Abstract
<title>Abstract</title> <p>Jupiter’s Great Red Spot (GRS) is the longest-lived large atmospheric vortex known in the Solar System. Its persistent identity coexists with secular shrinkage and continuous morphological variability, indicating a driven, dissipative, metastable structure rather than a static equilibrium. We develop a bounded mechanism model that connects established local fluid dynamics to slow structural evolution. Quasi-geostrophic or multilayer shallow-water equations remain the physical backend. A coherent-vortex feasible set is defined by hard gates on anticyclonic polarity, confinement between neighboring jets, continuity of a potential-vorticity (PV) transport barrier, finite vertical coherence, and the absence of catastrophic merger or filamentation. Only within this feasible set is the physical state projected onto reduced coordinates comprising normalized circulation, compactness, PV-barrier integrity, vertical coherence, aspect ratio, and jet-clearance margin. A reduced free-energy landscape then ranks legal coherent states and describes their slow relaxation. A forced complex saturation equation is introduced as a branch-admissible normal form for an isolated coherent mode, not as a replacement for the local atmospheric equations. The proposed longevity mechanism is the joint action of lateral confinement by zonal jets, suppressed cross-boundary mixing by an annular PV barrier, an increased disruption scale due to vertical coherence, and environmental energy input that offsets dissipation. These processes raise the effective escape barrier from the coherent-anticyclone basin and thereby permit secular shrinkage without loss of vortex identity. We formulate falsifiable predictions: lifetime and post-perturbation recovery should correlate more strongly with PV-barrier integrity and vertical coherence than with visible area alone; genuine destabilization should be preceded by a joint decline in boundary PV gradient, compactness recovery rate, vertical phase locking, and jet-clearance margin. This is a testable response-level mechanism hypothesis, not a direct empirical proof of the centuries-long lifetime and not a universal topological body force.</p>