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This thesis develops and applies novel methods to understand water mass variability and change in the global ocean. A water mass framework is developed that determines the diathermal and diahaline transformations of water between water mass classes from the temporal variations in their volumetric distribution. Water masses are defined in terms of their temperature and salinity. This reveals the roles of air--sea fluxes, mixing and transport changes. The second chapter explores the drivers of interannual variability in the overturning circulation in the North Atlantic subtropical gyre using the water mass framework. Variations in the volumetric distribution of water masses reveal that transport anomalies at the gyre boundaries predominantly set the volume and heat budget and that these transport anomalies are governed by Ekman pumping over the gyre. In the third and fourth chapters of this thesis the water mass framework is applied to observations of temperature and salinity in the Southern Ocean. Seasonal variations in the distribution of water masses reveal the imprint of the Southern Ocean overturning. This highlights the importance of seasonally varying air-sea fluxes in the formation of intermediate water at the expense of deep water, winter water and surface water. This reveals a diabatic pathway for the upwelling and conversion of deep water into intermediate water. Deep water is first cooled and freshened during the winter by mixing with overlying winter water triggered by a cabbeling instability. Sea ice-melt and surface heating then warm and freshen this seasonally formed water mass to create intermediate water during the summer months. These results suggest that the process of cabbeling could be a rate determining step in the global overturning circulation and the upwelling of deep waters. The fifth chapter of this thesis explores an alternative method to determine a volumetric distribution using individual Argo profiles. The volumetric distribution determined using this profile based estimate is compared to the distribution calculated using a geographically interpolated dataset. This comparison reveals that the interpolation scheme used to geographically grid Argo appears to artificially mix water masses toward the centre of the distribution.