Modeling landfast ice in the Beaufort-Chukchi Seas using CIOM

            The diminishing Arctic sea ice has shortened the ice season, in particular for the multiple year ice. This also affects the landfast ice along the Beaufort coast. Thus, the seasonal and interannual variability of landfast ice under the diminishing sea ice scenario in the PAR is an emerging topic.

            Landfast ice along the Chukchi and Beaufort coast is a seasonal phenomenon with interannual variability. 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. A 3.8-km resolution 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 using the daily NCEP forcing for the period 1990-2009. The SIC map (Fig. 1a) clearly shows what corresponds to simulated landfast ice attaching to the Alaskan Beaufort and Chukchi coast during the melt season as ice of high concentration. During spring, surface melting commences nearshore but ice concentrations first drops offshore as a result of complete melting 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 effective 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 the month, 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 (~1.5m) and thinner offshore (<1m) ice. Since the model is not explicitly simulating processes that contribute to landfast ice stabilization, in particular grounding of pressure ridges, other processes represented in the model can explain the formation and maintenance of landfast ice. These include a onshore northeast wind due to the Beaufort high pressure system; the eastward ACC and Beaufort Slope Current with its right-turning force due to the Coriolis effect, which advect the warmer Bering water; high resolution topography and bathymetry constraining ice motion in the coastal regions; and sea ice advection. However, these model-inherent factors that help keep landfast ice in place in nature deserve further investigation.

            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 for the period 1995-2005. 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, coupled 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 the ocean surface current and wind forcing. During the period of complete 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 with the velocity almost being zero), while the CIOM-simulated landfast ice still exhibits small movement. The difference is due to that 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.

            The CIOM-simulated landfast ice is generally consistent with landfast ice extent derived from satellite data. The CIOM reproduces the landfast ice boundary in January and February very well. However the model reproduces less landfast ice than the measured boundary in March. During April, a melting season, CIOM reasonably well reproduces landfast ice in general, but reproduces less ice from 147o-152 oW.  In May, CIOM reproduces more landfast ice between 140o-147 oW. In June, the model simulation compares very well with the measurement. 




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