Deep water formation and exchange rates in the Greenland and Norwegian Seas in the 1990s: inferences...
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Transcript of Deep water formation and exchange rates in the Greenland and Norwegian Seas in the 1990s: inferences...
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