Multi-parameter inconsistency of subsurface mesoscale eddies in the Kuroshio Extension

Subsurface mesoscale eddies are often assumed to satisfy geostrophic and hydrostatic balance, with pressure, vorticity, and stratification anomalies expected to align. Evidence starts to show that such alignment is sometimes violated in eddy-induced anomalies superimposed on large-scale background flow, yet the degree and key factors of such violation are poorly known. This study attempts to fill this gap by statistically assessing the consistency of pressure, vorticity and stratification anomalies in subsurface mesoscale eddies using reanalysis data in the Kuroshio Extension region. Eddies are identified based on closed contours of pressure, vorticity, and stratification anomalies, with their consistency evaluated via categorical normal ratios and area-matching scores. It is found that for subsurface eddies, the level of hydrostatic, i.e., pressure-stratification, (geostrophic, i.e., pressure-vorticity) consistency is only ~10–20% (~40–60%), signifying possible deviations from the largely-balanced basic state. Consistency improves with eddy age, peaking in mature eddies, but remains below classical expectations. Larger and younger eddies show the weakest consistency. Eddy deformation, reflected in shape bias and strain rate, disrupts consistency within the eddy anomaly field. These results underscore the need for multi-parameter diagnostics when studying eddy dynamics in the ocean interior.