scholarly journals Influence of Recent Stratification Changes on ENSO Stability in a Conceptual Model of the Equatorial Pacific

2013 ◽  
Vol 26 (13) ◽  
pp. 4790-4802 ◽  
Author(s):  
Sulian Thual ◽  
Boris Dewitte ◽  
Soon-Il An ◽  
Serena Illig ◽  
Nadia Ayoub
Keyword(s):  
Linear Stability ◽  
General Circulation ◽  
Southern Oscillation ◽  
General Circulation Models ◽  
Equatorial Pacific ◽  
Thermocline Depth ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Equatorial Pacific Ocean ◽  
Decadal Modulation ◽  
The Mean

Abstract Changes in the mean circulation of the equatorial Pacific Ocean partly control the strong decadal modulation of El Niño–Southern Oscillation (ENSO). This relationship is considered from the linear stability of a conceptual recharge/discharge model with parameters tuned from the observed mean state. Whereas decadal changes in the mean thermocline depth alone are usually considered in conceptual ENSO models, here focus is given to decadal changes in the mean stratification of the entire upper ocean (e.g., the mean thermocline depth, intensity, and thickness). Those stratification changes modify the projection of wind stress forcing momentum onto the gravest ocean baroclinic modes. Their influence on the simulated frequency and growth rate is comparable in intensity to the one of usual thermodynamic and atmospheric feedbacks, while they have here a secondary effect on the spatial structure and propagation of SST anomalies. This sensitivity is evidenced in particular for the climate shift of the 1970s in the Simple Ocean Data Assimilation (SODA) dataset, as well as in a preindustrial simulation of the Geophysical Fluid Dynamics Laboratory (GFDL) model showing stratification changes similar to the ones after 2000. Despite limitations of the linear stability approach, conclusions on the sensitivity to stratification may be extended to interpret the modulation and diversity of ENSO in observations and in general circulation models.

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.

The Equatorial Pacific Cold Tongue Bias in a Coupled Climate Model

2008 ◽  
Vol 21 (22) ◽  
pp. 5852-5869 ◽  
Author(s):  
Vasubandhu Misra ◽  
L. Marx ◽  
M. Brunke ◽  
X. Zeng
Keyword(s):  
General Circulation ◽  
Low Frequency ◽  
Coupled Model ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
Cold Tongue ◽  
Equatorial Pacific Ocean ◽  
Time Step ◽  
Coupling Interval ◽  
The Mean

Abstract A set of multidecadal coupled ocean–atmosphere model integrations are conducted with different time steps for coupling between the atmosphere and the ocean. It is shown that the mean state of the equatorial Pacific does not change in a statistically significant manner when the coupling interval between the atmospheric general circulation model (AGCM) and the ocean general circulation model (OGCM) is changed from 1 day to 2 or even 3 days. It is argued that because the coarse resolution of the AGCM precludes resolving realistic “weather” events, changing the coupling interval from 1 day to 2 or 3 days has very little impact on the mean coupled climate. On the other hand, reducing the coupling interval to 3 h had a much stronger impact on the mean state of the equatorial Pacific and the concomitant general circulation. A novel experiment that incorporates a (pseudo) interaction of the atmosphere with SST at every time step of the AGCM was also conducted. In this unique coupled model experiment, the AGCM at every time step mutually interacts with the skin SST. This skin SST is anchored to the bulk SST, which is updated from the OGCM once a day. Both of these experiments reduced the cold tongue bias moderately over the equatorial Pacific Ocean with a corresponding reduction in the easterly wind stress bias relative to the control integration. It is stressed from the results of these model experiments that the impact of high-frequency air–sea coupling is significant on the cold tongue bias. The interannual variation of the equatorial Pacific was less sensitive to the coupling time step between the AGCM and the OGCM. Increasing (reducing) the coupling interval of the air–sea interaction had the effect of weakening (marginally strengthening) the interannual variations of the equatorial Pacific Ocean. It is argued that the low-frequency response of the upper ocean, including the cold tongue bias, is modulated by the atmospheric stochastic forcing on the coupled ocean–atmosphere system. This effect of the atmospheric stochastic forcing is affected by the frequency of the air–sea coupling and is found to be stronger than the rectification effect of the diurnal variations of the air–sea interaction on the low frequency. This may be a result of a limitation in the coupled model used in this study in which the OGCM has an inadequate vertical resolution in the mixed layer to sustain diurnal variations in the upper ocean.

Tropical Indian Ocean Variability in the IPCC Twentieth-Century Climate Simulations*

2006 ◽  
Vol 19 (17) ◽  
pp. 4397-4417 ◽  
Author(s):  
N. H. Saji ◽  
S-P. Xie ◽  
T. Yamagata
Keyword(s):  
Twentieth Century ◽  
Indian Ocean ◽  
General Circulation ◽  
Rossby Waves ◽  
Southern Oscillation ◽  
Surface Wind ◽  
General Circulation Models ◽  
Thermocline Depth ◽  
Bjerknes Feedback ◽  
Intergovernmental Panel

Abstract The twentieth-century simulations using by 17 coupled ocean–atmosphere general circulation models (CGCMs) submitted to the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) are evaluated for their skill in reproducing the observed modes of Indian Ocean (IO) climate variability. Most models successfully capture the IO’s delayed, basinwide warming response a few months after El Niño–Southern Oscillation (ENSO) peaks in the Pacific. ENSO’s oceanic teleconnection into the IO, by coastal waves through the Indonesian archipelago, is poorly simulated in these models, with significant shifts in the turning latitude of radiating Rossby waves. In observations, ENSO forces, by the atmospheric bridge mechanism, strong ocean Rossby waves that induce anomalies of SST, atmospheric convection, and tropical cyclones in a thermocline dome over the southwestern tropical IO. While the southwestern IO thermocline dome is simulated in nearly all of the models, this ocean Rossby wave response to ENSO is present only in a few of the models examined, suggesting difficulties in simulating ENSO’s teleconnection in surface wind. A majority of the models display an equatorial zonal mode of the Bjerknes feedback with spatial structures and seasonality similar to the Indian Ocean dipole (IOD) in observations. This success appears to be due to their skills in simulating the mean state of the equatorial IO. Corroborating the role of the Bjerknes feedback in the IOD, the thermocline depth, SST, precipitation, and zonal wind are mutually positively correlated in these models, as in observations. The IOD–ENSO correlation during boreal fall ranges from −0.43 to 0.74 in the different models, suggesting that ENSO is one, but not the only, trigger for the IOD.

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.

scholarly journals Understanding ENSO Regime Behavior upon an Increase in the Warm-Pool Temperature Using a Simple ENSO Model

2011 ◽  
Vol 24 (5) ◽  
pp. 1438-1450 ◽  
Author(s):  
Baek-Min Kim ◽  
Soon-Il An
Keyword(s):  
Bifurcation Analysis ◽  
Southern Oscillation ◽  
Period Doubling ◽  
Warm Pool ◽  
Period Doubling Bifurcation ◽  
Enso Cycle ◽  
Thermocline Depth ◽  
Decadal Modulation ◽  
The Mean ◽  
Enso Model

Abstract The regime behavior of the low-order El Niño–Southern Oscillation (ENSO) model, according to an increase in the radiative–convective equilibrium sea surface temperature (SST; Tr), is studied to provide a possible explanation for the observed increase in ENSO irregularity characterized by decadal modulation. During recent decades, a clear increasing trend of the warm-pool SST has been observed. In this study, the increase in the warm-pool maximum SST is interpreted as an increase in Tr following previous studies. A bifurcation analysis with Tr as a control parameter is conducted to reveal that the degree of ENSO irregularity in the model is effectively controlled by the equilibrium states of the model. At a critical value of Tr, bifurcation analysis reveals that period-doubling bifurcation occurs and an amplitude-modulated ENSO emerges. At this point, a subcycle appears within the preexisting ENSO cycle, which initiates decadal modulation of ENSO. As Tr increases further, nested oscillations are successively generated, illustrating clear decadal modulation of ENSO. The qualitative regime changes revealed in this study are supported by the observation of regime shifts in the 1970s. With increasing Tr, the mean zonal SST gradient increases, and the model adjusts toward a “La Niña–like” mean state. Further constraint with shoaling of the mean thermocline depth and increasing stratification causes ENSO to exhibit stronger amplitude modulation. Furthermore, the timing of the period-doubling bifurcation advances with these two effects.

scholarly journals 2015 Bernhard Haurwitz Memorial Lecture: Model Diagnosis of El Niño Teleconnections to the Global Atmosphere–Ocean System

2016 ◽  
Vol 97 (6) ◽  
pp. 981-988 ◽  
Author(s):  
Ngar-Cheung Lau
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
World Ocean ◽  
General Circulation Models ◽  
Memorial Lecture ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Enso Events ◽  
The World

Abstract The El Niño–Southern Oscillation (ENSO) phenomenon is one of the most prominent modes of atmospheric and oceanic variability on interannual and interdecadal time scales. The essential ENSO signals originate from the tropical Pacific (TP). However, the impacts of ENSO are felt in many other parts of the world. Major ENSO events are accompanied by notable changes in the extratropical atmospheric circulation in both hemispheres, various monsoon systems located beyond the TP, global air temperature and precipitation patterns, and the sea surface temperature (SST) distribution throughout the World Ocean. During the past several decades, this author has participated in a sustained effort to study the processes contributing to these “teleconnections” between ENSO forcing in the TP and variations in the atmosphere–ocean system elsewhere. These investigations are based on a large suite of experiments with several generations of general circulation models (GCMs) at the Geophysical Fluid Dynamics Laboratory (GFDL). These experiments are specifically designed to reveal various facets of the teleconnections with ENSO. In this lecture, a summary is given of the principal findings of this series of model studies. An account is given of the train of thought underpinning the sequence of experiments described herein, so as to illustrate how certain experimental setups have been motivated by the problem or hypothesis at hand.

scholarly journals Sensitivity of the tropical climate to an interhemispheric thermal gradient: the role of tropical ocean dynamics

2018 ◽  
Vol 9 (1) ◽  
pp. 285-297 ◽  
Author(s):  
Stefanie Talento ◽  
Marcelo Barreiro
Keyword(s):  
Seasonal Cycle ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Thermocline Depth ◽  
Thermal Forcing ◽  
Tropical Ocean ◽  
Slab Ocean

Abstract. This study aims to determine the role of the tropical ocean dynamics in the response of the climate to extratropical thermal forcing. We analyse and compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first configuration the AGCM is coupled with a slab ocean model while in the second a reduced gravity ocean (RGO) model is additionally coupled in the tropical region. We find that the imposition of extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere with zero global mean) produces, in terms of annual means, a weaker response when the RGO is coupled, thus indicating that the tropical ocean dynamics oppose the incoming remote signal. On the other hand, while the slab ocean coupling does not produce significant changes to the equatorial Pacific sea surface temperature (SST) seasonal cycle, the RGO configuration generates strong warming in the central-eastern basin from April to August balanced by cooling during the rest of the year, strengthening the seasonal cycle in the eastern portion of the basin. We hypothesize that such changes are possible via the dynamical effect that zonal wind stress has on the thermocline depth. We also find that the imposed extratropical pattern affects El Niño–Southern Oscillation, weakening its amplitude and low-frequency behaviour.

scholarly journals Changes of Variability in Response to Increasing Greenhouse Gases. Part II: Hydrology

2009 ◽  
Vol 22 (22) ◽  
pp. 6089-6103 ◽  
Author(s):  
Richard T. Wetherald
Keyword(s):  
Soil Moisture ◽  
Greenhouse Gases ◽  
General Circulation ◽  
Circulation Model ◽  
Early Spring ◽  
Precipitation Rate ◽  
Late Winter ◽  
Annual Data ◽  
Geophysical Fluid Dynamics Laboratory ◽  
The Mean

Abstract This paper examines hydrological variability and its changes in two different versions of a coupled ocean–atmosphere general circulation model developed at the National Oceanic and Atmospheric Administration/Geophysical Fluid Dynamics Laboratory and forced with estimates of future increases of greenhouse gas and aerosol concentrations. This paper is the second part, documenting potential changes in variability as greenhouse gases increase. The variance changes are examined using an ensemble of 8 transient integrations for an older model version and 10 transient integrations for a newer model. Monthly and annual data are used to compute the mean and variance changes. Emphasis is placed on computing and analyzing the variance changes for the middle of the twenty-first century and compared with those found in the respective control integrations. The hydrologic cycle intensifies because of the increase of greenhouse gases. In general, precipitation variance increases in most places. This is the case virtually everywhere the mean precipitation rate increases and many places where the precipitation decreases. The precipitation rate variance decreases in the subtropics, where the mean precipitation rate also decreases. The increased precipitation rate and variance, in middle to higher latitudes during late fall, winter, and early spring leads to increased runoff and its variance during that period. On the other hand, the variance changes of soil moisture are more complicated, because soil moisture has both a lower and upper bound that tends to reduce its fluctuations. This is particularly true in middle to higher latitudes during winter and spring, when the soil moisture is close to its saturation value at many locations. Therefore, changes in its variance are limited. Soil moisture variance change is positive during the summer, when the mean soil moisture decreases and is close to the middle of its allowable range. In middle to high northern latitudes, an increase in runoff and its variance during late winter and spring plus the decrease in soil moisture and its variance during summer lend support to the hypothesis stated in other publications that a warmer climate can cause an increasing frequency of both excessive discharge and drier events, depending on season and latitude.

scholarly journals La Niña–like Mean-State Response to Global Warming and Potential Oceanic Roles

2017 ◽  
Vol 30 (11) ◽  
pp. 4207-4225 ◽  
Author(s):  
Tsubasa Kohyama ◽  
Dennis L. Hartmann ◽  
David S. Battisti
Keyword(s):  
Global Warming ◽  
Southern Oscillation ◽  
Forced Response ◽  
Equatorial Pacific ◽  
La Niña ◽  
The West ◽  
La Nina ◽  
The Mean ◽  
Mean State ◽  
Sst Gradient

Abstract The majority of the models that participated in phase 5 of the Coupled Model Intercomparison Project global warming experiments warm faster in the eastern equatorial Pacific Ocean than in the west. GFDL-ESM2M is an exception among the state-of-the-art global climate models in that the equatorial Pacific sea surface temperature (SST) in the west warms faster than in the east, and the Walker circulation strengthens in response to warming. This study shows that this “La Niña–like” trend simulated by GFDL-ESM2M could be a physically consistent response to warming, and that the forced response could have been detectable since the late twentieth century. Two additional models are examined: GFDL-ESM2G, which differs from GFDL-ESM2M only in the oceanic components, warms without a clear zonal SST gradient; and HadGEM2-CC exhibits a warming pattern that resembles the multimodel mean. A fundamental observed constraint between the amplitude of El Niño–Southern Oscillation (ENSO) and the mean-state zonal SST gradient is reproduced well by GFDL-ESM2M but not by the other two models, which display substantially weaker ENSO nonlinearity than is observed. Under this constraint, the weakening nonlinear ENSO amplitude in GFDL-ESM2M rectifies the mean state to be La Niña–like. GFDL-ESM2M exhibits more realistic equatorial thermal stratification than GFDL-ESM2G, which appears to be the most important difference for the ENSO nonlinearity. On longer time scales, the weaker polar amplification in GFDL-ESM2M may also explain the origin of the colder equatorial upwelling water, which could in turn weaken the ENSO amplitude.

scholarly journals Sensitivity of the tropical climate to an interhemispheric thermal gradient: the role of tropical ocean dynamics

2017 ◽  
Author(s):  
Stefanie Talento ◽  
Marcelo Barreiro
Keyword(s):  
Seasonal Cycle ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Thermocline Depth ◽  
Thermal Forcing ◽  
Tropical Ocean ◽  
Slab Ocean

Abstract. This study aims to determine the role of the tropical ocean dynamics in the response of the climate to an extratropical thermal forcing. We analyse and compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first configuration the AGCM is coupled with a slab ocean model while in the second a Reduced Gravity Ocean (RGO) model is additionally coupled in the tropical region. We find that the imposition of an extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere with zero global mean) produces, in terms of annual means, a weaker response when the RGO is coupled, thus indicating that the tropical ocean dynamics opposes the incoming remote signal. On the other hand, while the slab ocean coupling does not produce significant changes to the equatorial Pacific sea surface temperature (SST) seasonal cycle, the RGO configuration generates a strong warming in the centre-east of the basin from April to August balanced by a cooling during the rest of the year, strengthening the seasonal cycle in the eastern portion of the basin. We hypothesize that such changes are possible via the dynamical effect that zonal wind stress has on the thermocline depth. We also find that the imposed extratropical pattern affects El Niño Southern Oscillation, weakening its amplitude and low-frequency behaviour.

Sign in / Sign up

Export Citation Format

Share Document