Understanding The Occurrence Of Droughts And Floods During The Summer Monsoon Rainy Season In The Middle And Lower Reaches Of The Yangtze River Valley

The daily precipitation data at 720 stations over China for the 1957-2003 period during summer (May-August), ERA-40 reanalysis datasets and extended reconstructed sea surface temperature (ERSST) data taken from NOAA are used to investigate the droughts-floods coexistence (DFC) and droughts-floods abrupt alternation (DFA) phenomena during the normal summer monsoons in the middle and lower reaches of the Yangtze River valley. The variation and climatic features are analyzed and the potential affecting factors and the preceding signals are explored. First, the DFC and DFA climatic features are diagnosed statistically on different time scales. Then, the ocean-atmospheric features for the anomalous DFC and DFA years are demonstrated. Finally, some possible factors related to the DFC and DFA are suggested. The main results are as follows:1. Climatic Features of the Summer DFC during the Normal Summer Monsoons in the MLYRVThe results show that the occurrence of strong summer DFC displays an increasing trend from 1957 to 2000 and the anomalous subseasonal oscillation of the western Pacific subtropical high (WPSH) might be one factor contributing to the strong summer DFC in the MLYRV. The anomalously low-level westerly winds anomalies prevailing over the near-equatorial Indian Ocean and the western Pacific offer the necessary condition for the cross-equatorial propagation of Rossby wave from the Southern Hemisphere (SH) to the Northern Hemisphere (NH). Strong Southern Hemisphere annual mode (SAM) during the preceding November through January (NDJ) is often followed by the strong summer DFC in the MLYRV. In the preceding NDJ of the strong DFC summers in the MLYRV, high sea surface temperature (SST) in the oceanic areas from the Arabian Sea to the South China Sea (SCS) and El Nifio or the developing phase of El Nifio are often observed. All these phenomena offer some predictive signals for the strong summer DFC in the MLYRV. Nevertheless, the physical mechanism of the interannual variability of summer DFC is still not clear. Based on the results thus far, the cross-equatorial propagating of the preceding strong SAM signal from the SH to the NH, associated with the anomalous subseasonal oscillation of the WPSH, might also be a factor contributing to the strong summer DFC in the MLYRV. High SST developing in the tropical eastern Pacific might also play a role in the strong summer DFC in the MLYRV. This paper examines the summer DFC in the MLYRV on seasonal timescale. There is no doubt that if the timescale gets shorter, the possible mechanisms related to the DFC might be different.2. Climatic Features of the Summer Long-cycle Droughts-floods Abrupt Alternation (LDFA) in the MLYRVResults show that the summer droughts-to-floods (DTF) in the MLYRV often go with the more southward WPSH, negative vorticity, strong divergence, descending movements developing and the weak moisture transport in the low level, and the more southward position of the South Asia high (SAH), the westerly jets in the high level during May—June;While during July—August it usually accompanies with the northward shift of the WPSH, positive vorticity, strong convergence, ascending movements and strong moisture transport in the low level, and the northward shift of the SAH and the westerly jets in the high level. The summer floods-to-droughts (FTD) in the MLYRV is often related to the active cold air mass from the high latitude, positive vorticity, strong convergence, ascending movement developing and strong moisture transport in the low level, and the SAH over the Tibetan Plateau in the high level, while during July—August it is often connected with negative vorticity, strong divergence, descending movements developing and weak moisture transport in the low level, the remarkable northward shift of the WPSH, the SAH extending northeastward to North China and the easterly jets prevailing in the high level over the MLYRV. In addition, the summer LDFA in the MLYRV is significantly related with the Southern Hemisphere annual mode and the Northern Hemisphere annual mode in the preceding February.3. Climatic Features of the Summer Short-cycle Droughts-floods Abrupt Alternation (SDFA) in the MLYRVStrong and weak SDFA summers are often related to the south-north pattern and the middle pattern of surface soil humidity, respectively. The former is more likely to be influenced by the singularities of large scale atmospheric circulation, whereas the latter is often linked with the anomalous surface soil humidity. The SDFA phenomena also have significantly positive correlation with SAM in the preceding December, significantly negative correlation with NPO in the preceding January and NAO in the preceding March.