Model Depiction of FrontalLarge Scale Vertical Motions
Potential Temperature (Aqua), Convergence (Red), Divergence (Blue), Vertical Motion (Yellow Arrow).
Although this is a Northern Hemisphere polar front, the same concept applies in the Southern Hemisphere. Negative Divergence (regardless of hemisphere) is always convergence. Similarly, Positive Divergence is always divergence.
This chart shows a direct circulation, with the warm air rising and the cold air sinking behind the polar front. The strong low level convergence (red contour) between 30N 91W and 33N 92W, coupled with divergence aloft (blue dashed contours) contributes to the strong upward vertical motion.
Behind the front, an area of convergence lies above 300 hPa between 49N 98W and 38S 94W. The upper convergence is resulting in large scale downward vertical motion/subsidence, with a subtle warming of the isotherms between 300-700 hPa (note the subtle downward bulge of the potential temperature isotherms).