Abstract
Significant physical and biogeochemical alterations to the ocean have occurred in the recent past as a result of anthropogenic climate change, including warming and deoxygenation. A consequence of rising temperatures is the increasing frequency, intensity and duration of marine heatwaves (MHWs), which are defined as discrete and prolonged periods of anomalously warm temperatures (Hobday et al., 2016). Biogeochemical responses to MHWs remain unclear and there is a growing concern regarding extremes in marine biogeochemistry. Further, global dissolved oxygen concentrations [O2] have declined over the last 50 years. I use a high resolution 3-D physical-biogeochemical modelled dataset for the Mediterranean Sea (MS) from 1982-2100 to explore extremes in temperature and deoxygenation. The MS is a climate change hotspot and has experienced many MHWs as well as localised periods of hypoxia.MHW events in the Mediterranean Sea are one-in-a-hundred-day events in historical simulation and become one-in-six-day events by the end of the century under RCP4.5. By applying MHW definitions to [O2] model output, my work defines low-end extreme [O2] events, termed major oxygen lows (MOLs). I find evidence of MOLs in the historical simulation that develop into annual events between 80-200 metres depth with mean event duration and intensity increasing by a factor of 3 under the same moderate scenario. This is driven by a reduction in mean [O2] whereby the historical extreme threshold becomes the new normal in large portions of the basin. Furthermore, I find that increased MHW intensity enhances subsurface deoxygenation (<1 µmol l-1). Mediterranean regions that experience more intense MHWs also demonstrate stronger deoxygenation signals implying that a similar relationship could be found in other climatically sensitive areas of the ocean.
| Date of Award | 2 Dec 2021 |
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| Original language | English |
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| Supervisor | Oliver D Andrews (Supervisor) |