Influence of Westerly Wind Events stochasticity on El Niño amplitude: the case of 2014 vs. 2015 (bibtex)
by , , , , , , , , ,
Abstract:
The weak El Niño of 2014 was preceded by anomalously high equatorial Pacific Warm Water Volume (WWV) and strong Westerly Wind Events (WWEs), which typically lead to record breaking El Nino, like in 1997 and 2015. Here, we use the CNRM–CM5 coupled model to investigate the causes for the stalled El Niño in 2014 and the necessary conditions for extreme El Niños. This model is ideally suited to study this problem because it simulates all the processes thought to be critical for the onset and development of El Niño. It captures El Niño preconditioning by WWV, the WWEs characteristics and their deterministic behaviour in response to warm pool displacements. Our main finding is, that despite their deterministic control, WWEs display a sufficiently strong stochastic component to explain the distinct evolutions of El Niño in 2014 and 2015. A 100-member ensemble simulation initialized with early-spring equatorial conditions analogous to those observed in 2014 and 2015 demonstrates that early-year elevated WWV and strong WWEs preclude the occurrence of a La Niña but lead to El Niños that span the weak (with few WWEs) to extreme (with many WWEs) range. Sensitivity experiments confirm that numerous/strong WWEs shift the El Niño distribution toward larger amplitudes, with a particular emphasis on summer/fall WWEs occurrence which result in a five-fold increase of the odds for an extreme El Niño. A long simulation further demonstrates that sustained WWEs throughout the year and anomalously high WWV are necessary conditions for extreme El Niño to develop. In contrast, we find no systematic influence of easterly wind events (EWEs) on the El Niño amplitude in our model. Our results demonstrate that the weak amplitude of El Niño in 2014 can be explained by WWEs stochastic variations without invoking EWEs or remote influences from outside the tropical Pacific and therefore its peak amplitude was inherently unpredictable at long lead-time.
Reference:
Influence of Westerly Wind Events stochasticity on El Niño amplitude: the case of 2014 vs. 2015 (Martin Puy, Jérôme Vialard, Matthieu Lengaigne, Eric Guilyardi, Pedro N. DiNezio, Aurore Voldoire, Magdalena Balmaseda, Gurvan Madec, Christophe Menkes, Michael J. Mcphaden), In Climate Dynamics, 2017.
Bibtex Entry:
@Article{	  puy.ea_2017,
  author	= "Puy, Martin and Vialard, J{\'e}r{\^o}me and Lengaigne,
		  Matthieu and Guilyardi, Eric and DiNezio, Pedro N. and
		  Voldoire, Aurore and Balmaseda, Magdalena and Madec, Gurvan
		  and Menkes, Christophe and Mcphaden, Michael J.",
  title		= "Influence of Westerly Wind Events stochasticity on El
		  Ni{\~{n}}o amplitude: the case of 2014 vs. 2015",
  journal	= "Climate Dynamics",
  year		= "2017",
  month		= "Oct",
  day		= "05",
  abstract	= "The weak El Ni{\~{n}}o of 2014 was preceded by anomalously
		  high equatorial Pacific Warm Water Volume (WWV) and strong
		  Westerly Wind Events (WWEs), which typically lead to record
		  breaking El Nino, like in 1997 and 2015. Here, we use the
		  CNRM--CM5 coupled model to investigate the causes for the
		  stalled El Ni{\~{n}}o in 2014 and the necessary conditions
		  for extreme El Ni{\~{n}}os. This model is ideally suited to
		  study this problem because it simulates all the processes
		  thought to be critical for the onset and development of El
		  Ni{\~{n}}o. It captures El Ni{\~{n}}o preconditioning by
		  WWV, the WWEs characteristics and their deterministic
		  behaviour in response to warm pool displacements. Our main
		  finding is, that despite their deterministic control, WWEs
		  display a sufficiently strong stochastic component to
		  explain the distinct evolutions of El Ni{\~{n}}o in 2014
		  and 2015. A 100-member ensemble simulation initialized with
		  early-spring equatorial conditions analogous to those
		  observed in 2014 and 2015 demonstrates that early-year
		  elevated WWV and strong WWEs preclude the occurrence of a
		  La Ni{\~{n}}a but lead to El Ni{\~{n}}os that span the weak
		  (with few WWEs) to extreme (with many WWEs) range.
		  Sensitivity experiments confirm that numerous/strong WWEs
		  shift the El Ni{\~{n}}o distribution toward larger
		  amplitudes, with a particular emphasis on summer/fall WWEs
		  occurrence which result in a five-fold increase of the odds
		  for an extreme El Ni{\~{n}}o. A long simulation further
		  demonstrates that sustained WWEs throughout the year and
		  anomalously high WWV are necessary conditions for extreme
		  El Ni{\~{n}}o to develop. In contrast, we find no
		  systematic influence of easterly wind events (EWEs) on the
		  El Ni{\~{n}}o amplitude in our model. Our results
		  demonstrate that the weak amplitude of El Ni{\~{n}}o in
		  2014 can be explained by WWEs stochastic variations without
		  invoking EWEs or remote influences from outside the
		  tropical Pacific and therefore its peak amplitude was
		  inherently unpredictable at long lead-time.",
  issn		= "1432-0894",
  doi		= "10.1007/s00382-017-3938-9",
  url		= "https://doi.org/10.1007/s00382-017-3938-9"
}
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