Elevation-dependent sensible heat flux trend over the Tibetan Plateau and its possible causes

The present study documents the elevation-dependent sensible heat (SH) flux trend over the Tibetan Plateau (TP). The SH displays a decreasing trend over the TP above 2000 m with the magnitude of trend increasing with the elevation, but an increasing trend at low elevation stations. The above feature is more obvious in spring and summer. Surface wind speed is consistently the major contributor to the variation in SH trend from lower to higher altitude areas. Meanwhile, the role of the difference of ground-air temperature (Ts–Ta) in SH trend is enhanced above 2500 m regions. The SH variation associated with the change in Ts–Ta may be influenced primarily by the diminution in sunshine duration and snow depth at higher-altitude regions, and the latter is particularly important. The portion of the SH variation related to the change in surface wind speed is mainly attributed to the dynamic process related to the Pacific Decadal Oscillation (PDO). The warming in the northwestern Pacific in relation to the switch of the PDO from a warm phase to a cold phase in the recent decades causes divergence anomalies in the upper troposphere, which induces propagation of a wave pattern extending eastward until reaching the southwest TP. That leads to an enhancement in divergence of the upper troposphere and subsequently a boost in surface convergence and rising motion over southwest TP. Consequently, due to the easterly anomaly to the east of the convergence, surface wind speed is reduced over the central and eastern TP, especially in the higher altitude areas.