The singular vectors of a coupled ocean-atmosphere model of ENSO. II: Sensitivity studies and dynamical interpretation

A. M. Moore, Richard Kleeman

Research output: Contribution to journalArticle

Abstract

We have studied the singular vectors of an intermediate coupled ocean-atmosphere model of El Niño Southern Oscillation in an attempt to understand the factors which influence the growth of perturbations in the coupled system in the tropics. The singular vectors are important because they are the fastest growing perturbations that can exist in the system before nonlinearity becomes important. In this study, we have examined how the model singular-vectors are affected by variations in certain model parameters, some of which mathematically define the singular vectors. The important findings of this study can be summarized as follows: • The behaviour of the model singular-vectors can be understood in terms of a superposition of the linear modes of the coupled system which are non-orthogonal. In some cases, the gravest modes are oscillatory in time and this leads to resonances in the singular-vector growth-factors. • The singular-vector spectrum is generally dominated by its fastest-growing member whose wind structure is relatively insensitive to the parameters that define the singular vectors mathematically. This result is in accord with the findings of Part I, and suggests that, in the tropics, the coupled system has a preferred response to perturbations, and that this response will readily manifest itself if conditions are favourable. • Favourably configured perturbations can undergo rapid transient growth over the entire regime in which the coupled model is asymptotically stable (i.e. when there are no unstable modes in the system). In this regime, rapid transient growth can occur over a wide range of timescales from 1 week to 24 months. The results and ideas presented here have important ramifications for the way in which the real coupled ocean-atmosphere system will respond in the tropics to perturbations arising from stochastic forcing inherent in the system. The dynamical operators that govern the early stages of linear perturbation-development are non-normal which means that the ubiquitous stochastic noise present in the tropics can be amplified. The dynamical mechanisms by which this can occur are the same as those responsible for the growth of the system singular-vectors. The dominant singular-vectors of the coupled model bear a remarkable resemblance to the tropical-cyclone pairs that are often observed spanning the equator in the western Pacific Ocean accompanied by strong westerly wind bursts, and thought to be precursors of El Niño events. We propose that both westerly and easterly wind bursts occuring in nature are a preferred response of the coupled system in the tropics to its inherent stochastic noise component.

Original languageEnglish (US)
Pages (from-to)983-1006
Number of pages24
JournalQuarterly Journal of the Royal Meteorological Society
Volume123
Issue number540
StatePublished - Apr 1997

Fingerprint

El Nino-Southern Oscillation
atmosphere
ocean
perturbation
westerly
atmosphere-ocean system
Southern Oscillation
tropical cyclone
nonlinearity
tropics
timescale

Keywords

  • Atmosphere-ocean interaction
  • Oceanic mixed layer
  • Predictability
  • Sea-surface temperature
  • Seasonal forecasts
  • Singular vectors
  • Tropical meteorology

ASJC Scopus subject areas

  • Atmospheric Science
  • Earth and Planetary Sciences(all)
  • Environmental Science(all)

Cite this

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title = "The singular vectors of a coupled ocean-atmosphere model of ENSO. II: Sensitivity studies and dynamical interpretation",
abstract = "We have studied the singular vectors of an intermediate coupled ocean-atmosphere model of El Ni{\~n}o Southern Oscillation in an attempt to understand the factors which influence the growth of perturbations in the coupled system in the tropics. The singular vectors are important because they are the fastest growing perturbations that can exist in the system before nonlinearity becomes important. In this study, we have examined how the model singular-vectors are affected by variations in certain model parameters, some of which mathematically define the singular vectors. The important findings of this study can be summarized as follows: • The behaviour of the model singular-vectors can be understood in terms of a superposition of the linear modes of the coupled system which are non-orthogonal. In some cases, the gravest modes are oscillatory in time and this leads to resonances in the singular-vector growth-factors. • The singular-vector spectrum is generally dominated by its fastest-growing member whose wind structure is relatively insensitive to the parameters that define the singular vectors mathematically. This result is in accord with the findings of Part I, and suggests that, in the tropics, the coupled system has a preferred response to perturbations, and that this response will readily manifest itself if conditions are favourable. • Favourably configured perturbations can undergo rapid transient growth over the entire regime in which the coupled model is asymptotically stable (i.e. when there are no unstable modes in the system). In this regime, rapid transient growth can occur over a wide range of timescales from 1 week to 24 months. The results and ideas presented here have important ramifications for the way in which the real coupled ocean-atmosphere system will respond in the tropics to perturbations arising from stochastic forcing inherent in the system. The dynamical operators that govern the early stages of linear perturbation-development are non-normal which means that the ubiquitous stochastic noise present in the tropics can be amplified. The dynamical mechanisms by which this can occur are the same as those responsible for the growth of the system singular-vectors. The dominant singular-vectors of the coupled model bear a remarkable resemblance to the tropical-cyclone pairs that are often observed spanning the equator in the western Pacific Ocean accompanied by strong westerly wind bursts, and thought to be precursors of El Ni{\~n}o events. We propose that both westerly and easterly wind bursts occuring in nature are a preferred response of the coupled system in the tropics to its inherent stochastic noise component.",
keywords = "Atmosphere-ocean interaction, Oceanic mixed layer, Predictability, Sea-surface temperature, Seasonal forecasts, Singular vectors, Tropical meteorology",
author = "Moore, {A. M.} and Richard Kleeman",
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T1 - The singular vectors of a coupled ocean-atmosphere model of ENSO. II

T2 - Sensitivity studies and dynamical interpretation

AU - Moore, A. M.

AU - Kleeman, Richard

PY - 1997/4

Y1 - 1997/4

N2 - We have studied the singular vectors of an intermediate coupled ocean-atmosphere model of El Niño Southern Oscillation in an attempt to understand the factors which influence the growth of perturbations in the coupled system in the tropics. The singular vectors are important because they are the fastest growing perturbations that can exist in the system before nonlinearity becomes important. In this study, we have examined how the model singular-vectors are affected by variations in certain model parameters, some of which mathematically define the singular vectors. The important findings of this study can be summarized as follows: • The behaviour of the model singular-vectors can be understood in terms of a superposition of the linear modes of the coupled system which are non-orthogonal. In some cases, the gravest modes are oscillatory in time and this leads to resonances in the singular-vector growth-factors. • The singular-vector spectrum is generally dominated by its fastest-growing member whose wind structure is relatively insensitive to the parameters that define the singular vectors mathematically. This result is in accord with the findings of Part I, and suggests that, in the tropics, the coupled system has a preferred response to perturbations, and that this response will readily manifest itself if conditions are favourable. • Favourably configured perturbations can undergo rapid transient growth over the entire regime in which the coupled model is asymptotically stable (i.e. when there are no unstable modes in the system). In this regime, rapid transient growth can occur over a wide range of timescales from 1 week to 24 months. The results and ideas presented here have important ramifications for the way in which the real coupled ocean-atmosphere system will respond in the tropics to perturbations arising from stochastic forcing inherent in the system. The dynamical operators that govern the early stages of linear perturbation-development are non-normal which means that the ubiquitous stochastic noise present in the tropics can be amplified. The dynamical mechanisms by which this can occur are the same as those responsible for the growth of the system singular-vectors. The dominant singular-vectors of the coupled model bear a remarkable resemblance to the tropical-cyclone pairs that are often observed spanning the equator in the western Pacific Ocean accompanied by strong westerly wind bursts, and thought to be precursors of El Niño events. We propose that both westerly and easterly wind bursts occuring in nature are a preferred response of the coupled system in the tropics to its inherent stochastic noise component.

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