The Experts below are selected from a list of 17622 Experts worldwide ranked by ideXlab platform
Ronghua Zhang - One of the best experts on this subject based on the ideXlab platform.
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asymmetric evolution of el nino and La Nina the recharge discharge processes and role of the off equatorial sea surface height anomaly
Climate Dynamics, 2017Co-Authors: Zengzhen Hu, Bohua Huang, Arun Kumar, Ronghua ZhangAbstract:Observed oceanic and atmospheric anomalies in the tropical Pacific are analyzed to understand the symmetric and asymmetric characteristics between El Nino and La Nina evolutions. It is noted that the evolutions are Largely symmetric for El Nino and La Nina prior to and in their mature phase, but become asymmetric afterwards. It is further demonstrated that this asymmetry is due to the fact that, on average, the discharge process associated with El Nino is stronger than the recharge counterpart associated with La Nina. The symmetric and asymmetric evolution for different phases of El Nino-Southern OscilLation (ENSO) is consistent with the associated recharge and discharge processes that are linked with the evolution of anomalous ocean surface current and gradient of sea surface height anomalies (SSHAs). It is suggested that the evolution of the meridional gradient of SSHAs in the central and eastern equatorial Pacific is different between El Nino and La Nina years due to reLatively different rates of the SSHA changes on and off the equator. During the decay of a warm event, the equatorial SSHA quickly switches from positive to negative, meanwhile the off-equator areas are occupied by positive SSHAs. Such changes on the equator and in the off-equatorial regions make the concavity of the meridional SSHA favorable for growth and persistence of westward surface zonal current anomaly in the equatorial Pacific, then for the ENSO phase transition. During the decay of a cold event, the slow decrease of the negative SSHA on the equator combined with the disappearance of the negative SSHA in the off-equatorial regions makes the meridional concavity reverse easily and is unfavorable for the steady growth of the eastward surface zonal current anomaly and for the phase transition. Thus, the asymmetric evolution of ENSO is associated with both the atmosphere and ocean anomalies on the equator and off the equator.
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the roles of atmospheric wind and entrained water temperature t e in the second year cooling of the 2010 12 La Nina event
Climate Dynamics, 2017Co-Authors: Ronghua ZhangAbstract:An intermediate coupled model (ICM) yields a successful real-time prediction of the sea surface temperature (SST) evolution in the tropical Pacific during the 2010–12 La Nina event, whereas many other coupled models fail. It was previously identified that the thermocline effect on the SST (including vertical advection and mixing), as represented by water temperature entrained into the mixed Layer (Te) and its reLationship with the thermocline fluctuation, is an important factor that affects the second-year cooling in mid-Late 2011. Because atmospheric wind forcing is also important to ENSO processes, its role is investigated in this study within the context of real-time prediction of the 2010–12 La Nina event using the ICM in which wind stress anomalies are calcuLated using an empirical model as a response to SST anomalies. An easterly wind anomaly is observed to persist over the western-central Pacific during 2010–11, which acts to sustain a horse shoe-like Te pattern connecting Large negative subsurface thermal anomalies in the central-eastern regions off and on the equator. Sensitivity experiments are conducted using the ICM to demonstrate how its SST predictions are directly affected by the intensity of wind forcing. The second-year cooling in 2011 is not predicted to occur in the ICM if the easterly wind anomaly intensity is weakly represented below certain levels; instead, a surface warming can emerge in 2011, with weak SST variability. The results of the current study indicate that the intensity of interannual wind forcing is equally important to SST evolution during 2010–11 compared with that of the thermocline effect. To correctly predict the observed La Nina conditions in the fall of 2011, the ICM needs to adequately represent the intensity of both the wind forcing and the thermocline effects.
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a successful real time forecast of the 2010 11 La Nina event
Scientific Reports, 2013Co-Authors: Ronghua Zhang, Fei Zheng, Zhanggui WangAbstract:During 2010–11, a La Nina condition prevailed in the tropical Pacific. An intermediate coupled model (ICM) is used to demonstrate a real-time forecast of sea surface temperature (SST) evolution during the event. One of the ICM's unique features is an empirical parameterization of the temperature of subsurface water entrained into the mixed Layer (Te). This model provided a good prediction, particuLarly of the "double dip" evolution of SST in 2011 that followed the La Nina event peak in October 2010. Thermocline feedback, explicitly represented by the reLationship between Te and sea level in the ICM, is a crucial factor affecting the second cooling in 2011. Large negative Te anomalies were observed to persist in the central equatorial domain during 2010–11, inducing a cold SST anomaly to the east during July–August 2011 and leading to the development of a La Nina condition thereafter.
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effects of interannual salinity variability and freshwater flux forcing on the development of the 2007 08 La Nina event diagnosed from argo and satellite data
Dynamics of Atmospheres and Oceans, 2012Co-Authors: Fei Zheng, Ronghua ZhangAbstract:n n Oceanic salinity and its reLated freshwater flux (FWF) forcing in the tropical Pacific have been of increased interest recently due to their roles in the El Nino-Southern OscilLation (ENSO), the global climate and water cycle. A comprehensive data analysis is performed to illustrate the significant effects of interannual salinity variability and FWF forcing during the 2007/08 La Nina event using three-dimensional temperature and salinity fields from Argo profiles, and some reLated fields derived from the Argo and satellite-based data, including the mixed Layer depth (MLD), heat flux, freshwater flux, and buoyancy flux (Q(B)). It is demonstrated that during the developing phase of 2007/08 La Nina, a negative FWF anomaly and its associated positive sea surface salinity (SSS) anomaly in the western-central basin act to increase oceanic density and destabilize the upper ocean. At the same time, the negative FWF anomaly tends to reduce a positive Q(B) anomaly and deepen the mixed Layer (ML). These reLated oceanic processes act to strengthen the vertical mixing and entrainment of subsurface water at the base of ML, which further enhance cold sea surface temperature (SST) anomalies associated with the La Nina event, a demonstration of a positive feedback induced by FWF forcing.n
Lorenzo M Polvani - One of the best experts on this subject based on the ideXlab platform.
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why might stratospheric sudden warmings occur with simiLar frequency in el nino and La Nina winters
Journal of Geophysical Research, 2012Co-Authors: Chaim I Garfinkel, Amy H Butler, Darryn W Waugh, Margaret M Hurwitz, Lorenzo M PolvaniAbstract:[1] The effect of El Nino-Southern OscilLation (ENSO) on the frequency and character of Northern Hemisphere major mid-winter stratospheric sudden warmings (SSWs) is evaluated using a meteorological reanalysis data set and comprehensive chemistry-climate models. There is an apparent inconsistency between the impact of opposite phases of ENSO on the seasonal mean vortex and on SSWs: El Nino leads to an anomalously warm, and La Nina leads to an anomalously cool, seasonal mean poLar stratospheric state, but both phases of ENSO lead to an increased SSW frequency. A resolution to this apparent paradox is here proposed: the region in the North Pacific most strongly associated with precursors of SSWs is not strongly influenced by El Nino and La Nina teleconnections. In the observational record, both La Nina and El Nino lead to simiLar anomalies in the region associated with precursors of SSWs and, consistent with this, there is a simiLar SSW frequency in La Nina and El Nino winters. A simiLar correspondence between the penetration of ENSO teleconnections into the SSW precursor region and SSW frequency is found in the comprehensive chemistry-climate models. The inability of some of the models to capture the observed reLationship between La Nina and SSW frequency appears reLated to whether the modeled ENSO teleconnections result in extreme anomalies in the region most closely associated with SSWs. Finally, it is confirmed that the seasonal mean poLar vortex response to ENSO is only weakly reLated to the reLative frequency of SSWs during El Nino and La Nina.
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el nino La Nina and stratospheric sudden warmings a reevaluation in light of the observational record
Geophysical Research Letters, 2011Co-Authors: Amy H Butler, Lorenzo M PolvaniAbstract:[1] Recent studies have suggested that El Nino-Southern OscilLation (ENSO) may have a considerable impact on Northern Hemisphere wintertime stratospheric conditions. Notably, during El Nino the stratosphere is warmer than during ENSO-neutral winters, and the poLar vortex is weaker. Opposite-signed anomalies have been reported during La Nina, but are considerably smaller in amplitude than during El Nino. This has led to the perception that El Nino is able to substantially affect stratospheric conditions, but La Nina is of secondary importance. Here we revisit this issue, but focus on the extreme events that couple the troposphere to the stratosphere: major, mid-winter stratospheric sudden warmings (SSWs). We examine 53 years of reanalysis data and find, as expected, that SSWs are nearly twice as frequent during ENSO winters as during non-ENSO winters. Surprisingly, however, we also find that SSWs occur with equal probability during El Nino and La Nina winters. These findings corroborate the impact of ENSO on stratospheric variability, and highlight that both phases of ENSO are important in enhancing stratosphere-troposphere dynamical coupling via an increased frequency of SSWs.
Zesheng Chen - One of the best experts on this subject based on the ideXlab platform.
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roles of tropical sst anomalies in moduLating the western north pacific anomalous cyclone during strong La Nina decaying years
Climate Dynamics, 2017Co-Authors: Renguang Wu, Zesheng Chen, Yan DuAbstract:This study re-explores the roles of tropical SST anomalies in moduLating the anomalous cyclone over the Indo-western Pacific during strong La Nina decaying years via a series of numerical experiments. The results suggest that central to eastern equatorial Pacific cooling, western north Pacific (WNP) warming, and Indian Ocean cooling all contribute to the Indo-western Pacific cyclonic wind anomalies. The WNP SST anomalies pLay a role via a Rossby wave-type response. As the positive SST anomalies in the WNP decay, the local forcing effect decays as well. The Indian Ocean remote forcing seems to pLay an important role during boreal summer via cold Kelvin wave induced cyclonic shear and boundary Layer convergence. During strong La Nina decaying years, the negative SST anomalies in the equatorial central to eastern Pacific can maintain and persist into summer. Such cooling could induce anomalous Walker circuLation with ascending branch over the Maritime Continent to Philippines and descending branch over central to eastern Pacific, which links the WNP climate to the equatorial central and eastern Pacific SST anomalies. The convective heating over the WNP induced remotely by central to eastern Pacific cooling in turn forces atmospheric Rossby wave to its west, which may amplify the anomalous cyclone over the Indo-western Pacific.
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reLative importance of tropical sst anomalies in maintaining the western north pacific anomalous anticyclone during el nino to La Nina transition years
Climate Dynamics, 2016Co-Authors: Zesheng Chen, Renguang Wu, Jiabao WangAbstract:This study investigates the reLative importance of tropical Indian Ocean warming (IOW) and equatorial central to eastern Pacific cooling (EPC) in sustaining an anomalous Western North Pacific anticyclone (WNPAC) during the transition from an El Nino in the preceding winter to a La Nina in the subsequent summer through a suite of numerical experiments. The numerical results indicate that the WNPAC is maintained by a combined effect of IOW and EPC during the La Nina developing years. The contribution of IOW in maintaining the WNPAC sustains from spring to early summer, but appears to weaken after that as IOW decays. The role of IOW is via an eastward-propagating Kelvin wave induced Ekman divergence mechanism. The decay of IOW is because of reduction in downward soLar radiation associated with above normal precipitation in situ. As the cooling develops over central to eastern Pacific from spring to summer, EPC starts to contribute to the maintenance of the WNPAC during summer through stimuLating a Rossby wave response to its northwest. In this study, we have identified that the cooling over the central to eastern Pacific pLays an important role in sustaining the WNPAC during La Nina developing summers. This finding may help improve the prediction of the East Asian summer monsoon, which is closely associated with the WNPAC.
Insik Kang - One of the best experts on this subject based on the ideXlab platform.
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changes in el nino and La Nina teleconnections over north pacific america in the global warming simuLations
Theoretical and Applied Climatology, 2010Co-Authors: Soonil An, Insik KangAbstract:The change in the teleconnections of both El Nino and La Nina over the North Pacific and American regions due to a future greenhouse warming has been analyzed herein by means of diagnostics of the Intergovernmental Panel on Climate Change-Fourth Assessment Report (IPCC-AR4) coupled general circuLation models (CGCMs). Among the IPCC-AR4 CGCM simuLations, the composites of the eight-member multimodel ensemble are analyzed. In most CGCMs, the tropical Pacific warming due to the increase of CO2 concentration in the atmosphere promotes the main convection centers in the equatorial Pacific associated with both El Nino and La Nina to the east. The eastward shift of the convection center causes a systematic eastward shift of not only El Nino but also La Nina teleconnection patterns over the North Pacific and America, which is demonstrated in the composite maps of 500 hPa circuLation, surface temperature, and the precipitation against El Nino and La Nina, as observed in a comparison between the pre-industrial and CO2 doubling experiments. Thus, a systematic eastward migration of convection centers in the tropical Pacific associated with both El Nino and La Nina due to a future global warming commonly causes the eastward shift of the atmospheric teleconnection patterns over the Northern Hemisphere.
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el nino La Nina asymmetry in the coupled model intercomparison project simuLations
Journal of Climate, 2005Co-Authors: Soonil An, Insik KangAbstract:The El Nino–La Nina asymmetry was estimated in the 10 different models participating in the Coupled Model Intercomparison Project (CMIP). Large differences in the “asymmetricity” (a variance-weighted skewness) of SST anomalies are found between models and observations. Most of the coupled models underestimate the nonlinearity and only a few exhibit the positively skewed SST anomalies over the tropical eastern Pacific as seen in the observation. A significant association between the nonlinear dynamical heating (NDH) and asymmetricity in the model–ENSO indices is found, inferring that asymmetricity is caused mainly by NDH. Among the 10 models, one coupled GCM simuLates the asymmetricity of the tropical SST realistically, and its simuLation manifests a strong reLationship between the intensity and the propagating feature of ENSO—the strong ENSO events moving eastward and the weak ENSO events moving westward—which is consistent with the observation. Interestingly, the coupled general circuLation models, of which the ocean model is based on the one used by Bryan and Cox, commonly showed the reasonably positive skewed ENSO. The decadal changes in the skewness, variance, and NDH of the model-simuLated ENSO are also observed. These three quantities over the tropical eastern Pacific are significantly correLated to each other, indicating that the decadal change in ENSO variability is closely reLated to the nonlinear process of ENSO. It is also found that these decadal changes in ENSO variability are reLated to the decadal variation in the tropical Pacific SST, implying that the decadal change in the El Nino–La Nina asymmetry could manifest itself as a rectified change in the background state.
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preconditions for el nino and La Nina onsets and their reLation to the indian ocean
Geophysical Research Letters, 2005Co-Authors: Soonil An, Insik KangAbstract:[1] El Nino/La Nina onset is a challenging problem of ENSO prediction. In this study, we introduce two precursors of El Nino and La Nina onsets. One is the equatorial heat content, and the other is the Western Pacific (WP) wind. When the two precursors are considered together, both the El Nino and La Nina onsets can be highly predictable. In particuLar, the persistence of the WP wind is more important for the onset than sporadic wind events. The persistent WP westerly (easterly) wind tends to be concurrent with the Indian Ocean SST cooling (warming). The Indian Ocean SST anomaly is partly correLated to an ENSO event during the previous winter. We demonstrate that an asymmetric reLation between the Indian Ocean SST and ENSO can result in asymmetric progress of onset in the opposite ENSO phases.
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el nino and La Nina sea surface temperature anomalies asymmetry characteristics associated with their wind stress anomalies
Journal of Geophysical Research, 2002Co-Authors: Insik KangAbstract:[1] The asymmetric nature of El Nino and La Nina sea surface temperature (SST) anomalies is investigated by the use of National Centers for Environmental Prediction reanalysis data and various ocean and atmosphere models. It is demonstrated that the reLatively weak SST anomalies during La Nina compared with those of El Nino are reLated to the westward shift of wind stress anomalies by 10°–15°. The asymmetric characteristics of atmospheric responses are confirmed by the general circuLation model experiments with the two different SST anomalies, which have equal amplitude but are of opposite sign from each other. The experiments with an intermediate ocean model and a hybrid coupled model clearly show that the SST anomalies over the equatorial Pacific become weaker as the zonal wind stress shifts to the west. Not only the amplitude but also the oscilLation timescale of the SST anomaly is shown to be sensitive to the location of wind stress anomalies. The duration of La Nina, which is rather shorter than that of El Nino, is also reLated to the longitudinal dispLacement of the wind stress anomaly.
Jianping Li - One of the best experts on this subject based on the ideXlab platform.
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reexamining the reLationship of La Nina and the east asian winter monsoon
Climate Dynamics, 2019Co-Authors: Peng Zhang, Zhiwei Wu, Jianping LiAbstract:The northern and the southern modes are two distinct principle modes that dominate the winter mean surface air temperature (Ts) variations over East Asia (EA). The cold southern mode is represented by a significant cooling south of 45°N and is linked to La Nina events. An objective criterion, which could distinguish the spatial distributions and the maximum center of sea surface temperature anomaly (SSTA), is used to cLassify the La Nina events into two categories: mega-La Nina and equatorial La Nina. Their impacts are inspected onto the Ts southern mode. The mega-La Nina, featured by a significant K-shape warming in the western Pacific with the maximum SSTA cooling centered in the tropical central Pacific. As a response, an anomalous barotropic high is generated over North Pacific (NP) implying a weak zonal gradient between ocean and the EA continent, which induces a neutral Ts southern mode. The equatorial La Nina characterizes a significant cooling in the tropical eastern Pacific with convective descending motions shifting eastward to the east of the dateline. The resultant low-level circuLation anomalies show an anomalous subtropical NP low and a gigantic abnormal EA continent high. The strong zonal gradient results in significant northerly anomalies over EA from 55°N to southeastern China. Over the mid-upper troposphere, the anomalous subtropical NP low extends westward to the Korean PeninsuLa, leading to a strengthened and southward shifted EA trough. Such abnormal circuLation patterns favor the intrusion of cold air to southern EA and correspond to a strong Ts southern mode. The numerical results well validate the above processes and physical mechanisms.
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simuLated contrasting influences of two La Nina modoki events on aerosol concentrations over eastern china
Journal of Geophysical Research, 2017Co-Authors: Juan Feng, Jianping Li, Hong Liao, Yang YangAbstract:Using the Goddard Earth Observing System (GEOS)-Chem model driven by GEOS-4 assimiLated meteorological data, the potential influence of two La Nina Modoki events on aerosol concentrations over eastern China is explored. The results indicate that the impact of La Nina Modoki on aerosol concentrations differs between strong and moderate events. During the mature phase of the strong event of 1998/1999, an anomalous dipole pattern is observed over eastern China, i.e., with increased aerosol concentrations in the south and reduced concentrations in the north. The dipole pattern in the moderate event of 2000/2001 is reversed, with reduced aerosol concentrations in the south and increased concentrations in north China. Additionally, decreased aerosol concentrations are seen in spring of the decaying phase for the 1998/1999 event, while in the decaying spring of the 2000/2001 event, the dipole is reversed reLative to the mature phase. During the decaying summer, aerosol concentrations are found to increase over eastern China in the 1998/1999 event but not in the 2000/2001 event. These anomalous aerosol concentrations are mainly caused by changes in circuLation associated with the two La Nina Modoki events. In contrast, the role of wet deposition is observed to be limited during the lifespan of these two events. It is also found that the potential influence of the two La Nina Modoki events on aerosol concentrations can be up to 20% of the climatological mean. This suggests that La Nina Modoki has an important role in determining the distribution of aerosol concentrations over eastern China.
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the asymmetric influence of the two types of el nino and La Nina on summer rainfall over southeast china
Journal of Climate, 2013Co-Authors: Muhammad Afzaal Karori, Jianping LiAbstract:AbstractIn this study, the authors demonstrate that the two types of El Nino–Southern OscilLation (ENSO) have asymmetric features with respect to the impact of their positive and negative phases on boreal summer rainfall over the Yangtze River Valley (YRV) and South China (SC). The reLationship between rainfall over the YRV and the warm pool (WP) La Nina is positive and significant, whereas the reLationship with the WP El Nino is not. In the case of the cold tongue (CT) ENSO, its positive phase has a positive influence, while there is no significant reLationship with the negative phase. In contrast, rainfall over SC has a significant positive reLationship with WP El Nino, but a nonsignificant reLationship with WP La Nina. The positive phase of the CT ENSO has a significant negative influence on SC rainfall, while the negative phase has a nonsignificant impact. An asymmetric atmospheric response to the asymmetric sea surface temperature anomalies (SSTAs) was also observed in the lower troposphere. The loca...