The number and location of Jupiter's circumpolar cyclones explained by vorticity dynamics

The Westward Drift of Jupiter's Polar Cyclones Explained by a Center‐of‐Mass Approach

“…Since the discovery of Jupiter's polar cyclones, the role of vorticity gradient forces (or generalized β -drift), which takes into account both β and the relative vorticities of all neighboring cyclones, presented a consistent picture. These forces were used to explain the mean latitude of the cyclones and their number 19 , the oscillatory motion patterns of the cyclones 24 , and now their mean westward drifts as well. This series of studies, revealing different aspects of these forces, implies that these po- lar cyclones behave, to leading order, like discrete objects, linearly forced by a "spring-like" system driving them all in the poleward-westward direction while pushing them one from another.…”

“…This direction is poleward-westward for cyclones with background vorticity increasing poleward, but would flip to equatorward-eastward if the background vorticity increases equatorward (Fig. S1), such as in the case when the Jovian CPCs get too close to the shielded PCs 19,24 .…”

“…We only consider here shielded cyclones, i.e., cyclones with a ring of anticyclonic vorticity around them. In this shielded case, the interactions between cyclones are determined by the effect of the vorticity gradient of one cyclone on another 19,24 , rather than the advective interactions of one cyclone's tangential velocity on another. Indeed, the polar cyclones of Jupiter are suspected to be shielded based on observations 19 and on stability in shallow-water model simulations 23 .…”

“…The gradient of planetary vorticity, β , is the driver of β -drift. It was shown that if, in addition to β , the vorticity gradient of the PCs is accounted for when considering β -drift, then the Jovian CPCs are located at a latitude (approximately ±84 • ) where the net vorticity gradient vanishes, and thus the CPCs are in a stable equilibrium 19 . On Saturn, which is dynamically similar to Jupiter 20,21,22 , such equilibrium is currently unattainable, inhibiting stable CPCs 19 .…”

“…It was shown that if, in addition to β , the vorticity gradient of the PCs is accounted for when considering β -drift, then the Jovian CPCs are located at a latitude (approximately ±84 • ) where the net vorticity gradient vanishes, and thus the CPCs are in a stable equilibrium 19 . On Saturn, which is dynamically similar to Jupiter 20,21,22 , such equilibrium is currently unattainable, inhibiting stable CPCs 19 . For the PCs to have a vorticity gradient with the right direction to oppose β , they need to posses an anticyclonic "shielding" around them, as was shown using Shallow-Water (SW) simulations 23 .…”