Differentiating Eulerian and Lagrangian Tendencies in the Ocean Interior via a Dynamical Overturning Decomposition

Abstract

Repeat observations provide essential constraints on long-term changes in the ocean interior, such as warming and cooling trends. However, attributing these Eulerian signals to either reversible isopycnal heaving or irreversible diabatic transformations remains a fundamental challenge. This ambiguity arises not only from limited velocity observations, but also from the lack of a diagnostic framework capable of explicitly disentangling Lagrangian transformation from Eulerian variability. Recognizing that Eulerian and Lagrangian tendencies can be represented through distinct perspectives of the overturning circulations, we apply a dynamical overturning decomposition to state-of-the-art reanalysis products in the Atlantic, Indo-Pacific, and South China Sea to investigate their long-term behavior. The utility of this framework is supported by strong correlations between independently derived kinematic and thermodynamic indices, indicating a tight coupling between advective and isopycnal-heaving transports. Our analysis reveals distinct dynamical regimes in which similar Eulerian tendencies arise from fundamentally different Lagrangian processes. In particular, we identify (i) previously undiagnosed diapycnal downwelling, despite its prediction by bottom-intensified turbulent dissipation, (ii) coexistence of apparent Eulerian upwelling and diapycnal downwelling, (iii) an Atlantic sub-overturning cell linked to intermediate water-mass formation, and (iv) cases where Eulerian densification masks Lagrangian lightening. These results demonstrate that Eulerian trends alone can be misleading indicators of reversible and irreversible behaviors. By explicitly separating adiabatic heaving from diabatic processes, this framework offers a physically consistent diagnostic for interpreting interior ocean changes and establishes a dynamical basis for assessing the reversibility of such changes.