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Maddalen Iza, Natalia Calvo, and Elisa Manzini

has been paid to the stratospheric response to the cold ENSO phase (La Niña), and it is not clear yet whether La Niña can affect the NAE region through a stratospheric pathway. Some studies based on reanalysis and observational data have shown a polar stratospheric cooling during La Niña winters, although the response is either weak or not significant ( Mitchell et al. 2011 ; Free and Seidel 2009 ). It should be noted that the short record (and thus small signal-to-noise ratio) could be perhaps

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Pedro N. DiNezio and Clara Deser

1. Introduction A large fraction (35%–50%) of La Niña events last two years or longer ( Okumura and Deser 2010 ) in contrast to El Niño events, which rarely last longer than one year. The multiyear persistence of La Niña exacerbates its global climate impacts, especially in regions prone to drought. Several observational studies have documented the asymmetry in the duration of the two phases of El Niño–Southern Oscillation (ENSO) (e.g., Kessler 2002 ; Larkin and Harrison 2002 ; McPhaden and

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Bor-Ting Jong, Mingfang Ting, Richard Seager, and Weston B. Anderson

-level Rossby wave propagating from the equator to the extratropics across the Pacific–North America (PNA) region (e.g., Hoskins and Karoly 1981 ; Webster 1981 ). The low-frequency Rossby wave shifts the subtropical jet stream and storm track equatorward (poleward) during an El Niño (La Niña), subsequently influencing climate in remote regions including North America (e.g., Trenberth et al. 1998 ). Besides the direct tropical influence via Rossby wave propagation, midlatitude transient eddies also play

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Christopher F. O’Connor, Pao-Shin Chu, Pang-Chi Hsu, and Kevin Kodama

of most concern for rainfall variability in the tropical Pacific islands on seasonal time scales ( Ropelewski and Halpert 1987 ). Episodes are normally recognized through sea surface temperature (SST) anomalies in the equatorial Pacific region, most commonly in the Niño-3.4 region (5°S–5°N, 120°–170°W). The ENSO events are labeled as either a warm (El Niño) or cold (La Niña) phase, yet its amplitude varies across a continuum with essentially Gaussian statistics ( Trenberth 1997 ). Recent studies

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Jingzhi Su, Renhe Zhang, Tim Li, Xinyao Rong, J-S. Kug, and Chi-Cherng Hong

temperature anomalies (SSTAs) in the eastern equatorial Pacific is significantly larger during El Niño episodes than during La Niña episodes ( Burgers and Stephenson 1999 ). This asymmetric aspect of ENSO cannot be explained by the conceptual model mentioned earlier, in which ENSO is portrayed as a regular and periodic oscillation. An and Jin (2004 , hereafter AJ04) showed that nonlinear dynamical thermal advections could play important roles in the amplitude asymmetry between El Niño and La Niña. The

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Aoyun Xue, Wenjun Zhang, Julien Boucharel, and Fei-Fei Jin

temperature anomalies (SSTAs) in the equatorial Pacific, both leading to anomalies in the Niño-3.4 region of nearly 2.5°C, as shown in Fig. 1a . However, the two events greatly differed from each other during their decaying phases. After the 2015 El Niño onset, a weak La Niña event materialized as the SST anomalies barely reached the La Niña threshold ( ). Most state-of-the-art dynamical and statistical models also

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Bo Wu, Tim Li, and Tianjun Zhou

Asian trough and the intrusion of midlatitude cold air into the Philippine Sea might trigger the WNPAC ( Wang and Zhang 2002 ; Lau and Nath 2006 ). Third, the WNPAC results from the eastward movement of an anomalous anticyclone established over the northern Indian Ocean ( Chou 2004 ; Chen et al. 2007 ). Most previous studies assumed a symmetric circulation feature between El Niño and La Niña; namely, there is an anomalous anticyclone (cyclone) over the WNP during the El Niño (La Niña) mature

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Gary Meyers, Peter McIntosh, Lidia Pigot, and Mike Pook

Hemisphere winter. The pattern is in a positive phase when the sea surface temperature (SST) is anomalously cool in the east and warm in the west. The equatorial winds at this time are easterlies, with the wind coupled to the SST in the sense that it blows from the cooler toward the warmer waters. Some (but not all) positive IOD events occur during the same year as El Niño, and the same can be said about negative IOD events and La Niña ( Yamagata et al. 2004 ). This paper is concerned with developing a

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Sukyoung Lee

(enhanced convection over the western tropical Pacific warm pool) is associated with an eastward acceleration in the equatorial upper troposphere and with a warming of the Arctic during the winter. Similarly, one can ask if the Arctic winter would be warmer during La Niña than during El Niño because the tropical convection is more localized for La Niña. 1 Accordingly, in this study we address the following questions: Are Arctic surface temperatures higher for La Niña than for El Niño? If so, is the La

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Andrew Lorrey, Giovanni Dalu, James Renwick, Howard Diamond, and Marco Gaetani

convection zone ( Folland et al. 2002 ; Vincent et al. 2011 ). A more northward SPCZ position relative to climatology is observed in El Niño (positive IPO) episodes, while a southward bias is seen in La Niña (negative IPO) episodes. “Asymmetric” orientations of the SPCZ in a near-parallel alignment with the equator are also noted for very strong El Niños ( Vincent et al. 2011 ). SPCZ dynamics and associated precipitation variability play a major role in the climate of southwest Pacific Island nations

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