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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
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
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
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
-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
-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
-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
-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
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
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
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
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
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
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
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
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
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
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
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
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
