 |
A New Unstructured Grid Finite-Volume Arctic Ocean Model: AO-FVCOM |
The complex geometry of Arctic coastlines and steep bottom bathymetry along the continental slopes and ridges challenges the numerical model applications in addition to the problems of Arctic modeling discussed above. Numerous islands and narrow straits in the Arctic Ocean require a model capable of resolving irregular coastal geometry. In the Canadian Archipelago, for example, many islands have irregularly-shaped coastlines and are separated by passages with widths of ~5 km or less. Since these passages function as a network for water and ice exchange between the Arctic Ocean and North Atlantic Ocean [Maslowskiet al., 2000], resolving the coastal geometries of these islands and narrow passages is a critical need for a new Arctic Ocean model. One approach is to use a model with an unstructured grid that allows great flexibility in horizontal grid fitting and resolution.
|
Click here to view the full size of the image. The number shown in the image are the tidal amplitude. |
We, a team of University of Massachusetts-Dartmouth (UMASS) and Woods Hole Oceanographic Institution [the team members: Dr. Chen and Mr. G. Gao (UMASS) ; Drs. A. Proshutinsky and R. C. Beardsley (WHOI)], have developed a high-resolution, unstructured-grid, finite-volume coastal ocean model for the Arctic Ocean. This model was a spherical coordinate version of FVCOM (developed originally by Chen et al. (2003). FVCOM is a prognostic, free-surface hydrostatic 3-D model that solves the primitive equations using the finite-volume approach which guarantees local conservation of mass, momentum, heat, and tracer. The horizontal grid is constructed using unstructured triangular meshes while the terrain-following sigma-coordinate in used in the vertical. Originally developed for Cartesian coordinate applications, FVCOM was recently extended to include a spherical coordinate version (Chen et al., 2006b). This version allows us to construct unstructured triangular grid meshes without restriction from the meridional convergence of latitude and longitude and any need for “grid rotation”. The singularity at the North Pole is removed by using a spherical-polar stereographic projection nested grid at the pole.
The joint model development effort made by the UMASS/WHOI team meets the one of the major goals of the Arctic Ocean Model Intercomparison Project (AOMIP) in improving results of the Arctic Ocean dynamics and thermodynamics simulations for better reproduction of variability and changes of arctic climate (Proshutinsky et al., 2001,2005; Proshutinsky, 2006). The AOMIP studies (including papers presented in this special section) have shown that at least two obvious model improvements are possible at this stage of AOMIP development. The necessity to increase model horizontal and vertical resolutions is discussed by Pantellev et al. (2006) and by Golubeva and Platov [2006] (both publications are in this volume). The second model improvement recommendation is to include tidal motions directly in the model dynamics and to provide physically-based mixing rates that change horizontally and vertically depending on tidal currents and ice-tide interactions. In the absence of direct wind forcing, the tides in the Arctic Ocean provide mixing in the bottom boundary layer and at the ocean-ice surface due to tide-ice interaction. The importance of tidal forcing for sea ice and Arctic Ocean processes has been discussed by Proshutinsky (1993), Kowalik and Proshutinsky (1994) (K&P hereafter), Hibler et al., (2004, 2005) and Holloway and Proshutinsky (2006).
|
AO-FVCOM
AO-FVCOM covers the entire Arctic Ocean including the Canadian Archipelago, Hudson and Baffin Bays, the Labrador, Greenland, and Norwegian Seas, and the Demark Strait (see the image on the right). The bathymetric data are taken from two sources: the sub-Arctic region (up to 72 degree N) is represented by depths from the 2-minute Naval Oceanographic Office Digital Bathymetric Data Base–Variable resolution (DBDBV version 4.3 and the central Arctic bathymetry is obtained from The International Bathymetric Chart of the Arctic Ocean (IBCAO), a digital database that contains all available bathymetric data north of 64 degree N. These two data sets match very well in the overlapping region between 64 degree N and 72 degree N.
The model domain is bounded by three open boundaries across the Labrador and Greenland Seas in the Atlantic Ocean and across the Bering Strait connected to the Pacific Ocean. In the horizontal, the domain is configured with an unstructured triangular grid with a resolution varying from 1-3 km in the Canadian Archipelago, inlets and straits, and in and over the shelfbreak region to 10-15 km in the interior basins. Total numbers of triangular cells and nodes are 520,817 and 275,574. In the vertical, the domain is divided into 40 non-uniform s-layers, which corresponds to a vertical resolution of 1-10 m or less in the coastal region (where the water depth is shallower than 300 m) and about 10 m near the surface and bottom in the interior region (where the water is deeper than 4000 m). The minimum and maximum depths in the model is 5 m(applied along all coastlines)and 4977 m (at 44.5001 degree N, 169.2229 degree W).
|
Click here or the image to view the full size of the image for the unstructured triangular grid for FVCOM-Arctic.
|
Animation of Semidiurnal Tidal Elevation and Near-surface Tidal Currents
The semidiurnal tides are relatively small in the deep Arctic basins and larger in the Hudson Strait/Bay, southern Baffin Bay, the Denmark Strait west of Iceland, and the White Sea. Numerous amphidromic points occur along (or near to) the coast, indicating that the semidiurnal tidal phases at two very close locations can differ significantly. The M2 tidal motion around Iceland and Spitsbergen Island are characterized by the clockwise round-island wave.
Click here or the image on the right to view the full size of the animation
|
|
Enlarged View of M2 Tidal Elevation and Near-surface Tidal Current in Canada Archipelago
The Canadian Archipelago consists of numerous islands which are separated by narrow and deep straits. The bathymetry of this region is not well known because of the thick land-fast and pack ice. The model tidal amplitudes vary significantly in this region from a few to over 100 cm. Based on the 78 tide gauge station data, the mean and maximum tidal amplitudes are 41.2 (116.7) cm for M2 and 16.1 (48.1) cm for S2. During spring tides, the maximum tidal elevation of these four constituents exceeds 150 cm, which accounts for about 80% of the observed spring tide (with 96 tidal constituents) in this area.
Click here or the image on the right to view the full size of the animation.
|
|
Animation of Diurnal Tidal Elevation and Near-Surface Tidal Currents
The diurnal tidal amplitudes are roughly 10 times smaller than the semidiurnal tidal amplitudes. The diurnal K1 and O1 tides have similar horizontal patterns in amplitude but different phase distributions. Both diurnal tides are characterized by trains of trapped shelf waves along the Greenland shelfbreak and around Greenland as first described by Kowalik and Proshutinsky [1993, 1995], however, FVCOM co-tidal charts reveal additional locations for this phenomenon due to its much higher grid resolution. The model K1 and O1 phases differ significantly in the Greenland Sea and Arctic Basin. For example, around Spitsbergen Island, the K1 tide exhibits one node on the southwest and three nodes on the south side while the O1 tide has no nodes in these two areas. Along the Alaskan coast, the K1 has many nodes while the O1 has only one.
Click here or the image on the right to view the full-size of the animation.
|
|
Enlarged View of Diurnal Tidal Elevation and Near-surface Tidal Currents in Canadian Archipelago
Canadian Archipelago is the energy sink area for the diurnal tidal wave propagating from Baffin Bay and also one of the energy source the Arctic Ocean coast. The shelf of the Archipelago is characterized with the topographic-intensified wave consisting of a train of eddy-like current flow.
Click here or the image on the right to view the full-size of the animation.
|
|
Enlarged View of Diurnal Tidal Elevation and Near-surface Tidal Currents in Baffin Bay
The shelf of Baffin Bay, western coast of Greenland, is characterized by a slope-internsified coastal wave consisting of trains of "eddies". Sizes and intensities of these eddies are related to the 3D slope of the shelf.
Click here or the image on the right to view the full-size of the animation.
|
|
Unstructured grid Ice Model for the Arctic Ocean
Drs. Dupont, University Laval/Canada and Kliem at Danish Meteorological Institute/Denmark have implemented a finite-volume/finite-element ice model inside FVCOM. The ice model was built based on the formulation derived by Hibler III (1979), Zhang and Hibler III (1997) and Hunke and Dukowicz (1997), with inclusion of viscous-plastic rheology and thermodynamics suggested by Semtner (1976), Parkinson and Washington (1979) and Winton (2000). This model is solved numerically by an efficient implicit solver. An initial test was made for the Archipelago area, the results are very reasonable.
We are working on applying coupled FVOM-ice model to study the impact of the global-warming on the interannual variability of the ice coverage in the Arctic Ocean.
|
More results will be uploaded soon.
|
|
