Observed changes of global and western Pacific precipitation associated with global warming SST mode and mega-ENSO SST mode

Changes in the observed precipitation and moisture transport induced by anthropogenic forcing and natural variability were investigated. To separate into the anthropogenic and natural modes, the multi-variate EOF (MVEOF) analysis synthesized from three variables including of precipitation, SST, and moisture flux convergence is used. The precipitation pattern over the tropics has a tri-polar structure in anthropogenic mode but a zonal asymmetric structure near 150A degrees E in natural mode. The patterns of precipitation were determined in the principal modes of moisture flux convergence using MVEOF. Through a moisture budget analysis, the dynamic factor of moisture flux was found to play an important role in the precipitation changes. The natural and anthropogenic modes have zonal wave patterns of potential function associated with the dynamic factor: zonal wavenumber 1 and 2 structure, respectively. When the decadal ENSO is in mega-La Nia phase, natural and anthropogenic forcings represent a positive sign of potential function, which means increased precipitation over the tropical western Pacific (WP). In the tropics, anthropogenic forcing slightly strengthens westerly winds over the Indian Ocean and easterly winds over the WP, while weaken the easterly winds over the eastern Pacific. The natural variability strengthens the Walker circulation in La Nia phase. Therefore, during recent three decades, the slightly strengthening of the wind convergence near the WP by anthropogenic forcing and the strengthening of the Walker circulation by natural variability have caused the increasing precipitation over the WP.