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Tidally locked exoplanets likely host global atmospheric circulations with asuperrotating equatorial jet, planetary-scale stationary waves andthermally-driven overturning circulation. In this work, we show that each ofthese features can be separated from the total circulation by using a Helmholtzdecomposition, which splits the circulation into rotational (divergence free)and divergent (vorticity free) components. This technique is applied to thesimulated circulation of a terrestrial planet and a gaseous hot Jupiter. Forboth planets, the rotational component comprises the equatorial jet andstationary waves, and the divergent component contains the overturningcirculation. Separating out each component allows us to evaluate their spatialstructure and relative contribution to the total flow. In contrast withprevious work, we show that divergent velocities are not negligible whencompared with rotational velocities, and that divergent, overturningcirculation takes the form of a single, roughly isotropic cell that ascends onthe day-side and descends on the night-side. These conclusions are drawn forboth the terrestrial case and the hot Jupiter. To illustrate the utility of theHelmholtz decomposition for studying atmospheric processes, we compute thecontribution of each of the circulation components to heat transport from day-to night-side. Surprisingly, we find that the divergent circulation dominatesday-night heat transport in the terrestrial case and accounts for around halfof the heat transport for the hot Jupiter. The relative contributions of therotational and divergent components to day-night heat transport are likelysensitive to multiple planetary parameters and atmospheric processes, and meritfurther study.

The rotational and divergent components of atmospheric circulation on tidally locked planets