Wintertime observations in the central Arctic Ocean are sparse. We report a unique early winter data set containing atmospheric, hydrographic, and ocean velocity observations obtained during the first leg of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, with sea ice concentration at nearly 100%. One storm and four other periods of strong wind were encountered during the drift. This work focuses on the impact of the storm on oceanic variability in the upper 200 m. Variable oceanic surface mixed layers and pervasive horizontal density gradients were observed in the upper 100 m. Several anomalously large horizontal density gradients which lasted 1–2 days can be related to intrahalocline eddies. The nearly mixed upper 20-m water column and the deeper heavily stratified layer belonged to two separate dynamical regimes and appeared unaffected by the storm. Signatures of forced currents due to high ice drift speeds and velocity maxima resulting from intrahalocline eddies were seen above and below the base of the mixed layer, respectively. Near-inertial internal waves (NIWs) were detected among ice floes underneath the base of the mixed layer and were prevalent in the upper 100 m. Some NIWs were observed at depths deeper than 100 m with downward energy propagation. These results imply NIWs were generated at the surface. The single storm potentially injected half the overall near-inertial energy into the mixed layer, underlining the role of storm systems in the Arctic winter. Our results suggest that internal ice stress dampens momentum transfer from wind to the upper ocean, rendering wind forcing insufficient to erode preexisting stratification under full sea ice cover. This dampening contributes to the persistent thermohaline gradients observed. Instead, sea ice moving coherently in response to the winds acts as the main surface forcing driving upper-ocean processes such as NIWs.
Fang et al. (Thu,) studied this question.