Fig. 3.

Zoomed in streamline plots of the north polar regions of spherical surfaces at different depths: 0.95$r_o$ in A, 0.97$r_o$ in B, 0.99$r_o$ in C, and 0.993$r_o$ in D. All panels show the simulation case presented in Figs. 1 and 2. The figure highlights the gradual emergence of large coherent vortices as well as more coherent jets with depth. Note that the full white circle, representing a constant latitude, is located at 60°N of the equator in the local coordinates of each spherical surface. The 60°N latitude is omitted in D for clarity. Due to less coherent flow structures at progressively shallower depths (from A to D), flow streamlines become shorter and less surface filling, exposing more of the hypothetical white surface that is shown for highlighting the colored streamlines.