scholarly journals Decadal Modulation of ENSO in a Hybrid Coupled Model

2008 ◽  
Vol 21 (21) ◽  
pp. 5482-5500 ◽  
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.

scholarly journals Impact of Resolution on the Tropical Pacific Circulation in a Matrix of Coupled Models

2009 ◽  
Vol 22 (10) ◽  
pp. 2541-2556 ◽  
Author(s):  
Malcolm J. Roberts ◽  
A. Clayton ◽  
M.-E. Demory ◽  
J. Donners ◽  
P. L. Vidale ◽  
...  
Keyword(s):  
Coupled Model ◽  
Walker Circulation ◽  
Ocean Model ◽  
Tropical Pacific ◽  
Coupled Models ◽  
Model Stability ◽  
The Mean ◽  
The Impact ◽  
Mean State ◽  
The Tropical Pacific

Abstract Results are presented from a matrix of coupled model integrations, using atmosphere resolutions of 135 and 90 km, and ocean resolutions of 1° and 1/3°, to study the impact of resolution on simulated climate. The mean state of the tropical Pacific is found to be improved in the models with a higher ocean resolution. Such an improved mean state arises from the development of tropical instability waves, which are poorly resolved at low resolution; these waves reduce the equatorial cold tongue bias. The improved ocean state also allows for a better simulation of the atmospheric Walker circulation. Several sensitivity studies have been performed to further understand the processes involved in the different component models. Significantly decreasing the horizontal momentum dissipation in the coupled model with the lower-resolution ocean has benefits for the mean tropical Pacific climate, but decreases model stability. Increasing the momentum dissipation in the coupled model with the higher-resolution ocean degrades the simulation toward that of the lower-resolution ocean. These results suggest that enhanced ocean model resolution can have important benefits for the climatology of both the atmosphere and ocean components of the coupled model, and that some of these benefits may be achievable at lower ocean resolution, if the model formulation allows.

scholarly journals Regional impacts of ocean color on tropical Pacific variability

2009 ◽  
Vol 6 (1) ◽  
pp. 243-275 ◽  
Author(s):  
W. Anderson ◽  
A. Gnanadesikan ◽  
A. Wittenberg
Keyword(s):  
El Niño ◽  
El Nino ◽  
Pure Water ◽  
Ocean Color ◽  
Tropical Pacific ◽  
Shortwave Radiation ◽  
Penetration Length ◽  
The Mean ◽  
Mean State ◽  
The Tropical Pacific

Abstract. The role of the penetration length scale of shortwave radiation into the surface ocean and its impact on tropical Pacific variability is investigated with a fully coupled ocean, atmosphere, land and ice model. Previous work has shown that removal of all ocean color results in a system that tends strongly towards an El Niño state. Results from a suite of surface chlorophyll perturbation experiments show that the mean state and variability of the tropical Pacific is highly sensitive to the concentration and distribution of ocean chlorophyll. Setting the near-oligotrophic regions to contain optically pure water warms the mean state and suppresses variability in the western tropical Pacific. Doing the same above the shadow zones of the tropical Pacific also warms the mean state but enhances the variability. It is shown that increasing penetration can both deepen the pycnocline (which tends to damp El Niño) while shifting the mean circulation so that the wind response to temperature changes is altered. Depending on what region is involved this change in the wind stress can either strengthen or weaken ENSO variability.

scholarly journals Regional impacts of ocean color on tropical Pacific variability

Ocean Science ◽  
2009 ◽  
Vol 5 (3) ◽  
pp. 313-327 ◽  
Author(s):  
W. Anderson ◽  
A. Gnanadesikan ◽  
A. Wittenberg
Keyword(s):  
El Niño ◽  
El Nino ◽  
Pure Water ◽  
Ocean Color ◽  
Tropical Pacific ◽  
Shortwave Radiation ◽  
Penetration Length ◽  
The Mean ◽  
Mean State ◽  
The Tropical Pacific

Abstract. The role of the penetration length scale of shortwave radiation into the surface ocean and its impact on tropical Pacific variability is investigated with a fully coupled ocean, atmosphere, land and ice model. Previous work has shown that removal of all ocean color results in a system that tends strongly towards an El Niño state. Results from a suite of surface chlorophyll perturbation experiments show that the mean state and variability of the tropical Pacific is highly sensitive to the concentration and distribution of ocean chlorophyll. Setting the near-oligotrophic regions to contain optically pure water warms the mean state and suppresses variability in the western tropical Pacific. Doing the same above the shadow zones of the tropical Pacific also warms the mean state but enhances the variability. It is shown that increasing penetration can both deepen the pycnocline (which tends to damp El Niño) while shifting the mean circulation so that the wind response to temperature changes is altered. Depending on what region is involved this change in the wind stress can either strengthen or weaken ENSO variability.

scholarly journals Interactive Feedback between the Tropical Pacific Decadal Oscillation and ENSO in a Coupled General Circulation Model

2009 ◽  
Vol 22 (24) ◽  
pp. 6597-6611 ◽  
Author(s):  
Jung Choi ◽  
Soon-Il An ◽  
Boris Dewitte ◽  
William W. Hsieh
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Tropical Pacific ◽  
Thermocline Depth ◽  
Coupled General Circulation Model ◽  
The Mean ◽  
The Stability ◽  
Interactive Feedback ◽  
Mean State ◽  
The Tropical Pacific

Abstract The output from a coupled general circulation model (CGCM) is used to develop evidence showing that the tropical Pacific decadal oscillation can be driven by an interaction between the El Niño–Southern Oscillation (ENSO) and the slowly varying mean background climate state. The analysis verifies that the decadal changes in the mean states are attributed largely to decadal changes in ENSO statistics through nonlinear rectification. This is seen because the time evolutions of the first principal component analysis (PCA) mode of the decadal-varying tropical Pacific SST and the thermocline depth anomalies are significantly correlated to the decadal variations of the ENSO amplitude (also skewness). Its spatial pattern resembles the residuals of the SST and thermocline depth anomalies after there is uneven compensation from El Niño and La Niña events. In addition, the stability analysis of a linearized intermediate ocean–atmosphere coupled system, for which the background mean states are specified, provides qualitatively consistent results compared to the CGCM in terms of the relationship between changes in the background mean states and the characteristics of ENSO. It is also shown from the stability analysis as well as the time integration of a nonlinear version of the intermediate coupled model that the mean SST for the high-variability ENSO decades acts to intensify the ENSO variability, while the mean thermocline depth for the same decades acts to suppress the ENSO activity. Thus, there may be an interactive feedback consisting of a positive feedback between the ENSO activity and the mean state of the SST and a negative feedback between the ENSO activity and the mean state of the thermocline depth. This feedback may lead to the tropical decadal oscillation, without the need to invoke any external mechanisms.

scholarly journals Coupled Model Biases Breed Spurious Low‐Frequency Variability in the Tropical Pacific Ocean

2018 ◽  
Vol 45 (19) ◽  
Author(s):  
Dhrubajyoti Samanta ◽  
Kristopher B. Karnauskas ◽  
Nathalie F. Goodkin ◽  
Sloan Coats ◽  
Jason E. Smerdon ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
Low Frequency ◽  
Coupled Model ◽  
Tropical Pacific ◽  
Tropical Pacific Ocean ◽  
Model Biases ◽  
Low Frequency Variability ◽  
The Tropical Pacific

scholarly journals Rectification of El Niño–Southern Oscillation into Climate Anomalies of Decadal and Longer Time Scales: Results from Forced Ocean GCM Experiments

2014 ◽  
Vol 27 (7) ◽  
pp. 2545-2561 ◽  
Author(s):  
De-Zheng Sun ◽  
Tao Zhang ◽  
Yan Sun ◽  
Yongqiang Yu
Keyword(s):  
Climate Change ◽  
Time Scales ◽  
Wind Stress ◽  
Surface Wind ◽  
Tropical Pacific ◽  
Surface Wind Stress ◽  
Interannual Fluctuations ◽  
Mean State ◽  
The Tropical Pacific

Abstract To better understand the causes of climate change in the tropical Pacific on the decadal and longer time scales, the rectification effect of ENSO events is delineated by contrasting the time-mean state of two forced ocean GCM experiments. In one of them, the long-term mean surface wind stress of 1950–2011 is applied, while in the other, the surface wind stress used is the long-term mean surface wind stress of 1950–2011 plus the interannual monthly anomalies over the period. Thus, the long-term means of the surface wind stress in the two runs are identical. The two experiments also use the same relaxation boundary conditions, that is, the SST is restored to the same prescribed values. The two runs, however, are found to yield significantly different mean climate for the tropical Pacific. The mean state of the run with interannual fluctuations in the surface winds is found to have a cooler warm pool, warmer thermocline water, and warmer eastern surface Pacific than the run without interannual fluctuations in the surface winds. The warming of the eastern Pacific has a pattern that resembles the observed decadal warming. In particular, the pattern features an off-equator maximum as the observed decadal warming. The spatial pattern of the time-mean upper-ocean temperature differences between the two experiments is shown to resemble that of the differences in the nonlinear dynamic heating, underscoring the role of the nonlinear ocean dynamics in the rectification. The study strengthens the suggestion that rectification of ENSO can be a viable mechanism for climate change of decadal and longer time scales.

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

2008 ◽  
Vol 21 (4) ◽  
pp. 674-704 ◽  
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.

scholarly journals Interdecadal Change in the Tropical Pacific Precipitation Anomaly Pattern around the Late 1990s during Boreal Spring

2016 ◽  
Vol 29 (16) ◽  
pp. 5979-5997 ◽  
Author(s):  
Yuanyuan Guo ◽  
Zhiping Wen ◽  
Renguang Wu
Keyword(s):  
Precipitation Variability ◽  
Precipitation Anomaly ◽  
Tropical Pacific ◽  
Interdecadal Change ◽  
Spring Precipitation ◽  
Anomaly Pattern ◽  
Leading Mode ◽  
The Mean ◽  
Mean State ◽  
The Tropical Pacific

Abstract The leading mode of boreal spring precipitation variability over the tropical Pacific experienced a pronounced interdecadal change around the late 1990s. The pattern before 1998 features positive precipitation anomalies over the equatorial eastern Pacific (EP) with positive principle component years. The counterpart after 1998 exhibits a westward shift of the positive center to the equatorial central Pacific (CP). Observational evidence shows that this interdecadal change in the leading mode of precipitation variability is closely associated with a distinctive sea surface temperature (SST) anomaly pattern. The westward shift of the anomalous precipitation center after 1998 is in tandem with a similar shift of maximum warming from the EP to CP. Diagnostic analyses based on a linear equation of the mixed layer temperature anomaly exhibit that an interdecadal enhancement of zonal advection (ZA) feedback process plays a vital role in the shift in the leading mode of both the tropical Pacific SST and the precipitation anomaly during spring. Moreover, the variability of the anomalous zonal current at the upper ocean dominates the ZA feedback change, while the mean zonal SST gradient associated with a La Niña–like pattern of the mean state only accounts for a relatively trivial proportion of the ZA feedback change. It was found that both the relatively rapid decaying of the SST anomalies in the EP and the La Niña–like mean state make it conceivable that the shift of the leading mode of the tropical precipitation anomaly only occurs in spring. In addition, the largest variance of the anomalous zonal current in spring might contribute to the unique interdecadal change in the tropical spring precipitation anomaly pattern.

Successive Modulation of ENSO to the Future Greenhouse Warming

2008 ◽  
Vol 21 (1) ◽  
pp. 3-21 ◽  
Author(s):  
Soon-Il An ◽  
Jong-Seong Kug ◽  
Yoo-Geun Ham ◽  
In-Sik Kang
Keyword(s):  
General Circulation ◽  
Southern Oscillation ◽  
General Circulation Models ◽  
Tropical Pacific ◽  
Greenhouse Warming ◽  
Delayed Response ◽  
Subsurface Temperature ◽  
Climate System Model ◽  
The Mean ◽  
The Tropical Pacific

Abstract The multidecadal modulation of the El Niño–Southern Oscillation (ENSO) due to greenhouse warming has been analyzed herein by means of diagnostics of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled general circulation models (CGCMs) and the eigenanalysis of a simplified version of an intermediate ENSO model. The response of the global-mean troposphere temperature to increasing greenhouse gases is more likely linear, while the amplitude and period of ENSO fluctuates in a multidecadal time scale. The climate system model outputs suggest that the multidecadal modulation of ENSO is related to the delayed response of the subsurface temperature in the tropical Pacific compared to the response time of the sea surface temperature (SST), which would lead a modulation of the vertical temperature gradient. Furthermore, an eigenanalysis considering only two parameters, the changes in the zonal contrast of the mean background SST and the changes in the vertical contrast between the mean surface and subsurface temperatures in the tropical Pacific, exhibits a good agreement with the CGCM outputs in terms of the multidecadal modulations of the ENSO amplitude and period. In particular, the change in the vertical contrast, that is, change in difference between the subsurface temperature and SST, turns out to be more influential on the ENSO modulation than changes in the mean SST itself.

scholarly journals The Linear Response of ENSO to the Madden–Julian Oscillation

2005 ◽  
Vol 18 (13) ◽  
pp. 2441-2459 ◽  
Author(s):  
J. Zavala-Garay ◽  
C. Zhang ◽  
A. M. Moore ◽  
R. Kleeman
Keyword(s):  
Large Fraction ◽  
Surface Wind ◽  
Low Frequency ◽  
Coupled Model ◽  
Cumulative Effect ◽  
Enso Event ◽  
Kelvin Waves ◽  
Madden Julian Oscillation ◽  
Coupled Models ◽  
Stochastic Forcing

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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