Mode and Intermediate Water Formation Processes Captured by the ARGO Array.

It is becoming increasingly clear that dynamical processes arising from the nonlinear equation of state of seawater play an important role in the formation of mode and intermediate waters. Weakly stratified regions that contain a wide range of temperatures and salinities are prime locations for these dynamical processes: such regions exist at the interface between oceans that are permanently stratified by temperature (referred to as Alpha oceans) and those permanently stratified by salinity (Beta oceans). Whilst many hydrographic climatologies show these interfacial regions coincide with the headwaters of mode and intermediate waters, they fail to represent the nonlinear processes thought responsible for the latter, which is no surprise when considering the short time- and length-scales of these dynamical processes. The ARGO array of profiling floats provide near-global ocean temperature and salinity data at unprecedented resolution, and are ideal for identifying these nonlinear processes and quantifying their contribution to mode and intermediate water formation. Here, we employ 4 years of ARGO data to partition the upper 2km of the ocean into its Alpha, Beta and interfacial regions, and investigate the distribution and potential energy of instabilities arising from the nonlinear equation of state. Such instabilities overwhelmingly (>80%) occur in the interfacial regions between Alpha and Beta oceans, despite these interfacial regions containing only a third of the ocean volume. The potential energy reservoir of these instabilities is estimated to be similar to that of internal gravity waves (~1EJ). These findings suggest the interfacial regions between Alpha and Beta oceans operate as efficient pathways for mode and intermediate water formation, and thus facilitate the coupling of surface waters to the deep ocean.