Abstract:
Based on the monthly station-observed rainfall, HadISST 1.1 sea surface temperature (SST) and ERA5 atmospheric reanalysis datasets, the influence of the Indian Ocean SST basin mode (IOB) on the relationship between South China spring rainfall (SCSR) and ENSO in preceding winter is studied, and the possible mechanism of IOB affecting SCSR by regulating ENSO circulation anomalies is analyzed. The results show that: In preceding winter, above-normal (below-normal) SCSR usually occurs following the El Niño (La Niña) events associated with warm (cold) IOB phases; while the SCMR anomaly is ambiguous during El Niño or La Niña years with neutral IOB. The reason is that when El Niño occurs accompanied by warm IOB phases, the tropical Indian Ocean and the equatorial central-eastern Pacific Ocean are characterized by Sea-Surface Temperature Anomaly (SSTA), and SSTA persists from preceding winter to last spring. In the lower troposphere, positive SSTA in the central and eastern equatorial Pacific Ocean during spring induces an anomalous Western North Pacific Anticyclone (WNPAC). The positive SSTA in the tropical Indian Ocean induces an equatorial north-south antisymmetric circulation in the subtropical Indian Ocean, and anomalous easterly in the north of the equator favors the westward extension of the anomalous WNPAC. The anomalous westerly in the south of the equator the anomalous easterly from the equatorial western Pacific Ocean converge and rise in the eastern Indian Ocean, and the airflow sinks to the northwest Pacific Ocean, forming a meridional vertical circulation, which is conducive to the maintenance of WNPAC in spring. In the upper troposphere, the positive SSTA over the tropical Indian Ocean induces positive geopotential height anomalies over the tropical Indian Ocean associated with a strong pressure meridional gradient, resulting in an enhanced East Asian subtropical westerly jet, thus leading to anomalous divergence over South China. The westward extending and strengthening of WNPAC transports enough moisture to South China, and the moisture moves upward caused by the upper-tropospheric divergence, leading to above-normal SCSR. In contrast, during the El Niño events with neutral IOB, the SSTA decreases quickly and the WNPAC is not significant in the spring, resulting in indeterminate SCSR.