Deep water formation and exchange rates

in the Greenland and Norwegian Seas in the 1990s:

inferences from box model calculations

Abigail SpielerOral Examination Presentation

March 28, 2005

Outline

• Introduction• Box model design• Input functions• Box model

simulations• Scenarios• Conclusions

Norwegian S

ea

Greenland Sea

Eurasian Basin

Mid-Atlantic Ridge

Barents Sea

Kara Sea

Canadian Basin

Nan

sen

Bas

in

Amun

dsen

Bas

in

Fram Strait

(2600m)

Lom

onos

ov R

idge

(180

0m)

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.

qvolume

Jc

dt

dc

i

jij

j

i

• 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

ci = concentration in box i

Jji= volumetric flux from box j to box i

q = source/sink in box, e.g. radioactive decay

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.

CFC-12

CFC-11

Northern hemisphere atmospheric CFC-11 and CFC-12

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

Salinity of Greenland Sea Deep Water (below 2000m)

34.895

34.9

34.905

34.91

34.915

1990 1992 1994 1996 1998 2000 2002 2004

year

salinity

Potential temperature of Greenland Sea Deep Water (below 2000m)

-1.2

-1.1

-1

-0.9

-0.8

1990 1992 1994 1996 1998 2000 2002 2004

year

Cel

sius

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.

Fluxes, 1994-2005

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 ≈ 0.07Fluxes, 1979-1994Fluxes, 1940-1979

1979: GSUW flux reduced

1994: GSUW flux reduced,EBDW flux increased (to GSDW)

Salinity Potential temperature

EBBW EBBW

EBDW

EBDW

NSDW

NSDW

GSDWGSDW

Model simulation, ctd.

Model simulation, ctd.

GSDW

NSDW

EBDW

EBBW

Tritium

Model simulation, 3He

GSDW NSDW

EBDW EBBW

δ3He

3Hetritiogenic

δ3He

3Hetritiogenic

δ3He

3Hetritiogenic

δ3He

3Hetritiogenic

Model simulation, ctd.

CFC-11

CFC-12

GSDW

GSDW

NSDW NSDW

EBDWEBDW

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

3He/3Hage

Potentialtemp.

salinity

Scenario: fluxes constant after 1979, ctd.

• Predicted CFC-11 concentration is too high

• Good match with helium, tritium and age data

• Salinity and temperature increase in GSDW underestimated

3H

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, 1990-2020Fluxes, 1987-1990Fluxes, 1979-1987

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

δ3He

3He/3Hage

Potentialtemp.

salinity

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

3H

CFC-11

Scenario: pre-1979 fluxes restored in 2005

CFC and δ3He will remain sensitive to the renewal rate in the Greenland Sea for the near future.

δ3He3H salinity

potential

temp.

3He/3Hage

CFC-11

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

FresheningSubpolar Gyre

Freshening in LS