Recovery of sensible heating and its elevation amplification over and around the Tibetan Plateau since 2000s

摘要

Based on historical observations daily data for 1981-2016 from 130 meteorological stations over and around the Tibetan Plateau (TP), the trends of sensible heat flux (SH) and their elevation dependence were investigated. Results indicate that the SH over and around the TP experienced apparent trends' shift in approximately 2000, demonstrating noticeable reductions during 1981-2000 and pronounced recovery during 2001-2016 for the four seasons. The relation between elevations and trends in SH over and around TP has shown a feature known as "elevation amplification." Pronounced elevation-dependent reductions in SH can be discovered for the four seasons except winter during 1981-2000, and a substantially more significant enhance in SH was found in the higher elevation plateau compared to the lower elevation plateau during 2001-2016. The elevation-dependent trends of surface wind speed (V-0) influenced by the atmospheric circulation anomalies were the dominant factor driving the elevation-dependent trends of SH whenever during 1981-2000 and 2001-2016. During 1981-2000 (2001-2016), the areas to the north of TP warmed more strengthened (weaker) than the areas to the south of TP. It led to the anomalous temperature gradient and geopotential height gradient from the south (north) of TP to the north (south) of TP and resulted in decreasing (increasing) trends of the 500hPa subtropical westerlies over and around the TP. Furthermore, it caused the rate of reduction (increases) in V-0 amplified with elevation, because the higher altitude areas responded more strongly to the changes of atmosphere wind speed. As a result of the elevation-dependent reductions (increases) of V-0, the positive correlation between the subdued (enhanced) SH and elevation occurred in TP during 1981-2000 (2001-2016). The difference in ground-air temperature (Ts-Ta) was another factor influencing elevation dependence of SH trends during 2001-2016, which needs further investigations.