Abigail Spieler Oral Examination Presentation March 28, 2005 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

Outline Introduction Box model design Input functions Box model simulations Scenarios Conclusions

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

Arctic Ocean-Nordic Seas Thermohaline circulation

Vertical structure of the Greenland Sea gyre

Outline Introduction Box model design Input functions Box model simulations Scenarios Conclusions

Model Design

Model design, ctd. Water masses are represented by homogeneous boxes 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

Outline Introduction Box model design Input functions Box model simulations Scenarios Conclusions

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

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

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

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

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

Outline Introduction Box model design Input functions Box model simulations Scenarios Conclusions

Model simulation requirements Salinity and potential temperature increasing in GSDW

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

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

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

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

Model simulation, ctd. Tritium GSDW EBDW NSDW EBBW

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

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

Outline Introduction Box model design Input functions Box model simulations Scenarios Conclusions

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

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

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

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

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

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

Scenario: high GSUW flux, 1987-1990, ctd. 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

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

Outline Introduction Box model design Input functions Box model simulations Scenarios Conclusions

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

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