Using observational data, the East Asian-North Indian Ocean index (IEANI), which reflects a tropospheric thermal contrast, is found to correlate well with the East Asian summer monsoon (EASM) and associated rainfall over eastern China. Corresponding to a higher (lower) IEANI, the EASM at mid-latitudes is stronger (weaker), and there is more (less) rainfall in North China and less (more) rainfall near the Yangtze River valley. To investigate long-term variation in the EASM, we reconstructed the BC 665-AD 1985 IEANI based on reconstructed temperatures in Beijing and Tasmania, Australia. Over the past 2000 years, the reconstructed IEANI generally represents centennial-scale variations in the EASM and rainfall over eastern China. The correlation pattern between the reconstructed IEANI and rainfall over eastern China is similar to modern observations, implying that the correlation feature at centennial scales also occurred over the past 2000 years. With respect to longer-scale (several hundreds to one thousand years) IEANI variations and their correlations with rainfall, further verifications should be performed using various paleoclimatic proxy data.