The Average Shape of Sea Ice Ridge Keels.

Through analysis of over 64,000 ridge profiles identified from moored upward‐looking sonars, we identify a well‐defined average shape of pressure ridge keels that is concave or cusped, not triangular as widely assumed in other literature. On the basis of this average shape, we put forward a new, dimensional‐definition, of a pressure ridge cross‐section that follows a negative exponential form and allows an average pressure ridge cross section to be constructed with knowledge or choice of a single parameter. The horizonal asymptote of the profile represents the draft of the "background" ice in which the ridge is embedded. The draft of this background ice scales with keel depth and is typically greater than can be accounted through thermodynamic growth, indicating that ridges tend to be embedded in fields of mechanically thickened rubble. Using a variational ridge model we simulated keel shapes for a range of ridge building conditions. The model results agree well with the observations and indicate the cusped shape of an average ridge profile arises from the varying angle of horizontal shear in the ice cover when ridges form. The modeling results also explain the elongated tail of the area draft distribution of the pack. Plain Language Summary: Through analysis of over 64,000 sea ice ridge profiles identified from upward‐looking sonars, we identify a well‐defined average shape for ridge keels. The shape of keels tends to be concave and not triangular as commonly cited in existing literature. Because of this result, we put forward a new description of an average ridge cross‐section that has a concave shape. This new definition allows an average ridge cross section to be constructed with a value for a single parameter. It was observed the draft of the "background" ice, the ice cover on either side of a ridge, scales with keel depth and, on average, is greater than can be accounted through natural thickening of the ice due to freezing alone, indicating ridges tend to be embedded in rubble fields; fields of ice that result from ice that was fractured and compiled by movement of the ice cover. Using a mathematical sea ice ridge model, we simulated keels for a range of ridge building conditions. The modeling results agree with observations and indicate the concave shape of the average ridge profile arises from variations in the horizontal shearing of the ice cover when ridges form. Key Points: Paramters that define an average pressure ridge scale to one anotherOn average, sea ice ridge keels tend to have a concave (cusped) shape that scales to the draft of the background iceA varying angle of horizontal shear in the ice cover is the cause of the cusped shape of an average ridge keel [ABSTRACT FROM AUTHOR]