Abstract:

At the very largest global scale, including the Atlantic, Pacific, Indian and Southern Oceans, the observed overturning circulation is down-gradient, from high to low pressure, based on observations of the sea surface height and adjusted (absolute) steric height within the deep water layers and the Antarctic Bottom Water layer. Such down-gradient flow is a feature of theories of the largest circulation scale, and can be explained as a balance between pressure gradient and weak diffusivity in the down-“stream” direction (with geostrophic balance in the cross-“stream” direction). The observations are global maps of sea surface height derived from multiple data sets, and interior ocean maps (2000m, 3000m, 4000 m depth) based on hydrographic observations with geostrophic reference velocities selected for mass continuity and consistency with tracer observations.

The global overturn requires return of deep and abyssal waters to the sea surface, which occurs through both (i) nearly adiabatic upwelling in the Southern Ocean whose spiraling pathways we have documented, and (ii) global upwelling driven by diapycnal diffusion.  Diapycnal mixing is generally considered to be due to enhanced diffusivity over rough topography. Less studied is an observed shortcut for abyssal upwelling in the tropics. Older studies and very new abyssal turbulence measurements reveal abyssal equatorial turbulence in the Pacific. Enhanced abyssal tropical upwelling could drive the observed deep cyclonic gyres that flank the equator, particularly in the very wide Pacific and could possibly contribute to retention of abyssal waters for numerous cycles, further enhancing upwelling into the deep water layer.