Relationship between Droughts/Floods throughout a Year over the Dongting Lake Basin and Atmospheric Circulation and Sea Surface Temperature over Key Sea Areas

Droughts are the most common natural disasters with the most significant impact on human society. They are caused by water deficiency over extended periods. The Dongting Lake Basin is alternately controlled by winter, southwest, and southeast monsoons throughout a year, with meteorological droughts occurring each season. Additionally, the controlling factors for these droughts are distinct. The precipitation amount directly illustrates droughts and floods, affected by atmospheric circulation and water vapor conditions. Anomalies in atmospheric circulation are closely related to the evolution of Sea Surface Temperature (SST), which changes over long durations and is of significant importance for drought and flood forecasting over the basin. At present, studies that determine the linkage between SST and droughts/floods over the Dongting Lake Basin primarily show the statistical relationship between them; however, atmospheric circulation is the direct influencing factor of floods/droughts over the basin. Therefore, determining the relationship between meteorological elements and SST is conducive to revealing the mechanism of their statistical relationship. There are few studies on this research field in the Dongting Lake Basin. To determine the mechanism of the linkage between sea surface temperature and droughts/floods over Dongting Lake Basin and improve the understanding of forecast-improved factors for droughts/floods, this study analyzed the interannual evolution of droughts and floods in spring, summer, and autumn within the Dongting Lake Basin from 1960 to 2016 based on monthly precipitation data and NCEP/NCAR reanalysis data, investigated the distribution of global SST in typical drought/flood years, and studied the responses of meteorological factors (including precipitable water, sea level pressure, and wind fields at 850 hPa) to El Niño and Southern Oscillation (ENSO) and SST over key sea areas, using downscaling technologies and tendency analysis. Results show that in spring, the basin experienced interannual dry and wet alternations and an insignificant drying trend. In summer, droughts were slightly more severe than floods before 1990, and it was the wettest period from 1990 to the beginning of the 21st century. In autumn, the regional flood index (H) and regional drought index (G) remained almost stable, and typical drought and flood years appeared alternately. In spring, ENSO events in the preceding winter exerted a significant impact on droughts/floods in the basin. In addition, the SST over the southwest maritime continent (S1), the Masklin Islands (S2), and the Aleutian Islands (S3) all showed significant correlations with spring precipitation in the basin. These correlations last from the preceding winter to spring. The correlation between the SST at S3 and ENSO was weak. In summer, there was an insignificant statistical correlation between ENSO in the preceding winter and summer precipitation over the basin. The SST over the eastern Australian sea (S4) and the Bay of Bengal (S5) correlated with the summer precipitation in the basin from the preceding winter to summer; the SST signal over S5 was partially covered by the ENSO signal in the preceding winter. In autumn, the global SST had an approximately inverse phase compared to the typical drought and flood years of the basin. The SST anomaly in typical drought (flood) years of the basin was in the negative (positive) phase of the Indian Ocean Dipole (IOD) and La Niña (El Niño) pattern from the preceding summer to autumn; the two kinds of SST signals (IOD and ENSO) could independently affect droughts/floods in the basin. El Niño events in the preceding winter generated high pressures in the South China Sea and the east of the Philippines region in spring and summer, conducive to the transport of moisture from the South China Sea to the basin, resulting in greater precipitation in the basin. The high SST over the Nino3.4 region in the preceding summer also exerted a similar impact in the following autumn. When the SST at S3 was high in spring, the East Asian trough tended to be strong and westerly. In summer, a higher SST at S4 was likely to coincide with a weak East Asian summer monsoon. The mature phase of IOD in autumn was the dominant factor of droughts/floods over the basin