Contribution of the tropical western Atlantic thermal conditions during the preceding winter to summer temperature anomalies over the lower reaches of the Yangtze River basin-Jiangnan region

摘要

Using the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis and National Oceanic and Atmospheric Administration extended reconstructed sea surface temperature (SST) data sets, we examine the preceding factors in the Atlantic Ocean contributing to summer high temperature anomalies over the lower reaches of the Yangtze River basin-Jiangnan (LRYB-JN) region. We find that summer LRYB-JN surface air temperature (SAT) anomalies are highly correlated with, and physically connected to, the preceding winter SST and SAT anomalies in the tropical western Atlantic (TWA) region [(10 degrees-30 degrees N, 60 degrees-35 degrees W) and (0 degrees-10 degrees N, 50 degrees-35 degrees W)]. The effect of preceding winter thermal conditions in the TWA region can be explained by the following two mechanisms. Coupled with SST, positive TWA SAT anomalies can persist from the preceding winter to summer and excite a zonal wave train at middle latitudes extending from the Atlantic to Eurasia, with a deep and stable high pressure anomaly centred over the LRYB-JN region. As a result, high temperature anomalies occur over the LRYB-JN region. In addition to the above mechanism, the TWA SST-SAT coupled anomalies also exert an impact on summer LRYB-JN SAT by modulating the tropical Indian Ocean (TIO) SAT and SST variation, which causes summer LRYB-JN SAT anomalies. Given the contributions of TWA SAT anomalies, independent of and in concert with the TIO SAT variation, we define a preceding winter TWA-TIO SAT index, which measures the concurrent variation of TWA and TIO SATs. The preceding winter TWA-TIO index is more significantly correlated with the summer LRYB-JN SAT and therefore can reflect the latter better than the individual TWA or TIO SAT index.