Preliminary Study on the Effect of Intraseasonal Evolution of the Tropical Atlantic SST Anomalies on Summer Persistent Heatwave Events over the Area South of the Yangtze River

The area south of the Yangtze River is highly impacted by heatwave disasters. Previous studies have shown the effects of sea surface temperature anomalies (SSTAs) in several crucial sea regions (e.g., the tropical Atlantic, the North Indian Ocean, and the tropical central eastern Pacific) on summer mean air temperature anomalies over southern China on interannual or interdecadal time scales. However, little research exists focusing on the impacts of SSTAs in these key sea regions on the occurrences and maintenances of heatwave events over the area south of the Yangtze River on the intraseasonal time scale. For this reason, using observational data in gauge stations in China, the National Centers for Environmental Prediction–National Center for Atmosphere Research (NCEP/NCAR) reanalysis, and the National Oceanic and Atmospheric Administration (NOAA) SSTs, the present study focuses on the potential contribution of SSTAs in the tropical Atlantic to heatwave events over the area south of the Yangtze River by analyzing two summer heatwave events (occurred during July 21 to 31 and August 15 to 25) in 2016. In addition, based on the composites of multiple heatwave events during 1981–2016, the present study further explores a possible physical link explaining the effect of the intraseasonal evolution of SSTAs in the tropical Atlantic on heatwave events over the area south of the Yangtze River. The results indicate that the development and maintenance of warmer SSTAs in the tropical western Atlantic are conducive in stimulating a relatively stable Rossby wave train over Eurasia on the intraseasonal time scale. As a result, a deep high-pressure anomaly governs East Asia and its coastal areas, resulting in long-term persistent (≥8 days) heatwave events over the area south of the Yangtze River. This phased increase and maintenance of SSTAs in the tropical Atlantic and associated steady Rossby wave train are prior to the long-term persistent heatwave events, and the latter might occur in a month since the SSTs in the tropical Atlantic are significantly enhanced. On the intraseasonal time scale, there is a clearly phased increase in the SSTs in the North Indian Ocean, around 10 days after the significant increase in the SSTs in the tropical Atlantic Ocean. This implies that in addition to directly exciting the Rossby wave train across Eurasia, it is probable that SSTAs in the tropical Atlantic may affect the occurrence and maintenance of the heatwave events through modulating the phased variation of SSTAs in the North Indian Ocean on the intraseasonal time scale. Furthermore, during the period of El Niño decay and transition to La Niña, the synergistic intraseasonal variations accompanied with cooler SSTAs in the tropical central eastern Pacific and warmer SSTAs in the North Indian Ocean may also contribute to the long-term persistent heatwave events. The intraseasonal variations in SSTAs in the three key sea regions seem to be able to be used as precursory signals for the heatwave events. Nevertheless, the specific process explaining the intraseasonal effects of SSTAs in the three key regions, especially the joint effect of SSTAs in these regions and related physical process, requires further investigation in the future.