Evaluating recent and future changes in North Atlantic stratification with complementary energetics and water mass frameworks

Subpolar and high-latitude regions of the North Atlantic are subject to changing buoyancy and mechanical forcing, alongside changing heat and freshwater exchanges with subtropical and polar regions. Associated changes in water mass formation and circulation are accompanied by changes in upper ocean stratification, of consequence for the large-scale ocean circulation, air-sea interaction, and ocean biogeochemistry. Changes in water mass volumes, and the associated overturning circulation, have been extensively evaluated with the water mass transformation (WMT) framework. Changes in stratification may be quantified with the Potential Energy Anomaly (PEA) framework, which has been extensively applied to seasonally stratified shelf sea environments. The WMT and PEA frameworks in combination provide complementary and holistic insights, for understanding hydrographic changes in relation to selected drivers. These frameworks are used with high-resolution model ocean datasets, obtained from hindcast and coupled simulations, the latter in control mode and forced by rising greenhouse gas concentrations through the 20^th and 21^st centuries. For selected sub-regions of the subpolar North Atlantic, bound by OSNAP and neighbouring hydrographic sections, mapped stratification (PEA) anomalies are related to respective changes in surface heat and freshwater fluxes. Residual differences between buoyancy-forced and full PEA tendencies are attributed to vertical mixing and divergences of heat and freshwater transports. Changes in regional stratification are evaluated alongside corresponding rates of water mass transformation and associated volumetric variability, for selected water masses. Eulerian perspectives provided by the WMT and PEA frameworks further complement Lagrangian perspectives provided by particle tracking. In full combination, these diagnostics elucidate multiple drivers of change in the North Atlantic that have potentially far-reaching consequences for the wider Earth System.