The Effect of Mass Conservation
The life cycle of a cyclone or anticyclone is controlled by upper-level dynamics (i.e. by upper-level divergence and convergence) from its initial stage until its final dissipating stage.
Divergence aloft (below the tropopause) induces rising air due to mass conservation and results in surface convergence; conversely, convergence aloft induces sinking air and surface divergence (see Figure 1).
Figure 1: Reciprocal effect of upper-level divergence/convergence on surface convergence/divergence
Due to mass conservation, upper-level divergence (convergence) can trigger surface convergence (divergence). If upper-level divergence increases, more air from below has to replace the diverging air aloft and surface convergence increases. On the other hand, if upper-level divergence weakens, surface convergence will also decrease.
The described process triggers the evolution of cyclones as depicted in Figure 2.
Figure 2: Impact of upper-level divergence on surface cyclone pressure field
Note:
The superposition of upper-level divergence over a baroclinic boundary starts the development of a cyclone. The decrease or increase of upper-level divergence impacts the further evolution of the cyclone.