Based on your comments and your reference to a lack of supporting experimental evidence, I am quite sure that a simple free-convection flow explanation will not provide a full answer. Nevertheless, I thought that I might show you the key features of such free convection flow pattern, just in case it provides some clues. J. Fluid Mech. Today, I have scanned in the old images and annotated them a bit for clarity. Figure 1 Heated inclined plate viewed from above.
Black dye is released uniformly from the surface.
Regions of convergence at the surface are black streaks in direction of the main boundary layer flow up the plate. Longitudinal thermals along the surface of the inclined plate viewed from the bottom of the plate. The flow is helicoidal only near the surface of the plate. Once the thermal separates from the surface, it keeps rising in the neutrally buoyant fluid. This is different then the frequently observed helicoidal flows visualized by cloud streaks.