Pacific Meridional Modes without Equatorial Pacific Influence

Abstract:

Investigating Pacific Meridional Modes (PMM) without the influence of tropical Pacific variability is technically difficult if based on observations or fully coupled model simulations due to their overlapping spatial structures. To confront this issue, the present study investigates both North (NPMM) and South PMM (SPMM) in terms of their associated atmospheric forcing and response processes based on a mechanically decoupled climate model simulation. In this experiment, the climatological wind stress is prescribed over the tropical Pacific, which effectively removes dynamically coupled tropical Pacific variability (e.g., ENSO). Interannual NPMM in this experiment is forced not only by the North Pacific Oscillation but also by Aleutian low and a North Pacific tripole (NPT) pattern of variability, the latter of which also primarily forces decadal NPMM variability. Interannual and decadal variability of SPMM is primarily forced by the South Pacific Oscillation. In turn, both interannual and decadal NPMM variability can effectively induce an extratropical atmospheric response due to the northward displaced climatological Intertropical Convergence Zone, while this is not the case for SPMM variability. This mechanism, known as the SDC response, is most prominent in boreal summer and fall and influences Northern Hemisphere extratropical climate. Specifically, the SDC response-induced atmospheric teleconnection features a zonal wave-train in the Northern Hemisphere mid-latitudes, which results in surface warming over the western United States and Europe. Our results highlight a new pathway by which the NPMM feeds back to the extratropical climate, in addition to the equatorward influence on tropical Pacific variability.