Relative influences of the metocean forcings on the drifting ice pack and estimation of internal ice stress gradients in the Labrador Sea

Abstract

Understanding the relative influences of the metocean forcings on the drift of sea ice floes is a crucial component to the overall characterization of an ice environment and to developing an understanding of the factors controlling the ice dynamics. In addition, estimating the magnitude of the internal stress gradients on drifting sea ice floes generated by surrounding ice cover is important for modeling operations, informing the design of offshore structures and vessels in ice environments, and for the proper calibration of Discrete Element Models (DEM) of fields of drifting ice floes. In the spring of 2015 and 2016, four sea ice floes offshore Makkovik, Labrador were tagged with satellite‐linked ice tracking buoys along with one satellite‐linked weather station on each floe to transmit wind speed and direction. Twenty satellite‐linked Lagrangian surface ocean current tracking buoys were also deployed in the open water adjacent to the targeted ice floes. In this paper, the dynamics of the four ice floes are explored in terms of the relative proportions which were forced by the wind, current, sea surface topography, Coriolis, and internal stress gradients. The internal ice stress gradients are calculated as residuals between the observed accelerations of the floes as measured by the tracking buoys and the sums of the other metocean forcings. Results show that internal ice stress gradients accounted for up to 50% of the observed forcing on the floes, and may have reached up to around 0.19 kPa.

Evaluation of the relative contribution of meteorological and oceanic forces to the drift of ice islands offshore Newfoundland

“…The sensitivity analysis revealed that out of the four drag coefficients, the sea-ice force magnitudes on the ice island fragments were most sensitive to the water form drag coefficient, followed by the air form drag coefficient. The force records from the residual approach (Turnbull and others, 2017) also revealed that the sea-ice force magnitudes were almost insensitive to varying the skin drag coefficients for both air and water (Figs 13a, c). These results are expected given that the values of skin drag coefficients are significantly lower than form drag coefficients and that higher proportions of the fragments' walls were exposed to water than to air.…”

“…9.The concentration of sea ice surrounding the four ice island fragments during the analyzed drift periods, obtained from CIS daily ice charts.

Fig. 10.Sea-ice force magnitudes on the ice island fragments tracked by beacons 6640, 1700 and 5640, calculated using a residual approach (Turnbull and others, 2017) and Lichey and Hellmer's model (2001). The second y -axis (on the right) indicates the sea-ice concentration out of 10 (green lines) in the vicinity of the ice island fragments.

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“…While the ocean surface currents can sometimes be reasonably estimated from the wind conditions, the currents may vary significantly with depth. Turnbull and others (2017) used drifter buoys to directly measure the surface currents in the vicinity of the tracked ice floes, but the ocean currents in this study were extracted from CMEMS current data, which generally overestimate the measured current speed and could be wrong by 180° in azimuth (Brickman and Drozdowski, 2012). A small error in the ocean current estimate could make a large difference to the residual sea-ice force (Eqn (14)).…”

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Fig. 13.The residual sea-ice force magnitude on the ice island fragment tracked by beacon 1700, calculated using a residual approach (Turnbull and others, 2017) with varying skin ( c da and c dw ) and form ( c a and c w ) drag coefficient values for air (a, b) and water (c, d). Bold lines (in black) indicate the nominal values used in the study.

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“…The forces due to wind, current, Coriolis effect and sea surface tilt were calculated using reanalysis data and observed fragment drift velocity (from the tracking beacon data). The sea-ice force, however, was calculated using the residual approach that was used in Turnbull and others (2017), where the internal stress gradient force from surrounding ice on the drift of sea-ice floes was estimated. In this study, however, we used the residual approach (Turnbull and others, 2017) to estimate the influence of sea-ice force on the total force that governs the drift of ice islands, where the force from sea ice was calculated as the difference between the net force and the sum of other meteorological and oceanic forces on each fragment.…”