Long-term spatiotemporal variability in occurrences of wet and dry days across South Mongolian Plateau
Precipitation has been widely considered as the key point toward the apprehension of climate change, to which the arid and semi-arid southern Mongolian plateau is highly responsive. Using the daily data from 1960 to 2017 at 25 weather stations across the plateau, this study aimed to assess the spatial patterns and temporal trends in precipitation in terms of occurrences of wet and dry days. In this regard, four surrogate indices, namely frequencies of the occurrence of wet day following a dry day (F(d-w)), dry day following a wet day (F(w-d)), wet day following a wet day (F(w-w)), and dry day following a dry day (F(d-d)), were derived. The modified Mann-Kendall test, Sen's slope estimator, and sequential Mann-Kendall method were applied to examine trend significance, rate, and shifting year. The results indicated that the climate across the plateau was characterized by fairly long dry phases, short wet periods, and low frequent alternation of wet and dry spells, as well as an obvious northeast-southwest aridity increasing gradient. This study identified an overall decreasing trend for F(d-w), F(w-d) and F(w-w) and an increasing trend for F(d-d). The significant decreases in F(d-w) and F(w-d) concurrently occurred in the central part of the plateau, whereas the decrease in F(w-w) and the increase in F(d-d) were significant in the middle and northeastern parts. The tendency was most prominent in summer: this season's average annual decreasing rates of F(d-w), F(w-d), and F(w-w) were 0.072, 0.064, and 0.166%, respectively, while this season's average annual increasing rate of F(d-d) was 0.244%. A trend change point around the 1990s was found for half of the significant trends. Regionally mixed (increasing/decreasing) trends were detected for all the indices at temporal scales of dry season, spring, fall, and winter, only a few of which were statistically significant. It can be implied that over the southern Mongolian plateau, both duration and frequency of the precipitation events tended to decrease, along with longer-lasting dry spells and less frequent wetting-drying alternations in the past five decades. The overall drying trend was prominent across the central and northeastern parts, which was especially true in recent three decades. The advanced understanding of climate change provides a scientific base for sustaining such arid and semiarid hydro-ecosystems.