  Climatologies of the mixed layer depth (MLD) have been provided using several
definitions based on temperature/density thresholds or hybrid approaches. The
upper ocean pycnocline (UOP) that sits below the mixed layer base remains
poorly characterised, though this transition layer is an ubiquitous feature of
the ocean surface layer. Available hydrographic profiles provide near-global
coverage of the world's ocean and are used to build a seasonal climatology of
UOP properties -- intensity, depth, thickness -- to characterise the spatial
and seasonal variations of upper ocean stratification. The largest
stratification values $\mathcal{O}(10^{-3}\,s^{-2})$ are found in the
intertropical band, where seasonal variations of the UOP are also very small.
The deepest ($>$ 200 m) and least stratified $\mathcal{O}(10^{-6}\,s^{-2})$
UOPs are found in winter along the Antarctic Circumpolar Current (ACC) and at
high latitudes of the North Atlantic. The UOP thickness has a median value of
23 m with limited seasonal and spatial variations; only a few regions have UOP
thicknesses exceeding 35 m. The UOP properties allow the characterisation of
the upper ocean restratification that generally occurs in early spring and is
generally associated with large variability. Depending on the region, this
restratification may happen gradually as around the Rockall plateau or abruptly
as in the Kuroshio Extension. The UOP is also likely to merge intermittently
with the permanent pycnocline in winter. The upper limit of the UOP is
eventually consistent with MLD estimates, except in a few notable regions such
as in the Pacific Warm Pool where barrier layers are important, and during
wintertime at high latitudes of the North Pacific.
