Changing Sea-ice and the Bering Sea Ecosystem

Model and Schematic Circulation - Zhang - Stabeno etal 1999

Inset map shows schematic Bering Sea Circulation from Stabeno et al 1999. Rest of figure shows simulated Bering Sea surface velocity for August 2005 from the BESTMAS high resolution ocean-ice model (see below for details).

Changing Sea-ice and the Bering Sea Ecosystem
High Resolution Modeling studies of present and future Bering Sea circulation

Jinlun Zhang
(zhang@apl.washington.edu)
(homepage)

Rebecca Woodgate
(woodgate@apl.washington.edu)
(home page)

A contribution to the joint
NSF-NPRB
BSIERP-BEST Program

Funded by NSF Arctic Natural Sciences (grants ARC-0611967, ARC-1107327, ARC-1107187)
under the BEST (Bering Sea Ecosystem Study) Project

Polar Science Center, Applied Physics Laboratory,
University of Washington
1013 NE 40th Street, Seattle, WA 98105, U.S.A.

The Bering Sea - lying at the northern end of the Pacific Ocean and north of the Aleutian Chain - is the source of over 50% of the total US fish catch and the home to immense populations of birds and marine mammals. The Bering Sea ecosystem is strongly tied to the seasonal sea-ice, which influences the oceanic environment of the region and also provides a habitat for many species. Recent years have shown significant climate regime shifts in the Bering Sea. As part of the new NSF Bering Ecosystem Study (BEST), we plan to use a state-of-the-art numerical ocean-ice model to investigate prior (and predict future) changes in the Bering Sea ice cover and study the impacts of these changes on Bering Sea marine and eco-systems.

Science
Motivation

Science
Objectives

The BESTMAS coupled
ice-ocean model

RESULTS
- Bering Sea movies
- Bering Sea simulated data
(and how to plot it)

The Future of Arctic Sea-Ice?
(from the Polar Science Weekend at the Pacific Science Center, Seattle. March 2007)

SCIENCE MOTIVATION

      The Bering Sea is the source of over 50% of the total US fish catch and the home to immense populations of birds and marine mammals. This extraordinarily productive ecosystem is vulnerable to the significant climate regime shifts that have occurred over recent decades and will surely have substantial economic and social implications. These regime shifts are closely linked to a warming of the atmosphere and ocean, and the coincident retreating of the sea ice cover, both with strong interannual and decadal variability.
     The ice cover is a key player in the eastern Bering ecosystem. It affects ocean temperature, salinity, stratification, light distribution, and transport of nutrients and carbon, by modifying surface light availability and air-sea exchanges of heat, freshwater, momentum, and biogenic gasses. All these are important for ecosystem dynamics. Ice is also a vital habitat for many species. Whether the retreating ice cover and the warming regime shifts are due to natural climate variability or to anthropogenic climate change, for economic/social reasons it is essential to understand past change in the eastern Bering climate and ecosystem and to predict the timing and scope of future change.
      We propose a study of the historical and contemporary changes of the Bering Sea ice cover and the impacts of these changes on Bering Sea marine climate and physical-biological processes. Additionally, we will investigate future changes of the eastern Bering marine environment under global warming scenarios

SCIENCE OBJECTIVES

Our science objectives are to:
   1) Simulate the historical evolution and regime shifts of the eastern Bering ice-ocean system from 1970 to the present, to quantify the ice-ocean climate changes that have affected the ecosystem;
   2) Identify key linkages among the atmosphere, sea ice, and ocean, to understand mechanisms affecting physical forcings of biological processes;
   3) Examine the interactions between the Bering Sea climate and the Pacific and Arctic climate, to assess the Bering climate system’s vulnerability to and influence on hemispheric climate change;
   4) Estimate the impacts of projected future anthropogenic global climate change (including an ice-free regime) on the eastern Bering Sea system, specifically focusing on changes in ice-ocean climate and the physical forcings of biological systems, to provide a future change assessment to local and national stakeholders, including Native and fishing communities, and the general public.

The BESTMAS coupled ice-ocean model
Bering Ecosystem STudy ice-ocean Modeling and Assimilation System
Our studies will develop and run a state-of-the-art coupled ice-ocean model for synthesis and modeling of the eastern Bering Sea. Although significant progress has been made modeling Bering ocean processes (e.g. Hermann et al., Winsor et al., Wang; Clement), most of the previous models did not incorporate a sea-ice component.

Model Resolution
BESTMAS has a high resolution (min 2km, average 7km) grid focused on the eastern Bering Sea, and a model domain that extends to 35 deg N in the Pacific and the Atlantic.
This high resolution model is coupled to a global coupled ice-ocean model (resolution ~ 0.65 deg).

Ice and Ocean Models
    BESTMAS is a fully-coupled ice-ocean model.
    The Ice Model is based on the TED (thickness and enthalpy distribution model, Zhang & Rothrock) sea-ice model. It includes viscous plastic sea-ice rheology; has 12 categories each for undeformed ice, ridged ice, ice enthalpy and snow; simulates ridging and calculates thermodynamic growth/decay over each thickness category.
     The Ocean Model is based on the POP ocean model and includes isopyncal mixing (Gent McWilliams), K-profile vertical mixing parameterization (Large et al), implicit free surface. Although it is not (yet) practical to incorporate an interactive biological component, passive tracers will be used in the model to study advection of nutrient-rich waters. Tides play an important role in the Bering (60-90% of the horizontal kinetic energy. BESTMAS includes 8 primary tidal components, and tidal coefficients will be tested against observations.
      For BESTMAS model forcings and initial conditions, we will use
- mainly daily NCEP/NCAR Reanalysis data for surface atmospheric forcing, although ECMWF data will also be used for testing;
- observed monthly river runoff;
- Levitus initial conditions for temperature and salinity.
     Ice and Ocean model improvements will be implemented, including
= for sea-ice: - ice rafting, teardrop plastic sea-ice rheology, and lateral melting;
= for ocean: - latest version of the POP model, spatially varying anisotropic viscosity and partial bottom cells.
   BESTMAS will be developed to include data assimilation of sea ice concentration and satellite sea surface temperature (SST).
BESTMAS will be validated against satellite and insitu data, including hydrographic and mooring data.

Preliminary Results
Initial model runs are still in progress. Although thorough model-data validation is still to be done, preliminary results suggest the model ice-edge and the model water velocity are in reasonable agreement with observations. Inclusion of tidal forcings is also found to be essential to create the observed clockwise circulation around the Pribilof Islands.

Inset map shows schematic Bering Sea Circulation from Stabeno et al 1999. Rest of figure shows simulated Bering Sea surface velocity for August 2005 from the BESTMAS high resolution ocean-ice model

For use of any of these figures, please contact Jinlun Zhang (zhang@apl.washington.edu).

We gratefully acknowledge financial support for this work from the National Science Foundation (NSF).
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