Nonlinearity Modulating Intensities and Spatial Structures of Central Pacific and Eastern Pacific El Nio Events

This paper compares data from linearized and nonlinear Zebiak–Cane model, as constrained by observed sea surface temperature anomaly(SSTA), in simulating central Pacific(CP) and eastern Pacific(EP) El Nio. The difference between the temperature advections(determined by subtracting those of the linearized model from those of the nonlinear model),referred to here as the nonlinearly induced temperature advection change(NTA), is analyzed. The results demonstrate that the NTA records warming in the central equatorial Pacific during CP El Nio and makes fewer contributions to the structural distinctions of the CP El Nio, whereas it records warming in the eastern equatorial Pacific during EP El Nio, and thus significantly promotes EP El Nio during El Nio–type selection. The NTA for CP and EP El Nio varies in its amplitude,and is smaller in CP El Nio than it is in EP El Nio. These results demonstrate that CP El Nio are weakly modulated by small intensities of NTA, and may be controlled by weak nonlinearity; whereas, EP El Nio are significantly enhanced by large amplitudes of NTA, and are therefore likely to be modulated by relatively strong nonlinearity. These data could explain why CP El Nio are weaker than EP El Nio. Because the NTA for CP and EP El Nio differs in spatial structures and intensities, as well as their roles within different El Nio modes, the diversity of El Nio may be closely related to changes in the nonlinear characteristics of the tropical Pacific.     

Distinguished Effects of Interannual Salinity Variability on the Development of the Central-Pacific El Ni o Events

El Nio events in the central equatorial Pacific (CP) are gaining increased attention,due to their increasing intensity within the global warming context.Various physical processes have been identified in the climate system that can be responsible for the modulation of El Nio,especially the effects of interannual salinity variability.In this work,a comprehensive data analysis is performed to illustrate the effects of interannual salinity variability using surface and subsurface salinity fields from the Met Office ENSEMBLES (EN3) quality controlled ocean dataset.It is demonstrated that during the developing phase of an El Nio event,a negative sea surface salinity (SSS) anomaly in the western-central basin acts to freshen the mixed layer (ML),decrease oceanic density in the upper ocean,and stabilize the upper layers.These related oceanic processes tend to reduce the vertical mixing and entrainment of subsurface water at the base of the ML,which further enhances the warm sea surface temperature (SST) anomalies associated with the El Nio event.However,the effects of interannually variable salinity are much more significant during the CP-El Nio than during the eastern Pacific (EP) El Nio,indicating that the salinity effect might be an important contributor to the development of CP-El Nio events.     

Contrasting the Skills and Biases of Deterministic Predictions for the Two Types of El Nio

The tropical Pacific has begun to experience a new type of El Nio, which has occurred particularly frequently during the last decade, referred to as the central Pacific(CP) El Nio. Various coupled models with different degrees of complexity have been used to make real-time El Nio predictions, but high uncertainty still exists in their forecasts. It remains unknown as to how much of this uncertainty is specifically related to the new CP-type El Nio and how much is common to both this type and the conventional Eastern Pacific(EP)-type El Nio. In this study, the deterministic performance of an El Nio–Southern Oscillation(ENSO) ensemble prediction system is examined for the two types of El Nio. Ensemble hindcasts are run for the nine EP El Nio events and twelve CP El Nio events that have occurred since 1950. The results show that(1) the skill scores for the EP events are significantly better than those for the CP events, at all lead times;(2) the systematic forecast biases come mostly from the prediction of the CP events; and(3) the systematic error is characterized by an overly warm eastern Pacific during the spring season, indicating a stronger spring prediction barrier for the CP El Nio. Further improvements to coupled atmosphere–ocean models in terms of CP El Nio prediction should be recognized as a key and high-priority task for the climate prediction community.     

Distinguished Effects of Interannual Salinity Variability on the Development of the Central-Pacific El Nino Events

El Nio events in the central equatorial Pacific (CP) are gaining increased attention,due to their increasing intensity within the global warming context.Various physical processes have been identified in the climate system that can be responsible for the modulation of El Nio,especially the effects of interannual salinity variability.In this work,a comprehensive data analysis is performed to illustrate the effects of interannual salinity variability using surface and subsurface salinity fields from the Met Office ENSEMBLES (EN3) quality controlled ocean dataset.It is demonstrated that during the developing phase of an El Nio event,a negative sea surface salinity (SSS) anomaly in the western-central basin acts to freshen the mixed layer (ML),decrease oceanic density in the upper ocean,and stabilize the upper layers.These related oceanic processes tend to reduce the vertical mixing and entrainment of subsurface water at the base of the ML,which further enhances the warm sea surface temperature (SST) anomalies associated with the El Nio event.However,the effects of interannually variable salinity are much more significant during the CP-El Nio than during the eastern Pacific (EP) El Nio,indicating that the salinity effect might be an important contributor to the development of CP-El Nio events.     

A New Approach for Classifying Two Types of El Nio Events

正In recent decades, the typical El Nio events with the warmest SSTs in the tropical eastern Pacific have become less common, and a different of El Nio with the warmest SSTs in the central Pacific, which is flanked on the east and west by cooler SSTs, has become more fre-quent. The more recent type of El Nio was referred to as central Pacific El Nio, warm pool El Nio, or dateline El Nio, or the El Nio Modoki. Central Pacific El Nio links to a different tropical-to-extratropical teleconnection and exerts different impacts on climate, and several classification approaches have been proposed. In this study, a new classification approach is proposed, which is based on the linear combination (sum or difference) of the two leading Empirical Orthogonal Functions (EOFs) of tropical Pacific Ocean sea surface temperature anomaly (SSTA), and the typical El Ni o index (TENI) and the central El Nio index (CENI) are able to be derived by projecting the observed SSTA onto these combinations. This classification not only reflects the characteristics of non-orthogonality between the two types of events but also yields one period peaking at approximate two to seven years. In particular, this classification can distin-guish the different impacts of the two types of events on rainfall in the following summer in East China. The typical El Nio events tend to induce intensified rainfall in the Yangtze River valley, whereas the central Pacific El Nio tends to induce intensified rainfall in the Huaihe River valley. Thus, the present approach may be appropriate for studying the impact of different types of El Nio on the East Asian climate.     

Comparison of Constant and Time-variant Optimal Forcing Approaches in El Nio Simulations by Using the Zebiak–Cane Model

Model errors offset by constant and time-variant optimal forcing vector approaches(termed COF and OFV, respectively)are analyzed within the framework of El Nio simulations. Applying the COF and OFV approaches to the well-known Zebiak–Cane model, we re-simulate the 1997 and 2004 El Nio events, both of which were poorly degraded by a certain amount of model error when the initial anomalies were generated by coupling the observed wind forcing to an ocean component. It is found that the Zebiak–Cane model with the COF approach roughly reproduced the 1997 El Nio, but the 2004 El Nio simulated by this approach defied an ENSO classification, i.e., it was hardly distinguishable as CP-El Nio or EP-El Nio. In both El Nio simulations, substituting the COF with the OFV improved the fit between the simulations and observations because the OFV better manages the time-variant errors in the model. Furthermore, the OFV approach effectively corrected the modeled El Nio events even when the observational data(and hence the computational time) were reduced.Such a cost-effective offset of model errors suggests a role for the OFV approach in complicated CGCMs.     

Modulation of the Intraseasonal Variability of Pacific–Japan Pattern by ENSO

This study investigates how the El Ni?o–Southern Oscillation(ENSO) modulates the intraseasonal variability(ISV) of Pacific–Japan(PJ) teleconnection pattern. The PJ index during boreal summer is constructed from the empirical orthogonal function(EOF) of the 850-hPa zonal wind(U850) anomalies. Distinct periods of the PJ index are found during El Ni?o and La Ni?a summers. Although ISV of the PJ pattern is significant during 10–25 days for both types of summers, it peaks on Days 30 and 60 in El Ni?o and La Ni?a summers respectively. During El Ni?o summers, the 30-day ISV of PJ pattern is related to the northwestward propagating intraseasonal oscillation(ISO) over the western North Pacific(WNP), which is originated from the tropical Indian Ocean(IO). During La Ni?a summers,the 60-day ISV of PJ pattern is related to the northeastward propagating ISO from the tropical IO. The low-frequency ISV modes in both El Ni?o and La Ni?a summers are closely related to the boreal summer ISO(BSISO), and the high-frequency ISV modes over WNP are related to the quasi-biweekly oscillation. The underlying mechanisms for these different evolutions are also discussed.     

Variable and Robust East Asian Monsoon Rainfall Response to El Nio over the Past 60 Years(1957–2016)

Severe flooding occurred in southern and northern China during the summer of 2016 when the 2015 super El Nio decayed to a normal condition. However, the mean precipitation during summer(June–July-August) 2016 does not show significant anomalies, suggesting that — over East Asia(EA) — seasonal mean anomalies have limited value in representing hydrological hazards. Scrutinizing season-evolving precipitation anomalies associated with 16 El Nio episodes during 1957–2016 reveals that, over EA, the spatiotemporal patterns among the four categories of El Nio events are quite variable, due to a large range of variability in the intensity and evolution of El Nio events and remarkable subseasonal migration of the rainfall anomalies. The only robust seasonal signal is the dry anomalies over central North China during the El Nio developing summer. Distinguishing strong and weak El Nio impacts is important. Only strong El Nio events can persistently enhance EA subtropical frontal precipitation from the peak season of El Nio to the ensuing summer, by stimulating intense interaction between the anomalous western Pacific anticyclone(WPAC) and underlying dipolar sea surface temperature anomalies in the Indo-Pacific warm pool, thereby maintaining the WPAC and leading to a prolonged El Nio impact on EA. A weak El Nio may also enhance the post-El Nio summer rainfall over EA, but through a different physical process: the WPAC re-emerges as a forced response to the rapid cooling in the eastern Pacific. The results suggest that the skillful prediction of rainfall over continental EA requires the accurate prediction of not only the strength and evolution of El Nio, but also the subseasonal migration of EA rainfall anomalies.     

A New Approach for Classifying Two Types of El Nino Events

In recent decades, the typical El Nio events with the warmest SSTs in the tropical eastern Pacific have become less common, and a different of El Nio with the warmest SSTs in the central Pacific, which is flanked on the east and west by cooler SSTs, has become more fre-quent. The more recent type of El Nio was referred to as central Pacific El Nio, warm pool El Nio, or dateline El Nio, or the El Nio Modoki. Central Pacific El Nio links to a different tropical-to-extratropical teleconnection and exerts different impacts on climate, and several classification approaches have been proposed. In this study, a new classification approach is proposed, which is based on the linear combination (sum or difference) of the two leading Empirical Orthogonal Functions (EOFs) of tropical Pacific Ocean sea surface temperature anomaly (SSTA), and the typical El Ni o index (TENI) and the central El Nio index (CENI) are able to be derived by projecting the observed SSTA onto these combinations. This classification not only reflects the characteristics of non-orthogonality between the two types of events but also yields one period peaking at approximate two to seven years. In particular, this classification can distin-guish the different impacts of the two types of events on rainfall in the following summer in East China. The typical El Nio events tend to induce intensified rainfall in the Yangtze River valley, whereas the central Pacific El Nio tends to induce intensified rainfall in the Huaihe River valley. Thus, the present approach may be appropriate for studying the impact of different types of El Nio on the East Asian climate.     

Parallel Comparison of the 1982/83,1997/98 and 2015/16 Super El Nios and Their Effects on the Extratropical Stratosphere

This study uses multiple sea surface temperature(SST) datasets to perform a parallel comparison of three super El Ni os and their effects on the stratosphere. The results show that, different from ordinary El Ni os, warm SST anomalies appear earliest in the western tropical Pacific and precede the super El Ni o peak by more than 18 months. In the previous winter,relative to the mature phase of El Ni o, as a precursor, North Pacific Oscillation-like circulation anomalies are observed. A Pacific–North America(PNA) teleconnection appears in the extratropical troposphere during the mature phase, in spite of the subtle differences between the intensities, as well as the zonal position, of the PNA lobes. Related to the negative rainfall response over the tropical Indian Ocean, the PNA teleconnection in the winter of 1997/98 is the strongest among the three super El Ni os. The northern winter stratosphere shows large anomalies in the polar cap temperature and the circumpolar westerly, if the interferences from other factors are linearly filtered from the circulation data. Associated with the positive PNA response in a super El Ni o winter, positive polar cap temperature anomalies and circumpolar easterly anomalies,though different in timing, are also observed in the mature winters of the three super El Ni os. The stratospheric polar vortex in the next winter relative to the 1982/83 and 1997/98 events is also anomalously weaker and warmer, and the stratospheric circulation conditions remain to be seen in the coming winter following the mature phase of the 2015/16 event.     

Decadal Variation of the Impact of La Niña on the Winter Arctic Stratosphere

The impact of La Ni?a on the winter Arctic stratosphere has thus far been an ambiguous topic of research. Contradictory results have been reported depending on the La Ni?a events considered. This study shows that this is mainly due to the decadal variation of La Ni?a's impact on the winter Arctic stratosphere since the late 1970 s. Specifically,during the period1951–78,the tropospheric La Ni?a teleconnection exhibits a typical negative Pacific–North America pattern,which strongly inhibits the propagation of the planetary waves from the extratropical troposphere to the stratosphere,and leads to a significantly strengthened stratospheric polar vortex. In contrast,during 1979–2015,the La Ni?a teleconnection shifts eastwards,with an anomalous high concentrated in the northeastern Pacific. The destructive interference of the La Ni?a teleconnection with climatological stationary waves seen in the earlier period reduces greatly,which prevents the drastic reduction of planetary wave activities in the extratropical stratosphere. Correspondingly,the stratospheric response shows a less disturbed stratospheric polar vortex in winter.     

The most sensitive initial error modes modulating intensities of CP- and EP- El Niño events

The two types of El Niño events simulated by the Geophysical Fluid Dynamics Laboratory Climate Model version 2p1 (GFDL CM2p1) model and its “spring predictability barrier” (SPB) associations are examined. By conducting the ensemble hindcast experiments related to the sea temperature on the whole Pacific, both of the predictions for CP- and EP-El Niño show a significant SPB phenomenon, but the CP-El Niño features a much weaker SPB compared to the EP-El Niño. Further analyses revealed that, for CP-El Niño events, the initial sea temperature errors of the North Pacific with triple-like shape, referred to as negative Victoria Mode (VM) induces the largest prediction errors in Niño4 areas and modulates the intensities of CP-El Niño events. While for EP-El Niño events, the initial sea temperature errors in the subsurface layer of the western equatorial Pacific and the upper layer of the Southeast Pacific (15–30ºS) with the meridional mode induce the largest prediction errors in Niño3 areas and modulates the intensities of EP-El Niño events. Obviously, results stress that, in order to reduce final prediction errors and obtain better predictions in terms of intensity on the two types of El Niño events, we should mainly focus on initial sea temperature accuracy in not only the subsurface layer of the west equatorial Pacific but also the surface layer of southeast Pacific and the region covered by the VM-like mode in the North Pacific.     

Understanding the SAM influence on the South Pacific ENSO teleconnection

The relationship between the El Niño Southern Oscillation (ENSO) and the Southern Hemisphere Annular Mode (SAM) is examined, with the goal of understanding how various strong SAM events modulate the ENSO teleconnection to the South Pacific (45°–70°S, 150°–70°W). The focus is on multi-month, multi-event variations during the last 50 years. A significant (p < 0.10) relationship is observed, most marked during the austral summer and in the 1970s and 1990s. In most cases, the significant relationship is brought about by La Niña (El Niño) events occurring with positive (negative) phases of the SAM more often than expected by chance. The South Pacific teleconnection magnitude is found to be strongly dependent on the SAM phase. Only when ENSO events occur with a weak SAM or when a La Niña (El Niño) occurs with a positive (negative) SAM phase are significant South Pacific teleconnections found. This modulation in the South Pacific ENSO teleconnection is directly tied to the interaction of the anomalous ENSO and SAM transient eddy momentum fluxes. During La Niña/SAM+ and El Niño/SAM? combinations, the anomalous transient momentum fluxes in the Pacific act to reinforce the circulation anomalies in the midlatitudes, altering the circulation in such a way to maintain the ENSO teleconnections. In La Niña/SAM? and El Niño/SAM+ cases, the anomalous transient eddies oppose each other in the midlatitudes, overall acting to reduce the magnitude of the high latitude ENSO teleconnection.     

Contrasting global teleconnection features of the eastern Pacific and central Pacific El Niño events

Being triggered by different physical processes, the eastern Pacific (EP) and central Pacific (CP) El Niño events have several different teleconnection features around the globe. Using the ERA-Interim re-analysis monthly data during the period 1980–2016, the El Niño-Southern Oscillation (ENSO) teleconnections on the global scale and their statistical significance are investigated, with an emphasis on the contrasting features of the EP and CP El Niño events. With some exceptions, the EP El Niño and La Niña have generally similar teleconnection patterns with the reversed sign, while in some parts of the globe different and occasionally contrasting teleconnections of the EP and CP El Niño events are identified. Compared to the CP El Niño, more regions of the world are influenced by the statistically significant positive surface pressure anomalies during the EP El Niño, particularly over the Indian Ocean, tropical Atlantic and Northern Africa. It is found that the mid-tropospheric geopotential height anomalies across the globe are significantly different during the EP and CP El Niño events. Associated with different surface pressure and mid-tropospheric geopotential height anomalies, precipitation anomalies in many regions of the world are found different during the EP and CP El Niño events, particularly over the tropical Pacific, central to eastern equatorial Atlantic and the eastern Sahara. While central and eastern equatorial Atlantic experience statistically significant negative (positive) rainfall anomalies during the EP El Niño (La Niña), the CP El Niño does not have a strong influence on the amount of annual rainfall over the equatorial Atlantic. For the first time, statistically significant anomalously dry conditions are found over some parts of the Middle East and Southwest Asia during La Niña, and over the eastern Sahara during the EP El Niño.     

A note on the simulation of winter monsoon anomalies during an El Niño year

Summary This paper presents the results of the Florida State University atmospheric general circulation model that addresses the impact of sea surface temperature anomalies on an El Niño year. Northern Hemisphere winter season simulation. Specifically, our interest is in the simulation of seasonal winter monsoonal rainfall, the planetary scale divergent motions and the westerly wind anomalies of an El Niño year.The El Niño episode of 1982–1983 was interesting due to its higher than average amplitude and its overall evolution. By late 1982 the anomalous circulations associated with the sea surface temperature forcing had begun to take shape even though the anomalies did not attain their peak amplitude until February 1983. The atmosphere-ocean teleconnections set up a strong pattern of geopotential height anomalies during the Northern Hemisphere winter that coincides with El Niño conditions in the tropical Pacific Ocean.Wallace and Gutzler (1981) defined a Pacific North American (PNA) teleconnection pattern index based on data from within this region. The El Niño episode of 1982–1983 has been shown to be strong via the PNA Index and illustrates an importance for climate models to correctly simulate these teleconnections. The importance of the forced anomalies can be seen in the long-range forecasting of conditions over North America as well as the winter monsoon intensity and location.In this study, we utilize a general circulation model with a resolution of triangular truncation at 42 waves to investigate the effects of prescribed sea surface temperature anomalies. We are able to simulate the majority of the large-scale atmospheric response although on regional climatic scales some phase shifts seem apparent.With 7 Figures     

中东太平洋ITCZ对两类厄尔尼诺SST异常的敏感性试验

利用MPAS-A(The Model for Prediction Across Scales-Atmosphere)模式设计了中东太平洋热带辐合带CEP-ITCZ(Intertropical Convergence Zone over Central and Eastern Pacific)对两类厄尔尼诺SST(Sea Surface Temperature)异常的敏感性试验,通过试验结果与两类厄尔尼诺年实际大气异常的对比,初步解释了CEP-ITCZ在两类厄尔尼诺年产生不同异常的可能原因。通过CP-EL试验发现,热带太平洋SST异常的第一模态会使中东太平洋低层风场辐合增强,但对辐合带的位置影响不大,与中部型厄尔尼诺对CEP-ITCZ的影响基本一致。通过EP-EL试验发现,热带太平洋SST异常的第二模态会使中东太平洋低层风场产生较大异常,辐合带中心向南移动,辐合带明显减弱增宽,与东部型厄尔尼诺对CEP-ITCZ的影响基本一致。