Impact of El Nio on atmospheric circulations over East Asia and rainfall in China: Role of the anomalous western North Pacific anticyclone

This paper presents a review on the impact of El Nio on the interannual variability of atmospheric circulations over East Asia and rainfall in China through the anomalous anticyclone over western North Pacific(WNPAC). It explains the formation mechanisms of the WNPAC and physical processes by which the WNPAC affects the rainfall in China. During the mature phase of El Nio, the convective cooling anomalies over western tropical Pacific caused by the weakened convections trigger up an atmospheric Rossby wave response, resulting in the generation of the WNPAC. The WNPAC can persist from the winter when the El Nio is in its peak to subsequent summer, which is maintained by multiple factors including the sustained presence of convective cooling anomalies and the local air-sea interaction over western tropical Pacific, and the persistence of sea surface temperature anomalies(SSTA) in tropical Indian and tropical North Atlantic. The WNPAC can influence the atmospheric circulations over East Asia and rainfall in China not only simultaneously, but also in the subsequent summer after an El Nio year, leading to more rainfall over southern China. The current paper also points out that significant anomalies of atmospheric circulations over East Asia and rainfall over southern China occur in El Nio winter but not in La Nio winter, suggesting that El Nio and La Nio have an asymmetric effect. Other issues, including the impact of El Nio diversity and its impact as well as the relations of the factors affecting the persistence of the WNPAC with summer rainfall anomalies in China, are also discussed. At the end of this paper some issues calling for further investigation are discussed.     

Interannual modulation of intraseasonal sea level variability along the southern coast of Java

The intraseasonal variability(ISV) of sea level anomalies(SLAs) along the southern coast of Java and its interannual modulation were studied based on a gridded SLA product produced from the Archiving, Validation, and Interpretation of Satellite Oceanography dataset. This ISV is induced by the propagation of intraseasonal Kelvin waves derived from the central equatorial Indian Ocean(EIO). Wavelet analysis and empirical mode decomposition of intraseasonal SLAs along the southern coast of Java showed interannual variability, with weaker ISV events during El Ni years and positive Indian Ocean Dipole(IOD) years than during normal years. This interannual modulation of the ISV is influenced by the El Ni-Southern Oscillation teleconnection via the Walker Circulation and eastern Indian Ocean upwelling connected to IOD events. The anomalously weaker Walker Circulation during El Ni events generates anomalous surface easterlies over the central-eastern tropical Indian Ocean that produce upwelling Kelvin waves in the EIO and offshore water transport along the southern coasts of Sumatra and Java, resulting in negative SLAs along the southern coast of Java. These negative SLAs damp the positive SLAs induced by the eastward propagation of downwelling Kelvin waves from the central EIO during the following March–May of El Ni years. Similar features of SLAs and sea surface wind anomalies also occur during positive IOD years. Consequently, the sea level ISV along the southern coast of Java is weaker in El Ni and positive IOD years.     

A theoretical investigation of the tropical Indo-Pacific tripole mode

The El Ni o-Southern Oscillation(ENSO)phenomenon in the tropical Pacific has been a focus of ocean and climate studies in the last few decades.Recently,the short-term climate variability in the tropical Indian Ocean has attracted increasingly more attention,especially with the proposition of the Indian Ocean Dipole(IOD)mode.However,these phenomena are often studied separately without much consideration of their interaction.Observations reveal a striking out-of-phase relationship between zonal gradients of sea surface height anomaly(SSHA)and sea surface temperature anomaly(SSTA)in the tropical Indian and Pacific Oceans.Since the two oceans share the ascending branch of the Walker cells over the warm pool,the variation within one of them will affect the other.The accompanied zonal surface wind anomalies are always opposite over the two basins,thus producing a tripole structure with opposite zonal gradients of SSHA/SSTA in the two oceans.This mode of variability has been referred to as Indo-Pacific Tripole(IPT).Based on observational data analyses and a simple ocean-atmosphere coupled model,this study tries to identify the characteristics and physical mechanism of IPT with a particular emphasis on the relationships among ENSO,IOD,and IPT.The model includes the basic oceanic and atmospheric variables and the feedbacks between them,and takes into account the inter-basin connection through an atmospheric bridge,thus providing a valuable framework for further research on the short-term tropical climate variability.     

Observational analysis and numerical simulation of the interannual variability of the boreal winter Hadley circulation over the recent 30 years

The interannual variability of the boreal winter (DJF) Hadley Cell strength during 1979-2008 is investigated using NCEP/NCAR reanalysis data. The results of AMIP simulation of LASG/IAP AGCM GAMIL2.0 are compared against the reanalysis data. Both the reanalysis data and the simulation show that the interannual variability of the Hadley Cell strength has a non-uniform spatial distribution, as evidenced by the 1st Empirical Orthogonal Function (EOF) mode. The change of Hadley cell strength in the tropics is opposite to that in the subtropical regions. Our analysis indicates that a positive phase of EOF1 is associated with an El Ni o-like warmer equatorial central and eastern Pacific and a warmer southern Indian Ocean. Above features are also seen in the results of GAMIL2.0 simulation, indicating that the interannual variability of the Hadley Cell strength is driven by the tropical ocean variability. Our analysis also demonstrates that the contribution of the warmer central-eastern Pacific to the 1st EOF mode is larger than that of the South Indian Ocean. The SST forcing enhances the local Hadley circulation strength in the central Pacific and Africa (30°S-30°N, 150°E-90°W), while it weakens the local Hadley circulation in other regions (30°S-30°N, 90°-10°W). The western Pacific anticyclone remotely driven by the El Ni o forcing leads to a weakened local Hadley cell in the Northern Hemisphere, while the South Indian Ocean anticyclone driven by the remote El Ni o forcing and the local warmer SST anomalies in the southern Indian Ocean results in a weakened local Hadley Cell in the Southern Hemisphere. The enhancement of the Pacific local Hadley Cell is stronger (weaker) than that of the Atlantic, the western Pacific, and the southern Indian Ocean in the tropical (subtropical) part, thus for the zonal mean condition the strength of the total Hadley Cell is stronger (weaker) in the tropical (subtropical) limb. The amplitude of the Hadley Cell change in the Northern Hemisphere is stronger than that in the Southern Hemisphere. Hence the leading interannual variability mode of boreal winter Hadley Cell exhibits a non-uniform spatial pattern.     

Two interannual variability modes of the Northwestern Pacific Subtropical Anticyclone in boreal summer

Using the reanalysis data and 20th century simulation of coupled model FGOALS_gl developed by LASG/IAP, we identified two distinct interannual modes of Northwestern Pacific Subtropical Anticyclone (NWPAC) by performing Empirical Orthogonal Function (EOF) analysis on 850 hPa wind field over the northwestern Pacific in summer. Based on the associated anomalous equatorial zonal wind, these two modes are termed as "Equatorial Easterly related Mode" (EEM) and "Equatorial Westerly related Mode" (EWM), respectively. The formation mechanisms of these two modes are similar, whereas the maintenance mechanisms, dominant periods, and the relationships with ENSO are different. The EEM is associated with El Ni o decaying phase, with the anomalous anticyclone established in the preceding winter and persisted into summer through local positive air-sea feedback. By enhancing equatorial upwelling of subsurface cold water, EEM favors the transition of ENSO from El Ni o to La Ni a. The EWM is accompanied by the El Ni o events with long persistence, with the anomalous anticyclone formed in spring and strengthened in summer due to the warm Sea Surface Temperature anomalies (SSTA) forcing from the equatorial central-eastern Pacific. The model well reproduces the spatial patterns of these two modes, but fails to simulate the percentage variance accounted for by the two modes. In the NCEP reanalysis (model result), EEM (EWM) appears as the firstmode, which accounts for 35.6% (68.2%) of the total variance.     

Using CMIP5 model outputs to investigate the initial errors that cause the “spring predictability barrier” for El Nio events

Most ocean-atmosphere coupled models have difficulty in predicting the El Nio-Southern Oscillation(ENSO) when starting from the boreal spring season. However, the cause of this spring predictability barrier(SPB) phenomenon remains elusive. We investigated the spatial characteristics of optimal initial errors that cause a significant SPB for El Nio events by using the monthly mean data of the pre-industrial(PI) control runs from several models in CMIP5 experiments. The results indicated that the SPB-related optimal initial errors often present an SST pattern with positive errors in the central-eastern equatorial Pacific, and a subsurface temperature pattern with positive errors in the upper layers of the eastern equatorial Pacific, and negative errors in the lower layers of the western equatorial Pacific. The SPB-related optimal initial errors exhibit a typical La Ni-a-like evolving mode, ultimately causing a large but negative prediction error of the Nio-3.4 SST anomalies for El Nio events. The negative prediction errors were found to originate from the lower layers of the western equatorial Pacific and then grow to be large in the eastern equatorial Pacific. It is therefore reasonable to suggest that the El Nio predictions may be most sensitive to the initial errors of temperature in the subsurface layers of the western equatorial Pacific and the Nio-3.4 region, thus possibly representing sensitive areas for adaptive observation. That is, if additional observations were to be preferentially deployed in these two regions, it might be possible to avoid large prediction errors for El Nio and generate a better forecast than one based on additional observations targeted elsewhere. Moreover, we also confirmed that the SPB-related optimal initial errors bear a strong resemblance to the optimal precursory disturbance for El Nio and La Nia events. This indicated that improvement of the observation network by additional observations in the identified sensitive areas would also be helpful in detecting the signals provided by the precursory disturbance, which may greatly improve the ENSO prediction skill.     

Climatic significance of δ~(18)O records from an 80.36 m ice core in the East Rongbuk Glacier, Mount Qomolangma (Everest)

The δ18O variations in an 80.36 m ice core retrieved in the accumulation zone of the East Rongbuk Glacier, Mount Qomolangma (Everest), is not consistent with changes of air temperature from both southern and northern slopes of Himalayas, as well as these of the temperature anomalies over the Northern Hemisphere. The negative relationship between the δ18O and the net accumulation records of the ice core suggests the "amount effect" of summer precipitation on the δ18O values in the region. Therefore, the δ18O records of the East Rongbuk ice core should be a proxy of Indian Summer Monsoon intensity, which shows lower δ18O values during strong monsoon phases and higher values during weak phases.     

Processes involved in the second-year warming of the 2015 El Nio event as derived from an intermediate ocean model

The tropical Pacific experienced a sustained warm sea surface condition that started in 2014 and a very strong El Nio event in 2015. One striking feature of this event was the horseshoe-like pattern of positive subsurface thermal anomalies that was sustained in the western-central equatorial Pacific throughout 2014–2015. Observational data and an intermediate ocean model are used to describe the sea surface temperature(SST) evolution during 2014–2015. Emphasis is placed on the processes involved in the 2015 El Nio event and their relationships with SST anomalies, including remote effects associated with the propagation and reflection of oceanic equatorial waves(as indicated in sea level(SL) signals) at the boundaries and local effects of the positive subsurface thermal anomalies. It is demonstrated that the positive subsurface thermal anomaly pattern that was sustained throughout 2014–2015 played an important role in maintaining warm SST anomalies in the equatorial Pacific. Further analyses of the SST budget revealed the dominant processes contributing to SST anomalies during 2014–2015. These analyses provide an improved understanding of the extent to which processes associated with the 2015 El Nio event are consistent with current El Nio and Southern Oscillation theories.     

On the “spring predictability barrier” for strong El Nio events as derived from an intermediate coupled model ensemble prediction system

Using predictions for the sea surface temperature anomaly(SSTA) generated by an intermediate coupled model(ICM)ensemble prediction system(EPS), we first explore the "spring predictability barrier"(SPB) problem for the 2015/16 strong El Nio event from the perspective of error growth. By analyzing the growth tendency of the prediction errors for ensemble forecast members, we conclude that the prediction errors for the 2015/16 El Nio event tended to show a distinct season-dependent evolution, with prominent growth in spring and/or the beginning of the summer. This finding indicates that the predictions for the 2015/16 El Nio occurred a significant SPB phenomenon. We show that the SPB occurred in the 2015/16 El Nio predictions did not arise because of the uncertainties in the initial conditions but because of model errors. As such, the mean of ensemble forecast members filtered the effect of model errors and weakened the effect of the SPB, ultimately reducing the prediction errors for the 2015/16 El Nio event. By investigating the model errors represented by the tendency errors for the SSTA component,we demonstrate the prominent features of the tendency errors that often cause an SPB for the 2015/16 El Nio event and explain why the 2015/16 El Nio was under-predicted by the ICM EPS. Moreover, we reveal the typical feature of the tendency errors that cause not only a significant SPB but also an aggressively large prediction error. The feature is that the tendency errors present a zonal dipolar pattern with the west poles of positive anomalies in the equatorial western Pacific and the east poles of negative anomalies in the equatorial eastern Pacific. This tendency error bears great similarities with that of the most sensitive nonlinear forcing singular vector(NFSV)-tendency errors reported by Duan et al. and demonstrates the existence of an NFSV tendency error in realistic predictions. For other strong El Nio events, such as those that occurred in 1982/83 and 1997/98, we obtain the tendency errors of the NFSV structure, which cause a significant SPB and yield a much larger prediction error. These results suggest that the forecast skill of the ICM EPS for strong El Nio events could be greatly enhanced by using the NFSV-like tendency error to correct the model.     

Sea surface salinity-derived indexes for distinguishing two types of El Ni?o events in the tropical Pacific

In this study, sea surface salinity(SSS) indexes are derived from reanalysis and observational datasets to distinguish the two types of(Central Pacific(CP) and Eastern Pacific(EP)) El Ni?o events in the tropical Pacific. Based on the SSS anomalous spatial and temporal pointwise correlations with sea surface temperature(SST) indexes of two types of El Ni?o events, the key areas with SSS variations for EP and CP El Ni?o events are identified. For EP El Ni?o events, the key areas are located over an arcuate area centered at(0°, 130°E) and in the central equatorial Pacific covering(5°S–5°N, 175°W–158°W). For CP El Ni?o events, the key areas are located in the northeastern western Pacific covering(2°N, 142°E–170°E) and in the southeastern Pacific covering(20°S–10°S, 135°W–95°W). The key areas for EP and CP El Ni?o events in this study are not located near the dateline in the equatorial Pacific and differ from those obtained from the regression or composite methods.Accordingly, these key areas are used to construct SSS indexes, termed as the CP/EP El Ni?o SSS index(CSI/ESI), to distinguish EP and CP El Ni?o events independently. The SSS indexes are verified by different datasets over varying time periods and they can be adequately used to identify the two types of El Ni?o events and serve as another useful tool for monitoring ENSO. These analyses offer novel insight into how to represent the diversity of El Ni?o events.     

An intermediate coupled model for the tropical ocean-atmosphere system

An intermediate ocean-atmosphere coupled model is developed to simulate and predict the tropical interannual variability. Originating from the basic physical framework of the Zebiak-Cane(ZC) model, this tropical intermediate couple model(TICM) extends to the entire global tropics, with a surface heat flux parameterization and a surface wind bias correction added to improve model performance and inter-basin connections. The model well reproduces the variabilities in the tropical Pacific and Indian basins. The simulated El Ni?o-Southern Oscillation(ENSO) shows a period of 3–4 years and an amplitude of about 2°C, similar to those observed. The variabilities in the Indian Ocean, including the Indian Ocean basin mode(IOBM) and the Indian Ocean Dipole(IOD), are also reasonably captured with a realistic relationship to the Pacific. However, the tropical Atlantic variability in the TICM has a westward bias and is overly influenced by the tropical Pacific. A 47-year hindcast experiment using the TICM for the period of 1970–2016 indicates that ENSO is the most predictable mode in the tropics. Skillful predictions of ENSO can be made one year ahead, similar to the skill of the latest version of the ZC model, while a "spring predictability barrier" still exists as in other models. In the tropical Indian Ocean, the predictability seems much higher in the west than in the east. The correlation skill of IOD prediction reaches 0.5 at a 5-month lead, which is comparable to that of the state-of-the-art coupled general circulation models. The prediction of IOD shows a significant "winter-spring predictability barrier", implying combined influences from the tropical Pacific and the local sea-air interaction in the eastern Indian Ocean. The TICM has little predictive skill in the equatorial Atlantic for lead times longer than 3 months, which is a common problem of current climate models badly in need of further investigation.     

Mixed-layer water oscillations in tropical Pacific for ENSO cycle

The main modes of interannal variabilities of thermocline and sea surface wind stress in the tropical Pacific and their interactions are investigated,which show the following results.(1) The thermocline anomalies in the tropical Pacific have a zonal dipole pattern with 160°W as its axis and a meridional seesaw pattern with 6-8°N as its transverse axis.The meridional oscillation has a phase lag of about 90° to the zonal oscillation,both oscillations get together to form the El Ni?o/La Ni?a cycle,which be-haves as a mixed layer water oscillates anticlockwise within the tropical Pacific basin between equator and 12°N.(2) There are two main patterns of wind stress anomalies in the tropical Pacific,of which the first component caused by trade wind anomaly is characterized by the zonal wind stress anomalies and its corresponding divergences field in the equatorial Pacific,and the abnormal cross-equatorial flow wind stress and its corresponding divergence field,which has a sign opposite to that of the equatorial region,in the off-equator of the tropical North Pacific,and the second component represents the wind stress anomalies and corresponding divergences caused by the ITCZ anomaly.(3) The trade winds anomaly plays a decisive role in the strength and phase transition of the ENSO cycle,which results in the sea level tilting,provides an initial potential energy to the mixed layer water oscillation,and causes the opposite thermocline displacement between the west side and east side of the equator and also between the equator and 12°N of the North Pacific basin,therefore determines the amplitude and route for ENSO cycle.The ITCZ anomaly has some effects on the phase transition.(4) The thermal anomaly of the tropical western Pacific causes the wind stress anomaly and extends eastward along the equator accompanied with the mixed layer water oscillation in the equatorial Pacific,which causes the trade winds anomaly and produces the anomalous wind stress and the corresponding divergence in favor to conduce the oscillation,which in turn intensifies the oscillation.The coupled system of ocean-atmo-sphere interactions and the inertia gravity of the mixed layer water oscillation provide together a phase-switching mechanism and interannual memory for the ENSO cycle.In conclusion,the ENSO cycle essentially is an inertial oscillation of the mixed layer water induced by both the trade winds anomaly and the coupled ocean-atmosphere interaction in the tropical Pacific basin between the equator and 12°N.When the force produced by the coupled ocean-atmosphere interaction is larger than or equal to the resistance caused by the mixed layer water oscillation,the oscillation will be stronger or maintain as it is,while when the force is less than the resistance,the oscillation will be weaker,even break.     

Genesis of the 2014–2016 El Nio events

The 2015/2016 El Nio was one of the strongest El Nio events in history, and this strong event was preceded by a weak El Nio in 2014. This study systematically analyzed the dynamical processes responsible for the genesis of these events. It was found that the weak 2014 El Nio had two warming phases, the spring-summer warming was produced by zonal advection and downwelling Kelvin waves driven by westerly wind bursts(WWBs), and the autumn-winter warming was produced by meridional advection, surface heating as well as downwelling Kelvin waves. The 2015/2016 extreme El Nio, on the other hand, was primarily a result of sustained zonal advection and downwelling Kelvin waves driven by a series of WWBs, with enhancement from the Bjerknes positive feedback. The vast difference between these two El Nio events mainly came from the different amount of WWBs in 2014 and 2015. As compared to the 1982/1983 and 1997/1998 extreme El Nio events, the 2015/2016 El Nio exhibited some distinctive characteristics in its genesis and spatial pattern. We need to include the effects of WWBs to the theoretical framework of El Nio to explain these characteristics, and to improve our understanding and prediction of El Nio.     

Grain size records reveal variability of the East Asian Winter Monsoon since the Middle Holocene in the Central Yellow Sea mud area,China

Three cores(ZY-1,ZY-2 and ZY-3) retrieved from the Central Yellow Sea mud(CYSM) were analyzed in sensitive grain size and AMS 14 C dating to reconstruct the history of the East Asian Winter Monsoon(EAWM) since the Middle Holocene in the study area.The results show that these data provide a continuous history of the EAWM over the past 7.2 ka and that the EAWM can be divided into three periods:strong and highly fluctuating during 7.2-4.2 ka BP;moderate and relatively stable during 4.2-1.8 ka BP;and weakened during 1.8-0 ka BP.Compared with the East Asian Summer Monsoon(EASM) recorded in the previous studies,the evolutionary history of the EAWM broadly follows the orbital-derived winter insolation with a similar long-term step-decreased trend as the EASM.At the centennial scale,however,the EAWM intensified events correlate well with the EASM weakened events and the North Atlantic climatic variations(Bond events 0 to 5) within the dating error,most likely forced by the reduction of solar irradiance through changes in the oceanic-atmospheric circulation patterns.     

Seasonally evolving dominant interannual variability mode of air-sea CO_2 flux over the western North Pacific simulated by CESM1-BGC

We applied a season-reliant empirical orthogonal function(S-EOF) analysis based on the results of the Community Earth System Model, version 1-Biogeochemistry, to seasonal mean air-sea CO_2 flux over the western North Pacific(WNP)(0°–35°N, 110°E–150°E). The first leading mode accounts for 29% of the total interannual variance, corresponding to the evolution of the El Ni-Southern Oscillation(ENSO) from its developing to decaying phases. During the ENSO developing phase in the summer and fall, the contribution of surface seawater CO_2 partial pressure anomalies is greater than that of gas transfer/solubility anomalies, which contribute to increasing oceanic CO_2 uptake over the WNP. During the ENSO mature phase in the winter, the anomalous southwesterly northwest of the western North Pacific anticyclone(WNPAC) reduces the surface wind speed in the China marginal sea and thus decreases oceanic CO_2 uptake by reducing the gas transfer coefficient. In the subsequent spring, the WNPAC maintains with an eastward shift in position. The anomalous southwesterly warms sea surface temperatures in the China marginal sea by reducing evaporation and thus decreases oceanic CO_2 uptake by enhancing surface seawater CO_2 partial pressure. This process, rather than the effect of decreasing gas transfer coefficient, dominates CO_2 flux anomalies in the spring.     

Evolution of the 2015/16 El Nio and historical perspective since 1979

The 2015/16 El Nio developed from weak warm conditions in late 2014 and NINO3.4 reached 3℃ in November 2015. We describe the characteristics of the evolution of the 2015/16 El Nio using various data sets including SST, surface winds,outgoing longwave radiation and subsurface temperature from an ensemble operational ocean reanalyses, and place this event in the context of historical ENSO events since 1979. One salient feature about the 2015/16 El Nio was a large number of westerly wind bursts and downwelling oceanic Kelvin waves(DWKVs). Four DWKVs were observed in April-November 2015 that initiated and enhanced the eastern-central Pacific warming. Eastward zonal current anomalies associated with DWKVs advected the warm pool water eastward in spring/summer. An upwelling Kelvin wave(UWKV) emerged in early November 2015 leading to a rapid decline of the event. Another outstanding feature was that NINO4 reached a historical high(1.7℃), which was 1℃(0.8℃) higher than that of the 1982/83(1997/98) El Nio . Although NINO3 was comparable to that of the 1982/83 and 1997/98 El Nio , NINO1+2 was much weaker. Consistently, enhanced convection was displaced 20 degree westward, and the maximum D20 anomaly was about 1/3.1/2 of that in 1997 and 1982 near the west coast of South America.     

Evolution mechanisms of the intraseasonal oscillation associated with the Yangtze River Basin flood in 1998

Early in the1970s,Madden and Julian[1,2]first re-vealed the existence of eastward propagation of the intraseasonal(or30-60-day)oscillation(ISO)in the zonal wind fields over the tropics.Later on,the northward propagation of the ISO activities over the Indian Ocean was discovered by Yasunari[3,4],which accounts for the seasonal variations of the Indian summer monsoon.Similar studies were carried out on the low-frequency oscillation activities related to the ENSO and the East Asian summer …     

Tropical Indian Ocean Basin Mode recorded in coral oxygen isotope data from the Seychelles over the past 148 years

The tropical Indian Ocean(TIO) displays a uniform basin-wide warming or cooling in sea surface temperature(SST) during the decay year of El Niδo-Southern Oscillation(ENSO) events. This warming or cooling is called the tropical Indian Ocean Basin Mode(IOBM). Recent studies showed that the IOBM dominates the interannual variability of the TIO SST and has impacts on the tropical climate from the TIO to the western Pacific. Analyses on a 148-year-long monthly coral δ 18 O record from the Seychelles Islands demonstrate that the Seychelles coral δ 18 O not only is associated with the local SST but also indicates the interannul variability of the basin-wide SST in the TIO. Moreover, the Seychelles coral δ 18 O shows a dominant period of 3–7 years that well represents the variability of the IOBM, which in return is modulated by the inter-decadal climate variability. The correlation between the Seychelles coral δ 18 O and the SST reveals that the coral δ 18 O lags the SST in the eastern equatorial Pacific by five months and reaches its peak in the spring following the mature phase of ENSO. The spatial pattern of the first EOF mode indicates that the Seychelles Islands are located at the crucial place of the IOBM. Thus, the Seychelles coral δ 18 O could be used as a proxy of the IOBM to investigate the ENSO teleconnection on the TIO in terms of long-time climate variability.     

Contrast of tropical cyclone frequency in the western North Pacific between two types of La Nia events

According to the different pattern of sea surface temperature anomaly(SSTA) in the previous year of La Nia events,we categorized La Nia events into two types to investigate the different characteristics of tropical cyclone(TC) activity over the western North Pacific(WNP) in TC peak season of two types La Nia events.One type is following the previous El Nio event(La Nia I);the other is following the previous neutral phase or developing La Nia event(La Nia II).Results show that TC genesis frequency in the WNP during TC peak season of La Nia I is less than normal year,whereas it has no differences from normal year during La Nia II.The main reason is attributed to the different amplitude SSTA in the East Indian Ocean(EIO) and the western Pacific Ocean(WPO).Similar to the capacitor effect,strongly positive SSTA in the EIO-WPO during La Nia I triggers an equatorial baroclinic Kelvin wave,which intensifies the easterly in the lower troposphere and weakens the East Asian summer monsoon,and thus the TC frequency decreased during La Nia I.However,the easterly anomaly shows a weak response to the SSTA in the EIO-WPO during La Nia II,and there is no significant change in the environmental pattern over the WNP;so is TC frequency.The modulation of strong EIO-WPO SSTA on large-scale circulation over the WNP reduces the environmental barotropic energy conversion into synoptic-scale disturbances during La Nia I,and also suppresses TC disturbances.The understanding of two different types of La Nia events could help improve the seasonal prediction of TC activity in the WNP during La Nia.     

The unusual 2014–2016 El Ni?o events: Dynamics,prediction and enlightenments

The 2014–2016 El Ni?o events consist of a stalled El Ni?o event in the winter of 2014/2015 and a following extreme El Ni?o event in the end of 2015. Neither event was successfully predicted in operational prediction models. Because of the unusual evolutions of these events that rarely happened in the historical observations, few experience was ready for understanding and predicting the two El Ni?o events when they occurred. Also due to their specialties, considerable attention were attracted with aims to reveal the hidden mechanisms. This article reviews the recent progresses and knowledge that were obtained in these studies. Emerging from these studies, it was argued that the key factor that was responsible for the stalled El Ni?o in 2014 was the unexpected summertime Easterly Wind Surges(EWSs) or the lack of summertime Westerly Wind Bursts(WWBs). Most operational prediction models failed to reproduce such stochastic winds and thus made unrealistic forecasts. The two El Ni?o events awakened the research community again to incorporate the state-of-the-art climate models to simulate the stochastic winds and investigate their roles in the development of El Ni?o.