Modeling Landfast Ice in the Beaufort-Chukchi Seas Using CIOM

Landfast ice along the Chukchi and Beaufort coast is a seasonal phenomenon (Eicken et al. 2006; Mahoney et al. 2007). It is a great challenge for any coupled ice-ocean model to capture the dynamic and thermodynamic features of landfast ice, since many factors can affect the formation, anchoring, and melting of landfast ice, such as wind forcing, ocean currents, coastal topography and bathymetry, and model resolution. To this end, a 3.8-km resolution CIOM (Wang et al. 2d003, 2008; Jin et al. 2007) was used to investigate the seasonal and interannual variability of landfast ice in the Chukchi and Beaufort seas.

Figure 1 shows the CIOM-simulated climatology of June sea ice concentration (SIC) and thickness under the daily NCEP forcing for the period 1990-2009. The SIC map (Fig. 1a) clearly shows what corresponds to simulated landfast ice attached to the Alaskan Beaufort and Chukchi coast during the melt season as ice of high concentration. During spring, surface melt commences nearshore but ice concentrations first drops offshore as a result of complete melt and removal of thinner offshore ice in areas of higher open water concentrations that promote absorption of solar heat. This is also reflected in the small magnitude of ice velocity vectors superimposed on ice concentration in Fig. 1a; nevertheless, since landfast ice is not modelled explicitly, small residual velocities remain in some areas of effectively landfast ice. However, at the same time the strong contrast between stationary landfast ice and highly mobile ice just offshore from the landfast ice edge appears to be well captured. Even in mid-July, Beaufort landfast ice remains, not melting completely until the end of July, depending on weather (SAT and wind direction) conditions.

The simulated sea ice thickness map (Fig. 1b) in June shows some contrast in landfast ice thickness along the Beaufort and Chukchi coast and thinner offshore ice. Since the model is not explicitly simulating processes that contribute to landfast ice stabilization, in particular grounding of pressure ridges (Mahoney et al., 2007), other processes represented in the model drive the mechanics of formation and maintenance of landfast ice. These include the following factors (Wang et al. 2010c): 1) a northeast wind due to the Beaufort high pressure system, 2) the eastward ACW current with its right-turning force due to the Coriolis effect, 3) high resolution topography and bathymetry constraining ice motion in the coastal regions, and 4) internal sea ice stress. However, at this point it is unclear how these model-inherent factors relate to the processes that help keep landfast ice in place in nature.

Figure 2 shows the climatology (1990-2009) of the simulated landfast ice that was compared to observed landfast ice extents obtained from synthetic aperture radar satellite data (Eicken et al. 2006; Mahoney et al. 2007). In the model, landfast ice starts to form in autumn due to the Beaufort Gyre and anticyclonic winds induced by the Beaufort High, both of which push sea ice toward the Alaskan Beaufort coast, along with the thermal growth of sea ice along the shore. When sea ice completely covers the entire Arctic from December on, landfast ice is attached to shore, while pack ice offshore still moves with surface ocean current and wind forcing. During the period of continuous ice cover, the radar satellite data indicate completely stationary landfast ice with a clearly delineated boundary between pack ice and landfast ice (anchored to bottom and attached to shore; so the velocity is zero), while the CIOM-simulated landfast ice still exhibits small movement since the sea ice produced in CIOM is not resolving the anchoring of grounded pressure ridge keels that stabilize the landfast ice. Thus, more research is required to improve ice dynamics representation in coastal regions and landfast ice processes by formulating and including the relevant ice anchoring mechanisms in the model.

Nevertheless, the CIOM-simulated landfast ice is mostly consistent with landfast ice extent derived from satellite data. The CIOM reproduces the landfast ice boundary in January and February very well; however reproduces less landfast ice than the measured boundary. There are two approaches to distinguish landfast ice from pack ice in CIOM. One way is to define landfast ice by an ice velocity criterion that considers ice stationary below a given velocity threshold. Here, if the absolute ice velocity is at or less than 4 cm/s at water depths less than 35m, then grid cells are designated as landfast ice (Fig. 2). The second, prescriptive method stipulates that during the simulation, the wind stress, surface ocean current, and ice velocity are set to zero shoreward of the 35m isobaths, roughly corresponding to the extent of landfast in many areas (Fig. 1). This method is widely used in Baltic Sea ice simulations (Haapala et al. 2001; Meier, 2002a, b), but is not capable of representing spatial and interannual or seasonal landfast ice extent.

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