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With 1998 NCEP reanalysis data and the linear diagnostic equation for local meridional circulation, four main processes and the boundary effects with relatively large contributions to the intraseasonal oscillation of the vertical branches of the East Asian meridional circulation are quantitatively identified among all the processes involved in the quasi-primitive equations used in the derivation of the linear diagnostic equation. The numerical results show that the main processes with maximal contributions in the lower latitudes include the latent heating and vertical transports of sensible heat. These processes are mainly associated with the tropical convective activities and result in the low-frequency cyclones in the lower latitudes. The main processes with maximal contributions in the higher latitudes are the horizontal transports of westerly momentum and horizontal temperature advections. These processes are mainly associated with the fluctuations in the westerlies and result in the low-frequency cyclones in the higher latitudes. The low-frequency cyclones propagating away from lower latitudes not only interact with those from higher latitudes to enhance the lifting of the moist air in the Yangtze River Basin (YRB), but also leave room for the development of the low-frequency anticyclones over the South China Sea (SCS). The southwesterly in the northwestern quadrant of the SCS anticyclones provides the YRB with abundant moisture. This favorable moisture condition along with the enhanced rising motion in the YRB leads to the extraordinary flood in 1998.