Asymmetry of Atmospheric Responses to Two-Type El Ni?o and La Ni?a over Northwest Pacific

The mechanism for asymmetric atmospheric responses to the central Pacific(CP) El Ni?o and La Ni?a over the western North Pacific(WNP) is studied in this paper. The negative anomalies of rainfall over the key region of WNP are explained by diagnosing the column-integrated equations of moisture and moist static energy(MSE). It is revealed that the nonlinear advection of moist enthalpy is critical to introduce negative rainfall anomalies over the region. The anomalous easterly(westerly) in La Ni?a(CP El Ni?o) causes negative advection of anomalous moist enthalpy, inducing negative heating anomaly and an anticyclone anomaly in the WNP, which weakens(strengthens) the cyclone(anticyclone) in La Ni?a(CP El Ni?o). The MSE budget analysis shows a larger nonlinear term in CP El Ni?o than in eastern Pacific(EP) El Ni?o, inconsistent with the amplitudes of sea surface temperature anomalies. The reason is that the nonlinear term transforms to positive above 700 h Pa in EP El Ni?o, offsetting the negative advection below 700 h Pa and thus making the nonlinear term smaller. The nonlinear term is negative at low levels in CP El Ni?o, resulting in a larger nonlinear term. The stronger precipitation anomalies in the WNP during EP El Ni?o can be attributed to the linear moist enthalpy advection. The mean easterly wind at mid levels causes a larger(smaller) positive moist enthalpy advection in CP(EP) El Ni?o, due to a larger(smaller) moist enthalpy gradient, resulting in a positive(negative) linear moist enthalpy advection, which weakens(strengthens) the negative precipitation anomalies in the key region.     

Understanding the Development of the 2018/19 Central Pacific El Ni?o

A central Pacific(CP) El Ni?o event occurred in 2018/19. Previous studies have shown that different mechanisms are responsible for different subtypes of CP El Ni?o events(CP-I El Ni?o and CP-II El Ni?o). By comparing the evolutions of surface winds, ocean temperatures, and heat budgets of the CP-I El Ni?o, CP-II El Ni?o, and 2018/19 El Ni?o, it is illustrated that the subtropical westerly anomalies in the North Pacific, which led to anomalous convergence of Ekman flow and surface warming in the ...     

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.     

Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Ni?o Events

The inverse relationship between the warm phase of the El Ni?o Southern Oscillation(ENSO) and the Indian Summer Monsoon Rainfall(ISMR) is well established. Yet, some El Ni?o events that occur in the early months of the year(boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Ni?o(springtime)...     

Sensitivity Difference in the Extratropical Atmosphere to Two Types of El Nio Events

A comparison of sensitivity in extratropical circulation in the Northern Hemisphere(NH)and Southern Hemisphere(SH)is conducted through observational analyses and diagnostic linear model experiments for two types of El Nio events,the traditional El Nio with the strongest warmth in the eastern tropical Pacific(EP El Nio)and the El Nio Modoki with the strongest warmth in the central tropical Pacific(CP El Nio).It is shown that CP El Nio favors the occurrence of a negative-phase Northern Annular Mode(NAM),while EP El Nio favors that of the Pacific-North American(PNA)pattern.In SH,both EP and CP El Nio induce a negative phase Southern Annular Mode(SAM).However,the former has a greater amplitude,which is consistent with the stronger sea surface temperature(SST)warmth.The difference in the two types of El Nio events in NH may originate from the dependence of heating-induced extratropical response on the location of initial heating,which may be associated with activity of the stationary wave.In SH,the lack of sensitivity to the location of heating can be associated with weaker activity of the stationary wave therein.     

Impact of Pacific Decadal Oscillation on Frequency Asymmetry of El Nio and La Nia Events

正El Nio–Southern Oscillation(ENSO)is an abnormal sea surface warming or cooling phenomenon over the tropical Pacific,which also has severe global impact.Interestingly,ENSO characteristics are changing with climate change(e.g.,Collins et al.,2010).Therefore,it is expected that ENSO can be modulated on the decadal time scale,particularly when the tropical climate background state is fluctuating strongly     

Roles of Multi-Scale Disturbances over the Tropical North Pacific in the Turnabout of 1997-98 El Ni■o

The space-time features of major vorticity disturbances over the western North Pacific during the 1997-98 E1 Ni(?)o ranked as one of the strongest events on record was investigated in this study.We distinguished the different roles that these disturbances had on different timescales in causing the reversal or turnabout of the E1 Ni(?)o event.Remarkable differences in the various disturbances of synoptic,intraseasonal,and interannual timescales were found in the time evolution,propagation,and in their contributions to the changes in near- equatorial zonal flow,which was crucial to the demise of the warm sea surface temperature anomalies in the central-eastern Pacific.It is hypothesized that the westward-traveling synoptic and intraseasonal oscillations in the western North Pacific might be considered as a self-provided negative feedback from the E1 Ni(?)o and played an additional role in its reversal in comparison with other interannual internal and external forcings. In this case,the off-equatorial synoptic and intraseaonal fluctuations served as a stochastic forcing for the tropical ocean and gave rise to the aperiodicity or irregularity of the E1 Ni(?)o-Southern Oscillation.     

Meridional Wind Stress Anomalies over the Tropical Pacific and the Onset of El Ni?o Part Ⅱ: Dynamical Analysis

The data analyses in the first part of this study have shown that the sea surface temperature anomalies (SSTA) in the eastern equatorial Pacific are significantly correlated with the preceding anomalous convergence of the meridional wind stress near the equator. In order to understand the dynamical role of the convergent meridional wind stress anomalies in the El Nino occurring, an ideal wind stress which converges about the equator is set up based on the observations revealed in the first part. A simple dynamical model of tropical ocean is used to study the response of the tropical ocean to the convergent meridional wind stress. The results show that the convergent wind stress in the eastern equatorial Pacific is favorable for the occurrence of El Nino. When the convergent wind stress exerts on the tropical ocean, the westward propagating Rossby wave is excited, which, on the one hand, makes the mixed layer near the equator become thicker. On the other hand, the westward oceanic currents associated with the Rossby wave appear in the vicinity of the equator. The oceanic currents can drive the upper layer sea water to transfer to the west, which is favorable for the sea water to pile up in the western equatorial Pacific and to accumulate energy for the upcoming warm event.     

Are the teleconnections of Central Pacific and Eastern Pacific El Niño distinct in boreal wintertime?

A meteorological reanalysis dataset and experiments of the Goddard Earth Observing System Chemistry-Climate Model, Version 2 (GEOS V2 CCM) are used to study the boreal winter season teleconnections in the Pacific-North America region and in the stratosphere generated by Central Pacific and Eastern Pacific El Niño. In the reanalysis data, the sign of the North Pacific and stratospheric response to Central Pacific El Niño is sensitive to the composite size, the specific Central Pacific El Niño index used, and the month or seasonal average that is examined, highlighting the limitations of the short observational record. Long model integrations suggest that the response to the two types of El Niño are similar in both the extratropical troposphere and stratosphere. Namely, both Central Pacific and Eastern Pacific El Niño lead to a deepened North Pacific low and a weakened polar vortex, and the effects are stronger in late winter than in early winter. However, the long experiments do indicate some differences between the two types of El Niño events regarding the latitude of the North Pacific trough, the early winter polar stratospheric response, surface temperature and precipitation over North America, and globally averaged surface temperature. These differences are generally consistent with, though smaller than, those noted in previous studies.     

Simulation of the western North Pacific subtropical high in El Ni?o decaying summers by CMIP5 AGCMs

本文分析了耦合模式比较计划第五阶段(CMIP5)中的各大气环流模式对西北太平洋副热带高压系统在厄尔尼诺衰减期夏季的模拟。结果表明大部分模式都能够重现厄尔尼诺衰减期夏季的副高所伴随的大尺度环流的空间分布特征。大多数模式都可以合理地模拟出副高在厄尔尼诺衰减期夏季的位置相对于其气候平均态位置的向西偏移。这些模式对7至8月副高夏季季节内北跳的模拟强于对其东撤的模拟;几乎所有的模式都可以准确地模拟出副高的北跳,而只有三分之一的模式能够模拟出副高的东撤。模式中的副高在厄尔尼诺衰减期夏季位置的偏差很大程度上取决于其气候平均态位置的偏差。     

The Linkage between Two Types of El Ni?o Events and Summer Streamflow over the Yellow and Yangtze River Basins

It is generally agreed that El Ni?o can be classified into East Pacific(EP) and Central Pacific(CP) types. Nevertheless,little is known about the relationship between these two types of El Ni?o and land surface climate elements. This study investigates the linkage between EP/CP El Ni?o and summer streamflow over the Yellow and Yangtze River basins and their possible mechanisms. Over the Yellow River basin, the anomalous streamflow always manifests as positive(negative)in EP(CP) years, with a cor...     

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 study of climate model responses of the western Pacific subtropical high to El Niño diversity

Climate Dynamics - This study examines a climate model hindcast of the responses of the western Pacific subtropical high (WPSH) to three types of El Niño events: the Eastern Pacific (EP) El...     

Dependency of typhoon intensity and genesis locations on El Ni?o phase and SST shift over the western North Pacific

The effects of the El Ni?o-Southern Oscillation (ENSO) phase and the shifting of the ENSO sea surface temperature (SST) on the intensity of tropical cyclones (TC) have been extensively investigated in terms of TC genesis locations in the western North Pacific (WNP). To advance the hypothesis for a relation of genesis location–intensity that the TC formation location hints its intensity, two cases have been compared, which include the phase of the decaying El Ni?o turning over to La Ni?a (type I) and the phase that recovers to a neutral condition (type II). In addition, the shift of ENSO SST to the central Pacific warming (CPW) from the East Pacific warming (EPW) has been examined. The genesis potential index (GPI) and the accumulated cyclone energy have been applied to compare the differences between the ENSO phase and the TC formation location. It was apparent that ENSO influences the WNP typhoon formation location depending on the cycle of the ENSO phase. In addition, the typhoon activity was affected by the zonal shift of the El Ni?o SST. The CPW, which has maximum SST over the central Pacific, tends to have a persistently high GPI over the WNP in September–November and June–August, demonstrating that the formation locations of strong TCs significantly shift southeastward compared with the EPW having SST maximum over the eastern Pacific. CPW years revealed a distinguishable relationship between the TC formation location and the TC between the tropical depression (TD) + tropical storm (TS) and the intense typhoon of category 4?+?5.     

Impact of El Niño onset timing on the Indian Ocean: Pacific coupling and subsequent El Niño evolution

A relation between the timing of the El Niño onset and its subsequent evolution is examined by emphasizing its association with the Indian Ocean (IO) SST variation. Two types of El Niño events based on the timing of their onset are classified and their characteristics are examined and compared. In general, spring onset (SP) events grow greater in magnitude and their evolutions have a faster transition. On the contrary, summer onset (SU) events are relatively weaker in magnitude and have a slower transition. Moreover, in contrast to the SU events, the SP events have a strong tendency for accompanying an IO dipole and basin-wide type of warming pattern in the El Niño developing and mature phases, respectively. It is demonstrated here that the distinctive evolutions in transition phase of the two events are resulted from the difference in IO SST. The warm IO SST in the SP El Niño event, lead an anomalous easterlies over the western Pacific, which forces a fast termination of El Niño events.     

Theories on Formation of an Anomalous Anticyclone in Western North Pacific during El Ni?o: A Review

The western North Pacific anomalous anticyclone(WNPAC) is an important atmospheric circulation system that conveys El Ni?o impact on East Asian climate. In this review paper, various theories on the formation and maintenance of the WNPAC, including warm pool atmosphere–ocean interaction, Indian Ocean capacitor, a combination mode that emphasizes nonlinear interaction between ENSO and annual cycle, moist enthalpy advection/Rossby wave modulation, and central Pacific SST forcing, are discussed. It is concluded that local atmosphere–ocean interaction and moist enthalpy advection/Rossby wave modulation mechanisms are essential for the initial development and maintenance of the WNPAC during El Ni?o mature winter and subsequent spring. The Indian Ocean capacitor mechanism does not contribute to the earlier development but helps maintain the WNPAC in El Ni?o decaying summer.The cold SST anomaly in the western North Pacific, although damped in the summer, also plays a role. An interbasin atmosphere–ocean interaction across the Indo-Pacific warm pool emerges as a new mechanism in summer. In addition, the central Pacific cold SST anomaly may induce the WNPAC during rapid El Ni?o decaying/La Ni?a developing or La Ni?a persisting summer. The near-annual periods predicted by the combination mode theory are hardly detected from observations and thus do not contribute to the formation of the WNPAC. The tropical Atlantic may have a capacitor effect similar to the tropical Indian Ocean.     

Variability of the western Pacific warm pool structure associated with El Niño

Climate Dynamics - Sea surface temperature (SST) structure inside the western Pacific warm pool (WPWP) is usually overlooked because of its distinct homogeneity, but in fact it possesses a clear...     

An Introduction to the Integrated Climate Model of the Center for Monsoon System Research and Its Simulated Influence of El Ni?no on East Asian–Western North Pacific Climate

This study introduces a new global climate model—the Integrated Climate Model(ICM)—developed for the seasonal prediction of East Asian–western North Pacific(EA–WNP) climate by the Center for Monsoon System Research at the Institute of Atmospheric Physics(CMSR, IAP), Chinese Academy of Sciences. ICM integrates ECHAM5 and NEMO2.3 as its atmospheric and oceanic components, respectively, using OASIS3 as the coupler. The simulation skill of ICM is evaluated here, including the simulated climatology, interannual variation, and the influence of El Nińo as one of the most important factors on EA–WNP climate. ICM successfully reproduces the distribution of sea surface temperature(SST) and precipitation without climate shift, the seasonal cycle of equatorial Pacific SST, and the precipitation and circulation of East Asian summer monsoon. The most prominent biases of ICM are the excessive cold tongue and unrealistic westward phase propagation of equatorial Pacific SST. The main interannual variation of the tropical Pacific SST and EA–WNP climate—El Nińo and the East Asia–Pacific Pattern—are also well simulated in ICM, with realistic spatial pattern and period. The simulated El Nińo has significant impact on EA–WNP climate, as in other models. The assessment shows ICM should be a reliable model for the seasonal prediction of EA–WNP climate.     

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.