Part I observational study on a new mechanism for North Pacific Oscillation influencing the tropics

Abstract

The North Pacific Oscillation (NPO), a representative midlatitude atmospheric variability, plays an important role in the development of the El Niño-Southern Oscillation (ENSO). To explain this extratropical–tropical linkage, previous studies have focused on the atmospheric boundary layer processes coupled with the mixed-layer ocean. Different from the existing hypothesis, in this study, we propose a new mechanism to link the NPO to ENSO via upper-tropospheric teleconnections. Analyses of the wave activity flux show that wave energy associated with the NPO directly propagates from midlatitude to the tropics, modulating the tropical circulation. During the NPO event, this equatorward energy flux becomes pronounced after the NPO peak phase and persists for more than two weeks. As a result, when a positive NPO grows (here, north anticyclonic–south cyclonic circulation), upper-level easterly wind anomalies are situated along the equatorial Pacific. Accordingly, anomalous lower-level westerly winds simultaneously occur in the equatorial Pacific, contributing to the development of El Niño events. To demonstrate the wave energy propagation via the upper-level troposphere, a stationary wave model experiment was performed with an NPO-like barotropic vorticity forcing. The results show equatorward wave propagation consistent with the observation.

Potential impact of wintertime Arctic forcing on the subsequent sea surface temperature anomalies in the tropical eastern Pacific

“…It is noteworthy that the physical process linking the Arctic SHF and TEP SST changes is somewhat different from previous studies that examined how the spring subpolar signals triggers ENSO in the following winter [9,29,30,47]. They claimed that the negative phase of NPMM or NPO in spring [54,55] can induce a La Niña-like pattern in the subsequent winter.…”

“…We find that the equatorial westerly anomaly east of the dateline significantly reinforces after March (figures 4(b)-(d)), which may be excited by the equatorward Rossby waves propagation [47,48] and the combination of the WES feedback and tropical ocean dynamics [9,48,49]. The reinforced anomalous westerlies produce eastward-propagating equatorial downwelling Kelvin waves given the apparent eastward-propagation of the positive sea surface height anomalies (figure 6(a)).…”

Distinct Modulations of Northwest Pacific Tropical Cyclone Precipitation by Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation

“…Over the SCS, the negative AMO phase tends to induce a low‐level cyclonic circulation anomaly, increased upward motion (Figure 3c), and strengthened low‐level water vapor flux convergence (Figure 3d). These suggest a clear baroclinic response in the SCS, definitely favorable for more TCP (Zhao, Sung, et al., 2023). In response to the positive IPO phase (Figures 3e–3h), the tropical western Pacific is controlled by the low‐level westerly wind anomalies and the upper‐level easterly wind anomalies (Figures 3e and 3f, Figures S5e and S5f in Supporting Information ), with the magnitude much larger than that in the negative AMO phase (Figure 3b).…”

Impacts of Atmospheric Internal Variations on the Variability of Sea Surface Temperature based on the Hydra-SINTEX Model

“…The impact of the AIVs on the SST variability can be estimated with the differences in i.e., ENSO, IOD, Pacific-North American (PNA) teleconnection (Wallace & Gutzler, 1981), Kuroshio-Oyashio front mode (Qiu & Chen, 2010;Qiu & Kelly, 1993), Pacific Meridional Mode (Stuecker, 2018), and North Pacific Oscillation (Di Lorenzo et al, 2008;Zhao et al, 2023).…”