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(2023)ThesisSince 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.