An investigation of wind- and wave-driven dynamics in Antarctic sea ice from multiple types of buoy arrays

Womack, Ashleigh

An investigation of wind- and wave-driven dynamics in Antarctic sea ice from multiple types of buoy arrays

Thesis / Dissertation

2024

Publisher

University of Cape Town

Department

Department of Oceanography

Faculty

Faculty of Science

Abstract

The advance and retreat of Antarctic sea ice is the largest annually recurring event on Earth, and a significant portion of its natural variability appears to be associated with synoptic variability. Therefore, attaining a comprehensive understanding of both the dynamic and thermodynamic processes governing sea ice is vital. Moreover, this is particularly crucial for the Antarctic marginal ice zone (MIZ), where sea-ice behaviour is highly complex and where the interactions between the atmosphere and ocean are more variable. However, the paucity of observations in this region has meant that these drivers are poorly understood and modelled – specifically, the data on sea-ice drift, deformation, and concentration. Therefore, this thesis aims to enhance our understanding of the dynamic nature and response of the MIZ sea ice to extra-tropical cyclones, at both the synoptic and sub-daily scales, as well as to waves propagating into the Antarctic ice cover. This was done using multiple types of buoy arrays, which were operational in different regions around Antarctica and during different seasons and years. The thesis demonstrated that atmospheric (wind) forcing has a dominant physical control on ice drift. The transfer of momentum from winds at the lower (synoptic) frequencies has been determined to be the primary effect, with the initiation of inertial oscillations of sea ice as the secondary effect. Results indicated that there is a plausible correlation between the presence of cyclones and the onset of the inertial oscillations. However, it was also suggested that the penetration of storm-generated waves may trigger the inertial response of sea ice, or allow the underlying geostrophic currents to keep and sustain the weaker oscillations during the period of storm quiescence. An investigation into wave-ice interactions revealed that storm-generated waves have enough energy to propagate > 200 km into the ice cover. In the process, they are attenuated at a rate governed by the nature of sea ice, while the energy transferred in doing so leaves behind a wake of broken ice floes. This concomitant relationship is known to promote sea-ice growth during the winter months, when air temperatures begin to decrease. Additionally, it was demonstrated that the same waves-in-ice activity contributes to the break-up and ultimate retreat of the sea-ice cover, when air temperatures increase once again, and large floes are unable to reform after cyclonic events. This in turn impacts the ice cover's susceptibility for drift and deformation by winds and ocean currents. The evolution and spatial pattern of sea-ice drift and deformation are greatly affected by the balance between external forcing and local ice conditions. The results indicated that for sea ice close to the ice edge, the Antarctic Circumpolar Current (ACC) was able to provide a steady source of significant energy to the ice floes. This significantly modulated the relationship between ice motion and wind forcing, and caused the enhancement of the wind-driven drift. The resulting free-drift and deformation were primarily driven by large-scale atmospheric forcing. Remarkably, there was no discernable shift to the inertial frequency, in response to passing cyclones. Conversely, ice floes that drifted further from the ice edge and within a less advective ice-ocean system, showcased the expected energy cascade in their velocity power spectral density (PSD). However, counterintuitively, their corresponding deformation PSD demonstrated a strong de-coupling to the large-scale atmospheric forcing. Further analyses, encompassing existing deformation datasets from different regions around Antarctica revealed that, for similar spatial scales, the magnitude of sea-ice deformation varies between seasons, regions and the proximity to the sea-ice edge or the coastline. The observed variability in sea-ice drift and deformation indicates that the Antarctic ice cover is seasonally and regionally dissimilar. However, this study further demonstrates that the local ice conditions (i.e. sea-ice rheology), were also notably different during each buoy deployment. While these findings may lead to conclusions describing the Antarctic sea-ice cover as a non-coherent structure and unpredictable; it is necessary to acknowledge the persisting limitations in the availability of data, especially in contrast to the Arctic. Consequently, despite being the first to present a comprehensive review of sea-ice dynamics across various temporal and spatial scales, this thesis contends that the scarcity of buoy arrays in the Antarctic hinders in making a definitive assertion about sea-ice rheology, and hence, the ice cover's response to external forcing. Overall, this thesis confirms that the present concentration-based definition, where the sea-ice concentrations range between 15-80 %, is inadequate to describe the sea-ice cover in the Antarctic MIZ. There is an expectation that when sea ice attains 100 % coverage from space it becomes consolidated and subject to strong internal stresses. However, the thesis shows that the MIZ is not uniform and is rather comprised of a disarranged mixture of several ice types, with open-water fractions, that are continuously evolving. Additionally, free-drift conditions were found > 200 km from the ice edge during a winter expansion. This was a result of the complex interactions between sea ice, extra-tropical cyclones, and storm-generated waves.

Keywords

Physical oceanography

Reference:

Collections

PhD / Doctoral

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