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., the El Niño-Southern Oscillation). Interannual NPMM in this experiment is forced not only by the North Pacific Oscillation, but also by a North Pacific tripole (NPT) pattern of atmospheric internal variability, which primarily forces decadal NPMM variability. Interannual and decadal variability of the SPMM is partly forced by the South Pacific Oscillation. In turn, both interannual and decadal NPMM variability can excite atmospheric teleconnections over the Northern Hemisphere extratropics by influencing the meridional displacement of the climatological intertropical convergence zone throughout the whole year. Similarly, both interannual and decadal SPMM variability can also excite atmospheric teleconnections over the Southern Hemisphere extratropics by extending/shrinking the climatological South Pacific convergence zone in all seasons. Our results highlight a new poleward pathway by which both the NPMM and SPMM feed back to the extratropical climate, in addition to the equatorward influence on tropical Pacific variability.

Atmospheric Forcing of the Pacific Meridional Mode: Tropical Pacific‐Driven Versus Internal Variability

“…Specifically for the latter, the POGAs capture the less effective forcing role of AL variability in the PMM (Figures 1c and S3c). In addition, POGA results show the significant forcing roles of NPO and NPT variabilities in the PMM (Figures 1f, 1l and , ), consistent with previous findings (Amaya, 2019; Chiang & Vimont, 2004; Zhang et al., 2021). All the above results are significant at the 99% confidence level.…”

“…Previous studies have shown that the leading atmospheric driver of changes in trade wind strength is related to the North Pacific Oscillation (NPO; Rogers, 1981;Walker & Bliss, 1932), which is the second empirical orthogonal function (EOF) mode of sea level pressure (SLP) variability over the North Pacific. Its southern lobe over the Hawaiian Islands can effectively initiate the WES feedback and thus the PMM (Chiang & Vimont, 2004;Zhang et al, 2021). In addition to the NPO, a recent study by Zhang et al (2021) identified a new North Pacific atmospheric mode contributing to forcing the PMM, termed the North Pacific Tripole (NPT), which emerges as the fourth SLP EOF mode.…”

A Regime Shift in the Interhemispheric Teleconnection between the Yellow and East China Seas and The Southeastern Tropical Pacific in The Southern Hemisphere during The Boreal Summer

“…[21] Owing to the similarity between the SPMM and ENSO, studies on the meridional modes have devoted substantial efforts to remove the ENSO signals, for instance, by adopting linear regression to separate the cold tongue index from the reanalysis dataset (Chiang and Vimont 2004;Min et al 2017); by coupling a slab ocean mixed layer model to remove oceanic dynamics, thus pulling apart the Bjerknes feedback and Rossby wave adjustments (Zhang et al 2014a); by prescribing climatological winds which have no anomalous ENSO-related forcing to the ocean model (Larson et al 2018;Zhang et al 2021). We do not attempt to remove the oceanic dynamics, and the ENSOlike signature before the regime shift is unsurprising.…”

“…[36] The present study focused on the interannual variance of the SPMM due to a lack of reliable multidecadal SST time series. Previous studies, however, have underlined the critical role of stochastic forcing from weather systems in the Southern Hemisphere (Okumura 2013;Liguori and Di Lorenzo 2019;Zhang et al 2021), in tandem with a subsurface temperature anomaly driven by the subtropical shallow meridional subduction in the SPMM region (Gu and Philander 1997;Imada et al 2016), on driving decadal ENSO-like variability, IPO. proposed a nonlinear ENSO warming suppression (NEWS) mechanism to show that La Nina-like mean-state warming is also a possible candidate in a warm climate because of the nonlinear rectification effect (Jin et al 2003).…”

“…A shift in the flavor would trigger resultant interactions between SST and atmospheric variables such as wind, heat flux, and precipitation in the forcing region, possibly yielding a marginal teleconnection between the YECS and equatorial Pacific if studies do not consider the abrupt change of a forcing region. The second candidate that accounts for this discrepancy is the existence of ENSO precursors: North/South Pacific meridional modes (e.g., Vimont et al 2003;Chang et al 2007;Zhang et al 2014a;Min et al 2017;You and Furtado 2017;Larson et al 2018;Stuecker 2018;Zhang et al 2021). Approximately two or three seasons before an El Niño event reaches its mature phase, a weakening of the off-equatorial trade winds modulates latent heat fluxes, thereby exciting warm SST anomalies along the western coasts of North/South America (Vimont et al 2003;Chang et al 2007;Zhang et al 2014a).…”