Instantaneous phase shift of annual subsurface temperature cycles derived by the Hilbert-Huang transform

This study uses the Hilbert-Huang transform to compute the instantaneous (daily) phase shift between temperature signals at the ground surface and at a depth of 5m. This approach is not restricted to the stationary harmonic surface temperature assumptions invoked by analytical solutions. The annual cycles are extracted from the ground surface temperatures and the shallow subsurface temperatures at 5m depth recorded at the Hualien (23.98°N, 121.61°E) and Ilan (24.77°N, 121.75°E) meteorology stations of Central Weather Bureau in Taiwan from 1952 to 2008. Significant reductions in the phase shift and increases in the estimated thermal diffusivity from 1980s to 1990s are found and suggest that the recent warming of the Pacific Decadal Oscillation may affect heat transport in the subsurface environment. The marginal spectra of the instantaneous phase shifts and the precipitation intensity records at Hualien and Ilan reveal that precipitation may play a role in the evolution of seasonal variation in shallow subsurface heat transport.