The role of sea ice in mediating atmosphere-ice-ocean momentum transfer

Thesis (Ph.D.)--University of Washington, 2022The ongoing loss of Arctic sea ice prompts questions about changes in momentum transfer across the atmosphere-ice-ocean system and potential climate feedback mechanisms, but the role of the ice in mediating that process is not fully understood. To address knowledge gaps about atmosphere-ice-ocean momentum transfer, this study makes use of in situ measurements collected during two recent observational campaigns in the Beaufort Sea: the Marginal Ice Zone (MIZ) program and the Stratified Ocean Dynamics of the Arctic (SODA) program. The research is presented in two parts.Part I develops data processing methods for instrumentation deployed as part of the SODA program while part II uses data from both programs to evaluate controls of sea ice on the ocean surface stress and the associated response in the ocean surface mixed layer (ML). Measurements of upper ocean properties can be challenging in the Arctic Ocean due to environmental conditions, including the need for moored instruments to avoid contact with sea ice. Part I of this work describes methods for developing usable data products from upward-looking Acoustic Doppler Current Profiler (ADCP) measurements on the SODA moorings. This encompasses methods for creating combined data records from vertically-offset ADCPs on the same mooring chain and methods for using ADCPs to estimate surface ML depth and temperature. As ML depth and temperature measurements are not typically possible from subsurface moorings in the Arctic, the approach developed here provides considerable value and could be applied to other extant mooring records to recover information about ML property variability or trends. Building on these measurements, part II considers how sea ice properties, such as concentration and morphology, affect the transfer of momentum from the sea ice to the ocean and the associated upper-ocean dynamics. Ice-ocean drag coefficients, a measure of momentum transfer efficiency, calculated using a force-balance approach from the SODA mooring data show seasonal variations consistent with past observations, but at odds with previous model results.Tests of model parameterizations of drag based on direct observations of under-ice morphology reveal that the model-observation mismatch can be attributed to inaccuracies in empirical translations from bulk geometric properties to detailed geometry statistics, especially under-representation of floe sizes. The mooring measurements further show a seasonal upper-ocean response to surface stress, with minimal energy in ML inertial oscillations under sea ice cover. A simplified momentum budget for the coupled ice-ocean system shows that the seasonality stems from a combination of internal stress in the ice damping the oscillations and seasonal variability in the ocean ML depth. Finally, shipboard measurements from the MIZ program show the evolution of ice-edge-located freshwater front as it reacted to changes in the surface wind stress, raising questions about how sea ice might impact spatial heterogeneity of stress and how that might interact with frontal dynamics. This research shows that boundary layer dynamics and internal ice mechanics both play a role in mediating atmosphere-ice-ocean momentum transfer. The results suggest future trends of more "slippery" ice-ocean boundary layer, and decreasing internal sea ice stress, which could lead to increased sea ice drift speeds and more direct atmosphere-ocean coupling