Diverse Seasonal Patterns of Precipitation and Terrestrial Water Storage over the Indochina Peninsula: Roles of Monsoons, ENSO, and Topography

Being a transition zone of Asian monsoons, the water resources of the Indochina Peninsula (ICP) are highly sensitive to climate variability. While seasonal precipitation patterns have been documented, the distinct single-peak and double-peak modes—as specific characteristics of this seasonality—in both precipitation and terrestrial water storage (TWS), along with their climatic drivers, remain inadequately explored. This study investigates the long-term and seasonal variations of precipitation and TWS over the ICP from 2002 to 2017 using ERA5 and Gravity Recovery and Climate Experiment (GRACE) data. Principal component analysis reveals two dominant modes: a single-peak pattern (EOF1) across the entire ICP, peaking in June–August, and a double-peak pattern (EOF2) concentrated in the south, with peaks in May and September–October. We find that the Indian Monsoon dominates the single-peak mode, whereas the Western North Pacific Monsoon primarily modulates the double-peak mode. El Niño–Southern Oscillation (ENSO) significantly influences the double-peak mode, with a time lag of 3–5 months. Using Dijkstra's algorithm to trace moisture pathways, we identify the dominant atmospheric routes of ENSO's influence and quantify its propagation time, which ranges from 3 to 6 months under mean conditions but exhibits strong phase asymmetry: lengthened during El Niño (4–5 months) and shortened during La Niña (1–3 months). These temporal discrepancies are attributed to phase-dependent changes in the Walker circulation and its modulation of equatorial westerlies. Furthermore, the central ICP shows a unique increasing trend in TWS, likely due to its bowl-shaped topography enhancing water retention. This study provides an integrated dynamical explanation of hydroclimatic variability over the ICP, emphasising the interplay between monsoons, teleconnections, and local topography, with implications for improving regional water resource management and climate resilience.