Changes in Arctic sea ice thickness distribution in Fram Strait over the last three decades, 1990 – 2019

Fram Strait is an ideal location to monitor long-term changes of sea ice properties in the central Arctic since the major fraction of ice export from the Arctic occurs here. The Fram Strait Arctic Outflow observatory has been monitoring sea ice and ocean outflows at ~79°N for the last three decades. We examined changes of monthly mean sea ice thickness distributions obtained from upward looking sonars deployed in the observatory. We found that the thickness distributions can be reasonably approximated by lognormal functions except for fractions of very thin ice classes. We fitted the observed distributions with lognormal functions and used three parameters of the functions (modal thickness, modal peak height and variance) to describe the long-term changes of the thickness distribution. We found that these parameters exhibit a concurrent change and indicate a shift of the Arctic sea ice regime. The first regime is represented by a thick and deformed ice pack, described by thicker modal thickness with a smaller and more broad modal peak with larger variance of the distribution. The second regime has a thinner and more uniform ice cover, represented by thinner modal thickness with more compact distribution around the mode and smaller fraction of deformed ice. We examine factors causing this shift and introduce a stochastic sea ice thickening model which can explain the change of the ice thickness distribution.