Drivers of interannual mixed layer temperature variability and recent marine heatwaves on Campbell Plateau

Understanding the physical drivers of mixed layer temperature (MLT) variability and marine heatwaves (MHWs) in the Southern Ocean is important for predicting future warming and extreme events. On Campbell Plateau, south-east of New Zealand, two recent MHWs (2017-18 and 2022-23) exceeded typical summer ocean temperatures by more than 2 °C. The relative roles of atmospheric and oceanic mechanisms driving seasonal, interannual, and extreme MLT variability on Campbell Plateau remain poorly quantified. This thesis uses observational datasets and numerical model output to quantify the mechanisms governing MLT variability on Campbell Plateau. An eight-year record from a quasi-stationary Argo float (2016–2023), combined with satellite and reanalysis data, is used to construct an observationally constrained mixed-layer temperature budget (MLTB), quantifying contributions from air-sea heat fluxes, horizontal and vertical oceanic processes. Atmospheric composites and a Reynolds decomposition of anomalous air-sea heat flux complement this analysis to further understand synoptic forcing and the relative contribution of mixed layer shoaling and anomalous fluxes. Additionally, a spatially resolved MLTB, computed using Biogeochemical Southern Ocean State Estimate (B-SOSE) output (2013–2023) provides spatial context of the contribution of each mechanism over the onset of each MHW, providing a regional evaluation of the drivers of the 2017-18 and 2022-23 MHWs.Analyses of an observations-based MLTB reveal that the seasonal cycle of MLT on Campbell Plateau is largely a balance between summertime (wintertime) warming (cooling) from air-sea heat flux and year-round cooling by horizontal advection. The horizontal advection in primarily forced by Ekman transport rather than geostrophic transport. In contrast, interannual variability is primarily driven by fluctuations in air-sea heat flux, with shortwave radiation driving most of the observed year-to-year anomalies. An examination of an observational and model-derived MLTB during the two MHWs revealed that anomalous air-sea heat flux was the dominant term during the onset of both the MHWs. A further Reynolds decomposition of this anomalous air-sea heat flux term demonstrates that the 2017-18 MHW arose from a combination of anomalous surface fluxes and mixed-layer shoaling, whereas the 210-day 2022-23 event was driven almost entirely by anomalous shoaling. Composite maps of atmospheric variables show both events coincided with persistent high-pressure anomalies and reduced wind speeds, conditions that promote shoaling of the mixed layer.Together, these findings provide mechanistic evidence of the physical drivers of seasonal and interannual variability on the Campbell Plateau, as well as two recent MHWs. A key finding from this work is that persistent MHWs can arise from anomalous mixed-layer shoaling alone, underscoring the necessity of applying a Reynolds decomposition to correctly isolate the physical drivers of atmospherically forced MHWs.