Relationship between February–March Tropical Indian Ocean Sea Surface Temperature Anomaly and Onset Date of Spring Soaking Rain in Northeast China

We analyzed the climatic characteristics of early and late years of spring soaking rain (SSR) in Northeast China (NEC) and the relationship with sea surface temperature (SST) especially the tropical Indian Ocean SST forcing from the interannual time scale on the basis of the daily precipitation data from 1961 to 2019 at stations in NEC, monthly mean data of the National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) reanalysis, sea surface temperature (SST) data reconstructed using NOAA, and outgoing longwave radiation (OLR) data using statistical diagnostic methods such as causal analysis, correlation analysis and regression analysis. Results showed that the onset date of SSR and April precipitation were considerably consistent. The onset date of typical SSR was concentrated in mid–late April during the early years (1964, 1968, 1969, 1979, 1983, 1988, 1990, 1991, 1999, 2002, 2005, 2010, 2013, 2015, 2016) and in mid–late May during late years (1965, 1970, 1971, 1972, 1985, 1986, 1989, 1993, 1994, 1997, 2001, 2003, 2006, 2014, 2017, 2019). If 500-hPa geopotential height fields over Northeast Asia in April showed a “− +” anomalous circulation distribution from west to east with southerly winds and cyclonic circulation dominating in Northeast Asia, which were conducive to water vapor transport, SSR started early, and vice versa. The warm sea surface temperature anomaly (SSTA) in the tropical Indian Ocean during February–March was one of the important sources of stable influence in the early years of the SSR in NEC. The possible impact mechanism was that the Indian Ocean Basin warming (IOBW) in a positive phase was favorable to the anomalous anticyclone in Northwest Pacific during April and the 500-hPa atmospheric circulation anomaly over Northeast Asia was similar to that of the early years of SSR. The NEC was situated in the right of the 200-hPa westerly jet stream exit area with an enhanced vertical upward motion, resulting in increased precipitation.