Abigail Spieler Oral Examination Presentation March 28, 2005

1. Deep water formation and exchange rates in the Greenland and Norwegian Seas in the 1990s: inferences from box model calculations Abigail Spieler Oral Examination Presentation March 28, 2005

2. Outline • Introduction • Box model design • Input functions • Box model simulations • Scenarios • Conclusions

3. Canadian Basin Amundsen Basin Lomonosov Ridge (1800m) Nansen Basin Kara Sea Eurasian Basin Fram Strait (2600m) Barents Sea Greenland Sea Mid-Atlantic Ridge Norwegian Sea

4. Arctic Ocean-Nordic Seas Thermohaline circulation

5. Vertical structure of the Greenland Sea gyre

6. Outline • Introduction • Box model design • Input functions • Box model simulations • Scenarios • Conclusions

7. Model Design

8. Model design, ctd. ci = concentration in box i Jji= volumetric flux from box j to box i q = source/sink in box, e.g. radioactive decay • Water masses are represented by homogeneous boxes • Tracers conserved in deep water boxes • Surface water boxes represent boundary conditions of model • 1940-1980: assume steady state (Bönisch and Schlosser, 1995) and volume conservation • Integrate using forward differences in time

9. Outline • Introduction • Box model design • Input functions • Box model simulations • Scenarios • Conclusions

10. Tritium input • Natural 3H concentration in surface ocean ≈ 0.2 TU. Bomb peak in mid-1960s; half life = 12.43 years. • Precipitation is the main source of 3H in Atlantic-derived waters • For Norwegian and Greenland Sea surface waters, scale D-R curve to observations; exponential decay after 1974 North Atlantic surface water (Dreisigacker and Roether, 1978) Norwegian Sea Surface water

11. Tritium input, ctd. • The two components of GSUW are Greenland Sea Surface Water (GSSW) and Upper Arctic Intermediate Water • GSUW = 0.82*GSSW + 0.12*NSSW (lagged by 5 years) Greenland Sea Upper Water

12. Tritium input, ctd. • Barents Sea surface water consists of Atlantic-derived water with 3H/3He age ≈ 3 years, and river runoff. • BS = 0.004*river runoff (2 year lag) +0.996*NSSW (3 year lag) • 15% reduction of 3H in Atlantic-derived component due to radioactive decay. Barents Sea surface water

13. 100% of solubility in NSSW; 85% of solubility in GSUW and BS. Assume linear decline of CFC-11 and CFC-12 after 2005. CFC-11 and CFC-12 input functions Northern hemisphere atmospheric CFC-11 and CFC-12 CFC-12 CFC-11 CFC-11 in surface boxes

14. 3He inputs • Atmosphere (δ3Heatm ≡ 0) • Radioactive decay of 3H  3He • Produced in deep waters • Supplied to deep waters by BS and GSUW • Mantle source at spreading ridges • GSDW = 1.6 atoms cm-2 • NSDW = 1.0 atoms cm-2 • EBBW = 0.9 atoms cm-2

15. Outline • Introduction • Box model design • Input functions • Box model simulations • Scenarios • Conclusions

16. Model simulation requirements • Salinity and potential temperature increasing in GSDW

17. Model simulation requirements • Concentrations of CFC-11, CFC-12 and 3H in GSDW remain low

18. Model simulation requirements • δ3He of GSDW rapidly increasing in 1990s • Volume reduction in GSDW as upper boundary of GSDW moves downward.

19. Model simulation, continued • Steady state, with 0.47 flux from GSUW to GSDW, before 1979 (steady-state fluxes derived by Bönisch and Schlosser, 1995). • Flux from GSUW to GSDW reduced to 0.1 Sv, 1979-1994; volume reduced by 30%; upper boundary of GSDW descends to 2000m • 1994-2005: flux from GSUW to GSDW reduced to 0.03 Sv, while flux from EBDW to GSDW increases • Average GSUW flux to GSDW, 1979-2005 ≈ 0.07 Fluxes, 1994-2005 Fluxes, 1940-1979 Fluxes, 1979-1994

20. Model simulation, ctd. Salinity Potential temperature EBDW EBBW EBBW NSDW EBDW NSDW GSDW GSDW 1994: GSUW flux reduced, EBDW flux increased (to GSDW) 1979: GSUW flux reduced

21. Model simulation, ctd. Tritium GSDW EBDW NSDW EBBW

22. Model simulation, 3He GSDW NSDW δ3He δ3He 3Hetritiogenic 3Hetritiogenic EBDW EBBW δ3He δ3He 3Hetritiogenic 3Hetritiogenic

23. Model simulation, ctd. CFC-11 CFC-12 GSDW GSDW NSDW NSDW EBDW EBDW EBBW EBBW

24. Outline • Introduction • Box model design • Input functions • Box model simulations • Scenarios • Conclusions

25. Scenario: no flux reduction after 1979 • Much higher CFC-11 and tritium than observed • Warming and salinification trends not explained 3H CFC-11

26. Scenario: fluxes constant after 1979 • After 1979, flux from GSUW to GSDW reduced to 0.1 Sv • GSDW volume remains constant • Increased flux from EBDW to GSDW • Export to Atlantic from EBDW and NSDW reduced

27. Scenario: fluxes constant after 1979, ctd. δ3He salinity 3He/3H age Potential temp.

28. Scenario: fluxes constant after 1979, ctd. 3H • Predicted CFC-11 concentration is too high • Good match with helium, tritium and age data • Salinity and temperature increase in GSDW underestimated CFC-11

29. Scenario: three years of rapid ventilation in late 1980’s • From 1980-1987, flux from GSUW to GSDW = 0.1 Sv, GSDW volume decreases • From 1987-1990, flux from GSUW to GSDW restored to 0.47 Sv. • After 1990, zero flux from GSUW to GSDW while flux from EBDW to GSDW increases. • Average ventilation of GSDW is ≈0.85 Sv between 1979-2005 • volume reduced by 18% Fluxes, 1979-1987 Fluxes, 1990-2020 Fluxes, 1987-1990

30. Scenario: high GSUW flux, 1987-1990, ctd. δ3He salinity 3He/3H age Potential temp.

31. Scenario: high GSUW flux, 1987-1990, ctd. 3H • Good fit for helium and tritium data • Predicted CFC-11 too high • Rates of increase for GSDW salinity and temperature match observations • Deep water formation rate in GS varies from year to year CFC-11

32. Scenario: pre-1979 fluxes restored in 2005 salinity δ3He 3H 3He/3H age CFC-11 potential temp. CFC and δ3He will remain sensitive to the renewal rate in the Greenland Sea for the near future.

33. Outline • Introduction • Box model design • Input functions • Box model simulations • Scenarios • Conclusions

34. Conclusions • CFC concentrations in GSDW remained constant or declined in the late 1990’s, while GSDW temperature and salinity evolved towards EBDW • Model reproduces the warming and salinification trends and low transient tracer concentrations in GSDW between 1980 and 2005 • Average GSUW flux to GSDW between 1979-2005 ≈ 0.07-0.08 Sv • The rate of GSDW formation is variable from year to year during the period 1980-2002 • Most uncertainty with respect to modeled tracer concentrations in Eurasian Basin Deep water and Eurasian Basin Bottom Water

35. Loss of freshwater (sea ice) Increased glacial melting Increased river runoff Increased P – E Freshening in LS Increased freshwater inventory in GIN seas Freshening Subpolar Gyre Decrease in salinity in overflows Projected changes in freshwater fluxes: +0.05 Sv;