Diagnosing the development of seasonal stratification using the potential energy anomaly in the North Pacific

Abstract

Upper-ocean seasonal stratification (seasonal pycnocline and/or transition layer) is a ubiquitous feature and its vertical structure has large spatial variability. The density stratification regulates the stability of the upper ocean and thus can affect the oceanic response to atmospheric forcing and biogeochemical processes by modulating vertical mixing. In this study, we described the development of the seasonal stratification in terms of the stability of the water column, using the potential energy anomaly (PEA) as a metric based on Argo profiles. PEA budget analysis reveals that over most of the North Pacific, seasonal stratification develops under a vertical one-dimensional energy balance between an increase in PEA (i.e., a strengthening of the stratification) driven by atmospheric buoyancy forcing and a decrease in PEA associated with vertical mixing within the water column. Horizontal advection of PEA plays a significant role in the seasonal development of the stratification only in the regions of the western boundary current and equatorial current system south of 10°N. We find that, in addition to the total magnitude of the oceanic buoyancy gain, the balance between compositions of the atmospheric forcing (non-penetrating surface buoyancy forcing and penetrating radiative heating) is also important in explaining regional differences in the development of the seasonal stratification. The vertical diffusivity in the seasonal stratification estimated from the residual of the PEA budget is in the range from 5 × 10−5 m2 s−1 to 5 × 10−4 m2 s−1 and shows spatial and seasonal variability associated with local wind forcing.