Evaluation of ocean circulation models for the Bering Sea and Aleutian Islands Region

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Evaluation of ocean circulation models for the
Bering Sea and Aleutian Islands Region

• Albert J. Hermann1 and David L. Musgrave2
• National Oceanic Atmospheric Administration,
  Pacific Marine Environmental Laboratory, 7600
  Sand Point Way NE, Seattle, WA 98115 USA, (206)
  526-6495, Albert.J.Hermann_at_noaa.gov.
• 2 School of Fisheries and Ocean Sciences,
  Insitute of Marine Science P.O. Box 757220,
  Fairbanks AK 99775-7220 USA, [phone],
  musgrave_at_ims.uaf.edu2

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This workshop explored the present and future
state of ocean circulation modeling and
biological modeling of the Bering Sea and
Aleutian Island (BSAI) and the North Pacific

• Major topics
• 1) the present state of knowledge concerning the
  BSAI
• 2) the various types of circulation models which
  could be applied to the BSAI, with some
  assessment of their strengths and weaknesses
• 3) existing physical and biological models
• 4) adequacy of present forcing and bathymetry
  datasets
• 5) current status and future prospects for data
  assimilation
• 6) modeling needs of managers for this region
• 7) a timetable over which we might expect the
  development of improved models

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1. Present state of knowledge of the BSAI region

• Highly productive
• A big, broad shelf
• Passes
• wide, narrow, shallow, deep
• Canyons
• Cross-shelf flux
• Powerful tides
• A deep basin
• Ice

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A few highlights.

• Flows through passes spatially variable,
  strongly mixed, very important to biology
• Ice-edge blooms with possible Oscillating
  Control
• Distinct shelf regimes via tidal mixing
• Getting warmer, less ice!
• PDO was significant, but now other modes more
  important

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2. Classes of Ocean Circulation Models

• Pure tidal models
• Quasi-gestrophic models (simpler physics)
• Primitive equation models (hydrostatic but
  otherwise include all physics)
• Terrain-following coordinates
• Z-coordinate
• Layered coordinate
• Unstructured grid

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3. Existing physical and biological models
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3.1 Atmospheric models

• National Center for Environmental Prediction
  (NCEP) hindcasts
• Assimilated atmospheric data
• Easy to get!
• Biases include shortwave radiation (not enough
  stratus clouds)
• ECMWF hindcasts
• Better for Europe, not necessarily better for
  Bering
• Commercial product
• Community Climate Systems Program
• Offers global hindcasts and climate forecasts
• Better shortwave radiation (b/c assimilates cloud
  data/climatology)
• Regional models
• ETA downscales NCEP nowcasts
• NARR downscales NCEP hindcasts
• MM5 general tool for downscaling global winds

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3.2. Ice models

• Hibler model and its decendents
• Thermodynamics
• Melt/freeze, snow layers
• Dynamics
• Viscous-plastic solid
• Vary in number of ice/snow layers

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3.3 Circulation models

• Maslowski group
• N Hemisphere model based on MOM/POP
• Chao group
• N Pacific model based on ROMS
• Wang group
• Bering Sea model based on POM
• Curchitser/Hermann group
• Northeast Pacific model based on ROMS

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3.4. NPZ Models

• 3.4.1. 1D models
• NEMURO NPZD and fish
• Saury and herring
• Merico (2001) NPZD
• phytoplankton succession
• 3.4.2. 3D models
• Run both online and offline
• Examples
• Wang/Diehl NPZ model of Bering
• Hinckley et al NPZ models of CGOA
• Powell/Hermann NPZ for Northeast Pacific

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3.5 Individual-based Models

• Float-tracking plus behavior
• Very useful for individual fish species
• Can run online or offline
• Examples (from CGOA)
• Hinckley et al pollock model
• Rand salmon model

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3.6 Aggregated models

• Consider entire food web
• ECOPATH look at steady state
• ECOSIM add time
• ECOSPACE add space and time
• Can benefit from coupling with NPZD

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3.7 Fisheries models

• MSVPA
• Other stock assesment models

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4. Adequacy of models, forcing and bathymetry

• IDEALLY we would like
• Uniformly fine scale resolution or adaptive
  space/time resolution
• Numerically accurate/convergent
• Handle tides, subtidal flows, mixing all together
• Perfectly accurate bathymetry
• Perfectly accurate forcing hincasts and forecasts

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4.1 Numerics and resolution

• Terrain-following coordinates
• Tend to overemphasize bathymetry
• Need smoothed bathymetry
• Great for surface and bottom boundary layers
• Z-coordinate
• Less accurate/convergent numerics
• Consistent vertical spacing near the surface
• Distort bottom topography into stair steps
• Layered coordinate
• Good for the deep ocean
• bad for the shallow ocean (cant do tidal mixing)
• Unstructured grid
• Potentially powerful
• Hard to implement
• Relatively untested in the Bering Sea
• Danger of predetermining answer with choice of
  grid

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Crucial elements to get right

• Inflow/outflow through the Aleutian passes
• sets conditions in the Southeast BS
• Outflow through the Bering Strait
• Tidal mixing on the shelf
• Ice!

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4.2 Ice

• Hibler-based models are probably sufficient for
  the Bering Sea (no multiyear ice)
• Major uncertainties arise from shortwave
  radiation forcing need to improve

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4.3 Atmospheric forcing

• NCEP probably OK for winds (except for Aleutians)
• NCEP shortwave is badly biased
• CCSM is promising
• Need better bulk flux algorithms (e.g. to relate
  wind speed to wind stress)
• Extended range mesoscale forecasts are impossible
• Long range forecasts/scenarios are useful
• Downscaling is needed!

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4.4 Freshwater discharge

• Important in a few areas
• Data is essentially nonexistent!

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4.5 Tides

• Existing models can handle tidal and subtidal
  dynamics simultaneously
• This is crucial for the Bering Sea, as the two
  interact
• Tidal phasing may be biologically important, so
  want to get it right.

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4.6 Bathymetry

• OK on the shelf
• Need more data in the canyons
• Need much more data in the passes
• USGS eventually digitizing the Bering Sea charts

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4.7 NPZ models

• Need to get
• Pelagic/benthic gradients, north-south and
  cross-shelf
• Green Belt at the shelf break
• Important prey species for fish
• Jellyfish?
• IRON and other nutrients

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4.8 IBM models

• Need better data on fish movement and behaviour
• Need better data on space/time distribution
• Groundfish surveys have been very useful for
  modelers.

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4.9 Aggregated models

• Could benefit from NPZ results
• Aggregate NPZ by space, water mass, or biological
  regime?

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4.10 Fisheries models

• As with aggregated models, could make more
  spatially explicit

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4.11 Model coupling

• IDEAL integrate model might include
• IBMs of Multiple species and life stages
• NPZ with multiple size classes
• Feedback between IMB and NPZ!
• Long time scale simulations
• Web-accessible output and graphics

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5. Status, Needs and Prospects for Data
Assimilation

• T,S, nutrients in passes would be powerful
  constraint
• Skill assesment is difficult to do well
• Existing physical assimilation capabilities
• 3D variational assimilation (ROMS, Chao et al.)
• Weak constraint blend of data and model
• Useful for nowcasts, not as good for dynamical
  analysis
• Inexpensive to run
• 4D variational assimilation (ROMS, Moore et al.)
• Strong constraint adjustment of IC and BC for
  hindcasts
• Can be used for sensitivity analysis, indices!
• Can be expensive to run
• Possibilities for biological model optimization
• 4D variational assimilation (in ROMS)
• Genetic algorithms

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Data sources

• SMMR sattelite for ice cover
• TOPEX/POSEIDON/AVISO altimetry
• No information on the shelf
• Long term moorings
• Nice long time series should be continued
• Long spatial correlation scales make these
  representative of broad areas on the shelf
• XBT data very sparse prior to the 70s
• Hydro/mooring data sparse for the Western Bering
  Sea
• BASIS program in the Eastern Bering
• Global circulation models for ICs and BCs

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6. Needs of managers

• Mandates from
• National Environment Protection Act
• Marine Mammal Protection Act
• Endangered Species Act
• Stellar sea lion
• Sea otters
• Fur seals
• Right whales
• Fin whales
• Predictions of 5-10 years are of special interest
• Issues include
• Bycatch
• Indirect effects of fishing
• Phys-bio interactions
• Hindcasts of circulation and biology can help
  establish likely response to future change
• Need better indices!

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7. Estimated timetable of new model products and
projects

• See the report!

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Summary I

• The ideal circulation model would adequately and
  simultaneously resolve all the relevant scales of
  motion and phenomena in the BSAI, e.g.
• flows through the Aleutian Passes
• seasonal ice
• tidal mixing on the shelves.
• None of the present modeling approaches can
  rapidly and simultaneously capture all of these
  features for extended time periods on todays
  computers
• continuing advances in computer technology are
  expected to expand the limits of feasible
  simulations, at least doubling the possible
  spatial resolution for such runs before 2010.
• Both nested approaches with structured grids, and
  variable resolution approaches with unstructured
  grids, appear promising ways forward.

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Summary II

• Present ice model algorithms appear adequate for
  the Bering Sea.
• The accuracy of circulation hindcasts for the
  BSAI are limited by the paucity of data,
  especially as regards the passes.
• Long-term moorings and systematic hydrographic
  surveys, in conjunction with altimeter data, will
  help rectify this deficiency
• Effective mathematical approaches are now
  available in community model codes for effective
  assimilation of such data into hindcasts and
  nowcasts. Computer resources are still a limiting
  factor in the application of some of these codes.
  
• The atmospheric forcing datasets also have
  outstanding issues (e.g. biased shortwave
  radiation estimates), which limit the hindcast
  skill of BSAI simulations, and of ice in
  particular.

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Summary III

• The ideal scientific/management biological model
  might include
• multiple species and multiple life stage
  components
• specific species treated using spatially explicit
  IBMs
• coupled to multi-compartment NPZ and circulation
  models
• Proper feedback among different components
  especially challenging
• Intermediate step focus on coupling spatially
  explicit NPZ with spatially aggregated food web
  models.
• For all models, longer time scales are needed to
  aid in ecosystem-based management.
• Data gaps are even larger for the biology than
  for the physics of the BSAI
• sustained surveys (e.g. the NMFS groundfish
  surveys) have yielded much useful data for the
  quantification of food webs.

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Summary IV

• More collaborative development of both physical
  and biological models is recommended, as they
  will require substantial human resources.
• Human time to examine and interpret the output
  can be just as limiting as computer hardware
• One way to ease the development and
  interpretation of such multi-investigator models
  is to provide easy access to model output through
  web-based software.

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FIN!

• http//halibut.ims.uaf.edu/SALMON/BSIAModelWorksho
  p

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Features of the Bering Sea

• A big, broad shelf
• Passes
• wide, narrow, shallow, deep
• Canyons
• Cross-shelf flux
• Powerful tides
• A deep basin
• Ice
• High production
• Lots of fish!
• Climate change

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Foci of the workshop

• review existing modeling efforts in the BSAI
• assess strengths and weakness of the various
  types of ocean circulation models in accurately
  representing circulation, mixing and exchange due
  to
• forcing mechanisms (winds, tides, ice formation,
  river runoff)
• topographic features (coastline, shelf break,
  Aleutian Island passes)
• evaluate various monitoring and process studies
  that would improve the accuracy of the models
• describe pathway for using these models to
  develop products that would be useful for
  resource managers and users.