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Weddell Sea ocean model data: implications on basin to global scales

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Title: Weddell Sea ocean model data: implications on basin to global scales


1
Weddell Sea ocean model data implications on
basin to global scales
Michael Schodlok
H.Hellmer, M.Losch, J.Schwarz, D.Jansen,
J.Schroeter, M.Wenzel, G.Rohardt
2
Outline
  1. BRIOS Bremerhaven Regional Ice Ocean
    Simulations
  2. Weddell Sea water mass formation and spreading
  3. Iceberg Drift Modelling
  4. Additional Constraints in LSG
  5. Boundary Conditions in MITgcm

3
BRIOS
Bremerhaven Regional Ice Ocean Simulations
stand alone ocean models
coupled ice ocean model
coupled physical-ecosystem Model
tidal grid
4
spreading of bottom water masses
Formation areas
  • Weddell Sea
  • Ross Sea
  • Adélie Coast
  • Prydz Bay

5
Modelltopographie
1
6
2
4
5
3
3
2
4
1
5
6
6
BRIOS1.1
Ocean Modelling
  • ECMWF Reanalysis
  • 6 h TA, t, TD, CL
  • sea ice model
  • (Hibler, 1979, Lemke et al., 1990)
  • 30 d SST, SSS, t
  • ocean model
  • z - bottom following
  • 24 layers
  • ACC 130 Sv
  • initialised HASO
  • interaction
  • ice shelf ocean
  • ECMWF Reanalysis
  • 6 h TA, t, TD, CL
  • sea ice model
  • (Hibler, 1979, Lemke et al., 1990)
  • 30 d SST, SSS, t
  • ocean model
  • z - bottom following
  • 24 layers
  • ACC 130 Sv
  • initialised HASO
  • interaction
  • ice shelf ocean

high resolution part of circumpolar grid
resolution
7
Ice Shelf Ocean Interation
8
Water masses of the Weddell Sea
Reference Run monthly mean 1985-1993
pot. temperature C
Formation Processes
WW
salinity
9
sea ice extent
10
Surface Forcing
maximum sea-ice extent (September)
1990
1992
surface freezing point temperature (Model)
15 sea-ice concentration SSM/I (Heygster et
al., 1996)
11
Bottom water variability (modeled)
minimum sea ice extent
maximum sea ice extent
1990
1992
12
Bottom water variability (observed)
1998
2000
13
Circulation in the Weddell Sea
WSDW Transport (44W)
500 m
24 Sv Model Reference run
25 Sv Gordon et al., 2001 LADCP data
14
Passage transports
OP
PP
?WSDW ? 6.6 Sv
15
dense water masses in northwestern WS
Density differences 1992 - 1990
Freshwater input Larsen Ice Shelf
16
bottom water spreading
Simulation of ventilated bottom layer
Weddell Sea
17
mean particle trajectories
water mass spreading from convection sites on the
southern continental shelf
Mooring M MYRTLE (5943.7S, 5529.5W, 3690 m)
18
WSDW spreading
travel time OP - M 1.5 Jahre
after Rubython et al., 2001
passage transport
19
HSSW production
mixed layer depth off Filchner Ice Shelf

20
Summary model results
  • deep NW-Weddell shows complex spatial and
    temporal variability
  • sources in the inner Weddell determine
    characteristics, variability, and
    pathways of bottom water into the world abyss
  • source specifics (location, composition,
    intensity) are still speculative
  • contribution to AABW across SSR 6.6 Sv (50
    )

21
  1. Implications I Freshwater balance
  2. Implications II WS water mass impact on global
    ocean
  3. Implications III Ice Shelf interaction and
    global impacts

22
Rôle in ice-ocean system
About Icebergs
  • Calving at the ice fronts represents the main
    mass loss of the Antarctic ice sheet
  • Calving 2020 Gt/a gt 75 mSv
  • Basal Melting 910 Gt/a gt 28 mSv
  • In Weddell Sea 410 Gt/a gt 15 mSv

(Gladstone et al. 2001)
  • NCEP P-E 39 mSv
  • Melting icebergs provide cold freshwater during
    drift and final decay

23
Influences on Iceberg Evolution
  • Cryosphere
  • pre-calving evolution
  • collisions
  • sea ice
  • Atmosphere
  • temperature
  • precipitation
  • wind
  • Iceberg
  • Inherent ice dynamics
  • fracturing
  • Ocean
  • temperature
  • currents
  • Geo/Biosphere
  • Iceberg grounding

24
Freshwater input
25
Export Scenarios
Scenario 1 Total Export
Scenario 2 Partial Export
Scenario 3 No Export
26
Freshwater Export
Melt Rate 25 m/a Calved Volume 410
Gt/a Melted Volume 8 Freshwater Flux 32
Gt/a 1 mSv
27
Phytoplantkton bloom?
Ecosystem Modelling
  • coupled 1 D model
  • BIMAP ecosystem (Hense et al. 2003)
  • MITgcm OCGM (Losch, 2005)
  • iceberg freshwater/iron input
  • iron limitation 1.2 10-3 mmol/m-3

28
Summary
Iceberg drift in sea-ice cover gt 86 determined
by sea-ice movement, i.e. the wind field
iceberg melting 4 m/a 90 m/a, mainly freshwater
export
dust input indicates possible significant
contribution of iron, depending on dust iron
ratio
preliminary 1 D model results phytoplankton
increase with iceberg meltwater input
29
Outlook
combine observations and modell to estimate
freshwater input into WS
improve interaction iceberg ecosytem model with
iceshelf approach
improve model to 2d, 3d
sample icebergs for iron content
further SeaWiFS/MoDIS analysis
30
  1. Implications I Freshwater balance
  2. Implications II WS water mass impact on global
    ocean
  3. Implications III Ice Shelf interaction and
    global impacts

31
Improvements of global ocean?
LSG Large Scale Geostrophic model (Maier-Reimer
and Mikolajewisz, 1991)
  • Designed for climate studies,
  • time scales O(1000 yrs)
  • combined with adjoint method
  • (e.g. Wenzel and Schröter, 2002)
  • 2 x 2
  • assimilation period 1993-2001
  • monthly SSTs (Reynolds et al., 2002)
  • SSH anomalies TOPEX/Poseidon
  • Levitus T/S (WOA94)
  • 23 layers
  • spin up 1000 yrs
  • ACC weakly constrained to 135 Sv
  • seasonal cycle from 2 BRIOS sections

32
Look for global ocean state as close to obs as
possible Combine model and data using adjoint
method (variational optimization method) Based on
definition of costfunction J describing misfit
between data and model in Eulerian sense J
implicitly depends on control parameters that
have to be adjusted to achieve minimum cost Model
trajectory determined in space and time by
initial conditions and surface fields (tau, Ta,FW
) T, S, SSH subject to optimization Initial
velocities not directly modified, instead allow
to adjust to changed initial density structure
during 1st year Reduce number of control
parameters Hilbert empirical orthogonal function
decomposition for surface forcing fields
(special group of complex eofs) Advantage of
decomposition for representation of forcing Only
amplitudes of variable patterns that occur
naturally are allowed to be adjusted Unrealistic,
eg. Fluxes, cant correct local model
errors Disadvantage introduce spurious
teleconnections
33
9 year average of velocities
control experiment
additional constraints in Weddell Sea
34
improvements
  • double cell structure of Weddell Gyre
  • (Beckmann et al., 1999)
  • WG increase to gt30 Sv
  • 50 increase of SA subtropical gyre
  • Amundsen Bellingshausen Sea
  • ACC broadens, major shift to south at 150W

35
temporal mean
Salt Content
Heat Content
36
linear trend
Heat Content
Salt Content
37
  • Improvements due to redirection of ACC
  • -gt related to a change in density structure
  • -gt triggered by warmer saltier Amundsen
    Bellingshausen Sea
  • -gt only source ACC Weddell Sea circumpolar
    influence
  • transfer of cold, fresh water masses from
  • interior into adjacent basins
  • maximum trend 8 Wm-2 ,i.e., warming of
    0.035Ca-1
  • (similar to upper Ocean estimate by Gille 2002)
  • reduced trend in SO suggests that global
    simulations
  • (e.g. Manabe et al., 1991) could be less if
    high-latitude
  • processes are considered in more detail.

38
Improvements due to redirection of ACC -gt
Weddell Sea westward displacemnt,
strengthening of Weddell Gyre transfer of
cold, fresh water masses from interior into
adjacent basins -gt South Atlantic
circulation cooling freshening of eastern
Ross Sea due to meridional flow of
ACC Modification of ACC flow related to a change
in density structure triggered by warmer, saltier
ABS only source is ACC -gt indicating WS
characteristics influence circumpolar course of
current. Max trend in WEDEX 8 Wm-2 -gt warming
of 0.035Ca-1 (similar to upper Ocean estimate by
Gille 2002) However, reduced WEDEX trend to SDR
in SO suggests that global simulations (e.g.
Manabe et al., 1991) could be less if
high-latitude processes are considered in more
detail.
39
  1. Implications I Freshwater balance
  2. Implications II WS water mass impact on global
    ocean
  3. Implications III Ice Shelf interaction and
    global impacts

40
Ice Shelf - Ocean Interaction Parametrisation
of freshwater/heat fluxes on Antarctic
continental shelf or Prescribed Southern
Boundary Conditions?
41
Antarctic Peninsula
Observations
Disintegration of Larsen B Ice Shelf - 2002
Jan 30
Feb 17
Modis, NASA
Consequences for ocean circulation and surging of
ice streams ?
Mar 04
42
Ice shelf basal melting
Values in parentheses from Giovinetto Bentley
(1985)
43
Missing fresh water
BRIOS Model runs with and without ice shelf
cavities
Sea ice thinning
September, year 20
Hellmer 2004
44
Changes because of
45
Meridional Overturning
(sigma-2 vs. latitude)
minimum
minimum
AABW
Orsi et al., 1999
maximum density
maximum density
46
Global OGCM
  • MITgcm 2x2
  • Initialisation Levitus
  • Surface restoring Levitus
  • NCEP heatfluxes
  • KPP
  • Gent McWilliams 400 m2 s-1
  • allow freezing true
  • 23 Layers (ECCO2)
  • Integration time 2000 yrs
  • Model runs
  • BRIOS data restored in
  • Weddell Sea and Ross Sea
  • Levitus restored in
  • Arctic Ocean

47
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48
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49
Weddell Sea only
50
Weddell Sea only
temperature
salinity
160 m
1500 m
4000 m
51
Ross Sea only
Note colorbars are different to Weddell Sea only
52
Ross Sea only
temperature
salinity
160 m
1500 m
4000 m
53
Weddell Sea only
Ross Sea only
54
Dye Tracer
55
  • including Icebergs
  • no influence on deep and bottom water formation
    area
  • including hydrographic sections
  • improvement of Southern Ocean circulation
  • including Ice Shelf Water
  • should be represented in global models

56
Thanks
Thank you
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