Trends in upper tropospheric water vapour over the Tibetan Plateau from remote sensing

Water vapour in the upper troposphere is the dominant greenhouse gas and provides a large feedback mechanism for amplifying climate change. However, lack of high elevation observations has limited our understating of its long term variability. In this study, the homogenized time series of the upper tropospheric water vapour (UTWV) brightness temperature (BT) over the Tibetan Plateau (TP) during 1979–2012, developed through inter-satellite calibration of high-resolution infrared radiation sounder (HIRS) channel 12 clear-sky measurements, is examined. The HIRS UTWV BT above the TP is increasing (drying), with an annual rate of 0.6 K decade−1 (p < 0.05), with the largest increases above the western TP. On a seasonal basis, HIRS UTWV BT shows only slight changes during 1979–1995, but rapidly increases from 1996 to 2012. Thus, drying is concentrated during recent decades, consistent with reduced water vapour flux and relative humidity. The time series of HIRS UTWV BT is correlated with a Mongolian geopotential height index at 400 hPa and the South Asian summer monsoon index derived from National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis. Thus atmospheric circulation anomalies are a control of changes in UTWV. In summer, increasing horizontal divergence at 400 hPa has suppressed the upward motion of air from the lower to the upper troposphere, reducing conveyance of sensible heat to the upper troposphere and contributing to the drying observed.