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1. Introduction El Niño–Southern Oscillation (ENSO) is the dominant climate mode of variability in the tropical Pacific on interannual time scales, having a significant impact on global weather and climate. The onset of ENSO events generally occurs during boreal spring and summer, while most events peak in boreal winter, with the amplitudes decaying in the next spring. Although several negative feedbacks could explain how the coupled ocean–atmosphere system oscillates, for example, via a
1. Introduction El Niño–Southern Oscillation (ENSO) is the dominant climate mode of variability in the tropical Pacific on interannual time scales, having a significant impact on global weather and climate. The onset of ENSO events generally occurs during boreal spring and summer, while most events peak in boreal winter, with the amplitudes decaying in the next spring. Although several negative feedbacks could explain how the coupled ocean–atmosphere system oscillates, for example, via a
1. Introduction The El Niño–Southern Oscillation (ENSO)—a major source for interannual climate variability—affects weather and climate worldwide ( Ropelewski and Halpert 1987 ; Kiladis and Diaz 1989 ; Hoerling et al. 1997 ; Larkin and Harrison 2005 ; Sun and Bryan 2010 , Zhang et al. 2011 , 2014 ). The two phases of ENSO—El Niño and La Niña—which are defined as tropical Pacific anomalies relative to a long-term average, are not mirror images of each other: the strongest El Niño is
1. Introduction The El Niño–Southern Oscillation (ENSO)—a major source for interannual climate variability—affects weather and climate worldwide ( Ropelewski and Halpert 1987 ; Kiladis and Diaz 1989 ; Hoerling et al. 1997 ; Larkin and Harrison 2005 ; Sun and Bryan 2010 , Zhang et al. 2011 , 2014 ). The two phases of ENSO—El Niño and La Niña—which are defined as tropical Pacific anomalies relative to a long-term average, are not mirror images of each other: the strongest El Niño is
1. Introduction Major progress in understanding and modeling the coupled ocean–atmosphere interaction in the Tropics and the salient features of the El Niño–Southern Oscillation (ENSO) phenomenon has been made in the past two decades ( Bjerknes 1969 ; Rasmusson and Carpenter 1982 ; Philander et al. 1984 ; Cane 1984 ; Cane and Zebiak 1985 ; Schopf and Suarez 1988 ; Battisti and Hirst 1989 ; Philander 1990 ; Neelin et al. 1994 ; Jin 1996 , 1997a , b ; Weisberg and Wang 1997 ; Wallace
1. Introduction Major progress in understanding and modeling the coupled ocean–atmosphere interaction in the Tropics and the salient features of the El Niño–Southern Oscillation (ENSO) phenomenon has been made in the past two decades ( Bjerknes 1969 ; Rasmusson and Carpenter 1982 ; Philander et al. 1984 ; Cane 1984 ; Cane and Zebiak 1985 ; Schopf and Suarez 1988 ; Battisti and Hirst 1989 ; Philander 1990 ; Neelin et al. 1994 ; Jin 1996 , 1997a , b ; Weisberg and Wang 1997 ; Wallace
1. Introduction Since Bjerknes (1969) linked El Niño to the large-scale air–sea interaction in the tropical Pacific, El Niño–Southern Oscillation (ENSO) has been extensively studied. Many aspects of ENSO dynamics have been understood. For example, the rapid growth of an ENSO SST anomaly is linked to a positive feedback loop among SST, the surface winds, and ocean dynamics ( Bjerknes 1969 ). The transition between an El Niño event and a La Niño event has been associated with a linear
1. Introduction Since Bjerknes (1969) linked El Niño to the large-scale air–sea interaction in the tropical Pacific, El Niño–Southern Oscillation (ENSO) has been extensively studied. Many aspects of ENSO dynamics have been understood. For example, the rapid growth of an ENSO SST anomaly is linked to a positive feedback loop among SST, the surface winds, and ocean dynamics ( Bjerknes 1969 ). The transition between an El Niño event and a La Niño event has been associated with a linear
and natural resources, making the USAPIs particularly vulnerable to the impacts of climate variability ( Shea et al. 2001 ). Positioned in the tropical western Pacific, the climate of the USAPIs is strongly modulated by El Niño–Southern Oscillation (ENSO; Bjerknes 1966 , 1969 ; McPhaden et al. 2006 ). The effects of ENSO on the USAPIs include changes in seasonal rainfall ( Yu et al. 1997 ), tropical cyclone activity ( Lander 1994 ; Wang and Chan 2002 ), and sea surface temperature (SST; Wang
and natural resources, making the USAPIs particularly vulnerable to the impacts of climate variability ( Shea et al. 2001 ). Positioned in the tropical western Pacific, the climate of the USAPIs is strongly modulated by El Niño–Southern Oscillation (ENSO; Bjerknes 1966 , 1969 ; McPhaden et al. 2006 ). The effects of ENSO on the USAPIs include changes in seasonal rainfall ( Yu et al. 1997 ), tropical cyclone activity ( Lander 1994 ; Wang and Chan 2002 ), and sea surface temperature (SST; Wang
1. Introduction ENSO behaviors in observations and models have shown rich diversity and asymmetry. El Niños can peak at both the eastern Pacific (EP) and the central Pacific (CP) (e.g., Larkin and Harrison 2005 ; Ashok et al. 2007 ; Weng et al. 2007 ; Kao and Yu 2009 ; Kug et al. 2009 ; Taschetto and England 2009 ; Lee and McPhaden 2010 ; Newman et al. 2011 ; Takahashi et al. 2011 ; Karnauskas 2013 ; Capotondi et al. 2015 ; Fedorov et al. 2015 ; D. Chen et al. 2015 ). Extreme El
1. Introduction ENSO behaviors in observations and models have shown rich diversity and asymmetry. El Niños can peak at both the eastern Pacific (EP) and the central Pacific (CP) (e.g., Larkin and Harrison 2005 ; Ashok et al. 2007 ; Weng et al. 2007 ; Kao and Yu 2009 ; Kug et al. 2009 ; Taschetto and England 2009 ; Lee and McPhaden 2010 ; Newman et al. 2011 ; Takahashi et al. 2011 ; Karnauskas 2013 ; Capotondi et al. 2015 ; Fedorov et al. 2015 ; D. Chen et al. 2015 ). Extreme El
1. Introduction It has been well documented that El Niño–Southern Oscillation (ENSO) is important in global short-term climate variability and predictability ( National Research Council 2010 ), for example, for surface air temperature and precipitation variability in North America ( Ropelewski and Halpert 1987 ; Hu and Feng 2001 , 2012 ; Mo 2010 ; Deser et al. 2018 ; Peng et al. 2018 , 2019 ). For the seasonal climate variations in North America, the impact of ENSO is mainly through an
1. Introduction It has been well documented that El Niño–Southern Oscillation (ENSO) is important in global short-term climate variability and predictability ( National Research Council 2010 ), for example, for surface air temperature and precipitation variability in North America ( Ropelewski and Halpert 1987 ; Hu and Feng 2001 , 2012 ; Mo 2010 ; Deser et al. 2018 ; Peng et al. 2018 , 2019 ). For the seasonal climate variations in North America, the impact of ENSO is mainly through an
1. Introduction A lot of progress has been made over the last 20 yr in our understanding of, and ability to observe, model, and predict, tropical Pacific climate variability (e.g., see McPhaden et al. 1998 , and other articles of the J. Geophys. Res. TOGA special issue, 1998, Vol. 103, No. C7). The progress, however, has been primarily limited to ENSO and the annual cycle. While these phenomena explain the most variance, variability on other time scales is clearly evident ( Fig. 1 ). Our
1. Introduction A lot of progress has been made over the last 20 yr in our understanding of, and ability to observe, model, and predict, tropical Pacific climate variability (e.g., see McPhaden et al. 1998 , and other articles of the J. Geophys. Res. TOGA special issue, 1998, Vol. 103, No. C7). The progress, however, has been primarily limited to ENSO and the annual cycle. While these phenomena explain the most variance, variability on other time scales is clearly evident ( Fig. 1 ). Our
1. Introduction Over the course of the study of El Niño–Southern Oscillation (ENSO), models of varying amounts of complexity have been used. These results suggest that the deterministic part of ENSO is described by the first few coupled modes of variability in the equatorial Pacific (e.g., Anderson and McCreary 1985 ; Cane and Zebiak 1985 ; Suarez and Schopf 1988 ; Battisti and Hirst 1989 ; Philander 1990 ; Jin 1997 ; Neelin et al. 1998 ; Bejarano and Jin 2008 ). However, ENSO itself is
1. Introduction Over the course of the study of El Niño–Southern Oscillation (ENSO), models of varying amounts of complexity have been used. These results suggest that the deterministic part of ENSO is described by the first few coupled modes of variability in the equatorial Pacific (e.g., Anderson and McCreary 1985 ; Cane and Zebiak 1985 ; Suarez and Schopf 1988 ; Battisti and Hirst 1989 ; Philander 1990 ; Jin 1997 ; Neelin et al. 1998 ; Bejarano and Jin 2008 ). However, ENSO itself is
1. Introduction The El Niño–Southern Oscillation (ENSO) phenomenon dominates the climate fluctuations on the interannual time scale, with its period varying from 2 to 7 yr. It has been recognized as a result of the ocean–atmosphere interaction over the tropical Pacific. Since Bjerknes’ recognition of a new coupled ocean–atmosphere instability for ENSO ( Bjerknes 1969 ), theoretical studies have made great progress in understanding the dynamics of ENSO in terms of coupled instability of the
1. Introduction The El Niño–Southern Oscillation (ENSO) phenomenon dominates the climate fluctuations on the interannual time scale, with its period varying from 2 to 7 yr. It has been recognized as a result of the ocean–atmosphere interaction over the tropical Pacific. Since Bjerknes’ recognition of a new coupled ocean–atmosphere instability for ENSO ( Bjerknes 1969 ), theoretical studies have made great progress in understanding the dynamics of ENSO in terms of coupled instability of the
