ACCESS NUMBER 30818

STUDY TITLE: Coupled Ice-Ocean Modeling of the Arctic Ocean

REPORT TITLE: Numerical Simulation of Atmosphere/Ocean/Sea Ice Interaction in the Arctic

Ocean: 1982-1 996

CONTRACT NUMBER: 14-01-96-CT-30818

SPONSORING OCS REGION: Alaska Outer Continental Shelf Region

APPLICABLE PLANNING AREA(S): Beaufort Sea, Chukchi Sea, Hope Basin

FISCAL YEAR(S) OF PROJECT FUNDING: FY1996

COMPLETION DATE OF REPORT: October 2001

COST(S): $218,549

CUMULATIVE PROJECT COST: $218,549

PROJECT MANAGER: Dale B. Haidvogel

AFFILIATION OF PROJECT MANAGER: Rutgers,The State University of New Jersey

ADDRESS: Institute of Marine and Coastal Sciences, 71 Dudley Road, New Brunswick, NJ

08901-8521

PRINCIPAL INVESTIGATOR(S): Dale B. Haidvogel, Katherine S. Hedström, and Jennifer

Francis*

KEY WORDS: Numerical Modeling, Arctic Basin, Sea Ice, Beaufort Sea, Chukchi Sea, Climate

Change, TOVS

BACKGROUND: In an earlier study, a coupled ocean circulation! sea ice model was used to

simulate flow properties and sea ice evolution in the Western Arctic during the year 1983. The

coupled model -- based upon the semi-spectral primitive equation model of Haidvogel et al. and

the Hibler sea ice model -- was implemented on a uniform 20 km grid over the Western Arctic,

and utilized forcing by daily surface geostrophic winds and monthly averaged thermodynamic

fluxes. The results of the model simulation were shown to be consistent with the large-scale

circulation features of the Western Arctic. In particular, the Beaufort Gyre and its seasonal

variability in the sense and amplitude of its circulation were reproduced to the extent of our ability

to quantify them in the target year of 1983. The bulk properties of the ice distribution observed

during 1983 -- including ice concentration and thickness and its seasonal growth and retreat--

were also reproduced well by the coupled model to the degree that available observations

allowed model evaluation. Some systematic tendency for faster-than-observed melting in the

spring and summer months was seen in the model along the Siberian continental shelf. The

poorest model performance was obtained in the comparison between the model-inferred ice floe

("float) trajectories and the observed trajectories of surface floats released in the Arctic in 1983.

Mean motion of the simulated drifters was found to be comparable to those observed; however,

little agreement between individual sets of trajectories was obtained.

OBJECTIVES: The objectives of this study included:

Xthe implementation of a second-generation, coupled ice-ocean model for the Arctic

Ocean;

Xthe expansion of model geometry,relative to our previous studies, to include the entire

lateral extent and water column depth of the Arctic Ocean;

<the development and utilization of higher resolution and quality atmospheric forcing fields;

<the improvement of the parameterizations used for surface mixed layer dynamics; and

<the completion and analysis of a 15-year hindcast of Arctic Ocean circulation and sea ice

for the period from 1982 through 1996.
ACCESS NUMBER 30818

DESCRIPTION:

xThe newest version of the coupled ice-ocean model was implemented using the Regional

Ocean Modeling System (ROMS) -- a free-surface, generalized sigma coordinate,

primitive equation ocean circulation model -- and a revised version of the Hibler sea ice

model, now configured on an orthogonal, curvilinear, Arakawa C-grid. The improved sea

ice thermodynamics are a modified version of the Mellor and Kantha model.

<The new capabilities to utilize curvilinear grids enabled us to implement an efficient grid that

both covered the entire Arctic Basin and had enhanced resolution (<20 km) in the

Western Arctic Ocean.

>zThe Polar (Arctic) Pathfinder (Path-P) dataset used in this study, obtained from the 20-year

TOVS global dataset, has been reprocessed to 100 km by 100 km grid boxes in space

and to one Arctic-wide field per 24-hour period.

<Sur1ace vertical mixing is now parameterized with the K-Profile Parameterization (KPP)

scheme of Large et al.

<The resulting coupled model, forced with the daily Path-P datasets, produced a 15-year

hindcast of circulation and sea ice conditions for the period 1982-1996. Monthly mean

history records of all surface variables (surface ocean and ice) were retained for analysis.

STUDY RESULTS: The results of the 15-year coupled model hindcast are consistent with the

large-scale circulation features and sea ice distributions within the Arctic Ocean, to the extent that

quantitative datasets exist for validation. Importantly, the bulk properties of the sea ice

distribution observed during 1982-1996 -- including ice concentration, thickness and motion, and

their interannual variations -- are reproduced well by the coupled model. Several smaller-scale

disagreements with the observations occur primarily in two regions: directly adjacent to the

coastal boundary (e.g.) in the Western Arctic, and near the open boundary with the Atlantic

Ocean. The former defect is likely caused, in part, by omission of "fast ice" in the formulation of

the sea ice model. The latter is due to incomplete exchange of properties with the Atlantic Ocean

due to insufficiently "open" open boundaries.

The new Path-P datasets and the resulting coupled model hindcast clearly show considerable

interannual variability in atmospheric, oceanic and cryospheric variables. In addition to prominent

year-to-year variations in (e.g.) the strength of the Beaufort Gyre and the associated sea ice

motion, there are also strong progressive changes. In particular, the coupled model reproduces

the systematic variations in sea ice properties -- including ice concentration and thickness --

noted in several recent studies.

Despite the overall positive appraisal we give to the coupled model and its hindcast ability,

improvements in several areas have been identified as desirable:

Xlmproved model physics: Successive versions of the ROMS model have been perfected

since version 2 of our coupled model was implemented. These newer versions have

more efficient time-stepping, better quasi-monotone advection schemes, more accurate

pressure gradients with associated improvements in treatment of topography, and

additional capabilities for inclusion of riverine sources and tides. All of these new features

would be beneficial in future simulations.

<Highly parallel numerics: The latest circulation models are highly parallel, allowing efficient

computation of large problems on lOs to 100's of processors. Development of a highly

scalable coupled ice-ocean model should allow routine multi-decadal simulations on finer

spatial grids.

XEnhanced resolution [0(5) km] and steeper, less smoothed bathymetry: Both of these

features are now possible, given the preceding improvements in model software and

continuing enhancements in available computational resources.
ACCESS NUMBER 30818

<lce thermodynamics and mechanics: Additional ice categories and more vertical levels of

ice temperature, and improved ice rheology algorithms, are becoming available in more

recent sea ice model formulations.

<lmproved open boundary conditions: Although still a matter more of art than science, there

now exist better inflow/outflow treatments that should improve exchanges with (e.g.) the

Atlantic Ocean and Bering Sea.

As we note, progress on models and methods that has occurred in the past few years now make

many of these improvements possible.

STUDY PRODUCTS:

Hedstrom, K. S. 2000. Technical Manual for a Coupled Sea-Ice/Ocean Circulation Model (Version 2).

MMS 2000-047. USD01, MMS, Alaska OCS Region, Anchorage, AK., 2000.

Haidvogel, D. B., K. S. Hedstrom and J. Francis, 2001. Numerical Simulation of Atmosphere/OceanlSea

Ice Interaction in the Arctic Ocean: 1982-1996. OCS Study MMS 2001-069. Final Report Contract No.

14-35-01-96-CT-30818. USD01, MMS Alaska OCS Region: Anchorage, AK.

* Pis affiliation may be different than that listed for Project Manager(s)