Subtropical Mode Water and Permanent Pycnocline Properties in the World Ocean

Abstract A global reference state of the subtropical mode water and permanent pycnocline properties for the 2000–2015 period is presented. The climatology is obtained from a pattern recognition algorithm applied to stratification profiles from the Argo global array. The stratification features are identified as permanent upper ocean pycnostad and pycnocline even when the seasonal pycnocline is developed. The climatology shows that both Northern Hemisphere subtropical gyres have a qualitatively very similar stratification structure. The permanent pycnoclines in the North Atlantic and North Pacific show two deep centers colocated with thick subtropical and subpolar mode waters. These centers coincide with modes in the density and stratification space. These deep pycnocline centers are separated by a region with a shallower and thinner permanent pycnocline that is located downstream of Western Boundary Current Extensions and upstream of Eastern Subtropical Fronts. This feature creates a remarkable double‐bowl pattern at the basin scale. In the subtropical gyres of the South Atlantic and South Pacific Oceans, the mode water and permanent pycnocline structures are characterized by two modes in the density and stratification space that, unlike in the Northern Hemisphere, do not necessarily correspond to deep and thick permanent pycnocline regions. In the subtropical gyre of the South Indian Ocean, a single mode is found to correspond to a single center in the western part of the gyre. This study also shows that away from these deep centers where the pycnocline depth almost follows isopycnals, the permanent pycnocline experiences significant thermohaline gradients that are not density compensated.