Causes of intraseasonal diabatic heating variability over and near the Tibetan Plateau in boreal summer

Abstract

The structure and evolution features of the first two leading modes of the intraseasonal diabatic heating variability over the Tibetan Plateau (TP) during northern summer are investigated using reanalysis and observational data. Both of the leading modes present a dominant 10–30-day intraseasonal oscillation (ISO). The first mode is characterized by a perturbation center over the southern TP (STP), which remains quasi-stationary and is closely related to the low-latitude ISO. The associated low-latitude ISO is originated from the tropical western Pacific (WP) and propagates westward/northwestward toward northwestern India along the mean monsoon trough. The westward propagation near the South China Sea is mainly attributed to anomalous meridional vorticity advection and the advection of the planetary vorticity by ISO flow. The stationary feature of the perturbation over the STP is ascribed to the topographical features around the STP. The intraseasonal heating variability over the STP is attributed to the alternation of anticyclonic and cyclonic flow associated with the westward-propagating ISO perturbation originated from the tropical WP. The second leading mode is characterized by an east–west asymmetric structure over the TP. The intraseasonal diabatic heating anomaly propagates clockwise from the northwestern to eastern TP, while a heating anomaly with an opposite sign propagates from the southeastern to western TP. The mid-latitude Rossby wave trains play an essential role in forming the dipole structure. The wave trains propagate southeastward before reaching the TP and then eastward as they cross the TP. The source of anomalous water vapor over the TP is originated from lower latitudes. The upper- and lower-level wave trains are well coupled over the TP, exhibiting a baroclinic structure.