scholarly journals A Proposed Mechanism for the Asymmetric Duration of El Niño and La Niña

2011 ◽  
Vol 24 (15) ◽  
pp. 3822-3829 ◽  
Author(s):  
Yuko M. Okumura ◽  
Masamichi Ohba ◽  
Clara Deser ◽  
Hiroaki Ueda
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Surface Wind ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
La Niña ◽  
Western Pacific ◽  
Boreal Winter ◽  
La Nina

Abstract El Niño and La Niña exhibit significant asymmetry not only in their spatial structure but also in their duration. Most El Niños terminate rapidly after maturing near the end of the calendar year, whereas many La Niñas persist into the following year and often reintensify in boreal winter. Through atmospheric general circulation model experiments, it is shown that the nonlinear response of atmospheric deep convection to the polarity of equatorial Pacific sea surface temperature anomalies causes an asymmetric evolution of surface wind anomalies over the far western Pacific around the mature phase of El Niño and La Niña. Because of the eastward displacement of precipitation anomalies in the equatorial Pacific during El Niño compared to La Niña, surface winds in the western Pacific are more affected by SST forcing outside the equatorial Pacific, which acts to terminate the Pacific event.

scholarly journals Influence of the Seasonal Cycle on the Termination of El Niño Events in a Coupled General Circulation Model

2006 ◽  
Vol 19 (9) ◽  
pp. 1850-1868 ◽  
Author(s):  
Matthieu Lengaigne ◽  
Jean-Philippe Boulanger ◽  
Christophe Menkes ◽  
Hilary Spencer
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Circulation Model ◽  
Coupled System ◽  
Equatorial Pacific ◽  
Boreal Winter ◽  
Coupled General Circulation Model ◽  
El Niño Events ◽  
El Nino Events

Abstract In this study, the mechanisms leading to the El Niño peak and demise are explored through a coupled general circulation model ensemble approach evaluated against observations. The results here suggest that the timing of the peak and demise for intense El Niño events is highly predictable as the evolution of the coupled system is strongly driven by a southward shift of the intense equatorial Pacific westerly anomalies during boreal winter. In fact, this systematic late-year shift drives an intense eastern Pacific thermocline shallowing, constraining a rapid El Niño demise in the following months. This wind shift results from a southward displacement in winter of the central Pacific warmest SSTs in response to the seasonal evolution of solar insolation. In contrast, the intensity of this seasonal feedback mechanism and its impact on the coupled system are significantly weaker in moderate El Niño events, resulting in a less pronounced thermocline shallowing. This shallowing transfers the coupled system into an unstable state in spring but is not sufficient to systematically constrain the equatorial Pacific evolution toward a rapid El Niño termination. However, for some moderate events, the occurrence of intense easterly wind anomalies in the eastern Pacific during that period initiate a rapid surge of cold SSTs leading to La Niña conditions. In other cases, weaker trade winds combined with a slightly deeper thermocline allow the coupled system to maintain a broad warm phase evolving through the entire spring and summer and a delayed El Niño demise, an evolution that is similar to the prolonged 1986/87 El Niño event. La Niña events also show a similar tendency to peak in boreal winter, with characteristics and mechanisms mainly symmetric to those described for moderate El Niño cases.

scholarly journals The Influence of Wind-Induced Waves on ENSO Simulations

2021 ◽  
Vol 9 (5) ◽  
pp. 457
Author(s):  
Yao Hu ◽  
Xiaoxiao Tan ◽  
Youmin Tang ◽  
Zheqi Shen ◽  
Ying Bao
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
Heat Budget ◽  
Circulation Model ◽  
Wave Model ◽  
La Niña ◽  
La Nina ◽  
Wave Induced

We evaluated the influence of wind-induced waves on El Niño-Southern Oscillation (ENSO) simulations based on the First Institute of Oceanography-Earth System Model version 2 (FIO-ESM 2.0), a global coupled general circulation model (GCM) with a wave component. Two sets of experiments, the GCM, with and without a wave model, respectively, were conducted in parallel. The simulated sea surface temperature (SST) was cooled by introducing the wave model via the enhancement of the vertical mixing in the ocean upper layer. The strength of ENSO was intensified and better simulated with the inclusion of wave-induced mixing, particularly the La Niña amplitude. Furthermore, the simulated amplitude and spatial pattern of El Niño events were slightly altered with the wave model. Heat budget analyses revealed the intensification of La Niña events to be generally attributed to wave-induced vertical advection, followed by the zonal and meridional advection terms.

Interactions between the Responses of North American Climate to El Niño–La Niña and to the Secular Warming Trend in the Indian–Western Pacific Oceans

2008 ◽  
Vol 21 (3) ◽  
pp. 476-494 ◽  
Author(s):  
Ngar-Cheung Lau ◽  
Ants Leetmaa ◽  
Mary Jo Nath
Keyword(s):  
North American ◽  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Warming Trend ◽  
La Niña ◽  
Western Pacific ◽  
The North ◽  
La Nina ◽  
North American Climate

Abstract The modulation of El Niño and La Niña responses by the long-term sea surface temperature (SST) warming trend in the Indian–Western Pacific (IWP) Oceans has been investigated using a large suite of sensitivity integrations with an atmospheric general circulation model. These model runs entail the prescription of anomalous SST conditions corresponding to composite El Niño or La Niña episodes, to SST increases associated with secular warming in IWP, and to combinations of IWP warming and El Niño/La Niña. These SST forcings are derived from the output of coupled model experiments for climate settings of the 1951–2000 and 2001–50 epochs. Emphasis is placed on the wintertime responses in 200-mb height and various indicators of surface climate in the North American sector. The model responses to El Niño and La Niña forcings are in agreement with the observed interannual anomalies associated with warm and cold episodes. The wintertime model responses in North America to IWP warming bear a distinct positive (negative) spatial correlation with the corresponding responses to La Niña (El Niño). Hence, the amplitude of the combined responses to IWP warming and La Niña is notably higher than that to IWP warming and El Niño. The model projections indicate that, as the SST continues to rise in the IWP sector during the twenty-first century, the strength of various meteorological anomalies accompanying La Niña (El Niño) will increase (decrease) with time. The response of the North American climate and the zonal mean circulation to the combined effects of IWP forcing and La Niña (El Niño) is approximately equal to the linear sum of the separate effects of IWP warming and La Niña (El Niño). The summertime responses to IWP warming bear some similarity to the meteorological anomalies accompanying extended droughts and heat waves over the continental United States.

scholarly journals Sudden stratospheric warmings during El Niño and La Niña: sensitivity to model biases

2021 ◽  
Author(s):  
Nicholas L. Tyrrell ◽  
Juho M. Koskentausta ◽  
Alexey Yu. Karpechko
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
Circulation Model ◽  
La Niña ◽  
Model Biases ◽  
La Nina ◽  
Modelling Studies ◽  
The Impact

Abstract. The number of sudden stratospheric warmings (SSWs) per year is affected by the phase of the El Niño–Southern Oscillation (ENSO), yet there are discrepancies between the observed and modeled relationship. We investigate how systematic model biases may affect the ENSO-SSW connection. A two-step bias-correction process is applied to the troposphere, stratosphere or full atmosphere of an atmospheric general circulation model. ENSO type sensitivity experiments are then performed to reveal the impact of differing climatologies on the ENSO–SSW teleconnection. The number of SSWs per year is overestimated in the control run, and this statistic is improved when stratospheric biases are reduced. The seasonal cycle of SSWs is also improved by the bias corrections. The composite SSW responses in the stratospheric zonal wind, geopotential height and surface response are well represented in both the control and bias corrected runs. The model response of SSWs to ENSO phase is more linear than in observations, in line with previous modelling studies, and this is not changed by the reduced biases. However, the trend of more wave-1 events during El Niño years than La Niña years is improved in the bias corrected runs.

scholarly journals Predictability of the California Niño/Niña*

2015 ◽  
Vol 28 (18) ◽  
pp. 7237-7249 ◽  
Author(s):  
Takeshi Doi ◽  
Chaoxia Yuan ◽  
Swadhin K. Behera ◽  
Toshio Yamagata
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Regional Climate ◽  
Circulation Model ◽  
Seasonal Prediction ◽  
La Niña ◽  
Boreal Summer ◽  
La Nina ◽  
Basin Scale

Abstract Predictability of a recently discovered regional coupled climate mode called the California Niño (Niña) off Baja California and California is explored using a seasonal prediction system based on the Scale Interaction Experiment-Frontier, version 1 (SINTEX-F1) coupled ocean–atmosphere general circulation model. Because of the skillful prediction of basin-scale El Niño (La Niña), the California Niño (Niña) that co-occurs with El Niño (La Niña) with a peak in boreal winter is found to be predictable at least a couple of seasons ahead. On the other hand, the regional coupled phenomenon peaking in boreal summer without co-occurrence with El Niño (La Niña) is difficult to predict. The difficulty in predicting such an intrinsic regional climate phenomenon may be due to model deficiency in resolving the regional air–sea–land positive feedback processes. The model may also underestimate coastal Kelvin waves with a small offshore scale, which may play an important role in the generation of the California Niño/Niña. It may be improved by increasing horizontal resolution of the ocean component of the coupled model. The present study may provide a guideline to improve seasonal prediction of regional climate modes for important industrial as well as social applications.

scholarly journals Changes in Spread and Predictability Associated with ENSO in an Ensemble Coupled GCM

2006 ◽  
Vol 19 (17) ◽  
pp. 4378-4396 ◽  
Author(s):  
Renguang Wu ◽  
Ben P. Kirtman
Keyword(s):  
Surface Pressure ◽  
El Niño ◽  
El Nino ◽  
Tropical Pacific ◽  
La Niña ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Central Pacific ◽  
La Nina ◽  
Signal Change

Abstract The present study documents the influence of El Niño and La Niña events on the spread and predictability of rainfall, surface pressure, and 500-hPa geopotential height, and contrasts the relative contribution of signal and noise changes to the predictability change based on a long-term integration of an interactive ensemble coupled general circulation model. It is found that the pattern of the El Niño–Southern Oscillation (ENSO)-induced noise change for rainfall follows closely that of the corresponding signal change in most of the tropical regions. The noise for tropical Pacific surface pressure is larger (smaller) in regions of lower (higher) mean pressure. The ENSO-induced noise change for 500-hPa height displays smaller spatial scales compared to and has no systematic relationship with the signal change. The predictability for tropical rainfall and surface pressure displays obvious contrasts between the summer and winter over the Bay of Bengal, the western North Pacific, and the tropical southwestern Indian Ocean. The predictability for tropical 500-hPa height is higher in boreal summer than in boreal winter. In the equatorial central Pacific, the predictability for rainfall is much higher in La Niña years than in El Niño years. This occurs because of a larger percent reduction in the amplitude of noise compared to the percent decrease in the magnitude of signal from El Niño to La Niña years. A consistent change is seen in the predictability for surface pressure near the date line. In the western North and South Pacific, the predictability for boreal winter rainfall is higher in El Niño years than in La Niña years. This is mainly due to a stronger signal in El Niño years compared to La Niña years. The predictability for 500-hPa height increases over most of the Tropics in El Niño years. Over western tropical Pacific–Australia and East Asia, the predictability for boreal winter surface pressure and 500-hPa height is higher in El Niño years than in La Niña years. The predictability change for 500-hPa height is primarily due to the signal change.

scholarly journals Role of the Indo-Pacific Interbasin Coupling in Predicting Asymmetric ENSO Transition and Duration

2012 ◽  
Vol 25 (9) ◽  
pp. 3321-3335 ◽  
Author(s):  
Masamichi Ohba ◽  
Masahiro Watanabe
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
La Niña ◽  
La Nina ◽  
The Pacific ◽  
Enso Asymmetry ◽  
Sst Anomalies

Warm and cold phases of El Niño–Southern Oscillation (ENSO) exhibit a significant asymmetry in their transition/duration such that El Niño tends to shift rapidly to La Niña after the mature phase, whereas La Niña tends to persist for up to 2 yr. The possible role of sea surface temperature (SST) anomalies in the Indian Ocean (IO) in this ENSO asymmetry is investigated using a coupled general circulation model (CGCM). Decoupled-IO experiments are conducted to assess asymmetric IO feedbacks to the ongoing ENSO evolution in the Pacific. Identical-twin forecast experiments show that a coupling of the IO extends the skillful prediction of the ENSO warm phase by about one year, which was about 8 months in the absence of the IO coupling, in which a significant drop of the prediction skill around the boreal spring (known as the spring prediction barrier) is found. The effect of IO coupling on the predictability of the Pacific SST is significantly weaker in the decay phase of La Niña. Warm IO SST anomalies associated with El Niño enhance surface easterlies over the equatorial western Pacific and hence facilitate the El Niño decay. However, this mechanism cannot be applied to cold IO SST anomalies during La Niña. The result of these CGCM experiments estimates that approximately one-half of the ENSO asymmetry arises from the phase-dependent nature of the Indo-Pacific interbasin coupling.

scholarly journals Dynamics for El Niño-La Niña asymmetry constrain equatorial-Pacific warming pattern

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Michiya Hayashi ◽  
Fei-Fei Jin ◽  
Malte F. Stuecker
Keyword(s):  
El Niño ◽  
Climate Models ◽  
El Nino ◽  
Southern Oscillation ◽  
Equatorial Pacific ◽  
La Niña ◽  
Nonlinear Dynamical ◽  
La Nina ◽  
Warming Pattern ◽  
Enso Asymmetry

Abstract The El Niño-Southern Oscillation (ENSO) results from the instability of and also modulates the strength of the tropical-Pacific cold tongue. While climate models reproduce observed ENSO amplitude relatively well, the majority still simulates its asymmetry between warm (El Niño) and cold (La Niña) phases very poorly. The causes of this major deficiency and consequences thereof are so far not well understood. Analysing both reanalyses and climate models, we here show that simulated ENSO asymmetry is largely proportional to subsurface nonlinear dynamical heating (NDH) along the equatorial Pacific thermocline. Most climate models suffer from too-weak NDH and too-weak linear dynamical ocean-atmosphere coupling. Nevertheless, a sizeable subset (about 1/3) having relatively realistic NDH shows that El Niño-likeness of the equatorial-Pacific warming pattern is linearly related to ENSO amplitude change in response to greenhouse warming. Therefore, better simulating the dynamics of ENSO asymmetry potentially reduces uncertainty in future projections.

Ocean Chlorophyll-Induced Heating Feedbacks on ENSO in a Coupled Ocean Physics–Biology Model Forced by Prescribed Wind Anomalies

2018 ◽  
Vol 31 (5) ◽  
pp. 1811-1832 ◽  
Author(s):  
Rong-Hua Zhang ◽  
Feng Tian ◽  
Xiujun Wang
Keyword(s):  
Solar Radiation ◽  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
La Niña ◽  
La Nina ◽  
El Niño Events ◽  
Ocean Physics ◽  
El Nino Events

Ocean biology components affect the vertical redistribution of incoming solar radiation in the upper ocean of the tropical Pacific and can significantly modulate El Niño–Southern Oscillation (ENSO). The biophysical interactions in the region were represented by coupling an ocean biology model with an ocean general circulation model (OGCM); the coupled ocean physics–biology model is then forced by prescribed wind anomalies during 1980–2007. Two ocean-only experiments were performed with different representations of chlorophyll (Chl). In an interannual Chl run (referred to as Chlinter), Chl was interannually varying, which was interactively calculated from the ocean biology model to explicitly represent its heating feedback on ocean thermodynamics. The structure and relationship of the related heating terms were examined to understand the Chl-induced feedback effects and the processes involved. The portion of solar radiation penetrating the bottom of the mixed layer ( Qpen) was significantly affected by interannual Chl anomalies in the western-central equatorial Pacific. In a climatological run (Chlclim), the Chl concentration was prescribed to be its seasonally varying climatology derived from the Chlinter run. Compared with the Chlclim run, interannual variability in the Chlinter run tended to be reduced. The sea surface temperature (SST) differences between the two runs exhibited an asymmetric bioeffect: they were stronger during La Niña events but relatively weaker during El Niño events. The signs of the SST differences between the two runs indicated a close relationship with Chl: a cooling effect was associated with a low Chl concentration during El Niño events, and a strong warming effect was associated with a high Chl concentration during La Niña events.

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