The Linear Response of ENSO to the Madden–Julian Oscillation

Abstract The possibility that the tropical Pacific coupled system linearly amplifies perturbations produced by the Madden–Julian oscillation (MJO) is explored. This requires an estimate of the low-frequency tail of the MJO. Using 23 yr of NCEP–NCAR reanalyses of surface wind and Reynolds SST, we show that the spatial structure that dominates the intraseasonal band (i.e., the MJO) also dominates the low-frequency band once the anomalies directly related to ENSO have been removed. This low-frequency contribution of the intraseasonal variability is not included in most ENSO coupled models used to date. Its effect in a coupled model of intermediate complexity has, therefore, been studied. It is found that this “MJO forcing” (τMJO) can explain a large fraction of the interannual variability in an asymptotically stable version of the model. This interaction is achieved via linear dynamics. That is, it is the cumulative effect of individual events that maintains ENSOs in this model. The largest coupled wind anomalies are initiated after a sequence of several downwelling Kelvin waves of the same sign have been forced by τMJO. The cumulative effect of the forced Kelvin waves is to persist the (small) SST anomalies in the eastern Pacific just enough for the coupled ocean–atmosphere dynamics to amplify the anomalies into a mature ENSO event. Even though τMJO explains just a small fraction of the energy contained in the stress not associated with ENSO, a large fraction of the modeled ENSO variability is excited by this forcing. The characteristics that make τMJO an optimal stochastic forcing for the model are discussed. The large zonal extent is an important factor that differentiates the MJO from other sources of stochastic forcing.

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Southern Ocean Wind Stress in CMIP5 Models: Role of Wind Fluctuations

Journal of Climate ◽

10.1175/jcli-d-19-0466.1 ◽

2020 ◽

Vol 33 (4) ◽

pp. 1209-1226 ◽

Cited By ~ 2

Author(s):

Xia Lin ◽

Xiaoming Zhai ◽

Zhaomin Wang ◽

David R. Munday

Keyword(s):

Southern Ocean ◽

Time Scales ◽

Wind Stress ◽

Surface Wind ◽

Coupled Model ◽

Cmip5 Models ◽

Coupled Models ◽

Surface Wind Stress ◽

Stress Changes ◽

The Mean

AbstractThe Southern Ocean (SO) surface wind stress is a major atmospheric forcing for driving the Antarctic Circumpolar Current and the global overturning circulation. Here the effects of wind fluctuations at different time scales on SO wind stress in 18 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are investigated. It is found that including wind fluctuations, especially on time scales associated with synoptic storms, in the stress calculation strongly enhances the mean strength, modulates the seasonal cycle, and significantly amplifies the trends of SO wind stress. In 11 out of the 18 CMIP5 models, the SO wind stress has strengthened significantly over the period of 1960–2005. Among them, the strengthening trend of SO wind stress in one CMIP5 model is due to the increase in the intensity of wind fluctuations, while in all the other 10 models the strengthening trend is due to the increasing strength of the mean westerly wind. These discrepancies in SO wind stress trend in CMIP5 models may explain some of the diverging behaviors in the model-simulated SO circulation. Our results suggest that to reduce the uncertainty in SO responses to wind stress changes in the coupled models, both the mean wind and wind fluctuations need to be better simulated.

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The Impact of Extratropical Atmospheric Variability on ENSO: Testing the Seasonal Footprinting Mechanism Using Coupled Model Experiments

Journal of Climate ◽

10.1175/2010jcli3205.1 ◽

2010 ◽

Vol 23 (11) ◽

pp. 2885-2901 ◽

Cited By ~ 132

Author(s):

Michael A. Alexander ◽

Daniel J. Vimont ◽

Ping Chang ◽

James D. Scott

Keyword(s):

Heat Flux ◽

Coupled Model ◽

Enso Event ◽

Surface Heat ◽

Kelvin Waves ◽

Heat Fluxes ◽

Atmospheric Variability ◽

The Tropics ◽

Sst Anomalies ◽

The Tropical Pacific

Abstract Previous studies suggest that extratropical atmospheric variability influences the tropics via the seasonal footprinting mechanism (SFM), in which fluctuations in the North Pacific Oscillation (NPO) impact the ocean via surface heat fluxes during winter and the resulting springtime subtropical SST anomalies alter the atmosphere–ocean system over the tropics in the following summer, fall, and winter. Here, the authors test the SFM hypothesis by imposing NPO-related surface heat flux forcing in an atmospheric GCM coupled to a reduced gravity ocean model in the tropics and a slab ocean in the extratropics. The forcing is only imposed through the first winter, and then the model is free to evolve through the following winter. The evolution of the coupled model response to the forcing is consistent with the SFM hypothesis: the NPO-driven surface fluxes cause positive SST anomalies to form in the central and eastern subtropics during winter; these anomalies propagate toward the equator along with westerly wind anomalies during spring, reach the equator in summer, and then amplify, which leads to an ENSO event in the following winter. The anomalies reach the equator through a combination of thermodynamically coupled air–sea interactions, namely, the wind–evaporation–SST (WES) feedback and equatorial ocean dynamics. The initial off-equatorial anomaly propagates toward the equator through a relaxation of the climatological easterly winds south of the dominant SST anomalies, which leads to a reduction in upward latent heat flux. These westerly anomalies reach the equator during boreal summer, where they can excite downwelling equatorial Kelvin waves. The connection between off-equatorial variations and tropical ENSO-like conditions may also occur via the excitation of westward-propagating equatorial Rossby waves during spring, which reflect off of the western boundary as Kelvin waves, depressing the thermocline in the eastern Pacific during the following summer. NPO-related anomalies that form during the first winter in the tropical Pacific may also contribute to the development of an El Niño event in the following winter. The imposition of the NPO-related forcing caused warming in the ENSO region in ∼70% of the ensemble of 60 simulations; therefore, the response depends on the state of the tropical atmosphere–ocean system. For years where the control simulation was poised to develop into a neutral or negative ENSO event, the addition of the NPO heat fluxes tended to cause anomalous warming in the tropical Pacific in the following fall/winter; if the control was heading toward a warm ENSO event, the imposition of NPO forcing tends to reduce the amplitude of that event.

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A Model for the Interaction between 2-Day Waves and Moist Kelvin Waves*

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-0116.1 ◽

2012 ◽

Vol 69 (2) ◽

pp. 611-625 ◽

Cited By ~ 24

Author(s):

Fei Liu ◽

Bin Wang

Keyword(s):

Moisture Transfer ◽

Low Frequency ◽

Lower Troposphere ◽

Kelvin Waves ◽

Madden Julian Oscillation ◽

Middle Troposphere ◽

Si Model ◽

Multiscale Structures ◽

Comparable Magnitude ◽

Theoretical Results

Abstract The eastward-propagating tropical low-frequency disturbances, such as the moist Kelvin waves or the Madden–Julian oscillation (MJO), are often observed to experience convective enhancement when meeting with the westward-propagating 2-day waves. A scale interaction (SI) model is built to understand the nature of the interaction between the 2-day waves and moist Kelvin waves or MJO. In this model, the convective complex of moist Kelvin waves modulates the strength and location of the 2-day waves, which feed back through the upscale eddy transfer. An ageostrophic model describing the 2-day waves is first solved, and the resultant westward-propagating, backward-tilted disturbances are consistent with the observed 2-day waves. An explicit representation of eddy momentum transfer (EMT), eddy heating transfer (EHT), and eddy moisture transfer (EQT) arising from the 2-day waves is then formulated. The SI model shows that the 2-day waves in front of moist Kelvin waves produce an EMT accelerating the low-frequency easterly in the lower troposphere, an EHT cooling down the middle troposphere, and an EQT moistening the middle troposphere. These three transfer terms have comparable magnitude. Although the negative EHT tends to damp the moist Kelvin waves, both the EMT and EQT provide instability sources for the moist Kelvin waves. The 2-day waves also slow down the moist Kelvin waves, mainly through the advective effects of the EMT. So the unstable moist Kelvin waves may exhibit convective enhancement when meeting with the 2-day waves. The theoretical results presented here point to the need to further observe the multiscale structures within the moist Kelvin waves and the MJO.

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Meridional Energy Transport in the Coupled Atmosphere–Ocean System: Compensation and Partitioning

Journal of Climate ◽

10.1175/jcli-d-12-00133.1 ◽

2013 ◽

Vol 26 (18) ◽

pp. 7151-7166 ◽

Cited By ~ 33

Author(s):

Riccardo Farneti ◽

Geoffrey K. Vallis

Keyword(s):

Time Scales ◽

Energy Transport ◽

Climate Model ◽

Decadal Variability ◽

Low Frequency ◽

Coupled Model ◽

Meridional Overturning Circulation ◽

Radiation Budget ◽

Balance Model ◽

Coupled Models

Abstract The variability and compensation of the meridional energy transport in the atmosphere and ocean are examined with the state-of-the-art GFDL Climate Model, version 2.1 (CM2.1), and the GFDL Intermediate Complexity Coupled Model (ICCM). On decadal time scales, a high degree of compensation between the energy transport in the atmosphere (AHT) and ocean (OHT) is found in the North Atlantic. The variability of the total or planetary heat transport (PHT) is much smaller than the variability in either AHT or OHT alone, a feature referred to as “Bjerknes compensation.” Natural decadal variability stems from the Atlantic meridional overturning circulation (AMOC), which leads OHT variability. The PHT is positively correlated with the OHT, implying that the atmosphere is compensating, but imperfectly, for variations in ocean transport. Because of the fundamental role of the AMOC in generating the decadal OHT anomalies, Bjerknes compensation is expected to be active only in coupled models with a low-frequency AMOC spectral peak. The AHT and the transport in the oceanic gyres are positively correlated because the gyre transport responds to the atmospheric winds, thereby militating against long-term variability involving the wind-driven flow. Moisture and sensible heat transports in the atmosphere are also positively correlated at decadal time scales. The authors further explore the mechanisms and degree of compensation with a simple, diffusive, two-layer energy balance model. Taken together, these results suggest that compensation can be interpreted as arising from the highly efficient nature of the meridional energy transport in the atmosphere responding to ocean variability rather than any a priori need for the top-of-atmosphere radiation budget to be fixed.

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A Comparison of the Influence of Additive and Multiplicative Stochastic Forcing on a Coupled Model of ENSO

Journal of Climate ◽

10.1175/jcli3596.1 ◽

2005 ◽

Vol 18 (23) ◽

pp. 5066-5085 ◽

Cited By ~ 49

Author(s):

Cristina L. Perez ◽

Andrew M. Moore ◽

Javier Zavala-Garay ◽

Richard Kleeman

Keyword(s):

El Niño ◽

El Nino ◽

Southern Oscillation ◽

Low Frequency ◽

Coupled Model ◽

Deterministic System ◽

Low Frequency Oscillation ◽

Stochastic Forcing ◽

Integration Period ◽

Atmospheric Phenomena

Abstract A currently popular idea is that El Niño–Southern Oscillation (ENSO) can be viewed as a linear deterministic system forced by noise representing processes with periods shorter than ENSO. Also, there is observational evidence to suggest that the Madden–Julian oscillation (MJO) acts to trigger and/or amplify the warm phase of ENSO in this way. The feedback of the slower process, ENSO, to higher-frequency atmospheric phenomena, of which a large part of the variability in the intraseasonal band is due to the MJO, has received little attention. This paper considers the hypothesis that the probability of an El Niño event is modified by high MJO activity and that, in turn, the MJO is regulated by ENSO activity. If this is indeed the case, then viewing ENSO as a low-frequency oscillation forced by additive stochastic noise would not present a complete picture. This paper tests the above hypothesis using a stochastically forced intermediate coupled model by allowing ENSO to directly influence the stochastic forcing. The model response to a variety of stochastic forcing types is found to be sensitive to the type of forcing applied. When the model is operated beyond its intrinsic Hopf bifurcation, its probability distribution function (PDF) is fundamentally altered when the stochastic forcing is changed from additive to multiplicative. The model integration period also influences the shape of the PDF, which is also compared to the PDF derived from observations. It is found that multiplicative stochastic forcing reproduces some measures of the observations better than the additive stochastic forcing.

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Modeling the Mechanisms of Linear and Nonlinear ENSO Responses to the Pacific Meridional Mode

Journal of Climate ◽

10.1175/jcli-d-16-0090.1 ◽

2016 ◽

Vol 29 (24) ◽

pp. 8745-8761 ◽

Cited By ~ 15

Author(s):

Erin E. Thomas ◽

Daniel J. Vimont

Keyword(s):

Southern Oscillation ◽

Surface Wind ◽

Coupled Model ◽

Enso Event ◽

Ocean Dynamics ◽

Large Variability ◽

Enso Events ◽

The Pacific ◽

Meridional Mode ◽

Pacific Meridional Mode

Abstract Interactions between the Pacific meridional mode (PMM) and El Niño–Southern Oscillation (ENSO) are investigated using the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) and an intermediate coupled model (ICM). The two models are configured so that the CESM simulates the PMM but not ENSO, and the ICM simulates ENSO but not the PMM, allowing for a clean separation between the PMM evolution and the subsequent ENSO response. An ensemble of CESM simulations is run with an imposed surface heat flux associated with the North Pacific Oscillation (NPO) generating a sea surface temperature (SST) and wind response representative of the PMM. The PMM wind is then applied as a forcing to the ICM to simulate the ENSO response. The positive (negative) ensemble-mean PMM wind forcing results in a warm (cold) ENSO event although the responses are not symmetric (warm ENSO events are larger in amplitude than cold ENSO events), and large variability between ensemble members suggests that any individual ENSO event is strongly influenced by natural variability contained within the CESM simulations. Sensitivity experiments show that 1) direct forcing of Kelvin waves by PMM winds dominates the ENSO response, 2) seasonality of PMM forcing and ENSO growth rates influences the resulting ENSO amplitude, 3) ocean dynamics within the ICM dominate the ENSO asymmetry, and 4) the nonlinear relationship between PMM wind anomalies and surface wind stress may enhance the La Niña response to negative PMM variations. Implications for ENSO variability are discussed.

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Decadal Modulation of ENSO in a Hybrid Coupled Model

Journal of Climate ◽

10.1175/2008jcli1933.1 ◽

2008 ◽

Vol 21 (21) ◽

pp. 5482-5500 ◽

Cited By ~ 19

Author(s):

Robert J. Burgman ◽

Paul S. Schopf ◽

Ben P. Kirtman

Keyword(s):

Time Scales ◽

Low Frequency ◽

Coupled Model ◽

Tropical Pacific ◽

Stochastic Forcing ◽

Decadal Modulation ◽

The Mean ◽

Frequency Changes ◽

Mean State ◽

The Tropical Pacific

Abstract Decadal variations in the amplitude of El Niño and the Southern Oscillation have been the subject of great interest in the literature for the past decade. One theory suggests that ENSO is best described as a stable system driven by linear dynamics and that stochastic atmospheric forcing is responsible for the development and modulation of ENSO on interannual as well as decadal time scales. Another theory suggests that ENSO is driven by strong nonlinear coupled feedbacks between the ocean and atmosphere and low frequency changes in ENSO amplitude are driven by decadal changes in the tropical Pacific mean state. Unfortunately, the observed record is too short to collect reliable statistics for such low frequency behavior. A hybrid coupled model composed of a simple statistical atmosphere coupled to the Poseidon isopycnal ocean model has been developed for the study of ENSO decadal variability. The model simulates realistic ENSO variability on interannual and decadal time scales with negligible climate drift over 1000 years. Through analysis and experimentation the authors show that low frequency changes in the atmospheric “weather noise” drive changes in the tropical Pacific mean state leading to changes in the amplitude of ENSO on decadal time scales. Additional model simulations suggest that, while predictability is limited by the presence of atmospheric noise, there are extended periods when the coupled instability, strengthened by changes in the mean state, is insensitive to noise on interannual time scales. The relationship between decadal modulation of ENSO and mean state changes resides somewhere between the linear damped stochastically forced theory and the strongly unstable theory. Unlike the strongly unstable system, changes in ENSO amplitude on longer time scales are determined by the stochastic forcing. The stochastic forcing is not necessary in this model to sustain ENSO; however, its presence is crucial for low frequency changes in the mean state of the tropical Pacific. The strong relationship between the mean state and ENSO amplitude modulation in the model is in opposition to the linear damped stochastically forced theory. The fact that changes in the tropical Pacific mean state lead directly to changes in ENSO amplitude and predictability has positive implications for predictability.

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The weak volcanic response in climate models related to low-frequency Pacific variability

10.5194/egusphere-egu21-572 ◽

2021 ◽

Author(s):

Giorgio Graffino ◽

Jonathan Gregory

Keyword(s):

Ad Hoc ◽

Climate Models ◽

Function Analysis ◽

Low Frequency ◽

Coupled Model ◽

Volcanic Eruptions ◽

Delayed Response ◽

Coupled Models ◽

The Pacific ◽

Volcanic Forcing

<p>Volcanic eruptions are among the most important naturally occurring cause of climate variability. Their effect can outlive the residence time of the volcanic aerosol in the stratosphere, due to the intervention of the ocean as heat reservoir. Coupled models exhibit deficiencies and uncertainties in their response to volcanic forcing as well as multiannual variability. We have investigated a possible link by analysing experiments included in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP), along with several ad-hoc model simulations, in comparison with observational reanalyses and reconstructions. We introduce a novel technique to analyse the delayed response of sea surface temperature (SST) and mean sea level pressure (MSLP) in the Pacific Ocean to large volcanic eruptions, complemented with with an empirical orthogonal function analysis. Our study shows that coupled models are not able to reproduce the observed SST response to volcanic forcing, which has the shape of the cold phase of the Interdecadal Pacific Oscillation (IPO), and that their MSLP response is too weak. On the other hand, the observed MSLP response is reproduced by atmosphere-only simulations forced with realistic 20th-century SST.</p>

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Sensitivity of Hybrid ENSO Models to Unresolved Atmospheric Variability

Journal of Climate ◽

10.1175/2007jcli1188.1 ◽

2008 ◽

Vol 21 (15) ◽

pp. 3704-3721 ◽

Cited By ~ 32

Author(s):

J. Zavala-Garay ◽

C. Zhang ◽

A. M. Moore ◽

A. T. Wittenberg ◽

M. J. Harrison ◽

...

Keyword(s):

General Circulation ◽

Temporal Structure ◽

Low Frequency ◽

General Circulation Models ◽

Hybrid Models ◽

Kelvin Waves ◽

Clear Correlation ◽

Frequency Content ◽

Atmospheric Variability ◽

Coupled Models

Abstract A common practice in the design of forecast models for ENSO is to couple ocean general circulation models to simple atmospheric models. Therefore, by construction these models (known as hybrid ENSO models) do not resolve various kinds of atmospheric variability [e.g., the Madden–Julian oscillation (MJO) and westerly wind bursts] that are often regarded as “unwanted noise.” In this work the sensitivity of three hybrid ENSO models to this unresolved atmospheric variability is studied. The hybrid coupled models were tuned to be asymptotically stable and the magnitude, and spatial and temporal structure of the unresolved variability was extracted from observations. The results suggest that this neglected variability can add an important piece of realism and forecast skill to the hybrid models. The models were found to respond linearly to the low-frequency part of the neglected atmospheric variability, in agreement with previous findings with intermediate models. While the wind anomalies associated with the MJO typically explain a small fraction of the unresolved variability, a large fraction of the interannual variability can be excited by this forcing. A large correlation was found between interannual anomalies of Kelvin waves forced by the intraseasonal MJO and the Kelvin waves forced by the low-frequency part of the MJO. That is, in years when the MJO tends to be more active it also produces a larger low-frequency contribution, which can then resonate with the large-scale coupled system. Other kinds of atmospheric variability not related to the MJO can also produce interannual anomalies in the hybrid models. However, when projected on the characteristics of Kelvin waves, no clear correlation between its low-frequency content and its intraseasonal activity was found. This suggests that understanding the mechanisms by which the intraseasonal MJO interacts with the ocean to modulate its low-frequency content may help to better to predict ENSO variability.

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The Roles of Atmospheric Stochastic Forcing (SF) and Oceanic Entrainment Temperature (Te) in Decadal Modulation of ENSO

Journal of Climate ◽

10.1175/2007jcli1665.1 ◽

2008 ◽

Vol 21 (4) ◽

pp. 674-704 ◽

Cited By ~ 27

Author(s):

Rong-Hua Zhang ◽

Antonio J. Busalacchi ◽

David G. DeWitt

Keyword(s):

El Niño ◽

El Nino ◽

Coupled Model ◽

Tropical Pacific ◽

Coupled Systems ◽

Subsurface Water ◽

Coupled Models ◽

Stochastic Forcing ◽

Svd Analysis ◽

The Tropical Pacific

Abstract The El Niño–Southern Oscillation (ENSO) has been observed to exhibit decadal changes in its properties; the cause and implication of such changes are strongly debated. Here the authors examine the influences of two particular attributors of the ocean–atmospheric system. The roles of stochastic forcing (SF) in the atmosphere and decadal changes in the temperature of subsurface water entrained into the mixed layer (Te) in modulating ENSO are compared to one another using coupled ocean–atmosphere models of the tropical Pacific climate system. Two types of coupled models are used. One is an intermediate coupled model (ICM) and another is a hybrid coupled model (HCM), both of which consist of the same intermediate ocean model (IOM) with an empirical parameterization for Te, constructed via singular value decomposition (SVD) analysis of the IOM simulated historical data. The differences in the ICM and HCM are in the atmospheric component: the one in the ICM is an empirical feedback model for wind stress (τ), and that in the HCM is an atmospheric general circulation model (AGCM; ECHAM4.5). The deterministic component of atmospheric τ variability, representing its signal response (τSig) to an external SST forcing, is constructed statistically by an SVD analysis from a 24-member ensemble mean of the ECHAM4.5 AGCM simulations forced by observed SST; the SF component (τSF) is explicitly estimated from the ECHAM4.5 AGCM ensemble and HCM simulations. Different SF representations are specified in the atmosphere: the SF effect can be either absent or present explicitly in the ICM, or implicitly in the HCM where the ECHAM4.5 AGCM is used as a source for SF. Decadal changes in the ocean thermal structure observed in the late 1970s are incorporated into the coupled systems through the Te parameterizations for the two subperiods before (1963–79) and after (1980–96) the climate shift (T63–79e and T80–96e), respectively. The ICM and HCM simulations well reproduce interannual variability associated with El Niño in the tropical Pacific. Model sensitivity experiments are performed using these two types of coupled models with different realizations of SF in the atmosphere and specifications of decadal Te changes in the ocean. It is demonstrated that the properties of ENSO are modulated differently by these two factors. The decadal Te changes in the ocean can be responsible for a systematic shift in the phase propagation of ENSO, while the SF in the atmosphere can contribute to the amplitude and period modulation in a random way. The relevance to the observed decadal ENSO variability in the late 1970s is discussed.

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