The Pacific meridional mode (PMM) is the ocean-atmosphere coupled variability in the subtropical eastern Pacific. The one in the subtropical northeastern Pacific is referred to as the north PMM (NPMM), and its counterpart in the subtropical southeastern Pacific is called the south PMM (SPMM). Both the NPMM and SPMM strongly interact with the equatorial Pacific variability, thus impacting the predictability of equatorial Pacific climate. The two PMMs vary on a range of timescales, including interannual and decadal timescales, while the latter of which paid little attention in the literature. Here we investigate the decadal variability of the NPMM and SPMM based on a mechanically decoupled experiment. In this experiment, air-sea mechanical coupling is disengaged only in the tropical Pacific by prescribing the climatological wind stress. Without the interactive ocean dynamics, the equatorial Pacific variability is substantially reduced. This experiment thus can be used to study the decadal NPMM and SPMM variability without the equatorial Pacific influence. Our result shows that their variabilities exist in the experiment, suggesting their possible origin from the extratropics. Further investigation on their formation mechanism indicates that the decadal NPMM is not driven by the North Pacific Oscillation, a well-known mechanism for the NPMM formation, but is forced stochastically by a northwest-southeast oriented tripole pattern of sea level pressure variability. We refer to it as the North Pacific tripole (NPT) mode. The decadal SPMM is suggested to be stochastically forced by the South Pacific Oscillation. After the decadal NPMM and SPMM formation, they can also feed back to the atmosphere. These feedback processes are not only confined locally over the subtropical Pacific, but also manifest as atmospheric teleconnections emanating poleward to the Northern and Southern Hemisphere extratropics, respectively. These teleconnections are excited through influencing the Intertropical Convergence Zone and South Pacific Convergence Zone, respectively. Consequently, both decadal NPMM and SPMM have the potential to impact the extratropical climate. Given the model bias such as in simulating their spatial patterns, further study on the atmospheric forcing and response processes associated with the decadal PMM variability is needed based on observations and targeted model experiments.

Decadal variability,Pacific Meridional Mode,Atmospheric processes