**4. Discussion**

Summer SH over the TP had a significant influence on the regional and Asian climate. Therefore, comprehending and predicting the variability of summer SH is of grea<sup>t</sup> importance. Using the monthly remote sensing and reanalysis data with a period of 2001–2018, the features of dominant modes of SH were investigated in this study. EOF analysis showed that an interdecadal weakening of SH appeared over almost the whole TP from 1981–2018 in the leading mode. This is in grea<sup>t</sup> agreemen<sup>t</sup> with previous studies. Duan et al. [17] indicated that SH over the TP exhibited a decreasing trend since the 1980s, which was mainly induced by the weakening of the westerly jet. In recent years, SH has presented an increasing trend as the result of enhanced surface wind speed [18,38]. However, it was thought that the variations of SH were long-term trends due to global warming and its hiatus, while this study considered that the different trends of SH might be a manifestation of the interdecadal variation. Based on satellite data and observations, recent studies [23] have found that SH has increased slightly over the TP from around 2001. To reach more accurate SH over the TP, the introduction of satellite data will be carried out in the future, which would overcome the shortage of accuracy in reanalysis data. The EOF2 pattern of summer SH over the TP was characterized by a zonally asymmetric pattern with positive (negative) SH anomalies in the western (eastern) TP. This indicated that positive SH anomalies in the western TP are usually accompanied by negative SH anomalies in the eastern TP on an interannual time scale. Based on meteorological data and satellite products, a recent study found that the long-term variations of summer evapotranspiration (ET) over the TP present a dipole pattern, with an increasing trend in the eastern TP and a decreasing trend in the western TP [29]. It is worth studying the cause of opposite spatial patterns between SH and ET over the TP.

Further analysis showed that the interannual variation of SH had a tight connection to the stationary teleconnection pattern NAENA, which was the second leading mode of the upper-tropospheric meridional wind anomalies over the Eurasian continent in summer. The interdecadal weakening of summer SH over the TP was mainly due to the decreased westerly wind anomalies over the TP, which were induced by the anticyclonic circulation to the northeast of the TP. The atmospheric circulation anomalies linked to the interdecadal weakening of SH were characterized by a teleconnection wave train, which was similar to that linked to the interdecadal phase shift of SRP. This indicated that the decadal shift of the phase in SRP plays a crucial role in the interdecadal weakening of SH. Han et al. [38] suggested that SRP is a key factor influencing summer atmospheric heat over the inner TP on interdecadal time scales, which is consistent with our conclusion. A recent study proposed that the predictability of the TP rainfall also originates from SRP [39]. Therefore, the interdecadal variation of SRP can significantly impact the climate variations of the TP on interdecadal and multidecadal time scales and acts as a potential predictor for the climate variations for TP. Previous studies have demonstrated that the phase of the AMO is likely to provide some prediction potential for the interdecadal variations of SRP [35]. The impacts of mid-high atmospheric circulation systems on the TP call for further studies.
