Mixed layer depth (MLD) and near-surface stratification (derived from bulk and subskin density) are crucial variables for defining stability within the upper ocean. Conventional paradigms generally describe the relationship between MLD and stratification for the subsurface layer or the upper ocean below 5 m, where a weak stratification is associated with a deeper MLD, and a strong stratification with a shallower MLD. In this study, satellite-based and in situ-based monthly data from 2011 to 2021 reveal a positive relationship between monthly-averaged near-surface stratification (above the bulk layer) and MLD. That is, on a monthly timescale, a weaker near-surface stratification co-occurs with a shallower MLD, and vice versa. This observed pattern deviates from the conventional expectation. Analysis shows that this phase synchronization is primarily driven by density variations in the bulk layer, which exceed those in the subskin layer. Further investigation using wind stress and buoyancy flux data indicates that MLD deepening aligns with enhanced stratification above the bulk layer. These findings refine the traditional understanding by emphasizing the distinct roles of vertical layers (subskin vs bulk). They underscore the necessity to incorporate layer-specific dynamics when using satellite-derived stratification to parameterize MLD dynamics in climate models. Such refinement is critical for optimizing predictions of ocean-atmosphere interactions and climate variability. Future work should leverage the fusion of satellite and in situ data to advance multi-layer ocean stability diagnostics.

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