Seasonal sea-ice advance and retreat in the Southern Ocean: Sensitivity and response to climate variability

This thesis is an investigation of the sensitivity and response of Southern Ocean sea-ice advance and retreat to climate variability. Variability in sea-ice influences the global heat budget, surface albedo, and consequently ocean and atmosphere circulation. Regionally, sea-ice advance and retreat strongly affect the physical environment, dramatically changing ocean-atmosphere exchanges of momentum, heat, gases, and hence ocean mixed layer properties (e.g., depth, light availability, nutrients). In turn, these seasonal changes in the ocean-atmosphere-ice environment significantly affect the marine ecosystem. It is shown that the timing of sea-ice advance and retreat are highly variable, as are winter sea-ice duration, extent, concentration and vertical thickness. Ice-atmosphere interactions during sea-ice advance and retreat show that on daily to weekly timescales atmospheric synoptic variability determines the rate and magnitude of sea-ice advance and retreat, thereby also defining winter sea-ice extent and duration. The spatio-temporal variability of sea-ice advance and retreat shows that the advance more so than the retreat determines winter ice season duration, and that the retreat and subsequent advance co-vary more widely indicating that coherent anomalies are produced during spring-to-autumn (i.e., over summer). Further, trends in sea-ice advance, retreat and consequently ice season duration were detected in just two regions of the Southern Ocean: the greater Antarctic Peninsula region including the southern Bellingshausen Sea, and the western Ross Sea region. The overall significance of this thesis work therefore includes an investigation into the nature of these trends: the identification of when seasonally trends are being initiated (over the spring-to-autumn period), and how climatically they occur in just two Southern Ocean regions (the intensification of the high-latitude response to La Nina events). The inter-seasonal sea-ice feedbacks also help to explain the rapid winter warming in the Antarctic Peninsula region. A later advance and an earlier retreat result in a shorter sea-ice season, coincident with decreases in winter concentration, and by inference, thickness. These inter-seasonal feedbacks are in turn related to increased ocean winter heat flux and cloudiness, both of which serve to amplify winter warming. The impacts of regional sea-ice trends on the marine ecosystem are complex, but the net effect is perhaps best reflected in the decreasing and increasing populations of the ice-obligate Adelie penguin in the Antarctic Peninsula and Ross Sea regions respectively.