scholarly journals El Niño in a Coupled Climate Model: Sensitivity to Changes in Mean State Induced by Heat Flux and Wind Stress Corrections

2007 ◽  
Vol 20 (10) ◽  
pp. 2273-2298 ◽  
Author(s):  
Hilary Spencer ◽  
Rowan Sutton ◽  
Julia M. Slingo
Keyword(s):  
Heat Flux ◽  
Wind Stress ◽  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Seasonal Forecasts ◽  
Coupled Models ◽  
The Mean ◽  
Flux Corrections ◽  
Mean State

Abstract Here the factors affecting the mean state and El Niño variability in the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) are examined with and without heat flux or wind stress corrections. There is currently little confidence in the prediction of El Niño for seasonal forecasts or climate change due to the inaccuracies in coupled models. If heat flux or wind stress corrections could reduce these biases then forecasts might be improved. Heat flux corrections have unexpected effects on both the mean state and variability of HadCM3. HadCM3 is found to be very sensitive to the corrections imposed. If heat flux corrections are imposed Tropics wide then easterlies in the eastern equatorial Pacific are increased leading to localized steep east–west gradients in the thermocline or “thermocline jumps,” which appear to suppress propagation of heat from the west to the east and hence suppress strong El Niños so that ENSO variability is weak. In contrast, if heat flux corrections are imposed only within 10° of the equator, an atmospheric teleconnection from the cold subtropical SST biases intensifies the ITCZ and weakens the equatorial easterlies. As a result, the thermocline jumps are flattened and strong El Niños occur very frequently. Neither heat flux correction procedure improves the representation of El Niño. Wind stress corrections alone have a small impact on the coupled model. Some of the SST warm biases are reduced, but the variability is not altered significantly.

scholarly journals The 1997/98 Summer Rainfall Season in Southern Africa. Part II: Model Simulations and Coupled Model Forecasts

2009 ◽  
Vol 22 (13) ◽  
pp. 3802-3818 ◽  
Author(s):  
Bradfield Lyon ◽  
Simon J. Mason
Keyword(s):  
Indian Ocean ◽  
Southern Africa ◽  
El Niño ◽  
General Circulation ◽  
Climate Models ◽  
El Nino ◽  
Seasonal Forecasts ◽  
Coupled Models ◽  
El Niño Events ◽  
El Nino Events

Abstract This is the second of a two-part investigation of rainfall in southern Africa during the strong El Niño of 1997/98. In Part I it was shown that widespread drought in southern Africa, typical of past El Niño events occurring between 1950 and 2000, generally failed to materialize during the 1997/98 El Niño, most notably during January–March (JFM) 1998. Here output from three atmospheric general circulation models (AGCMs) forced with observed sea surface temperatures (SSTs) and seasonal forecasts from three coupled models are examined to see to what extent conditions in JFM 1998 could have potentially been anticipated. All three AGCMs generated widespread drought conditions across southern Africa, similar to those during past El Niño events, and did a generally poor job in generating the observed rainfall and atmospheric circulation anomaly patterns, particularly over the eastern and southern Indian Ocean. In contrast, two of the three coupled models showed a higher probability of wetter conditions in JFM 1998 than for past El Niño events, with an enhanced moisture flux from the Indian Ocean, as was observed. However, neither the AGCMs nor the coupled models generated anomalous stationary wave patterns consistent with observations over the South Atlantic and Pacific. The failure of any of the models to reproduce an enhanced Angola low (favoring rainfall) associated with an anomalous wave train in this region suggests that the coupled models that did indicate wetter conditions in JFM 1998 compared to previous El Niño episodes may have done so, at least partially, for the wrong reasons. The general inability of the climate models used in this study to generate key features of the seasonal climate over southern Africa in JFM 1998 suggests that internal atmospheric variability contributed to the observed rainfall and circulation patterns that year. With the caveat that current climate models may not properly respond to SST boundary forcing important to simulating southern Africa climate, this study finds that the JFM 1998 rainfall in southern Africa may have been largely unpredictable on seasonal time scales.

scholarly journals Stratospheric Communication of El Niño Teleconnections to European Winter

2009 ◽  
Vol 22 (15) ◽  
pp. 4083-4096 ◽  
Author(s):  
C. J. Bell ◽  
L. J. Gray ◽  
A. J. Charlton-Perez ◽  
M. M. Joshi ◽  
A. A. Scaife
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
Polar Vortex ◽  
Circulation Model ◽  
Active Role ◽  
Late Winter ◽  
The Mean ◽  
Mean State

Abstract The stratospheric role in the European winter surface climate response to El Niño–Southern Oscillation sea surface temperature forcing is investigated using an intermediate general circulation model with a well-resolved stratosphere. Under El Niño conditions, both the modeled tropospheric and stratospheric mean-state circulation changes correspond well to the observed “canonical” responses of a late winter negative North Atlantic Oscillation and a strongly weakened polar vortex, respectively. The variability of the polar vortex is modulated by an increase in frequency of stratospheric sudden warming events throughout all winter months. The potential role of this stratospheric response in the tropical Pacific–European teleconnection is investigated by sensitivity experiments in which the mean state and variability of the stratosphere are degraded. As a result, the observed stratospheric response to El Niño is suppressed and the mean sea level pressure response fails to resemble the temporal and spatial evolution of the observations. The results suggest that the stratosphere plays an active role in the European response to El Niño. A saturation mechanism whereby for the strongest El Niño events tropospheric forcing dominates the European response is suggested. This is examined by means of a sensitivity test and it is shown that under large El Niño forcing the European response is insensitive to stratospheric representation.

scholarly journals Weakened Anomalous Western North Pacific Anticyclone during an El Niño–Decaying Summer under a Warmer Climate: Dominant Role of the Weakened Impact of the Tropical Indian Ocean on the Atmosphere

2018 ◽  
Vol 32 (1) ◽  
pp. 213-230 ◽  
Author(s):  
Chao He ◽  
Tianjun Zhou ◽  
Tim Li
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Tropical Indian Ocean ◽  
Tropospheric Temperature ◽  
Coupled Models ◽  
Subtropical Anticyclone ◽  
Mean State

Abstract The western North Pacific subtropical anticyclone (WNPAC) is the most prominent atmospheric circulation anomaly over the subtropical Northern Hemisphere during the decaying summer of an El Niño event. Based on a comparison between the RCP8.5 and the historical experiments of 30 coupled models from the CMIP5, we show evidence that the anomalous WNPAC during the El Niño–decaying summer is weaker in a warmer climate although the amplitude of the El Niño remains generally unchanged. The weakened impact of the sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) on the atmosphere is essential for the weakened anomalous WNPAC. In a warmer climate, the warm tropospheric temperature (TT) anomaly in the tropical free troposphere stimulated by the El Niño–related SSTA is enhanced through stronger moist adiabatic adjustment in a warmer mean state, even if the SSTA of El Niño is unchanged. But the amplitude of the warm SSTA over TIO remains generally unchanged in an El Niño–decaying summer, the static stability of the boundary layer over TIO is increased, and the positive rainfall anomaly over TIO is weakened. As a result, the warm Kelvin wave emanating from TIO is weakened because of a weaker latent heating anomaly over TIO, which is responsible for the weakened WNPAC anomaly. Numerical experiments support the weakened sensitivity of precipitation anomaly over TIO to local SSTA under an increase of mean-state SST and its essential role in the weakened anomalous WNPAC, independent of any change in the SSTA.

scholarly journals Role of Air–Sea Interaction in the Long Persistence of El Niño–Induced North Indian Ocean Warming*

2009 ◽  
Vol 22 (8) ◽  
pp. 2023-2038 ◽  
Author(s):  
Yan Du ◽  
Shang-Ping Xie ◽  
Gang Huang ◽  
Kaiming Hu
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Northwest Pacific ◽  
Basin Scale ◽  
The Mean ◽  
Second Peak

Abstract El Niño induces a basin-wide increase in tropical Indian Ocean (TIO) sea surface temperature (SST) with a lag of one season. The north IO (NIO), in particular, displays a peculiar double-peak warming with the second peak larger in magnitude and persisting well through the summer. Motivated by recent studies suggesting the importance of the TIO warming for the Northwest Pacific and East Asian summer monsoons, the present study investigates the mechanisms for the second peak of the NIO warming using observations and general circulation models. This analysis reveals that internal air–sea interaction within the TIO is key to sustaining the TIO warming through summer. During El Niño, anticyclonic wind curl anomalies force a downwelling Rossby wave in the south TIO through Walker circulation adjustments, causing a sustained SST warming in the tropical southwest IO (SWIO) where the mean thermocline is shallow. During the spring and early summer following El Niño, this SWIO warming sustains an antisymmetric pattern of atmospheric anomalies with northeasterly (northwesterly) wind anomalies north (south) of the equator. Over the NIO as the mean winds turn into southwesterly in May, the northeasterly anomalies force the second SST peak that persists through summer by reducing the wind speed and surface evaporation. Atmospheric general circulation model experiments show that the antisymmetric atmospheric pattern is a response to the TIO warming, suggestive of their mutual interaction. Thus, ocean dynamics and Rossby waves in particular are important for the warming not only locally in SWIO but also on the basin-scale north of the equator, a result with important implications for climate predictability and prediction.

scholarly journals Diagnosing Relationships between Mean State Biases and El Niño Shortwave Feedback in CMIP5 Models

2018 ◽  
Vol 31 (4) ◽  
pp. 1315-1335 ◽  
Author(s):  
Samantha Ferrett ◽  
Matthew Collins ◽  
Hong-Li Ren
Keyword(s):  
El Niño ◽  
El Nino ◽  
Coupled Model ◽  
Heat Fluxes ◽  
Cmip5 Models ◽  
Coupled Models ◽  
El Niño Events ◽  
Mean State ◽  
Cloud Response ◽  
El Nino Events

The rate of damping of tropical Pacific sea surface temperature anomalies (SSTAs) associated with El Niño events by surface shortwave heat fluxes has significant biases in current coupled climate models [phase 5 of the Coupled Model Intercomparison Project (CMIP5)]. Of 33 CMIP5 models, 16 have shortwave feedbacks that are weakly negative in comparison to observations, or even positive, resulting in a tendency of amplification of SSTAs. Two biases in the cloud response to El Niño SSTAs are identified and linked to significant mean state biases in CMIP5 models. First, cool mean SST and reduced precipitation are linked to comparatively less cloud formation in the eastern equatorial Pacific during El Niño events, driven by a weakened atmospheric ascent response. Second, a spurious reduction of cloud driven by anomalous surface relative humidity during El Niño events is present in models with more stable eastern Pacific mean atmospheric conditions and more low cloud in the mean state. Both cloud response biases contribute to a weak negative shortwave feedback or a positive shortwave feedback that amplifies El Niño SSTAs. Differences between shortwave feedback in the coupled models and the corresponding atmosphere-only models (AMIP) are also linked to mean state differences, consistent with the biases found between different coupled models. Shortwave feedback bias can still persist in AMIP, as a result of persisting weak shortwave responses to anomalous cloud and weak cloud responses to atmospheric ascent. This indicates the importance of bias in the atmosphere component to coupled model feedback and mean state biases.

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 Interdecadal Relationship between the Mean State and El Niño Types*

2013 ◽  
Vol 26 (2) ◽  
pp. 361-379 ◽  
Author(s):  
Pei-Hsuan Chung ◽  
Tim Li
Keyword(s):  
El Niño ◽  
El Nino ◽  
Atmospheric Precipitation ◽  
Frequent Occurrence ◽  
Central Pacific ◽  
Cp El Niño ◽  
Budget Analysis ◽  
Ep El Niño ◽  
The Mean ◽  
Mean State

Abstract The interdecadal change of the mean state and two types of El Niño was investigated based on the analysis of observational data from 1980 to 2010. It was found that easterly trades and sea surface temperature (SST) gradients across the equatorial Pacific undergo a regime change in 1998/99, with enhanced trades and a significant cooling (warming) over tropical eastern (western) Pacific in the later period. Accompanying this mean state change is more frequent occurrence of central Pacific (CP) El Niño during 1999–2010. The diagnosis of air–sea feedback strength showed that atmospheric precipitation and wind responses to CP El Niño are greater than those to the eastern Pacific (EP) El Niño for given a unit SST anomaly (SSTA) forcing. The oceanic response to the same wind forcing, however, is greater in the EP El Niño than in the CP El Niño. A mixed layer heat budget analysis reveals that zonal advection (thermocline change induced vertical advection) primarily contributes to the CP (EP) El Niño growth. The role of the mean SST zonal gradient in El Niño selection was investigated through idealized numerical experiments. With the increase of the background zonal SST gradient, the anomalous wind and convection response to a specified EP or CP SSTA shift to the west. Such a difference results in a bifurcation of maximum SSTA tendency, as shown from a simple ocean model. The numerical results support the notion that a shift to the La Niño–like interdecadal mean state is responsible for more frequent occurrence of CP-type El Niño.

scholarly journals Representing El Niño in Coupled Ocean–Atmosphere GCMs: The Dominant Role of the Atmospheric Component

2004 ◽  
Vol 17 (24) ◽  
pp. 4623-4629 ◽  
Author(s):  
E. Guilyardi ◽  
S. Gualdi ◽  
J. Slingo ◽  
A. Navarra ◽  
P. Delecluse ◽  
...  
Keyword(s):  
El Niño ◽  
General Circulation ◽  
State Of The Art ◽  
El Nino ◽  
Dominant Role ◽  
Power Spectra ◽  
General Circulation Models ◽  
Coupled Models ◽  
Model Improvement

Abstract A systematic modular approach to investigate the respective roles of the ocean and atmosphere in setting El Niño characteristics in coupled general circulation models is presented. Several state-of-the-art coupled models sharing either the same atmosphere or the same ocean are compared. Major results include 1) the dominant role of the atmosphere model in setting El Niño characteristics (periodicity and base amplitude) and errors (regularity) and 2) the considerable improvement of simulated El Niño power spectra—toward lower frequency—when the atmosphere resolution is significantly increased. Likely reasons for such behavior are briefly discussed. It is argued that this new modular strategy represents a generic approach to identifying the source of both coupled mechanisms and model error and will provide a methodology for guiding model improvement.

scholarly journals The Effect of the Galápagos Islands on ENSO in Forced Ocean and Hybrid Coupled Models

2008 ◽  
Vol 38 (11) ◽  
pp. 2519-2534 ◽  
Author(s):  
Kristopher B. Karnauskas ◽  
Raghu Murtugudde ◽  
Antonio J. Busalacchi
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
El Niño ◽  
Time Scale ◽  
El Nino ◽  
Galapagos Islands ◽  
Galápagos Islands ◽  
Coupled Models ◽  
Zonal Wind Stress ◽  
Sst Anomaly

Abstract An ocean general circulation model (OGCM) of the tropical Pacific Ocean is used to examine the effects of the Galápagos Islands on the El Niño–Southern Oscillation (ENSO). First, a series of experiments is conducted using the OGCM in a forced context, whereby an idealized El Niño event may be examined in cases with and without the Galápagos Islands. In this setup, the sensitivity of the sea surface temperature (SST) anomaly response to the presence of the Galápagos Islands is examined. Second, with the OGCM coupled to the atmosphere via zonal wind stress, experiments are conducted with and without the Galápagos Islands to determine how the Galápagos Islands influence the time scale of ENSO. In the forced setup, the Galápagos Islands lead to a damped SST anomaly given an identical zonal wind stress perturbation. Mixed layer heat budget calculations implicate the entrainment mixing term, which confirms that the difference is due to the Galápagos Islands changing the background mean state, that is, the equatorial thermocline as diagnosed in a previous paper. In the hybrid coupled experiments, there is a clear shift in the power spectrum of SST anomalies in the eastern equatorial Pacific. Specifically, the Galápagos Islands lead to a shift in the ENSO time scale from a biennial to a quasi-quadrennial period. Mechanisms for the shift in ENSO time scale due to the Galápagos Islands are discussed in the context of well-known paradigms for the oscillatory nature of ENSO.

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.

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