From Geothermal Anomaly to Layer Oscillation: Geodynamic Effects of Gravitational Torque from the Obliquity of the Ecliptic

[Purpose] Earth's internal circulation (core dynamo, mantle convection) and external circulation (atmospheric circulation, water cycle, seasonal variation) have been studied separately within disciplinary boundaries, and the mechanical coupling pathway between them remains unidentified. This paper aims to deduce the existence and functional role of this pathway. [Method] Starting from two geometric facts—the obliquity of the ecliptic and the Earth's ellipsoidal shape—combined with observational conclusions regarding the Earth's layered non-rigid structure, this paper progressively derives through classical mechanical logic that the Earth necessarily sustains periodic north-south gravitational torque during its orbital revolution. On this basis, combined with the tendency of layered non-rigid systems to form self-oscillation through long-term evolution, this paper demonstrates the torque's role as an external reference frequency that modulates internal self-frequencies, thereby producing annual relative oscillation among layers. Finally, the functional role of this oscillation between internal and external circulation is analyzed. [Results] 1. The Earth's layered structure tends to form independent self-oscillation frequencies (self-frequencies) for each layer through long-term evolution; the solar gravitational torque, as an annual periodic external reference, modulates and phase-locks each layer's self-frequency; 2. Different coupling strengths between each layer's self-frequency and the external reference produce inter-layer phase differences, resulting in annual north-south relative oscillation; 3. This oscillation is neither a byproduct of internal circulation nor a driven item of external circulation, but rather the rhythmic synchronization mechanism between them; 4. Multi-layer observational data exhibit features consistent with this derivation. Observations of quasi-periodic oscillation in the equatorial stratosphere of gas giant planets (Jupiter) provide independent astronomical evidence for the planetary self-oscillation tendency. [Conclusion] The annual north-south layer oscillation is the product of coupling between Earth's internal self-frequencies and the external reference frequency, and also the mechanical coupling mechanism between Earth's internal and external circulation. The role of the solar gravitational torque is not to provide oscillation energy, but to serve as an annual periodic frequency anchor—each layer of Earth's interior has formed its own self-frequency through long-term evolution; the external reference frequency modulates each layer's self-frequency, and differences in coupling strength produce inter-layer phase differences, resulting in relative oscillation. Without this pathway, Earth's interior and exterior constitute two decoupled systems; with it, Earth maintains its structural homeostasis as an integrated whole. Within the global framework of the author's series "From Three-Body to Steady State: Linear Balance from a Global Perspective," this oscillation is a specific instantiation of "dynamic linear balance" at the planetary scale.