Jan-Erik Tesdal, Ryan Abernathey 

Variation in upper ocean heat content is a critical factor in understanding global climate variability. By using temperature anomaly budgets in a physically consistent ocean state estimate we describe the balance between atmospheric forcing and ocean transport mechanisms for different depth horizons and at varying temporal and spatial resolutions. The processes controlling local variations in ocean heat content differ in relevance depending on region and time scale. The advection term dominates in the tropical oceans, while forcing is most relevant at higher latitudes and in parts of the subtropics. When integrating over greater depths, the forcing signal clearly weakens and advective heat convergence becomes more dominant. Temporal aggregation shows that advection becomes the principal term that determines variability at longer timescales. Ocean heat variability is due to anomalies in circulation, while the effect of anomalies in temperature are constrained to specific regions and increase in relevance with temporal aggregation. Even though there is a shift in the relative importance of forcing and advection with coarser horizontal resolution, the overall balance between the budget terms is remarkably insensitive to the spatial scale. A novel cluster analysis was used to identify regions with similar underlying mechanisms relevant to ocean heat content variability. Advection-driven regions coincide with strong currents such as western boundary currents, the Antarctic Circumpolar Current and the tropics, while regions with a strong forcing signal are defined by shallower wintertime mixed layers and weak velocity fields. The vast majority of the ocean includes significant contributions by both forcing and advection.