablation. The pancakes and multiyear ice #oes were embedded in a dense frazil ice layer that was measured as being approximately 10 cm thick (Brandon, 1995). Wadhams and Wilkinson (1999), on the basis of structural analysis, suggest that the pancakes at the South Island could be up to 35 d old. This would imply that once the pancakes were formed and after the initial haline- and wind-driven convection, conditions would have been stable within the Odden. Near-surface potential temperature}salinity data from the mean casts of three 1-h deployments of the shallow CTD and a simultaneous deep CTD station from Polarstern (station 40) (Fig. 6) show that the mixed layer was approximately 30 dbar thick. The most obvious feature was a `kneea in the pro"les at 65 dbar, potential temperature !1.8523C and salinity 34.665 PSU (Fig. 6). This knee is not a direct product of the mixing of any local water masses present but rather implies that the surface waters had advected into the region. Lacking nutrient tracer data, the origin of the surface water is indeterminate; however, the proximity of this station to warmer, more saline water to the northwest (Fig. 2) would suggest exchange with these waters. The CTD data in Fig. 6 show no impact from melt water from the pancakes. This will be discussed below. 3.2. The North Island of the Odden Wadhams and Wilkinson (1999) show that the only ice types at this station, pancake and frazil ice, were very similar to the ice observed on 14 February 1993 from the Northern Horizon. Wadhams and Wilkinson (1999), again using structural analysis of the ice, estimate the maximum age of the pancakes as approximately 5 d. Fig. 7 shows the mean of pro"le for almost 1 h of shallow CTD measurements made beneath the pancake ice cover. All the measurements were within the 100 dbar-thick mixed layer that was revealed by a CTD station (Fig. 8). Both the temperature and salinity of the surface waters decreased towards the surface (Fig. 7). At this station the ratio of the coe$cient of haline expansion to the thermal coe$cient of expansion, b/a, was greater than 30.0 and thus the small salinity gradient determined the stable density structure shown in Fig. 7c. Moore and Wallace (1988) have shown that the addition of ice melt water to a water mass means that the water will follow a line in potential temperature}salinity space
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Fig. 6. The potential temperature-salinity plot of the near-surface data from three 1-h Seacat SBE-19 deployments and deep CTD station 40 on 3 April 1993.
given by d¹ ¸ " ds c (s!s ) N
(5)
and ¸ is the latent heat of ice salinity s given by
s ¸ "¸ 1! , s
(6)
where ¸ is the latent heat of pure ice (3.34;105 J kg\). A line de"ned by Eq. (5) is shown in Fig. 7d, a potential temperature against salinity plot, as a dotted line that clearly matches the potential temperature}salinity data. This indicates that the surface waters are principally derived at this stage in the winter by the addition of melt water from the decaying pancake ice. The increase in the gradient of the potential temperature in the surface of 4 dbar is because of the addition of the pancake melt water is mixed over this shallow depth by surface wave energy.
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Fig. 7. The mean pro"le from one hour of shallow data recorded at the North Island of the Odden The mean is the solid line and the dotted line is one standard deviation from the mean. 7(d) is a potential temperature}salinity pro"le for the mean cast along with a `melt linea as calculated from the formula of Moore and Wallace (1988).
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Fig. 8. The potential density referenced to the surface at the shallowest hydrographic bottle from the Polarstern ARKTIS IX deep CTD stations, 23 March to 18 April 1993.
3.3. Ice melt rates We can calculate the rate of addition of melt water to the surface waters demonstrated by Fig. 7d and use the empirical model by McPhee (1992), which is based on results from the MIZEX and CEAREX experiments. We can then estimate the possible further lifetime of the pancakes. McPhee (1992) demonstrated that instead of solving the full boundary-layer formulation for the #uxes at the ice}ocean interface, a simple drag law can be used to parameterize the ice}ocean temperature #ux. The ice}ocean temperature #ux, F , (3C m s\) is then given by F "c u (¹!¹ ), O
(7)
where c is a transfer coe$cient estimated from CEAREX data as 0.0055, ¹ is the ocean temperature, ¹ is the melting point of the sea ice, and u is the ice}ocean skin O friction given by u ,q, where q is the surface wind stress. Steele and Morison (1993) O showed that under similar conditions, u varies between 0.01 and 0.02 m s\, and for O these calculations a representative value of 0.01 is used. Eq. (7) is now multiplied by the speci"c heat of sea water, C , and the density of the water, o , to give Q , the heat N #ux from the ocean to the ice (W m\) Q "C o F . N
(8)
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Table 1 The heat #ux and melt rate at the ice}ocean interface Place and date
F (3C m s\)
Q (W m\)
dh /dt (cm d\) G
South Island: 3 April 1993 North Island: 10 April 1993
1.5;10\ 3.3;10\
6.3 13.7
0.2 0.8
Finally, to calculate an ice melt rate from this heat #ux we divide Eq. (8) by the latent heat, ¸ , and density, o , of the sea ice to give dh Q " . (9) dt ¸o The calculated melt rate from Eq. (9) will be an underestimate as the model assumes melting only from the underside of the pancakes, and excludes the frazil ice. In reality the #oes also melt from the sides and, as the #oe diameter decreases, the signi"cance of lateral melt increases (Steele, 1992). The rate of #oe melting is also a function of wave energy, with higher wave energy increasing the speed of melting (Wadhams et al., 1979). The results of Eqs. (7), (9) with local values of the input parameters are presented in Table 1. For both islands of the Odden the melt rate from the underside of the pancakes is low. Calculated melt rates suggest that the pancakes at the South Island could survive for a further 100 d, but only 20 d at the North Island, assuming similar atmospheric and oceanic conditions. This would be an extremely long lifetime for the pancakes at the South Island, and is clearly physically unrealistic. The contradiction is solved by calculating the full heat budget equation (1) with the addition of Q , which reveals that at this stage in the winter the controlling factor in the heat budget is the incident short-wave radiation. Brandon (1995) and Wadhams and Wilkinson (1999) have noted the existence of melt pools on the surface of the pancakes at the South Island. These pools reduce the surface albedo of the ice and would have a signi"cant impact on increasing the speed of the pancake decay. The calculation, however, does show the stable nature of the pancakes at the South Island once formed, the ice at the North Island being more transient.
4. Discussion To discuss our results on the hydrographic structure beneath the Odden ice tongue we "rst must put the ice tongue into context in the role of formation of the important water mass, Greenland Sea Deep Water (GSDW). The Odden ice tongue has been implicated in the formation of GDSW (e.g., Clarke et al., 1990; Rudels, 1990), and here we encounter a conundrum. The density of the shallowest hydrographic bottle at the surface of the Greenland Sea (calculated from the data in Fig. 2) reveals that the densest surface water encountered in the Greenland Sea was in a pool of open water to the northwest of the Odden. This region is frequently ice free in winter, and is called Nordbukta.
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Wadhams (1981) described the development of the Odden as being initially a broad swath of ice across the Greenland Sea before the ice retreats and Nordbukta develops. This scenario has been con"rmed by passive microwave data (e.g., Visbeck et al., 1995), although in some years, like 1993, the Odden only forms above the JMC in the southern part of the Greenland Gyre. When the Odden initially develops in the open ocean, conditions mean that ice generation will follow the scheme detailed in Section 2.3 and develop into pancakes, the multiyear ice exported from Fram Strait tending to remain in the EGC (Vinje and Finneka sa, 1986). To facilitate this ice formation across the sea (and ultimately limit its thickness) there must be a su$cient salinity strati"cation at the surface to limit the cooling required for ice formation. This strati"cation can arise in two ways; the fresh water could be a remnant from the previous winter or it could arise from a greater than average ice export from the Arctic Ocean. Small anomalies of the multiyear ice export through Fram Strait are common (N. Davis, personal communication, 1995), and once in the Nordic Basin it would be the following winter before the resulting fresh-water anomaly would have advected around the Greenland Gyre to the region of the Nordbukta. When combined with the conditions demonstrated in Figs. 3 and 4 this strati"cation would allow ice to grow over the entire Greenland Sea, inducing haline-driven convection. There are now two possibilities to remove the pancake ice from the region of Nordbukta. The "rst is that the haline-driven convection brings warmer LAIW towards the surface, the stored heat melting the ice layer initially grown and representing no net input of salt. Another mechanism is through prevailing northerly winter winds exporting ice to the south. Using a one-dimensional ice}ocean model, Visbeck et al. (1995) have demonstrated that ice export in this way is possibly a prerequisite for deep convection. In reality the opening of Nordbukta most likely would result from a mixture of these two processes. Further convection in the now-open Nordbukta would deepen the mixed layer. Schott et al. (1993) have shown that once the ice cover is removed from Nordbukta the mixed layer deepens rapidly. Simultaneous acoustic tomography showed that this deepening to be coupled to cooling extending from the surface to the intermediate layers of the Greenland Sea (Worcester et al., 1993). Once the intermediate layers are cooled, for deeper convection and the "nal stage of GSDW formation, the dependence of compressibility on temperature may become signi"cant (BudeH us et al., 1993). The haline strati"cation in the JMC is greater than that in the central Greenland Sea (Brandon, 1995); thus more ice can grow without releasing heat from the deeper water. Schott et al. (1993) showed, with data from a mooring that in the JMC close to the South Island, that conditions throughout the winter were similar to the ARKTIS IX/1 data, i.e. reasonably stable. Here the mixed layer would deepen slowly throughout the winter after the initial pancake-ice formation.
5. Summary Conditions such as those presented above cool the surface of the ocean su$ciently to enable ice formation. The ice forms as frazil slicks, which then develop into
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pancakes. Depending on the severity of a storm or the degree of haline strati"cation, the Odden can grow across the entire Greenland Sea. This ice formation can be self-destructive, as may be the case in the Nordbukta region of the Odden. In the region of the JMC, such as the South Island, the greater strati"cation means that the pancakes can survive. After initial pancake formation, the salt #ux into the surface layer will decrease as the ice develops slowly over a winter. The decay of the pancakes in late winter in the more stable region is driven at the surface by short wave radiation and not from the ocean}ice heat #ux. As previous authors have suggested (Rudels, 1990; Carsey and Roach, 1994) sea ice is an important component in the Greenland Sea system. However, the fate of the ice is the relevant factor. In the Nordbukta region, depending on suitable conditions, this may lead to eventual deep-water formation; in the southern Greenland Sea Gyre the ice is stable and melt water returned locally.
Acknowledgements The authors thank J. Meincke and G. BudeH us for the CTD bottle data and Steven Wells, Eleanor Prussen and David Crane for their extensive help during the ARKTIS IX/1 expedition. This work was funded by the Commission of the European Communities under contract MAS2-CT93-0057 of the MAST-II programme.
References Aagaard, K., Carmack, E.C., 1994. The arctic ocean and climate: a perspective. In: Johannessen, O.M., Muench, R.D., Overland, J.E. (Eds.), The Polar Oceans and their Role in Shaping the Global Environment. American Geophysical Union, Washington, DC, pp. 5}20. Backhaus, J.O., KaK mpf, J., 1999. Simulations of sub-mesoscale oceanic convection and ice}ocean interactions in the Greenland Sea. Deep-Sea Research II 46, 1427}1455. Bourke, R.H., Paquette, R.G., Blythe, R.F., 1992. The Jan Mayen Current of the Greenland Sea. Journal of Geophysical Research 97 (C5), 7241}7250. Brandon, M.A., 1995. Winter surface water mass modi"cation in the Greenland Sea. Ph.D. Thesis. University of Cambridge, Cambridge, 246 pp. BudeH us, G., Maul, A.-A., Krause, G., 1993. Variability in the Greenland Sea as revealed by a repeated high spatial resolution conductivity-temperature-depth survey. Journal of Geophysical Research 98 (C6), 9985}10000. Carsey, F.D., Roach, A.T., 1994. Oceanic convection in the Greenland Sea Odden region as interpreted by satellite data. In: Johannessen, O.M., Muench, R.D., Overland, J.E. (Eds.), The Polar Oceans and their Role in Shaping the Global Environment. American Geophysical Union, Washington, DC, pp. 211}222. Cavalieri, D.J., Martin, S., 1994. The contribution of Alaskan, Siberian, and Canadian coastal polynyas to the cold halocline layer of the Arctic Ocean. Journal of Geophysical Research 99 (C9), 18343}18362. Clarke, R.A., Swift, J.H., Reid, J.L., Koltermann, K.P., 1990. The formation of Greenland Sea Deep Water: double di!usion or deep convection? Deep-Sea Research 37 (9), 1385}1424. Gloersen, P., 1990. A time lapse motion study of the `Oddena during 1978}1987 as observed with the Nimbus 7 scanning multichannel microwave radiometer. CRREL Monograph 90 (1), 177}179. Gloersen, P., Campbell, W.J., Cavalieri, D.J., Comiso, J.C., Parkinson, C.L., Zwally, H.J., 1992. Arctic and Antarctic Sea ice, 1978}1987: satellite passive-microwave observations and analysis. National Aeronautics and Space Administration, Report of NASA SP-511, 290 pp.
M.A. Brandon, P. Wadhams / Deep-Sea Research II 46 (1999) 1301}1318
1317
Johannessen, J.A., Johannessen, O.M., Svendsen, E., Shuchman, R., Manley, T., Campbell, W.J., Josberger, E.G., Sandven, S., Gascard, J.C., Olaussen, T., Davidson, K., Van Leer, J., 1987. Mesoscale eddies in the Fram Strait marginal ice zone during the 1983 and 1984 marginal ice zone experiments. Journal of Geophysical Research 92 (C7), 6754}6772. Jones, H., Marshall, J., 1993. Convection with rotation in a neutral ocean: a study of deep-ocean convection. Journal of Physical Oceanography 23, 1009}1039. KaK mpf, J., Backhaus, J.O., 1999. Ice}ocean interactions during shallow convection under conditions of steady winds: three-dimensional numerical studies. Deep-Sea Research II 46, 1335}1355. Killworth, P.D., 1983. Deep convection in the world ocean. Reviews of Geophysics and Space Physics 21 (1), 1}26. Lange, M.A., Ackley, S.F., Wadhams, P., Dieckmann, G.S., Eicken, H., 1989. Development of sea ice in the Weddell Sea. Journal of Glaciology 12, 92}96. Martin, S., Kau!man, P., 1981. A "eld and laboratory study of wave damping by grease ice. Journal of Glaciology 27, 283}314. Maxworthy, T., Narimousa, S., 1994. Unsteady turbulent convection into a homogenous rotating #uid with oceanographic applications. Journal of Physical Oceanography 24 (5), 865}887. Maykut, G.A., 1986. Surface heat and mass balance. In: Untersteiner, N. (Ed.), The Geophysics of Sea Ice. Plenum Press, New York, pp. 395}463. McPhee, M.G., 1992. Turbulent heat #ux in the upper ocean under sea ice. Journal of Geophysical Research 97 (C4), 5365}5379. Meincke, J., Verch, N., Purschel, W., Baering, S., Dengler, M., Senet, C., BudeH us, G., Darnell, C., Johnson, J., Gascard, J.C., Rouault, C., 1994. Physical oceanography during ARKIX/1. Berichte zur Polarforschung 134, 53}57. Moore, R.M., Wallace, D.W.R., 1988. A relationship between heat transfer to sea ice and temperaturesalinity properties of Arctic Ocean waters. Journal of Geophysical Research 93 (C1), 565}571. Nansen, F., 1902. The oceanography of the north polar basin. The Norwegian North Polar Expedition, 1893}1896, 3(9), 427 pp. Parkinson, C.L., Washington, W.M., 1979. A large scale numerical model of sea ice. Journal of Geophysical Research 84 (C1), 311}337. Pawlowicz, R., 1995. A note on the seasonal cycles of temperature and salinity in the upper waters of the Greenland Sea Gyre from historical data. Journal of Geophysical Research 100 (C3), 4715}4726. Rudels, B., 1990. Haline convection in the Greenland Sea. Deep-Sea Research 37 (9), 1491}1511. Schmitz Jr, W.J., 1995. On the interbasin-scale thermohaline circulation. Reviews of Geophysics 33 (2), 151}173. Schott, F., Visbeck, M., Fischer, J., 1993. Observations of vertical currents and convection in the Central Greenland Sea during the winter of 1988}1989. Journal of Geophysical Research 98 (C8), 14401}14421. Steele, M., 1992. Sea ice melting and #oe geometry in a simple ice}ocean model. Journal of Geophysical Research 97 (C11), 17729}17738. Steele, M., Morison, J.H., 1993. Hydrography and vertical #uxes of heat and salt northeast of Svalbard in autumn. Journal of Geophysical Research 98 (C6), 10013}10024. Tadross, M., Brandon, M.A., 1993. A report on the sea ice conditions encountered by M.V. Northern Horizon 27.1.93 } 19.2.93. Scott Polar Research Institute, Sea Ice Group Technical Report, 20 pp. van Aken, H.M., BudeH us, G., HaK hnel, M., 1995. The anatomy of the Arctic Frontal Zone in the Greenland Sea. Journal of Geophysical Research 100 (C8), 15999}16014. Vieho!, T., Eicken, H., Ramseier, R., Wadhams, P., 1993. Sea-ice conditions during ARKIX/1a, b with RV `Polarsterna: shipboard observations and satellite imagery. Alfred Wegener Institut-FuK r Polar-undMeeresforschung, Berichte aus dem Fachbereich Physik, 43. Vinje, T.E., 1977. Sea ice conditions in the European sector of the marginal seas of the Arctic, 1966}1975, As rbok Norsk Polarinstitutt, 1975, pp. 163}174. Vinje, T.E., Finneka sa, "., 1986. The ice transport through the Fram Strait. Skrifter NR 186, Norsk Polarinstitutt, Oslo. Visbeck, M., Fischer, J., Schott, F., 1995. Preconditioning the Greenland Sea for deep convection: ice formation and ice drift. Journal of Geophysical Research 100 (C9), 18489}18502.
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Wadhams, P., 1981. The ice cover in the Greenland and Norwegian Seas. Reviews of Geophysics and Space Physics 19 (3), 345}393. Wadhams, P., Gill, A.E., Linden, P.F., 1979. Transects by submarine of the East Greenland Polar Front. Deep-Sea Research 26, 1311}1327. Wadhams, P., Holt, B., 1991. Waves in frazil ice and pancake ice and their detection in Seasat Synthetic Aperture Radar imagery. Journal of Geophysical Research 91 (C5), 8835}8852. Wadhams, P., Wilkinson, J.P., 1999. The physical properties of sea ice in the Odden ice tongue. Deep-Sea Research II 46, 1275}1300. Worcester, P.F., Lynch, J.F., Morawitz, W.M.L., Pawlowicz, R., Sutton, P.J., Cornuelle, B.D., Johannessen, O.M., Munk, W.H., Owens, W.B., Schumann, R., Spindel, R.C., 1993. Evolution of the large scale temperature "eld in the Greenland Sea during 1988}89 from tomographic measurements. Geophysical Research Letters 20 (20), 2211}2214.