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1. Seasonal Dependence of the Pacific–North American Teleconnection Associated with ENSO and Its Interaction with the Annual Cycle

   https://doi.org/10.1175/JCLI-D-23-0148.1  

   Hu, Suqiong; Zhang, Wenjun; Jin, Fei-Fei; Hong, Li-Ciao; Jiang, Feng; Stuecker, Malte F. (October 2023, Journal of Climate)

   The Pacific–North American (PNA) teleconnection pattern is one of the prominent atmospheric circulation modes in the extratropical Northern Hemisphere, and its seasonal to interannual predictability is suggested to originate from El Niño–Southern Oscillation (ENSO). Intriguingly, the PNA teleconnection pattern exhibits variance at near-annual frequencies, which is related to a rapid phase reversal of the PNA pattern during ENSO years, whereas the ENSO sea surface temperature (SST) anomalies in the tropical Pacific are evolving much slower in time. This distinct seasonal feature of the PNA pattern can be explained by an amplitude modulation of the interannual ENSO signal by the annual cycle (i.e., the ENSO combination mode). The ENSO-related seasonal phase transition of the PNA pattern is reproduced well in an atmospheric general circulation model when both the background SST annual cycle and ENSO SST anomalies are prescribed. In contrast, this characteristic seasonal evolution of the PNA pattern is absent when the tropical Pacific background SST annual cycle is not considered in the modeling experiments. The background SST annual cycle in the tropical Pacific modulates the ENSO-associated tropical Pacific convection response, leading to a rapid enhancement of convection anomalies in winter. The enhanced convection results in a fast establishment of the large-scale PNA teleconnection during ENSO years. The dynamics of this ENSO–annual cycle interaction fills an important gap in our understanding of the seasonally modulated PNA teleconnection pattern during ENSO years. 

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2. Equatorial Western–Central Pacific SST Responsible for the North Pacific Oscillation–ENSO Sequence

   https://doi.org/10.1175/JCLI-D-23-0434.1  

   Hu, Suqiong; Zhang, Wenjun; Watanabe, Masahiro; Jiang, Feng; Jin, Fei-Fei; Chen, Han-Ching (June 2024, Journal of Climate)

   Abstract El Niño–Southern Oscillation (ENSO), the dominant mode of interannual variability in the tropical Pacific, is well known to affect the extratropical climate via atmospheric teleconnections. Extratropical atmospheric variability may in turn influence the occurrence of ENSO events. The winter North Pacific Oscillation (NPO), as the secondary dominant mode of atmospheric variability over the North Pacific, has been recognized as a potential precursor for ENSO development. This study demonstrates that the preexisting winter NPO signal is primarily excited by sea surface temperature (SST) anomalies in the equatorial western–central Pacific. During ENSO years with a preceding winter NPO signal, which accounts for approximately 60% of ENSO events observed in 1979–2021, significant SST anomalies emerge in the equatorial western–central Pacific in the preceding autumn and winter. The concurrent presence of local convection anomalies can act as a catalyst for NPO-like atmospheric circulation anomalies. In contrast, during other ENSO years, significant SST anomalies are not observed in the equatorial western–central Pacific during the preceding winter, and correspondingly, the NPO signal is absent. Ensemble simulations using an atmospheric general circulation model driven by observed SST anomalies in the tropical western–central Pacific can well reproduce the interannual variability of observed NPO. Therefore, an alternative explanation for the observed NPO–ENSO relationship is that the preceding winter NPO is a companion to ENSO development, driven by the precursory SST signal in the equatorial western–central Pacific. Our results suggest that the lagged relationship between ENSO and the NPO involves a tropical–extratropical two-way coupling rather than a purely stochastic forcing of the extratropical atmosphere on ENSO. 

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3. Atmospheric Rivers Linking Tropical Forcing to Winter Precipitation over Southern China

   https://doi.org/10.1175/JCLI-D-26-0035.1  

   Zhang, Yingxian; Ding, Qinghua; Yang, Song; Lu, Mengqian; Zhou, Botao; Liu, Yanju; Chao, Qingchen; Xiao, Chan; Wang, Guofu (June 2026, Journal of Climate)

   Abstract Significant winter [December–February (DJF)] precipitation over southern China (SC) is one of the key features of the East Asian winter monsoon, accounting for nearly 20% of annual precipitation in the area. While oceanic drivers of its interannual variability are extensively studied, the influence of atmospheric rivers (ARs), contributing approximately 30%–40% of the climatological wintertime precipitation in SC, remains unclear. Additionally, how seasonal forecast models capture the impact of tropical sea surface temperature (SST) variations on winter precipitation through ARs requires further investigation using objective metrics. This study identifies a tropical SST pattern involving El Niño–Southern Oscillation (ENSO), the Indian Ocean dipole, and the SST anomalies over the western North Pacific (WNP), whose coevolving structure rapidly develops from the preceding summer to winter. This anomalous SST configuration generates a hemispheric-scale circulation pattern from the tropics to the subtropics, which enhances vertical wind shear and meridional moisture transport over SC, favoring increased AR intrusion into the region. Consequently, significant precipitation anomalies occur particularly near SC along 20°–30°N, explaining over 50% of the interannual DJF precipitation variability. These ENSO-driven precipitation changes, mediated by AR activity, are reasonably predicted by two operational seasonal forecast systems, suggesting that ENSO and its interaction with WNP SST anomalies serve as the primary sources of forecast skill for winter ARs and SC precipitation. Furthermore, a screening scheme based on the observed SST and circulation states during October and November preceding the target winter is developed to determine the years in which the dynamical model forecast skill for SC DJF precipitation is higher than in other winters. Significance StatementA coupling pattern between atmospheric rivers (ARs), accounting for one-third of December–February (DJF) precipitation in climatology, and large-scale circulation plays a critical role in shaping the winter (DJF) precipitation variability in southern China (SC). This mode, characterized by a low pressure anomaly over central China and more active ARs steered into the region, explains at least 50% of the region’s total DJF precipitation variability. El Niño–Southern Oscillation (ENSO), along with related sea surface temperature anomalies in the western North Pacific, acts as the primary driver of this large-scale circulation–AR coupling. As this ENSO-driven process is reasonably well captured by current seasonal forecast systems, this process could be the main source enabling an improved seasonal forecast of SC winter precipitation. 

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4. A Critical Role for North Pacific Meridional Mode in the ENSO Response to Orbital Precession

   https://doi.org/10.1175/JCLI-D-25-0065.1  

   Persch, Cole F; Sanchez, Sara C (November 2025, Journal of Climate)

   Abstract Orbital precession modifies the intensity of the annual cycle at millennial time scales and is a major external driver of El Niño–Southern Oscillation (ENSO) variability in both proxy records and climate model simulations. We examine precession’s influence on ENSO through a subtropical pathway, the Pacific meridional mode (PMM), using a suite of NCAR Community Earth System Model, version 1.2 (CESM1.2), experiments that simulate five precessional extremes: perihelion at autumnal equinox (AE), winter solstice (WS), vernal equinox (VE), summer solstice (SS), and zero eccentricity (E0). We investigate mechanisms that may moderate the PMM’s influence on ENSO such as the strength of midlatitude stochastic forcing via the North Pacific Oscillation, changes in the climatological mean state, and the wind–evaporation–sea surface temperature (SST) (WES) feedback. We find that orbital precession strongly influences PMM variability, the PMM’s ability to trigger El Niño events, and ENSO diversity. Precessional extremes characterized by a more southerly intertropical convergence zone (ITCZ) and stronger trade winds (WS and AE) have more variable PMM behavior and a PMM that is more effective at triggering El Niño events, particularly central Pacific events. Precessional extremes characterized by a more northerly ITCZ and weaker trade winds (SS and VE) have reduced PMM variability and a PMM that is a less-reliable precursor to El Niño events. The PMM response to precession is driven by variations in surface wind fields that moderate the strength of WES feedback, the mechanism by which PMM anomalies grow and propagate. Understanding the sensitivity of ENSO to subtle shifts in the mean state contextualizes past variability and aids in anticipating future change. Significance StatementOrbital precession alters the seasonal distribution of solar insolation around Earth and has vast impacts on the climate system including El Niño–Southern Oscillation (ENSO). Here, we examine how precession influences ENSO through a subtropical ENSO precursor: the Pacific meridional mode (PMM). We use climate models to test how the influence of warm PMM events on El Niño events may vary under extreme states of orbital precession. We find precession moderates the variability of the PMM, the ability of the PMM to trigger El Niño events, and the spatial diversity of El Niño events. These results help us to interpret past climatic changes and understand the sensitivity of the tropical Pacific to small variations in external forcing. 

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5. Non-ENSO Precursors for Northwestern Pacific Summer Monsoon Variability with Implications for Predictability

   https://doi.org/10.1175/JCLI-D-23-0169.1  

   Zhang, Pengcheng; Xie, Shang-Ping; Kosaka, Yu; Lutsko, Nicholas J (January 2024, Journal of Climate)

   Abstract The influence of El Niño–Southern Oscillation (ENSO) in the Asian monsoon region can persist through the post-ENSO summer, after the sea surface temperature (SST) anomalies in the tropical Pacific have dissipated. The long persistence of coherent post-ENSO anomalies is caused by a positive feedback due to interbasin ocean–atmospheric coupling, known as the Indo-western Pacific Ocean capacitor (IPOC) effect, although the feedback mechanism itself does not necessarily rely on the antecedence of ENSO events, suggesting the potential for substantial internal variability independent of ENSO. To investigate the respective role of ENSO forcing and non-ENSO internal variability, we conduct ensemble “forecast” experiments with a full-physics, globally coupled atmosphere–ocean model initialized from a multidecadal tropical Pacific pacemaker simulation. The leading mode of internal variability as represented by the forecast-ensemble spread resembles the post-ENSO IPOC, despite the absence of antecedent ENSO forcing by design. The persistent atmospheric and oceanic anomalies in the leading mode highlight the positive feedback mechanism in the internal variability. The large sample size afforded by the ensemble spread allows us to identify robust non-ENSO precursors of summer IPOC variability, including a cool SST patch over the tropical northwestern Pacific, a warming patch in the tropical North Atlantic, and downwelling oceanic Rossby waves in the tropical Indian Ocean south of the equator. The pathways by which the precursors develop into the summer IPOC mode and the implications for improved predictability are discussed. 

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