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 Are Simulated Megadroughts in the North American Southwest Forced?*

2014 ◽  
Vol 28 (1) ◽  
pp. 124-142 ◽  
Author(s):  
Sloan Coats ◽  
Jason E. Smerdon ◽  
Benjamin I. Cook ◽  
Richard Seager
Keyword(s):  
North American ◽  
General Circulation ◽  
Southern Oscillation ◽  
Tropical Pacific ◽  
American Southwest ◽  
Climate System Model ◽  
Multidecadal Variability ◽  
The North ◽  
North American Southwest ◽  
The Tropical Pacific

Abstract Multidecadal drought periods in the North American Southwest (25°–42.5°N, 125°–105°W), so-called megadroughts, are a prominent feature of the paleoclimate record over the last millennium (LM). Six forced transient simulations of the LM along with corresponding historical (1850–2005) and 500-yr preindustrial control runs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are analyzed to determine if atmosphere–ocean general circulation models (AOGCMs) are able to simulate droughts that are similar in persistence and severity to the megadroughts in the proxy-derived North American Drought Atlas. Megadroughts are found in each of the AOGCM simulations of the LM, although there are intermodel differences in the number, persistence, and severity of these features. Despite these differences, a common feature of the simulated megadroughts is that they are not forced by changes in the exogenous forcing conditions. Furthermore, only the Community Climate System Model (CCSM), version 4, simulation contains megadroughts that are consistently forced by cooler conditions in the tropical Pacific Ocean. These La Niña–like mean states are not accompanied by changes to the interannual variability of the El Niño–Southern Oscillation system and result from internal multidecadal variability of the tropical Pacific mean state, of which the CCSM has the largest magnitude of the analyzed simulations. Critically, the CCSM is also found to have a realistic teleconnection between the tropical Pacific and North America that is stationary on multidecadal time scales. Generally, models with some combination of a realistic and stationary teleconnection and large multidecadal variability in the tropical Pacific are found to have the highest incidence of megadroughts driven by the tropical Pacific boundary conditions.

scholarly journals A Pacific Centennial Oscillation Predicted by Coupled GCMs*

2012 ◽  
Vol 25 (17) ◽  
pp. 5943-5961 ◽  
Author(s):  
Kristopher B. Karnauskas ◽  
Jason E. Smerdon ◽  
Richard Seager ◽  
Jesús Fidel González-Rouco
Keyword(s):  
Time Scale ◽  
General Circulation ◽  
Southern Oscillation ◽  
Heat Content ◽  
General Circulation Models ◽  
Wave Pattern ◽  
Tropical Pacific ◽  
Equatorial Pacific ◽  
Centennial Time Scale ◽  
The Tropical Pacific

Abstract Internal climate variability at the centennial time scale is investigated using long control integrations from three state-of-the-art global coupled general circulation models. In the absence of external forcing, all three models produce centennial variability in the mean zonal sea surface temperature (SST) and sea level pressure (SLP) gradients in the equatorial Pacific with counterparts in the extratropics. The centennial pattern in the tropical Pacific is dissimilar to that of the interannual El Niño–Southern Oscillation (ENSO), in that the most prominent expression in temperature is found beneath the surface of the western Pacific warm pool. Some global repercussions nevertheless are analogous, such as a hemispherically symmetric atmospheric wave pattern of alternating highs and lows. Centennial variability in western equatorial Pacific SST is a result of the strong asymmetry of interannual ocean heat content anomalies, while the eastern equatorial Pacific exhibits a lagged, Bjerknes-like response to temperature and convection in the west. The extratropical counterpart is shown to be a flux-driven response to the hemispherically symmetric circulation anomalies emanating from the tropical Pacific. Significant centennial-length trends in the zonal SST and SLP gradients rivaling those estimated from observations and model simulations forced with increasing CO2 appear to be inherent features of the internal climate dynamics simulated by all three models. Unforced variability and trends on the centennial time scale therefore need to be addressed in estimated uncertainties, beyond more traditional signal-to-noise estimates that do not account for natural variability on the centennial time scale.

Evaluating the performance of CMIP6 models in the tropical Atlantic: mean state, variability, and remote impacts

2020 ◽  
Author(s):  
Ingo Richter ◽  
Hiroki Tokinaga
Keyword(s):  
General Circulation ◽  
General Circulation Models ◽  
Tropical Pacific ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Westerly Wind ◽  
Equatorial Atlantic ◽  
Wide Range ◽  
Mean State ◽  
The Tropical Pacific

<p>General circulation models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are examined with respect to their ability to simulate the mean state and variability of the tropical Atlantic, as well as its linkage to the tropical Pacific. While, on average, mean state biases have improved little relative to the previous intercomparison (CMIP5), there are now a few models with very small biases. In particular the equatorial Atlantic warm SST and westerly wind biases are mostly eliminated in these models. Furthermore, interannual variability in the equatorial and subtropical Atlantic is quite realistic in a number of CMIP6 models, which suggests that they should be useful tools for understanding and predicting variability patterns. The evolution of equatorial Atlantic biases follows the same pattern as in previous model generations, with westerly wind biases during boreal spring preceding warm sea-surface temperature (SST) biases in the east during boreal summer. A substantial portion of the westerly wind bias exists already in atmosphere-only simulations forced with observed SST, suggesting an atmospheric origin. While variability is relatively realistic in many models, SSTs seem less responsive to wind forcing than observed, both on the equator and in the subtropics, possibly due to an excessively deep mixed layer originating in the oceanic component. Thus models with realistic SST amplitude tend to have excessive wind amplitude. The models with the smallest mean state biases all have relatively high resolution but there are also a few low-resolution models that perform similarly well, indicating that resolution is not the only way toward reducing tropical Atlantic biases. The results also show a relatively weak link between mean state biases and the quality of the simulated variability. The linkage to the tropical Pacific shows a wide range of behaviors across models, indicating the need for further model improvement.</p>

Role of the Atmospheric and Oceanic Circulation in the Tropical Pacific SST Changes

2008 ◽  
Vol 21 (10) ◽  
pp. 2019-2034 ◽  
Author(s):  
Jingzhi Su ◽  
Huijun Wang ◽  
Haijun Yang ◽  
Helge Drange ◽  
Yongqi Gao ◽  
...  
Keyword(s):  
Tropical Pacific ◽  
Hadley Cell ◽  
Convective Precipitation ◽  
Delayed Response ◽  
Positive Temperature ◽  
Subsurface Temperature ◽  
Tropical Ocean ◽  
Temperature Anomalies ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract A coupled climate model is used to explore the response of the tropical sea surface temperature (SST) to positive SST anomalies in the global extratropics. The main model results here are consistent with previous numerical studies. In response to prescribed SST anomalies in the extratropics, the tropical SSTs rise rapidly and reach a quasi-equilibrium state within several years, and the tropical subsurface temperatures show a slow response. The annual-mean Hadley cell, as well as the surface trades, are weakened. The weakened trades reduce the poleward Ekman transports in the tropical ocean and, furthermore, lead to anomalous positive convergences of heat transport, which is the main mechanism for maintaining the tropical Pacific SST warming. The process of an extratropical influence on the tropics is related to both the atmospheric and oceanic circulations. The intertropical convergence zone (ITCZ) moves southward and eastward in the Pacific, corresponding to a reduction of the Hadley circulation and Walker circulation. At the same time, convective precipitation anomalies are formed on the boundary of the climatological ITCZ, while the climatological mean convections centered in the Southeast Asia region are suppressed. The largely delayed response of the tropical subsurface temperature cannot be explained only by the strength change of the subtropical cells (STCs), but can be traced back to the slow changing of subsurface temperature in the extratropics. In the extratropical oceans, warming and freshening reduce the surface water density, and the outcropping lines of certain isopycnal layers are moved poleward. This poleward movement of outcropping lines can weaken the positive temperature anomalies, or even lead to negative anomalies, on given isopycnal layers. Displayed on time-dependent isopycnal layers, positive subsurface temperature anomalies are present only in the region after subduction, and are subsequently replaced by negative temperature anomalies in the deep tropics regions. The noticeable features of the density compensation of temperature and salinity indicate that diapycnal processes play an important role in the equatorward transport of the temperature and salinity anomalies from the midlatitude.

scholarly journals A CGCM Study on the Interaction between IOD and ENSO

2006 ◽  
Vol 19 (9) ◽  
pp. 1688-1705 ◽  
Author(s):  
Swadhin K. Behera ◽  
Jing Jia Luo ◽  
Sebastien Masson ◽  
Suryachandra A. Rao ◽  
Hirofumi Sakuma ◽  
...  
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Southern Oscillation ◽  
Walker Circulation ◽  
Tropical Pacific ◽  
Boreal Summer ◽  
Enso Variability ◽  
The Indian Ocean ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract An atmosphere–ocean coupled general circulation model known as the Scale Interaction Experiment Frontier version 1 (SINTEX-F1) model is used to understand the intrinsic variability of the Indian Ocean dipole (IOD). In addition to a globally coupled control experiment, a Pacific decoupled noENSO experiment has been conducted. In the latter, the El Niño–Southern Oscillation (ENSO) variability is suppressed by decoupling the tropical Pacific Ocean from the atmosphere. The ocean–atmosphere conditions related to the IOD are realistically simulated by both experiments including the characteristic east–west dipole in SST anomalies. This demonstrates that the dipole mode in the Indian Ocean is mainly determined by intrinsic processes within the basin. In the EOF analysis of SST anomalies from the noENSO experiment, the IOD takes the dominant seat instead of the basinwide monopole mode. Even the coupled feedback among anomalies of upper-ocean heat content, SST, wind, and Walker circulation over the Indian Ocean is reproduced. As in the observation, IOD peaks in boreal fall for both model experiments. In the absence of ENSO variability the interannual IOD variability is dominantly biennial. The ENSO variability is found to affect the periodicity, strength, and formation processes of the IOD in years of co-occurrences. The amplitudes of SST anomalies in the western pole of co-occurring IODs are aided by dynamical and thermodynamical modifications related to the ENSO-induced wind variability. Anomalous latent heat flux and vertical heat convergence associated with the modified Walker circulation contribute to the alteration of western anomalies. It is found that 42% of IOD events affected by changes in the Walker circulation are related to the tropical Pacific variabilities including ENSO. The formation is delayed until boreal summer for those IODs, which otherwise form in boreal spring as in the noENSO experiment.

scholarly journals Improving SST Anomaly Simulations in a Layer Ocean Model with an Embedded Entrainment Temperature Submodel

2006 ◽  
Vol 19 (18) ◽  
pp. 4638-4663 ◽  
Author(s):  
Rong-Hua Zhang ◽  
Antonio J. Busalacchi ◽  
Raghuram G. Murtugudde
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Tropical Pacific ◽  
Subsurface Water ◽  
Subsurface Temperature ◽  
Sst Anomaly ◽  
The Impact ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract In this study, an improved sea surface temperature (SST) anomaly (SSTA) solution for the tropical Pacific is presented by explicitly embedding into a layer ocean general circulation model (OGCM) a separate SSTA submodel with an empirical parameterization for the temperature of subsurface water entrained into the ocean mixed layer (Te). Instead of using subsurface temperature directly from the OGCM, Te anomalies for the embedded SSTA submodel are calculated from a historical data-based empirical procedure in terms of sea level (SL) anomalies simulated from the OGCM. An inverse modeling approach is first adopted to estimate Te anomalies from the SSTA equation using observed SST and simulated upper-ocean currents from the OGCM. A relationship between Te and SL anomalies is then obtained by utilizing an empirical orthogonal function (EOF) analysis technique. The empirical Te parameterization optimally leads to a better balanced depiction of the subsurface effect on SST variability by the mean upwelling of anomalous subsurface temperature and vertical mixing in the equatorial Pacific. As compared with a standard OGCM simulation, SSTA simulations from the embedded submodel exhibit more realistic variability, with significantly increased correlation and reduced SSTA errors due to the optimized empirical Te parameterization. In the Niño-3 region (5°S–5°N, 150°–90°W), the anomaly correlation and root-mean-square (RMS) error of the simulated SST anomalies for the period 1963–96 from the standard OGCM are 0.74° and 0.58°C, while from the embedded SSTA submodel they are 0.94° and 0.29°C in the Te-dependent experiment, and 0.86° and 0.41°C in the experiment with one-dependent-year data excluded, respectively. Cross validation and sensitivity experiments to training periods for building the Te parameterization are made to illustrate the robustness and effectiveness of the approach. Moreover, the impact on simulations of SST anomalies and El Niño are examined in hybrid coupled atmosphere–ocean models (HCMs) consisting of the OGCM and a statistical atmospheric wind stress anomaly model that is constructed from a singular value decomposition (SVD) analysis. Results from coupled runs with and without embedding the SSTA submodel are compared. It is demonstrated that incorporating the embedded SSTA submodel in the context of an OGCM has a significant impact on performance of the HCMs and the behavior of the coupled system, with more realistic simulations of interannual SST anomalies (e.g., the amplitude and structure) in the tropical Pacific.

scholarly journals Megadroughts in Southwestern North America in ECHO-G Millennial Simulations and Their Comparison to Proxy Drought Reconstructions*

2013 ◽  
Vol 26 (19) ◽  
pp. 7635-7649 ◽  
Author(s):  
Sloan Coats ◽  
Jason E. Smerdon ◽  
Richard Seager ◽  
Benjamin I. Cook ◽  
J. F. González-Rouco
Keyword(s):  
North American ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Tropical Pacific ◽  
Drought Severity ◽  
The North ◽  
Hydroclimate Variability ◽  
And Control ◽  
The Tropical Pacific

Abstract Simulated hydroclimate variability in millennium-length forced transient and control simulations from the ECHAM and the global Hamburg Ocean Primitive Equation (ECHO-G) coupled atmosphere–ocean general circulation model (AOGCM) is analyzed and compared to 1000 years of reconstructed Palmer drought severity index (PDSI) variability from the North American Drought Atlas (NADA). The ability of the model to simulate megadroughts in the North American southwest is evaluated. (NASW: 25°–42.5°N, 125°–105°W). Megadroughts in the ECHO-G AOGCM are found to be similar in duration and magnitude to those estimated from the NADA. The droughts in the forced simulation are not, however, temporally synchronous with those in the paleoclimate record, nor are there significant differences between the drought features simulated in the forced and control runs. These results indicate that model-simulated megadroughts can result from internal variability of the modeled climate system rather than as a response to changes in exogenous forcings. Although the ECHO-G AOGCM is capable of simulating megadroughts through persistent La Niña–like conditions in the tropical Pacific, other mechanisms can produce similarly extreme NASW moisture anomalies in the model. In particular, the lack of low-frequency coherence between NASW soil moisture and simulated modes of climate variability like the El Niño–Southern Oscillation, Pacific decadal oscillation, and Atlantic multidecadal oscillation during identified drought periods suggests that stochastic atmospheric variability can contribute significantly to the occurrence of simulated megadroughts in the NASW. These findings indicate that either an expanded paradigm is needed to understand multidecadal hydroclimate variability in the NASW or AOGCMs may incorrectly simulate the strength and/or dynamics of the connection between NASW hydroclimate variability and the tropical Pacific.

scholarly journals The Predictability of Tropical Pacific Decadal Variability: Insights from Attractor Reconstruction

2019 ◽  
Vol 76 (3) ◽  
pp. 801-819 ◽  
Author(s):  
Nandini Ramesh ◽  
Mark A. Cane
Keyword(s):  
General Circulation ◽  
Decadal Variability ◽  
General Circulation Models ◽  
Tropical Pacific ◽  
The Other ◽  
Intermediate Complexity ◽  
Pacific Decadal Variability ◽  
Attractor Reconstruction ◽  
Climatic Impacts ◽  
The Tropical Pacific

Abstract Tropical Pacific decadal variability (TPDV), though not the totality of Pacific decadal variability, has wide-ranging climatic impacts. It is currently unclear whether this phenomenon is predictable. In this study, we reconstruct the attractor of the tropical Pacific system in long, unforced simulations from an intermediate-complexity model, two general circulation models (GCMs), and the observations with the aim of assessing the predictability of TPDV in these systems. We find that in the intermediate-complexity model, positive (high variance, El Niño–like) and negative (low variance, La Niña–like) phases of TPDV emerge as a pair of regime-like states. The observed system bears resemblance to this behavior, as does one GCM, while the other GCM does not display this structure. However, these last three time series are too short to confidently characterize the full distribution of interdecadal variability. The intermediate-complexity model is shown to lie in highly predictable parts of its attractor 37% of the time, during which most transitions between TPDV regimes occur. The similarities between the observations and this system suggest that the tropical Pacific may be somewhat predictable on interdecadal time scales.

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 The Tropical Pacific ENSO–Mean State Relationship in Climate Models over the Last Millennium

2020 ◽  
Vol 33 (17) ◽  
pp. 7539-7551
Author(s):  
D. Allie Wyman ◽  
Jessica. L. Conroy ◽  
Christina Karamperidou
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Global Climate ◽  
General Circulation Models ◽  
Tropical Pacific ◽  
Last Millennium ◽  
Climate Simulations ◽  
Intercomparison Project ◽  
The Relationship ◽  
The Tropical Pacific

AbstractENSO and the mean zonal sea surface temperature gradient (dSST) of the tropical Pacific are important drivers of global climate and vary on decadal to centennial time scales. However, the relationship between dSST and ENSO cannot be assessed with the short instrumental record, and is uncertain in proxy data, with intervals of both stronger and weaker ENSO postulated to occur with overall strong dSST in the past. Here we assess the ENSO–dSST relationship during the last millennium using general circulation models (GCMs) participating in phase 3 of the Paleoclimate Modeling Intercomparison Project. Last millennium GCM simulations show diversity in the strength and direction of the ENSO–dSST relationship. Yet, the models that best simulate modern tropical Pacific climate frequently have a more negative ENSO–dSST correlation. Thus, last millennium tropical Pacific climate simulations support the likelihood of enhanced ENSO during decadal to centennial periods of reduced tropical Pacific dSST. However, the alternating directional ENSO–dSST relationship in all model simulations suggests that this relationship is not constant through time and is likely controlled by multiple mechanisms.

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