In a changing climate, a description of the permanent pycnocline, which is globally coherent and independant from any thermohaline properties, is required in order to provide a reference state for the identification of the warm water sphere variability. Using a recently developped pattern recognition method applied to stratification profiles from Argo in situ data, this study presents such a reference state for the subtropical latitudes and the 2000-2015 period. With this method, a mode water core is recognised as a stratification minimum that is, possibly temporarely, trapped below the seasonal pycnocline. The permanent pycnocline is recognised as the stratification maximum localised below that mode water core. 

First, we will show that away from deep/thick regions where the pycnocline depth almost follows isopycnals, we observe significant thermohaline gradients, which are not density compensated along the permanent pycnocline surface. Thus, at the basin scale, the permanent pycnocline cannot be approximated by an isopycnal surface.

Second, we will show that all subtropical gyres, but that of the Indian Ocean, show remarkably similar stratification structures that have a particularity presented here for the first time. The permanent pycnocline shows two deep/thick centres colocated with thick subtropical and subpolar mode waters. These deep/thick pycnocline centres are separated by a shallower/thinner pycnocline region that is located downstream of Western Boundary Current Extensions and upstream of Eastern Subtropical Fronts. This creates a double-bowl pattern at the basin scale that is a first order deviation from the classic single-bowl wind-driven gyre pattern with deepening isopycnal depths Westward and equatorward. We will document the observed structure and attempt to provide an explanation for it based on large-scale potential vorticity gradient along isopycnal surfaces and associated conservation principles.