A shift in cloud cover over the southeastern Tibetan Plateau since 1600: evidence from regional tree-ring δ18O and its linkages to tropical oceans

• The composite δ18O chronology shows different behavior before and after 1860.
• Tree-ring δ18O was significantly negatively correlated with summer cloud cover.
• The climate was cold and cloudy before 1860, but became sunnier and warmer later.
• Tree-ring δ18O networks can highlight low-frequency regional climatic signals.

We studied the climatic significance and potential forcing of regional tree-ring δ18O across the southeastern Tibetan Plateau from 1600 to 2009. Three normalized tree-ring δ18O time series showed high signal coherence at both low and high frequencies. A 400 years composite tree-ring δ18O chronology in southeastern Tibetan Plateau was established by averaging Z-score method. The composite δ18O chronology showed that climate changed since 1860, causing a shift in the hydroclimatic regime and in the influence of regional atmospheric circulation. Spatial correlation analysis revealed that tree-ring δ18O was significantly negatively correlated with moisture conditions, and especially with cloud cover during wet summers; the strongest correlation was found for the June to August period. Our regional cloud cover reconstruction for June–August period between 26°N and 34°N and between 89°E and 103°E explained 37.6% of the variation from 1953 to 2009. The climate conditions from 1600 to 1860 (the Little Ice Age) were cold and cloudy, but thereafter became sunnier and warmer. Hydroclimatic variations in this region are primarily affected by atmospheric processes in the tropical Indian Ocean and in the East Pacific Ocean, linked to the El Niño–Southern Oscillation (ENSO). The cloud cover over the southeastern Tibetan Plateau is modulated more strongly by sea surface temperatures in the Indian Ocean than over the Pacific Ocean, suggesting low-frequency responses to anomalous ocean warming. The impact of ENSO on regional tree-ring δ18O series was temporally unstable. Our comparisons among different proxies highlight that a tree-ring δ18O network has great potential to reveal common low-frequency climatic signals in monsoon Asia over long time scales.