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In recent decades, long-term changes of the Tibetan Plateau (TP) water vapor and the associated mechanisms have not been investigated fully. This study aims to examine the interdecadal change of summer TP water vapor using the monthly mean European Centre for Medium-Range Weather Forecasts interim reanalysis during 1979-2014. The results show a drier phase in the TP during 1979-94, with a subsequent wetter phase, which suggests an interdecadal variation of summer TP water vapor around the middle of the 1990s. This interdecadal variation is mainly due to a significant change of the water vapor export on the eastern boundary of the TP, which is closely associated with a summer atmospheric circulation anomaly near Lake Baikal. When a cyclonic (an anticyclonic) anomaly occurs near Lake Baikal, there is less (more) water vapor over the TP. On the interdecadal scale, the atmospheric circulation anomaly near Lake Baikal is related to an extratropical large-scale anomalous wave train over the northwestern Atlantic-East Asian region, with an eastward propagation of the anomalous wave energy from the Atlantic to East Asia. Climate model simulations further demonstrate an impact of sea surface temperature (SST) anomalies in the northwestern Atlantic on the anomalous wave train. Both the extratropical tropospheric anomalous wave train and the anomalous atmospheric circulation near Lake Baikal are successfully simulated by changing the summer northwestern Atlantic SST. Therefore the warming northwestern Atlantic is an important factor contributing to the wetting TP in recent decades.