Meridional circulation in the Sun and stars

TL;DR

This paper uses mean-field hydrodynamics to explain the origin and properties of meridional circulation in stellar convection zones, highlighting the roles of centrifugal and buoyancy forces and boundary layer effects.

Contribution

It provides a theoretical analysis of meridional flow driven by non-conservative forces, emphasizing boundary layer dynamics and thermal wind balance deviations.

Findings

Meridional flow is driven by centrifugal and buoyancy forces.

Flow velocities are largest near convection zone boundaries.

Theoretical predictions are not conclusively supported or refuted by helioseismic or numerical data.

Abstract

Mean-field hydrodynamics advanced to clear explanations for the origin and properties of the global meridional flow in stellar convection zones. Qualitative arguments and analysis of basic equations both show that the meridional circulation is driven by non-conservative centrifugal and buoyancy forces and results from a slight disbalance between these two drivers. The deviations from the thermal wind balance are relatively large near the boundaries of convection zones. Accordingly, the meridional flow attains its largest velocities in the boundary layers and decreases inside the convection zone. This picture, however, is neither supported nor dismissed by the conflicting results of recent helioseismic soundings or 3D numerical experiments. The relevant physics of the differential temperature and its possible relation to the solar oblateness are briefly discussed.