**4. Discussion**

As was shown above, the nighttime precipitation intensity dominates the variations of night precipitation and then precipitation amount in the SRTR. Previous studies have investigated the interdecadal variability of regional precipitation in the SRTR. For example, Shang et al. (2021) found that the increased precipitation in the cold season over the SRTR is associated with the strengthened easterly anomalies over the TP and water vapor meridional transport enhancement from tropical oceans and the South China Sea [34]. Zhao et al. (2021) found the heavy precipitation events, which mainly contribute to the interannual increasing trend of summer precipitation over the SRTR, could be largely attributed to the enhanced moisture sources from the neighboring northeastern areas of the SRTR [35]. It is necessary to diagnose the changes in water vapor between the two decades.

Figure 10 shows the difference between the climatology of water vapor and its divergence between the two decades. For the changes in annual average and different seasons,

the variation in atmospheric water vapor convergence and dispersion is in good agreemen<sup>t</sup> with the variation in the nighttime precipitation (Figure 8). For example, in SON and July, the increase in water vapor transported from the south side and the decrease in water vapor on the north side result in a distribution characteristic of a dry north and a wet south, despite the spatial gradients being different. In MAM and DJF, the water vapor variation is not evident, and with the strong decrease of vertical velocity (figure not shown), the nighttime precipitation of the SRTR generally presents a reduction trend (Figure 8). For JJA, a clearly drying trend is shown, although the pattern varies from month to month. Basically, the water vapor transport increases from the south ocean in the monsoon period, while it shows a decreasing trend in the monsoon retreat period. It is worth noting that the substantial drying of the atmospheric water vapor content and decrease of vertical upward motion in July in the past two decades over the whole SRTR has led to a decrease in summer precipitation, which are different from other studies focusing on longer time periods [12,35–37].

**Figure 10.** Differences between climatology of vertical water vapor (Vectors, kg/(m.s)) and its divergence (Shaded, kg/(m2.s)) in (**a**) annual average, (**b**) June-July-August (JJA) average, (**c**) March-

April-May (MAM) average, (**d**) June average, (**e**) September-October-November (SON) average, (**f**) July average, (**g**) December-January-Februrary (DJF) average, and (**h**) August average from 2010 to 2019 and 2001 to 2010.
