Abstract. During the winter of; 2019/2020, as the Multidisciplinary drifting Observatory for the Study of; Arctic Climate (MOSAiC) project started its work, the Arctic Oscillation (AO) experienced some of its largest shifts, ranging from a highly negative index in November 2019 to an extremely; positive index during January–February–March (JFM) 2020. The permanent; positive AO phase for the 3 months of JFM 2020 was accompanied by a; prevailing positive phase of the Arctic Dipole (AD) pattern. Here we analyze; the sea ice thickness (SIT) distribution based on CryoSat-2/SMOS satellite-derived data augmented with results from the hindcast simulation by the; fully coupled Regional Arctic System Model (RASM) from November 2019 through; March 2020. A notable result of the positive AO phase during JFM 2020 was; large SIT anomalies of up to 1.3 m that emerged in the Barents Sea (BS),; along the northeastern Canadian coast and in parts of the central Arctic; Ocean. These anomalies appear to be driven by nonlinear interactions between; thermodynamic and dynamic processes. In particular, in the Barents and Kara; seas (BKS), they are a result of enhanced ice growth connected with low-temperature anomalies and the consequence of intensified atmospherically driven; sea ice transport and deformations (i.e., ice divergence and shear) in this; area. The Davies Strait, the east coast of Greenland and the BS regions are; characterized by convergence and divergence changes connected with thinner; sea ice at the ice borders along with an enhanced impact of atmospheric wind forcing.; Low-pressure anomalies that developed over the eastern Arctic during JFM 2020 increased northerly winds from the cold Arctic Ocean to the BS and; accelerated the southward drift of the MOSAiC ice floe. The; satellite-derived and simulated sea ice velocity anomalies, which compared; well during JFM 2020, indicate a strong acceleration of the Transpolar Drift; relative to the mean for the past decade, with intensified speeds of up to 6 km d−1. As a consequence, sea ice transport and deformations driven by; atmospheric surface wind forcing accounted for the bulk of the SIT anomalies,; especially in January 2020 and February 2020. RASM intra-annual ensemble; forecast simulations with 30 ensemble members forced with different; atmospheric boundary conditions from 1 November 2019 through 30 April 2020 show a pronounced internal variability in the sea ice volume, driven; by thermodynamic ice-growth and ice-melt processes and the impact of dynamic surface winds on sea ice formation and deformation. A comparison of the respective; SIT distributions and turbulent heat fluxes during the positive AO phase in; JFM 2020 and the negative AO phase in JFM 2010 corroborates the conclusion; that winter sea ice conditions in the Arctic Ocean can be significantly; altered by AO variability.;
