Differences in Intraseasonal Activity of Eurasian Subtropical Zonal Wave Train and Associated Indian Summer Rainfall in Two Opposite AMO Phases

By using daily variables extracted from the NCEP/NCAR reanalysis, this study compares the intraseasonal activities of the Eurasian summer subtropical wave train during the two opposite phases of the Atlantic Multidecadal Oscillation (AMO). Significant difference is found based on pentadic mean 200-hPa meridional wind component. The associated intraseasonal variability of Indian summer precipitation is also different during the two AMO phases. Lead-lag regressions of 500-hPa wind field and vertical motion are then conducted to illustrate the AMO's modulation on the subtropical wave train and the Indian summer rainfall. The results suggest that during the AMO negative phase, the intraseasonal evolution of the positive-phase subtropical wave train has positive centers over the Great Britain and the Black-Caspian Sea but negative centers over the North Atlantic south of Greenland, the Mediterranean and north of South Asia in 200-hPa height. This pattern of wave train induces anomalous subsidence and reduced rainfall in central and northwestern India. It is opposite for the negative-phase subtropical wave train. In contrast, during the positive AMO phase, positive-phase subtropical wave train originates from the North Atlantic south of Iceland and propagates toward northeastern South Asia via southern Denmark, western Russia and central Asia. The intraseasonal evolution of this wave train leads to anomalous convergence and increased rainfall in most areas of India. It is opposite for the negative-phase subtropical wave train. This study highlights the AMO's modulation on intraseasonal variability of Indian summer rainfall through influencing the Eurasian summer subtropical wave train.