Permafrost thawing under overlaying salt water.

Thawing of permafrost substantially affects the local environment and global energy balance. When salt water overlays permafrost, Rayleigh-Darcy (R-D) instability emerges because of the density mismatch and regulates melting (thawing) dynamics. Contrary to expectations that a higher Rayleigh number () would amplify instability, our experiments revealed fingering and stable melting fronts at low and high , respectively. We attribute the occurrence of the two melting patterns to the interplay between two competing flow structures: local circumfluence modulated by front perturbation and transversal chaotic mixing. We propose theories that rationalize the melting pattern transition and finger-scale evolution. In addition, the classic mass transport theory for R-D convection drastically underestimates the melting rate and misses key variable(s). The presence of fingering patterns and accelerated dynamics may have led to earlier penetration of the permafrost layer than previously anticipated. These findings have implications for understanding similar processes in magma migration, carbon sequestration, and subsurface energy recovery.