1. Polynya dynamics and export of dense water Results from the Storfjord laboratory Peter M. Haugan 1,2, Ragnheid Skogseth 3, Ilker Fer 2,1, Lars H. Smedsrud 2,1 1 Geophysical Institute, University of Bergen, Norway 2 Bjerknes Centre for Climate Research, Bergen 3 University Centre on Svalbard Observed polynya opening and closing, salt rejection and build-up, outflow, entrainment and mixing from satellite and in situ data Interannual variability Future use of the ”laboratory” www.gfi.uib.no, www.bjerknes.uib.no, www.unis.nowww.bjerknes.uib.no ProClim project web site www.noclim.org

2. Some references Skogseth, R., I. Fer and P.M. Haugan 2005. Dense-water production and outflow from an Arctic coastal polynya in Storfjorden. In Helge Drange, Trond Dokken, Tore Furevik, Ruediger Gerdes and Wolfgang Berger (Ed.): The Nordic Seas: An Integrated Perspective, volume 158 of AGU Geophysical Monograph, pages 73-88. American Geophysical Union Skogseth, R., P.M. Haugan and M. Jakobsson 2005. Watermass transformations in Storfjorden. Cont. Shelf Research 25, 667-695. Skogseth, R., P.M. Haugan and J. Haarpaintner 2004. Ice and Brine Production in Storfjorden From Four Winters of Satellite and in Situ Observations. J. geoph. Res. 109, C10008, doi:10.1029/2004JC002384. Fer, I., R. Skogseth and P.M. Haugan 2004. Mixing of the Storfjorden (Svalbard Archipelago) overflow inferred from density overturns. J. geoph. res. Vol. 109, C01005, doi:10.1029/2003JC001968. Fer, I., R. Skogseth, P.M. Haugan and P. Jaccard 2003. Observations of the Storfjorden overflow. Deep-Sea Research Part I, Vol 50 (10-11) pp 1283-1303. Fer, I. 2006. Scaling turbulent dissipation in an Arctic fjord. Accepted in Deep-Sea Res. II. Smedsrud, L.H. and R. Skogseth 2006. Field measurements of Arctic grease ice properties and processes. Accepted in Cold Regions Science and Technology doi:10.1016/j.coldregions.2005.11.002 Smedsrud, L.H., Budgell, W.P, Jenkins, A.D. and Ådlandsvik, B. 2006 Fine scale sea ice modelling of the Storfjorden polynya. Accepted in Annals of Glaciology Vol. 44. Haarpaintner, J., J.-C. Gascard and P.M. Haugan 2001. Ice production and brine formation in Storfjorden. J. geoph. res. 106(C7) 14001-14013. Haarpaintner, J., P.M. Haugan and J.-C. Gascard 2001. Interannual variability of the Storfjorden (Svalbard) ice cover and ice production observed by ERS-2 SAR. Ann. Glaciol. 33, 430-436. Haarpaintner, J., J. O'Dwyer, J.-C. Gascard, P.M. Haugan, U. Schauer, and S. Østerhus 2001. Seasonal transformation of water masses, circulation and brine formation observed in Storfjorden, Svalbard. Ann. Glaciol. 33, 437-443. Thanks also to Jørg Haarpaintner, student at UNIS 97/98, PhD at Univ. of Paris 2001

3. Storfjord Arctic Ocean coastal polynyas supply the basins with dense deep water help maintaining the Arctic halocline Exported dense water contributes to the meridional overturning circulation

4. UNIS Longyearbyen

5. Storfjorden polynya April 6 2001

6. Storfjorden 190 km long, 190m deep Cyclonic Coastal Current (East Spitsbergen C. – Sørkapp C.) Access to Atlantic Water and main shelf break Strong tidal currents in northern sounds (4-5 m/s and 2-3 m/s, respectively) Area about 13  10 3 km 2 and volume of 850 km 3 Roughly 16% of the area is deeper than the 120m sill; the corresponding volume is 5% of the total

7. Formation of brine-enriched shelf water (BSW) shelf water Surface advection (Polar Front Water) Surface advection through the sounds (Arctic) Diffusion /convectionDiffusion /convection Diffusion /convectionDiffusion /convection Sill 77ºN Advection from South Out-flow Pack IceFast Ice B ri n e re le as e d u ri n g fr ee zi n g Storfjorden Polynya 78°N shelf water Surface advection (Polar Front Water) Surface advection through the sounds (Arctic) Diffusion /convectionDiffusion /convection Diffusion /convectionDiffusion /convection Sill 77ºN Advection from South Out-flow Pack IceFast Ice B ri n e re le as e d u ri n g fr ee zi n g Storfjorden Polynya 78°N Brine enriched shelf water Surface advection (Polar Front Water) Surface advection through the sounds (Arctic) Diffusion / convection Sill 77ºN Advection from South (AW) Out-flow Pack IceFast Ice Brine release during freezing Storfjorden Polynya 78°N ERS-2 SARInterpretation (Modified from Haarpaintner, Gascard & Haugan, JGR, 2001)

8. Observed polynya widths during 2000 and 2001 Error bars show spread in widths from satellite data at different locations along polynya. Note that polynya is defined here as both open water, frazil and thin ice. Modelled polynya widths in fully drawn line.

9. Observed areal fractions Fast ice (white) Polynya (black) Pack ice (grey) during 5 winters from ERS-2 SAR November – May Haarpaintner et al. (JGR, 2001): Winter 98 Haarpaintner et al. (Annals of Glaciol., 2001): Winters of 98 and 99 Skogseth et al. (JGR, 2004): 4 winters from 98-2001, revised model and 32 year simulation Skogseth et al. (AGU Monograph, 2005): 5 winters including 2002

10. Simple polynya model Approach: –Satellite observations are used to describe polynya width defined by distance from shore to pack ice. –A wind driven polynya width model is matched to the observations using winds from a nearby weather station. Opening and closing factors for the model are found. –An accompanying wind and heat flux driven open water width model is used to distinguish open water (frazil ice) zone from thin ice zone within polynya. –Total ice freezing within polynya region (and in pack ice/fast ice regions) over the whole Storfjord basin is computed. [Nilsen, Weigel & Skogseth (in prep. 2006): 7 winters including 2004, show that opening factor is closely related to wind stress curl and regional ice conditions in the Barents Sea]

11. Polynya and open water width models Polynya width PW, Open water width OW (in polynya), Wind U n, B 1 = 0.02, B 2 = 0.04: Frazil ice growth, F net from full heat budget Growth of thin (H t ) and fast/pack ice (H fp ),  : freezing-degree-seconds, h s : snow thickness (only for fast/pack ice) Mass conservation of thin ice, Areas given as (Width) x (Length= 48 km) A: open/close factor  0 : preferred direction  n : actual wind direction h c : collection thickness h c = (1.0m + 0.1s·U n )/15

12. Salt release from modeled ice production and observed surface salinity Most in winter 2002 Least in winter 1999 About 1-1.5 Mt of released mass of salt each year Assuming 69% immediate release during freezing and 10 % from aging while still in basin

13. Observed BSW salinity Varies interannually by more than 1, winter 2002 is the most saline. [Model also gives highest salt release in 2002.]

14. Strong northeasterly wind component  high polynya activity and ice production in Storfjorden large ice transport from the Arctic into the Barents Sea lower air temperature  increases the ice growth and ice cover in the western Barents Sea the surface water will be fresher and the water column more stable the following summer  inhibit large production of BSW the next winter Northerly wind Ice from Arctic

15. High NAO low polynya activity and ice production in Storfjorden less ice transport from the Arctic into the Barents Sea higher air temperature  decreases the ice growth and ice cover in the western Barents Sea the surface water will be more saline and the water column less stable the subsequent summer  favors large production of high salinity BSW the next winter Wind Ice Weak northeasterly wind component 

16. Observations of mixing of the Storfjorden overflow

17. Hydrographic conditions (summer) Fresh, cold melt water mixed with Arctic water advected with coastal current down to 65-75 m. Stronger inflow of Atlantic water in the exchange zone with a core at 60-70 m depth. Overflow of BSW in the exchange zone. Overflow ceases during autumn. Census and overview of seasonal changes in Skogseth et al. (CSR, 2005)

18. May 2001

19. Two layered overflow: May 2001 Identified as two layers: 1)a lower layer (~15 m) with relatively uniform vertical structure 2)an upper, thick mixing layer (~30 m) with larger vertical density gradient The widening of the lower layer is comparable to Ekman veering (friction from the bottom) CTD and LADCP data covered nearly 37 km of the plume path with starting point about 68 km downstream of the sill

20. Survey-averaged profiles from 2002 Thorpe scale analysis

21. Mixing estimated from CTD data 2002: Thorpe scale analysis

22. Effect of stratification Note: Exponent here -1.2 ± 0.3. Later study with microstructure measurements (Fer, 2006) gives -1.4 ± 0.2 and confirms validity of Thorpe scale method

23. Some conclusions from overflow studies Most of the overflow takes place on the western side of the sill entrance with a geostrophic transport of 0.01-0.21 Sv Transport doubled from 0.06 to 0.12 Sv along the observed path Observed core of the plume is captured by the predicted path from Killworth’s simple model (local equilibrium, constant Rio) Shear induced mixing at the dense plume and ambient water interface is responsible for the upper layer of the overflow. Bottom generated turbulence homogenizes the lower part of the plume and hence, is responsible for the structure of the lower layer of the overflow. The plume gains a considerable amount of heat from the overlying Atlantic Water. Heat flux estimates from finestructure compare well with the rate of change of heat in its core.

24. Summary Storfjorden contributes 6-15% of total ice production and 3- 20% of dense water formation in Arctic coastal polynyas: Proximity to Atlantic Water and main shelf break Recurrent polynya Time scale of BSW accumulation and flushout < 1 year Quantifiable interannual variability -> ”Laboratory” Simple approach provided fundamental understanding, but source->basin->export scheme has complexities. Identified possibilities for feedback and oscillations involving wind, external ice conditions and source waters affecting total ice production, BSW volume and BSW salinity. Quantified mixing processes affecting gravity current.

25. Further work Plans: Winter field work in 2006 addressing freezing/salt release mechanism and ice thickness distribution Summer field work in 2006 addressing ambient mixing and effects on overflow and gravity current Outflow (bottom ADCP) moorings planned through 2008 Coupled atmosphere-ice-ocean modelling in progress Issues: Role of tides in addition to wind as forcing for polynya opening (-> current meters near northern sounds) Internal cascading in the basin (-> distributed sensors) Maintaining time series of satellite data, CTD and other data for improved understanding of interannual variability

26. Salt content in newly frozen sea ice

27. Lars H. Smedsrud taking samples of frazil ice Winter field work continuing in 2006

28. Some references Skogseth, R., I. Fer and P.M. Haugan 2005. Dense-water production and outflow from an Arctic coastal polynya in Storfjorden. In Helge Drange, Trond Dokken, Tore Furevik, Ruediger Gerdes and Wolfgang Berger (Ed.): The Nordic Seas: An Integrated Perspective, volume 158 of AGU Geophysical Monograph, pages 73-88. American Geophysical Union Skogseth, R., P.M. Haugan and M. Jakobsson 2005. Watermass transformations in Storfjorden. Cont. Shelf Research 25, 667-695. Skogseth, R., P.M. Haugan and J. Haarpaintner 2004. Ice and Brine Production in Storfjorden From Four Winters of Satellite and in Situ Observations. J. geoph. Res. 109, C10008, doi:10.1029/2004JC002384. Fer, I., R. Skogseth and P.M. Haugan 2004. Mixing of the Storfjorden (Svalbard Archipelago) overflow inferred from density overturns. J. geoph. res. Vol. 109, C01005, doi:10.1029/2003JC001968. Fer, I., R. Skogseth, P.M. Haugan and P. Jaccard 2003. Observations of the Storfjorden overflow. Deep-Sea Research Part I, Vol 50 (10-11) pp 1283-1303. Fer, I. 2006. Scaling turbulent dissipation in an Arctic fjord. Accepted in Deep-Sea Res. II. Smedsrud, L.H. and R. Skogseth 2006. Field measurements of Arctic grease ice properties and processes. Accepted in Cold Regions Science and Technology doi:10.1016/j.coldregions.2005.11.002 Smedsrud, L.H., Budgell, W.P, Jenkins, A.D. and Ådlandsvik, B. 2006 Fine scale sea ice modelling of the Storfjorden polynya. Accepted in Annals of Glaciology Vol. 44. Haarpaintner, J., J.-C. Gascard and P.M. Haugan 2001. Ice production and brine formation in Storfjorden. J. geoph. res. 106(C7) 14001-14013. Haarpaintner, J., P.M. Haugan and J.-C. Gascard 2001. Interannual variability of the Storfjorden (Svalbard) ice cover and ice production observed by ERS-2 SAR. Ann. Glaciol. 33, 430-436. Haarpaintner, J., J. O'Dwyer, J.-C. Gascard, P.M. Haugan, U. Schauer, and S. Østerhus 2001. Seasonal transformation of water masses, circulation and brine formation observed in Storfjorden, Svalbard. Ann. Glaciol. 33, 437-443. Thanks also to Jørg Haarpaintner, student at UNIS 97/98, PhD at Univ. of Paris 2001