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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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Michael J. McPhaden
and
Christina Karamperidou

two days, on 24 November 1934, presumably from botulism ( Treherne 1983 ). As he lay dying, he reportedly said “that it would be very funny indeed if he as a vegetarian was going to die of meat poisoning” ( Wittmer 1989 ). A story that has not been told, and the subject of this article, is the role that ­climate ­variability, and in particular, La Niña, 1 played in the ­Galapagos ­Affair. We will show, using reconstructed sea surface ­temperatures (SSTs), the Twentieth Century Reanalysis

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Weston Anderson
,
Benjamin I. Cook
,
Kim Slinski
,
Kevin Schwarzwald
,
Amy McNally
, and
Chris Funk

-based multiyear forecasts of Niño-3.4 anomalies demonstrate skill up to 16 months in advance ( Ham et al. 2019 ). Such forecasts of tropical Pacific SSTs are relevant for forecasting drought in East Africa during both the October–December and March–May season. Indeed, operational NMME precipitation forecasts are more skillful during strong ENSO years in East Africa ( Shukla et al. 2019 ). La Niña events in particular may be predictable multiple years in advance provided the correct initial conditions ( Wu

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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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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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Hyein Jeong
,
Hyo-Seok Park
,
Jasti S. Chowdary
, and
Shang-Ping Xie

La Niña, the cold phase of El Niño–Southern Oscillation (ENSO) in the equatorial Pacific, has pronounced global impacts on weather and climate. La Niña events typically last less than a year but can sometimes last for several years. Multiyear La Niña events often induce cool and wet climate, especially over global lands ( Min et al. 2013 ; Luo et al. 2017 ), and can slow down the rate of global warming ( Kosaka and Xie 2013 ; England et al. 2014 ; Watanabe et al. 2014 ). The most recent La

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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 ( http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_disc_dec2016/ensodisc.pdf ). Most state-of-the-art dynamical and statistical models also

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Na Wen
and
Laurent Li

 al. 2002, 2003 ), the “recharge and discharge” effect of the tropical Indian Ocean ( Yang et al. 2007 ; Xie et al. 2009 ), and the possible bridging role of the tropical Atlantic SST anomalies ( Rong et al. 2010 ). La Niña is the antiphase of El Niño in the ENSO cycle but not its simple mirror. There are asymmetries in many aspects, such as amplitude, event evolutional course, and associated atmospheric responses ( Zhou et al. 2014 ; McPhaden and Zhang 2009 ; Dommenget et al. 2013 ; Okumura 2019

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