Decadal Surge in Tibetan Plateau 20–30-Day Oscillation Intensity (1995–2005) Linked to Summer NAO Structure and Baroclinic Energy Conversion

Abstract

The 20–30-day intraseasonal oscillation (ISO) over the Tibetan Plateau (TP) significantly modulates summer precipitation, accounting for over 10% of regional rainfall variability. This study identifies a decadal intensification of the summer 20–30-day ISO during 1995–2005 and examines its multidecadal covariation with the summer North Atlantic Oscillation (SNAO). The intensification coincides with a positive phase and eastward-displaced SNAO, which facilitates enhanced Rossby wave propagation toward the central and eastern TP along the Eurasian waveguide. Rather than directly forcing intraseasonal variability, the SNAO modulates the local dynamical environment and vertical motion, thereby regulating the growth of the TP ISO. Energetic diagnostics reveal that baroclinic energy conversion plays a central role in sustaining the oscillation. Crucially, the SNAO-induced forcing provides compensatory energy in the upper troposphere, preserving the vertical coherence and deep-tropospheric structure of the ISO. These findings indicate that the decadal surge arises from frequency-selective energy allocation and optimized growth conditions, highlighting the critical role of teleconnection morphology in shaping the TP hydroclimate.