Sea-ice turbulent dynamics derived from Lagrangian buoys tracks

The Arctic, a central place to our global climate, is mainly covered by sea ice. The motion of sea ice is important as it exports freshwater from the central Arctic to the North Atlantic. The motion itself is on a large scale driven by atmospheric and oceanic patterns. Yet, fluctuating velocities are superimposed on this mean circulation. These fluctuating velocities represent atmospheric and oceanic turbulence as well as internal sea ice dynamics. One main property of the sea-ice fluctuating velocity is its high intermittency induced by the fracturing of sea ice. To understand the dynamical properties of the fluctuating velocities, we study the kinematic energy using Lagrangian statistics derived from observed trajectories, inspired by classical multi-scale analysis from fluid turbulence. We use trajectories from the International Arctic Buoy Program (IABP), covering a 40-years period of the sea-ice covered central Arctic, both in summer and winter seasons. The energy spectra is found to follow the Kolmogorov -5/3 scaling, which is classically observed for fluid turbulence. The cross-correlated acceleration-velocity structure function (Sau) provides information on the energy cascade between spatial scales. While noisy, the computed Sau is negative in the 10-1000 km scales, which would imply a direct energy cascade (from large to small scales). The imprint of the atmospheric and oceanic turbulence must present a direct energy cascade. However, as the characteristics of the sea-ice fluctuating velocity fields are also impacted by the sea ice internal physics, such a negative cascade could result from the energy transfer from large-scale forcing fields down to the smallest fractures in the sea ice. While the statistics of the sea-ice turbulence are similar over the whole 40-years period, differences are found between the old and modern data. These changes point towards a transition of dynamical regime that the Arctic sea ice is undergoing.