Icebergs, Ice Shelves and Sea Ice: A ROMS Study of the Southwestern Ross Sea for 2001-2003

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Icebergs, Ice Shelves and Sea Ice A ROMS Study
of the Southwestern Ross Sea for 2001-2003
Michael S. Dinniman John M. Klinck Center for
Coastal Physical Oceanography Old Dominion
University Walker O. Smith, Jr. Virginia
Institute of Marine Science College of William
and Mary
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Outline of Presentation

• Motivation for study
• Describe circulation model and ice shelf
  modeling
• Ross Ice Shelf basal melt
• Changes in HSSW production
• Iceberg effects in McMurdo Sound
• Conclusions

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Motivation

• Large interannual variability in the observed
  sea ice recently (2001-2003) at least partially
  due to several large icebergs (C-19 and B-15)
• Difficult to model with dynamic sea ice model
• Development of high resolution (5 km) regional
  ocean circulation model to examine physical
  environment and marine ecosystems

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Image courtesy of AMRC U. Wisc. (Jan 2003)
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Ross Sea Model

• ROMS (Regional Ocean Modeling System)
• - Free surface, hydrostatic, primitive equation
  ocean general circulation model in
  terrain-following coordinates
• 5 km grid spacing, 24 vertical levels
• Bathymetry from ETOP05 and BEDMAP
• Ice Cavities (Ice thickness from BEDMAP)
• - Mechanical and thermodynamic effects
• Includes macro-nutrients and nutrient uptake

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Ice Shelf Modeling

• PGF calculation assumes the ice shelf has no
  flexural rigidity and pressure at the base comes
  from the floating ice
• Thermodynamics viscous sublayer model

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Idealized Test Cases (ISOMIP D. Holland et al.)
ROMS
MOM2 K. Grosfeld
MICOM D. Holland
Idealized Test Case Start w/ uniform water at
-1.9 C, 34.4 psu and integrate for 30 yrs. All
models have 0.1 Sv. of overturning
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Circulation Model (cont.)

• Imposed sea ice
• - Set model ice concentration to SSM/I 25km
  data
• - Heat and salt fluxes computed from
  thermodynamic calculation of ice freezing or
  melting, but ice is not accumulated or
  transported
• Bulk flux algorithm (COARE 2.0) for open water
• Daily wind stress and wind speed from a blend of
  QSCAT data and NCEP analyses

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Experiments

• Model is initialized in mid-September and spun up
  for 6 years with a 2-year repeating cycle of
  daily winds and monthly climatologies of sea ice
  and atmospheric values
• Three simulations continue from the spin up
  forced by daily winds for 9/2001 9/2003
• - VARICE Uses observed sea ice for
  9/2001-9/2003
• - CLMICE Uses climatological sea ice
• - ICEBERG VARICE Stationary B-15A

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Mean annual average basal melt rate
(2nd year) CLMICE 14.0 cm/yr VARICE 12.6
cm/yr
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Climatology Data courtesy of Chrissy Stover and
Alex Orsi
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The difference in salt flux over time is close to
zero except for winter 2002. Even in winter
2002 the difference in advection is
more important than the vertical diffusion.
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Ice Draft (No Iceberg)
Ice Draft (Iceberg)
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30m temp (1/22/02) ICEBERG
30m temp (1/22/02) VARICE
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30m temp (1/17/03) ICEBERG
30m temp (1/17/03) CLMICE
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Extra sea ice in eastern McMurdo in Feb. 2002,
but much more in Feb. 2003 even after the Ross
Sea Polynya opened up
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The iceberg blocked some of the Ross Sea Polynya
heat from entering McMurdo Sound. However,
a bigger effect was the limited opening of
the polynya in summer 2002-2003 (due to C-19).
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Conclusions

• Interannual sea ice differences can have an
  effect on Ice Shelf Water and High Salinity Shelf
  Water implications for large-scale thermohaline
  circulation
• Icebergs B-15A and C-19 both had an effect on the
  advection of warm surface water into McMurdo
  Sound large icebergs can potentially greatly
  alter local environmental conditions and local
  ecosystems

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Future Plans

• Tides
• Dynamic sea-ice (previous talk)
• Bio-optical primary production model
• Better bathymetry
• AMPS forcing

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Acknowledgements

• BEDMAP data courtesy of the BEDMAP consortium
• Computer facilities and support provided by the
  Center for Coastal Physical Oceanography
• Financial support from the U.S. National Science
  Foundation (OPP-03-37247).

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Annual average basal melt rate (cm/yr) CLMICE
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Summer Average (20m, CLMICE)
Summer Average (20m, ICEBERG)