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(2023) Huguenin-Virchaux, MauriceThesisSince the 1970s the ocean has absorbed over 90% of the excess heat trapped in the Earth system due to increasing greenhouse gases. However, sparse observations limit our understanding of the processes driving this heat uptake and its regional patterns. In this thesis, three numerical modelling projects demonstrate how ocean warming has played out over the last 50 years, including how it is affected by El Niño-Southern Oscillation (ENSO), the Earth's dominant mode of interannual climate variability. Part 1 of this thesis investigates recent multi-decadal ocean heat content trends basin-by-basin, including what proportion of the total trend is forced by atmospheric surface warming, surface wind changes or both. The analysis reveals that Southern Ocean heat uptake accounts for almost all the planet’s ocean warming since the 1970s, thereby controlling the rate of climate change. This heat uptake is facilitated in almost equal parts by both warming of the atmosphere and changes in the surface winds. An integral part of forecasting ENSO is the analysis of the Pacific warm water volume (WWV), the volume of water above 20°C between 5°S and 5°N of the equator. This is because WWV variations lead ENSO events by 6-8 months. WWV variability is thought to be dominated by adiabatic advection of warm water into and out of the equatorial latitude band. Part 2 uses a complete heat budget to illustrate that WWV changes associated with diabatic processes (surface heat fluxes and vertical mixing) are also important. ENSO impacts remote regions around the globe, including West Antarctica through its atmospheric teleconnections to the Amundsen Sea. Subsurface warming associated with ENSO in this region has the potential to affect basal melting of West Antarctic ice shelves, yet our knowledge of the oceanic ENSO response here remains limited. Part 3 reveals that during El Niño, the Amundsen Sea Low and coastal easterlies in West Antarctica weaken and reduce the poleward Ekman transport of cold waters across the shelf break. Consequently, warm Circumpolar Deep Water (CDW) flows onto the continental shelf to balance this mass deficit. The La Niña shelf circulation response is largely opposite and inhibits cross-shelf upwelling of CDW. This has implications for global sea level rise as basal melting can reduce the buttressing of the ice sheets behind the West Antarctic ice shelves.
The ideal long-term record: Using baleen to understand changes in whale feeding and spatial movements through time(2023) Dedden, AdelaideThesisThe stable isotopes of nitrogen (δ15N) and carbon (δ13C) within whale baleen plates provide an indirect measure of individual resource use, migration strategy, and physiological changes at an interannual level. Predictable annual changes in whale dietary input and/or physiological condition (e.g., annual fasting periods) cause periodic oscillations in nitrogen and carbon stable isotope values within a baleen plate. However, there is a high degree of interannual variability in stable isotope oscillation patterns observed, both within an individual, as well as within and between populations, making interpretations difficult. This interannual variability may be driven by changes in resource utilisation, movements between different regions, and/or a difference in fasting endurance. However, methods to quantify variability in stable isotope patterns and what drives these differences each year remain unknown. In my thesis, I develop new methodology to quantify variability in baleen stable isotope patterns, and then applied these methods to examine some of the potential drivers behind baleen stable isotope variability. Overall, I show that large-scale climate cycles and oceanographic signals (like SST) are associated with changes in baleen stable isotope patterns. I also show, for some species, that baleen stable isotope patterns vary between males and females of the same species and thus, sex differences may also drive inter-individual variability within a population. Additionally, I infer the resource use and movement patterns of the smallest baleen whale, the pygmy right whale, commonly left out among comparisons with other baleen whale species. I find evidence to suggest they remain in temperate waters between southern Australia and the Subtropical Convergence, where they likely rely on euphausiid and copepod species to sustain their feeding and breeding requirements. Despite the lack of research on pygmy right whales, I show that their annual presence in coastal waters make them a relatively accessible species to study. Previously, the stable isotope patterns within baleen have been valuable in identifying feeding and movement patterns within individuals. However, the need to explore how these patterns relate to changes in the environment as well as meaningful ways to quantify interannual variability in stable isotope patterns (i.e., changes in resource use and movement strategies) will be essential forecasting tools within a changing climate.