Spatiotemporal Variation And Multivariate Controls Of Short-Cycle Drought-Flood Abrupt Alternation In The North-South Transitional Zone Of China

The north-south transitional zone of China is recognized for its diverse natural disasters,high intensity,and severe damage.Research into drought and flood changes between adjacent months in this region is crucial for effective disaster prevention and rational planning of water resources.However,due to uncertainties in empirical coefficient and threshold selection,inadequate drought standards,and incomplete reflection of the severity of short-cycle droughtflood abrupt alternation events(s-DFAA events,including drought to flood(DTF)and flood to drought(FTD))in existing short-cycle drought-flood abrupt alternation events index(SDFAI)formula,accurately capturing the changing characteristics of these events is not possible.This paper focuses on the north-south transitional zone of China,proposing a revised SDFAI(R-SDFAI)that incorporates custom drought standards based on the standardized precipitation index(SPI)of 32 meteorological stations.The effectiveness of R-SDFAI in this region is analyzed.Then R-SDFAI classifies s-DFAA events into four levels-mild,moderate,severe,and extreme-enabling better characterization of such events.Through the use of multiple statistical methods,the paper explores the spatiotemporal evolution characteristics of s-DFAA events in the region from 1960 to 2018,the large-scale atmospheric circulation characteristics of typical drought-flood abrupt alternation periods,and the multidimensional relationship between s-DFAA events and their potential driving factors(i.e.,precipitation,relative humidity,sunshine duration,temperature,evaporation;Arctic Oscillation,AO;NINO3.4 SSTA Index,NINO3.4;Total Sunspot Number Index,TSNI;Pacific Decadal Oscillation Index,PDO)as well as their combinations.The main research findings are as follows:(1)By incorporating custom drought standards(0.5),it improves the accuracy of identifying s-DFAA events without omitting any s-DFAA events and over-selecting non-sDFAA events.Additionally,it corrects some of the unreasonable aspects of the original SDFAI and provides a better representation of the severity of s-DFAA events.Overall,the RSDFAI is a more reliable and effective tool for assessing changes in drought and flood in the north-south transitional zone of China.(2)The analysis of temporal evolution characteristics of s-DFAA events indicates that FTD events are the dominant event type in the north-south transitional zone.These abrupt alternations in drought and flood events are observed throughout the year,but they are most frequent during midsummer(flood season),with DTF and FTD events accounting for 42.5%and 45.1% of total events,respectively.Furthermore,the overall trend of the s-DFAA events,as well as the interannual sequence of events across four levels,exhibit an insignificant downward trend.In the interdecadal changes,s-DFAA events also show a downward trend,with a higher frequency observed in the 1970 s and 1990 s.The Morlet wavelet analysis reveals that the primary periods of the overall frequency of DTF and FTD events in the north-south transitional zone are similar in time scale.The first and second main periods of DTF events occur on the 30-year and 17-year scales,respectively,while the first and second main periods of the FTD events occur on the 30-year and 14-year scales,respectively.(3)The analysis of spatial evolution characteristics of s-DFAA events reveals that the distribution of DTF and FTD events follows a pattern of more events in the east and fewer in the west,typically bounded by the Jialing River.Additionally,as the level of s-DFAA events increases,the susceptible range of mild,moderate,severe and extreme DTF and FTD events continues to narrow eastwards.In addition,the s-DFAA events in the western regions have relatively lower frequency and severity,as evidenced by their small R-SDFAI values.While these events are more frequent in the central-eastern regions,their severity level remains relatively low.In contrast,the eastern region stands out with frequent and severe s-DFAA events.Further analysis indicates a significant negative correlation between the frequency of s-DFAA events in the north-south transitional zone and altitude(p<0.01).Areas with elevations below 1500 meters are particularly vulnerable to such events,with mild,moderate,and severe DTF events and high incidence of FTD events mainly concentrated near 1000 meters above sea level.And the lower the altitude,the greater the likelihood of experiencing extreme DTF and FTD events.(4)The analysis of the potential driving forces of s-DFAA events reveals that during typical drought-flood abrupt alternation periods,the western Pacific subtropical high and the South Asian high "extend west to east and move in opposite directions" during flooding periods and "retreat east to west and move in opposite directions" during drought periods,with overall water vapor transport conditions showing opposite changes.Considering the autocorrelation,the wavelet transform coherence and multiple wavelet coherence results of precipitation and its combination were excluded.Analysis shows meteorological and teleconnection factors have significant effects on s-DFAA events at large scales(>8 years),but their driving mechanisms fundamentally differ.Among the meteorological factors,the single relative humidity,and its associated combination,have a relatively highest percentage of significant coherent area(PASC)ranging from 17.47 to 29.46.In contrast,each teleconnection factor,along with its combination,is irreplaceable,and their PASC always increases with the number of variables ranging from 8.3 to 10.29,12.38 to 18.65,24.02 to30.58,and 40.88 for a single,two,three,and four factors,respectively.