The Pacific Meridional Modes (PMMs) are the leading ocean-atmosphere coupled modes in the subtropical northeastern (NPMM) and southeastern (SPMM) Pacific, respectively, and have been suggested to be key precursors to equatorial Pacific variability. Previous studies pointed out that both PMMs-related sea surface temperature (SST) anomalies are primarily driven by net surface heat flux variations during their equatorward evolution. However, whether ocean heat advective processes would play a role during the evolution remains unclear. To address this issue, we perform an ocean mixed-layer heat budget analysis based on observations and three ocean reanalysis datasets, and then reveal the effect of ocean advections on the evolution by comparing a fully coupled dynamic ocean model (DOM) to a slab ocean model (SOM). Our results suggest that for the NPMM evolution, ocean advections—primarily by anomalous meridional Ekman heat advections driven by mean and anomalous zonal wind stresses—play a damping role in the south of the NPMM, resulting in the NPMM shifts poleward but still freely propagates westward from preceding boreal winter to the following summer. This finding challenges the traditional view that the NPMM propagates equatorward through the wind-evaporation-SST feedback. For the SPMM evolution, ocean advections play a damping role in the center of the SPMM from boreal spring to summer, as well as an intensification role in the southwest Pacific during summer. However, the effect of the intensification on the SPMM evolution is hard to be revealed due to the strong simulation bias of the SPMM evolution in the DOM.

Pacific meridional mode,Ocean advection