Arctic sea ice anomalies during the MOSAiC winter 2019/20
Authors: Dethloff Klaus , Maslowski Wieslaw , Hendricks Stefan , Lee Younjoo J. , Goessling Helge F. , Krumpen Thomas , Haas Christian , Handorf Dörthe , Ricker Robert , Bessonov Vladimir , Cassano John J. , Kinney Jaclyn Clement , Osinski Robert , Rex Markus , Rinke Annette , Sokolova Julia , Sommerfeld Anja 
Published: March 2022

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.