the roles of tropical and subtropical wind stress anomalies in the el nino modoki onset the el nino modoki is separated into two types el nino modoki i and el nino modoki ii which are characterized by substantially different mechanism according to comparisons between the evolution of air sea coupled processes and heat budget analyses this study discovered the distinct dynamical mechanism of the onset of el nino modoki ii compared to that of el nino modoki i for el nino modoki i westerly wind anomalies in the western pacific appear earlier than warm sea surface temperature sst anomalies in the central tropical pacific in addition zonal advection is responsible for changes in mixed layer temperature in the nino 4 region during the developing phase in spring and summer however warm sst anomalies accompany easterly anomalies in the central tropical pacific and there are no westerly anomalies in the western tropical pacific during the developing phase in spring and summer for el nino modoki ii for el nino modoki ii ekman feedback is the major contributor to changes in mixed layer temperature according to reanalysis data the 2 5 layer model results and coupled model simulations anomalous westerly wind stress in the subtropical north pacific and easterly wind stress in the central eastern tropical pacific can lead to equatorward and westward oceanic current anomalies thus enhancing meridional and zonal convergences at upper layer because anomalous convergence at upper layer can inhibit upwelling motion in the central tropical pacific sst anomalies in the central tropical pacific tend to be warm even without anomalous westerlies in the western pacific

the vertical structure of temperature in the tropics different flavors of el nino to explore the vertical coherence of the vertical temperature structure in the atmosphere an analysis is performed of the full three dimensional spatial structure of the temperature field monthly mean anomalies from the 40 yr ecmwf re analysis era 40 for a core region of the tropics from 30 n to 30 s with results projected globally the focus is on the first three empirical orthogonal functions eofs two of which have primary relationships to el nino southern oscillation enso and feature rather different vertical structures the second eof 2 also has a weak enso signature but a very complex vertical structure and reflects mainly nonlinear trends some real but also some in large part spurious and associated with problems in assimilating satellite data the dominant pattern eof 1 in its positive sign features highly coherent zonal mean warming throughout the tropical troposphere from 30 n to 30 s that increases in magnitude with height to 300 hpa drops to zero about 100 hpa at the tropopause and has reverse sign to 30 hpa with peak negative values at 70 hpa spatially at low levels it shows warmth throughout most of the tropics although with weak or slightly opposite sign in the western tropical pacific and a strong reversed sign in the pacific subtropics coherent wave structures below 700 hpa at higher latitudes cancel out in the zonal mean however the structure becomes more zonal above about 700 hpa and features off equatorial maxima straddling the equator in the eastern pacific in the upper troposphere with opposite sign at 100 hpa as a signature of a forced rossby wave the corresponding sea level pressure pattern is similar to but more focused in equatorial regions than the southern oscillation pattern the time series highlights the 1997 98 el nino along with those in 1982 83 and 1986 87 and the 1988 89 la nina and correlates strongly with global mean surface temperatures missing however is the prolonged sequence of three successive el nino events in the early 1990s which are highlighted in eof 3 as part of a mainly lower frequency decadal variation that features modest zonal mean warming below 700 hpa cooling from 700 to 300 hpa and warming above 300 hpa peaking at 100 hpa and extending from 40 n to 50 s spatially at the surface this pattern is dominated by southern oscillation wave 1 structures throughout the tropics and especially the subtropics the regional temperature structures are coherent throughout the troposphere with strongest values in the pacific and extending well into the extratropics with a sign reversal at and above 100 hpa strong rossby wave signatures are featured in the troposphere with a distinctive quadrupole pattern that reverses at 100 hpa the vertical coherence of all patterns suggests that they should be apparent in broad layer satellite temperature records but that stratospheric anomalies are not independent the quite different three dimensional structure of these different patterns highlights the need to consider the full structure outside of the pacific and at all vertical levels in accounting for impacts of enso and how they relate to the global mean

the 1998 2000 la nina in the context of historically strong la nina events 1 contrary to the impacts ascribed to strong cold el nino southern oscillation enso events la nina during the last half of the twentieth century the 2 year long la nina during 1998 2000 was accompanied by abnormally warm winter conditions over most of north america in this study climate anomalies associated with the 1998 2000 la nina are compared with the composite of the previous seven strong la nina events since 1950 despite the presence of a moderately strong la nina analyses of atmospheric circulation and thermal features show that the pacific north american sector climate was associated with a zonally elongated ridge in the extratropical north pacific and warmer than normal sea surface temperatures ssts in the western tropical pacific that extended into the north pacific midlatitudes during 1998 2000 event relative to the composite of the previous seven strong la ninas anomalous walker circulation was considerably stronger and shifted westward with the ascending branch of the cell located near 110 e during the 1998 2000 event this was associated with stronger convection and hence positive precipitation anomalies in the western pacific the relationship between ssts and 500 hpa geopotential heights over the north pacific shows that the differences in thermal and circulation features between the 1998 2000 la nina and the seven strong la ninas bear a close resemblance to a coupled mode of extratropical ocean atmosphere variability as revealed by singular value decomposition analysis this non enso coupled mode of variability which relates the interannual variability in ssts with 500 hpa heights was prominent during the winter of 1998 2000 the processes related to this mode are different from those involved in the establishment of extratropical atmospheric circulation and sst anomalies associated with typical la nina events

investigation of a large scale mode of ocean atmosphere variability and its relation to tropical pacific sea surface temperature anomalies abstract by examining the linearly coupled atmospheric and oceanic signals related to interannual variability in sea surface temperatures and upper air wind fields a hemispheric scale ocean atmosphere teleconnection mode is isolated that is significantly correlated with equatorial pacific ssts 12 months later the interannual component of this teleconnection mode is related to a basin scale dipole in the upper air wind fields stretching across the extratropical pacific with additional anomalies extending from the eastern tropical pacific over north america and into the atlantic basin in addition it is related to variability in the sst field with warm anomalies found over the tropical subtropical western pacific as well as the equatorial eastern pacific also there are related cold anomalies over the extratropical central north pacific that extend down into the central subtropical tropical pacific diagnostic studies investigating the ocean atmosphere structure for this mode of variability indicate th

a closer look at the central pacific el nino and warm pool migration events from 1982 to 2011 national science foundation nsf 11 582 nasa physical oceanography program epscor noaa sea grant

contrasting impacts of two types of enso on the boreal spring hadley circulation the possible influences of two types of enso i e the canonical enso and enso modoki em on hadley circulation hc during the boreal spring are investigated during 1979 2010 el ni no events are featured with a symmetric pattern in equatorial zonal mean sea surface temperature anomalies ssta with a maximum around the equator in contrast the zonal mean ssta associated with el ni modoki events shows an asymmetric structure with a maximum around 108n the contrasting underlying thermal structures corresponding with enso and em have opposite impacts on the simultaneous hc in el ni years a symmetric anomalous meridional circulation is seen with enhanced rising around the equator and anomalousdescentatabout158nand208s incontrast anasymmetricequatorialmeridionalcirculationisobserved for el ni modoki years with anomalous ascent around 108n and descent at about 108s and 208n the contrasting meridional circulation anomalies within enso and em are caused by their different meridional ssta structure this result is theoretically explained indicating that anomalous meridional circulation is subject to the meridional ssta gradient moreover the observed results are reproduced in numerical experiments driven by anomalous warming in the eastern and central pacific ocean thus the authors conclude that the anomalous hc linked to enso and em is induced by the accompanying meridional gradient in zonal mean ssta

a new method for identifying the pacific south american pattern and its influence on regional climate variability abstractthe pacific south american psa pattern is an important mode of climate variability in the mid to high southern latitudes it is widely recognized as the primary mechanism by which el nino southern oscillation enso influences the southeast pacific and southwest atlantic and in recent years has also been suggested as a mechanism by which longer term tropical sea surface temperature trends can influence the antarctic climate this study presents a novel methodology for objectively identifying the psa pattern by rotating the global coordinate system such that the equator a great circle traces the approximate path of the pattern the identification algorithm utilizes fourier analysis as opposed to a traditional empirical orthogonal function approach the climatology arising from the application of this method to era interim reanalysis data reveals that the psa pattern has a strong influence on temperature and precipitation variability over west antarctica and the antarctic peninsula and on sea i

the enso related west pacific sea surface temperature gradient abstractel nino southern oscillation enso events are accompanied by an anomalous zonal sea surface temperature sst gradient over the west pacific ocean defined here as the west pacific sst gradient wpg the wpg is defined as the standardized difference between area averaged sst over the central pacific ocean nino 4 region and west pacific ocean 0 10 n 130 150 e while the direction of the wpg follows enso cycles the magnitude of the gradient varies considerably between individual el nino and la nina events in this study el nino and la nina events are grouped according to the magnitude of the wpg and tropical sst circulations and precipitation are examined for the period 1948 2011 until the 1980s the wpg showed little trend as the west and central pacific warmed at similar rates however the west pacific has recently warmed faster than the central pacific which has resulted in an increased wpg during la nina events the temporal evolution and distribution of tropical pacific sst as we

heartbeat of the southern oscillation explains enso climatic resonances the el ni no southern oscillation enso nonlinear oscillator phenomenon has a far reaching influence on the climate and human activities the up to 10 year quasi period cycle of the el ni no and subsequent la ni na is known to be dominated in the tropics by nonlinear physical interaction of wind with the equatorial waveguide in the pacific long term cyclic phenomena do not feature in the current theory of the enso process we update the theory by assessing low 10 years and high 10 years frequency coupling using evidence across tropical extratropical and pacific basin scales we analyze observations and model simulations with a highly accurate method called dominant frequency state analysis dfsa to provide evidence of stable enso features the observational data sets of the southern oscillation index soi north pacific index anomaly and enso sea surface temperature anomaly as well as a theoretical model all confirm the existence of long term and short term climatic cycles of the enso process with resonance frequencies of 2 5 3 8 5 12 14 61 75 180 years this fundamental result shows long term and short term signal coupling with mode locking across the dominant enso dynamics these dominant oscillation frequency dynamics defined as enso frequency states contain a stable attractor with three frequencies in resonance allowing us to coin the term heartbeat of the southern oscillation due to its characteristic shape we predict future enso states based on a stable hysteresis scenario of short term and long term enso oscillations over the next century

recharge oscillator mechanisms in two types of enso the el ni oscillation enso tends to behave arguably as two different types or flavors in recent decades one is the canonical cold tongue type enso with major sea surface temperature anomalies ssta positionedoverthe easternpacific theotheris awarm pool typeensowith sstacenteredin the central pacific near the edge of the warm pool in this study the basic features and main feedback processes of these two types of enso are examined it is shown that the interannual variability of upper ocean heat content exhibits recharge discharge processes throughout the life cycles of both the cold tongue ct and warm pool wp enso types through a heat budget analysis with focus on the interannual frequency band the authors further demonstrate that the thermocline feedback plays a dominant role in contributing to the growth and phase transitions of both enso types whereas the zonal advective feedback contributes mainly to their phase transitions the westward shift of the ssta center of the wp enso and the presence of significant surface easterly wind anomalies over the far eastern equatorial pacific during its mature warm phase are the two main factors that lead to a reduced positive feedback for the eastern pacific ssta nevertheless both the wp and ct enso can be understood to a large extent by the recharge oscillator mechanism

reconstructing enso the influence of method proxy data climate forcing and teleconnections in this study we compare three newly developed independent nino3 4 sea surface temperature sst reconstructions using data from 1 the central pacific corals 2 the texmex region of the usa tree rings and 3 other regions in the tropics corals and an ice core which are teleconnected with central pacific ssts in the 20th century although these three reconstructions are strongly calibrated and well verified inter proxy comparison shows a significant weakening in inter proxycoherenceinthe 19thcentury thisbreakdownin commonsignalcouldberelatedtoinsufficient data dating errors in some of the proxy records or a breakdown in el nino southern oscillation s enso s influence on other regions however spectral analysis indicates that each reconstruction portraysenso likespectralproperties superposedepochanalysisalsoshowsthateachreconstruction shows a generally consistent el nino like response to major volcanic events in the following year while duringyearst 4tot 7 la nina like conditions prevail theseresults suggest thateachofthe series expresses enso like behaviour but this behaviour does not appear to be spatially or temporally consistent this result may reflect published observations that there appear to be distinct types of enso variability depending on location within the tropical pacific future work must address potential dating issues within some proxies i e sampling of multiple coral heads for one location as well as assessing the time stability of local climate relationships with central pacific ssts more emphasis is needed on sampling new and extending old coral proxy records from the crucial central and eastern tropical pacific region copyright 2009 john wiley sons ltd and crown copyright 2009

integration of tibetan plateau ice core temperature records and the influence of atmospheric circulation on isotopic signals in the past century abstract temperature signals in ice core 18o on the tibetan plateau tp particularly in the central and southern parts continue to be debated because of the large scale of atmospheric circulation this study presents ten ice core 18o records at an annual resolution with four malan muztagata guliya and dunde in the northern three puruogangri geladaindong tanggula in the central and three noijin kangsang dasuopu east rongbuk in the southern tp integration shows commonly increasing trends in 18o in the past century featuring the largest one in the northern a moderate one in the central and the smallest one in the southern tp which are all consistent with ground based measurements of temperature the influence of atmospheric circulation on isotopic signals in the past century was discussed through the analysis of el nino southern oscillation enso and of possible connections between sea surface temperature sst and the different increasing trends in both ice core 18o and temperature particularly el nino and the corresponding warm bay of bengal bob sst enhance the tp ice core isotopic enrichment while la nina or corresponding cold bob sst causes depletion this thus suggests a potential for reconstructing the enso history from the tp ice core 18o

characterizing the onset and demise of the indian summer monsoon an objective index of the onset and demise of the indian summer monsoon ism is introduced this index has the advantage of simplicity by using only one variable which is the spatially averaged all india rainfall a reliably observed quantity for more than a century the proposed onset index is shown to be insensitive to all historic false onsets by definition now the seasonal mean rainfall anomalies become a function of variations in onset and demise dates rendering their monitoring to be very meaningful this new index provides a comprehensive representation of the seasonal evolution of the ism by capturing the corresponding changes in large scale dynamic and thermodynamic variables we also show that the interannual variability of the onset date of the ism is associated with el nino southern oscillation enso with early late onsets preceded by cold warm enso

the impact of eastern pacific versus central pacific el ninos on the north equatorial countercurrent in the pacific ocean 1 using a three dimensional ocean reanalysis data product the influence of two types of the el ninos on the north equatorial countercurrent necc across the pacific basin is examined in this study eastern pacific el ninos ep el ninos exert a significant impact on the interannual changes of the necc whereas central pacific el ninos cp el ninos have little influence from the developing to mature phase of the ep el ninos that occursfrequently in the latter half of a calendar year the necc tends to intensify and shift southward while the ep el nino related position changes of the necc is out of phase with its seasonal position changes the ep el nino related intensity variability of the necc is in phase with its seasonal modulation during both types of el ninos the el nino modulated wind stress curl forcing changes the upper ocean thickness modifying the necc intensity and central position the weaker and shorter fluctuations of wind stress curl forcing during the cp el nino events induce irregular and insignificant changes of the necc

afro eurasian intermediate frequency teleconnection and modulation by enso abstractthe afro eurasian intermediate frequency atmospheric teleconnection conveys meteorological signals zonally leads to various atmospheric variations and causes extreme events along its path

ice core evidence of westerly and monsoon moisture contributions in the central tibetan plateau stable isotopes are a primary tool for inferring past temperature changes and atmospheric moisture variability from ice cores a 33 m ice core representing the period 1850 2004 was retrieved from the tanggula mountains central tibetan plateau 5743 m a s l in august 2005 annual average stable isotope d 18 o dd values generally increase during the period while the second order parameter of deuterium excess d excess generally decreases high annual average d excess values 18 2 throughout the ice core suggest a significant contribution of continental recycled moisture d excess values shift from relatively higher values during 1850 1940 to lower values since the 1940s annual isotope values and reconstructed accumulation are compared with climate indices local station temperature records and northern india monsoon precipitation significant correlation is observed between d 18 o and the southern oscillation nino3 4 and dipole mode indices annual average d excess values revealed a significant negative correlation with the dipole mode index results suggest a relatively greater contribution of westerly dominated continental moisture prior to the 1940s and an increase in the contribution of moisture evaporated under more humid conditions since the 1940s

anomalous tropical ocean circulation associated with la nina modoki 1 el nino modoki is a variant of el nino characterized by warming around the dateline flanked by anomalous cooling in the east and west however the opposite phase la nina modoki has received little attention because the prominent cooling of sea surface temperature sst during major la nina events is observed in the central pacific and thus it is difficult to define the two different types of cold events from the sst anomaly pattern here we demonstrate that cold events in 2000 and 2008 can be clearly distinguished from traditional la nina events using surface currents derived from satellite observations during 2000 and 2008 anomalous zonal currents in the equatorial pacific demonstrate divergence with westward currents west of the dateline and eastward currents east of it these currents are opposite to the circulation pattern during the 2004 el nino modoki event an empirical orthogonal function eof analysis of surface currents for the period 1993 2009 shows a circulation anomaly pattern similar to that in 2000 2004 and 2008 in the second eof the first eof is consistent with traditional el nino la nina events and does not exhibit a current reversal along the equator our results also indicate that strong cyclonic anticyclonic circulation anomalies occur in the tropical western pacific around 5 n 15 n during the 2004 2000 and 2008 el nino la nina modoki events and during the strong traditional el nino la nina events of 1997 1998 these circulation anomalies are related to an sst gradient in the western and central pacific

influence of the meridional shifts of the kuroshio and the oyashio extensions on the atmospheric circulation abstract the meridional shifts of the oyashio extension oe and of the kuroshio extension ke as derived from high resolution monthly sea surface temperature sst anomalies in 1982 2008 and historical temperature profiles in 1979 2007 respectively are shown based on lagged regression analysis to significantly influence the large scale atmospheric circulation the signals are independent from the enso teleconnections which were removed by seasonally varying asymmetric regression onto the first three principal components of the tropical pacific sst anomalies the response to the meridional shifts of the oe front is equivalent barotropic and broadly resembles the north pacific oscillation western pacific pattern in a positive phase for a northward frontal displacement the response may reach 35 m at 250 hpa for a typical oe shift a strong sensitivity since the associated sst anomaly is 0 5 k however the amplitude but not the pattern or statistical significance strongly depends on the lag and an

detecting decadal changes in enso using neural networks the present manuscript analyzes monthly equatorial pacific indices by using a specific neural algorithm the so called self organizing maps soms the main result is a change found in the nature of the transitions between cold to warm and warm to cold extreme events from 1950 to present around the late 1970s som is an unsupervised clustering technique which allows one to reduce high dimensional data space in this case three indices over 636 months in terms of a smaller set of three dimensional reference vectors 100 characterizing pertinent situations these reference vectors which are displayed on a two dimension map are closely related by a topological relationship leading us to discriminate la nina conditions from the opposite el nino conditions in a second step a hierarchical agglomerative clustering hac method is used to further group the reference vectors into a small number of clusters 12 whose spatial and temporal characteristics can be analyzed and interpreted in terms of physical parameters schematically these 12 clusters can be divided into two warm clusters six neutral or transition clusters and four cold clusters in each particular group warm neutral cold the clusters mainly differ from each other by the amplitude of the anomalies their spatial patterns and their temporal variability some clusters are found to be strongly linked to the boreal spring period while others have barely any records during that season other clusters are associated with records mainly observed either prior to or after 1980 this suggests that the method is able to identify changes in the variability of the tropical pacific basin observed on decadal time scales 1976 climate shift in our case each monthly record can be summarized by the cluster to which it belongs the temporal evolution of this value during extreme enso events shows similar patterns persistence in specific clusters and transition between groups of clusters associated with comparable el nino or la nina events the methodology described in the present study som plus hac is suggested to be useful both for seasonal enso predictability and for the detection of decadal changes in enso behavior

interannual modality of upper ocean temperature 4d structure revealed by argo data abstractusing the newly available decade long argo data for the period 2004 13 a detailed study is carried out on deriving four dimensional 4d modality of sea temperature in the upper ocean with emphasis on its interannual variability in terms of amplitude phase and periodicity three principal modes with central periodicities at 19 2 33 8 and 50 3 months have been identified and their relationship with el nino southern oscillation enso is investigated yielding a number of useful results and conclusions 1 a striking tick shaped pipe like feature of interannual variability maxima which is named the nino pipe in this paper has been revealed within the 10 s 10 n upper pacific ocean 2 the pipe core extends downward from 50 m at 130 e to 250 m near the date line before tilting upward to the sea surface at about 275 e coinciding nicely with the pathway of the pacific equatorial undercurrent euc 3 the double peak zonal modality pattern of the nino pipe in the upper pacific is echoed in

observations and mechanisms of a simple stochastic dynamical model capturing el nino diversity abstractel nino southern oscillation enso has significant impact on global climate and relevance for seasonal forecasts recently a simple modeling framework was developed that captures the enso diversity where state dependent stochastic wind bursts and nonlinear advection of sea surface temperature are coupled to a simple ocean atmosphere model that is otherwise deterministic linear and stable in this article the coupled model is compared with observations using reanalysis data over the last 34 yr where the observed non gaussian statistics and the overall mechanisms of enso are both captured by the model then the formation mechanisms of both the central pacific cp and the traditional el nino in the model are systematically studied first ocean rossby waves induced by easterly trade wind anomalies facilitate the heat content buildup then the reflected ocean kelvin waves and the nonlinear advection lead to positive sst anomalies in the cp region and create a cp el nino second two formation

role of off equatorial sst in el nino teleconnection to east asia during el nino decaying spring observational evidences are presented to show significant differences in pacific east asian pea teleconnections during the el nino decaying period depending on the northern off equatorial sea surface temperature sst conditions on the basis of the northern off equatorial central pacific sst nocp index we separate el nino events into two groups distinctive differences are observed between the two groups not only in tropical features but also in mid latitude teleconnections in particular the nocp index is closely related to the strength of the local feedback between the sst anomalies and the anticyclonic flow over the western north pacific wnp during the ensuing spring of the el nino events because the wnp anticyclone tends to persist until boreal spring in the case of negative nocp diabatic cooling due to the reduced precipitation over the wnp induces anticyclonic flow over the kuroshio extension region via a rossby wave response which leads to warm and wet conditions over east asia however because the wnp anticyclone decays early in the case of positive nocp cyclonic circulation exists over the north pacific which leads to cold and dry conditions over east asia consequently the maintenance of the wnp anticyclone during the el nino decaying period may explain the discrepancy in the pea teleconnections in various types of el nino accordingly the nocp index could be a possible predictor for boreal spring climate conditions over east asia

sea surface chlorophyll signature in the tropical pacific during eastern and central pacific enso events 1 recent analyses of physical measurements show the existence of a central pacific type of el nino cpen with a sea surface temperature warming pattern distinct from that of the classical eastern pacific el nino epen in this study we analyze the surface chlorophyll signature of el nino southern oscillation enso using monthly maps of satellite derived chlorophyll anomalies between september 1997 and december 2010 we identify five typical enso structures the first structure describes the lonely 1997 1998 epen of the period the second and third represent la nina the fourth illustrates intermediate conditions and the fifth characterizes cpen during the 1997 1998 epen a large eastward shift of the oligotrophic warm pool and a reduction of equatorial upwelling result in negative chlorophyll anomalies east of 170 e between 10 s and 10 n during the four cpen events a reduced eastward shift yields negative chlorophyll anomalies in the equatorial band within about 160 e and 160 w only westward surface current in the central basin limits the expansion of the anomaly core negative chlorophyll anomalies that extend eastward from the equatorial anomaly core probably result from reduced upward iron fluxes linked to the deepening of the equatorial undercurrent during la nina the westward expansion of the equatorial upwelling results in positive chlorophyll anomalies west of the date line away from the equatorial band advection of oligotrophic warm pool waters by enhanced eastward countercurrents drives negative anomalies within 8 10 n and toward the marquesas islands during cpen while reduced countercurrents lead to positive chlorophyll anomaly during la nina

linear trends in sea surface temperature of the tropical pacific ocean and implications for the el nino southern oscillation a principal component decomposition of monthly sea surface temperature sst variability in the tropical pacific ocean demonstrates that nearly all of the linear trends during 1950 2010 are found in two leading patterns the first sst pattern is strongly related to the canonical el nino southern oscillation enso pattern the second pattern shares characteristics with the first pattern and its existence solely depends on the presence of linear trends across the tropical pacific ocean the decomposition also uncovers a third pattern often referred to as enso modoki but the linear trend is small and dataset dependent over the full 61 year record and is insignificant within each season enso modoki is also reflected in the equatorial zonal sst gradient between the nino 4 region located in the west central pacific and the nino 3 region in the eastern pacific it is only in this zonal sst gradient that a marginally significant trend arises early in the northern hemisphere spring march may during el nino and la nina and also in the late summer july september during el nino yet these sst trends in the zonal gradient do not unequivocally represent an enso modoki like dipole because they are exclusively associated with significant positive sst trends in either the eastern or western pacific with no corresponding significant negative trends insignificant trends in the zonal sst gradient are evident during the boreal wintertime months when enso events typically mature given the presence of positive sst trends across much of the equatorial pacific ocean using fixed sst anomaly thresholds to define enso events likely needs to be reconsidered

enso like interdecadal variability 1900 93 a number of recent studies have reported an enso like eof mode in the global sea surface temperature sst field whose time variability is marked by an abrupt change toward a warmer tropical eastern pacific and a colder extratropical central north pacific in 1976 77 the present study compares this pattern with the structure of the interannual variability associated with the enso cycle and documents its time history back to 1900 the analysis is primarily based on the leading eofs of the sst anomaly and anomaly deviation fields in various domains and the associated expansion coefficient or principal component time series which are used to construct global regression maps of sst sea level pressure slp and a number of related variables the use of anomaly deviations i e departures of local sst anomalies from the concurrent global mean sst anomaly reduces the influence of global mean sst trends upon the structure of the eofs and their expansion coefficient time series an important auxiliary time series used in this study is a southern oscillation index based on marine surface observations by means of several different analysis techniques the time variability of the leading eof of the global sst field is separated into two components one identified with the enso cycle related variability on the interannual timescale and the other a linearly independent residual comprising all the interdecadal variability in the record the two components exhibit rather similar spatial signatures in the global sst slp and wind stress fields the sst signature in the residual variability is less equatorially confined in the eastern pacific and it is relatively more prominent over the extratropical north pacific the corresponding slp signature is also stronger over the extratropical north pacific and its counterpart in the cold season 500 mb height field more closely resembles the pna pattern the amplitude time series of the enso like pattern in the residual variability reflects the above mentioned shift in 1976 77 as well as a number of other prominent features including a shift of opposite polarity during the 1940s

abrupt shift in subsurface temperatures in the tropical pacific associated with changes in el nino radiocarbon 14c content of surface waters inferred from a coral record from the galapagos islands increased abruptly during the upwelling season july through september after the el nino event of 1976 sea surface temperatures ssts associated with the upwelling season also shifted after 1976 the synchroneity of the shift in both 14c and sst implies that the vertical thermal structure of the eastern tropical pacific changed in 1976 this change may be responsible for the increase in frequency and intensity of el nino events since 1976

el nino phases embedded in asian and north american drought reconstructions abstract the amplitude of el nino southern oscillation enso varies substantially at each phase of its evolution affecting the timing and patterns of atmospheric teleconnections around the globe instrumental records are too short to capture the full behavior of enso variability here we use the well validated monsoon asia drought atlas mada and north america drought atlas nada for the past 700 years and show that tree ring records from different regions represent tropical sea surface temperature sst conditions at various phases of enso three modes of tree ring based summer drought variability are found to be correlated with enso summer droughts over the maritime continent and southwest north america na and a dipole mode between central and south asia a lagged correlation analysis is performed to determine the time when precipitation and temperature anomaly imprints on summer droughts as recorded in tree rings drought anomalies in the maritime continent and southwest na represent enso at the developing and peak phases respectively while those over central south asia are associated with tropical wide sst anomalies including the indian ocean at the decay phase of enso thus proxy records from different regions can provide valuable information on long term behavior of enso at different phases

the response of enso flavors to mid holocene climate implications for proxy interpretation the response of el nino southern oscillation enso to mid holocene boundary conditions remains an open question paleoclimate proxies and climate model simulations do not agree in the magnitude of the reduction of enso variability while recent proxy evidence from fossil corals from the central pacific show that the reduction in mid holocene enso variability compared to the end of the twentieth century is not different from the reduction during other holocene time intervals this is inconsistent with the interpretation of lake and ocean sediment records from the eastern pacific which show a significant reduction compared to all other holocene periods in order to reconcile the seemingly conflicting proxy evidence from the eastern and central pacific we hypothesize that enso remained active during the mid holocene however there was a change in the spatial pattern of the sea surface temperature anomalies also known as enso flavors using national center for atmospheric research s community climate system model version 4 forced with mid holocene orbital conditions we find that the frequency of occurrence of the strongest eastern pacific ep events decreases in the mid holocene and their variance is reduced by 30 while the frequency of central pacific cp events slightly increases and their variance does not change we also find a shift in the seasonality of ep events but not in that of cp events lastly mid holocene ep events develop more slowly and decay faster the differential response of enso flavors to mid holocene forcing is remotely forced by the west pacific where a weakening of the trade winds in early boreal spring in the mid holocene initiates an anomalous downwelling annual kelvin wave which reaches the eastern pacific during the enso development season weakens the upper ocean stratification and results in reduced enso upwelling feedback the simulated reduction in the ep flavor versus the cp flavor in the mid holocene is consistent with proxy evidence the teleconnection patterns of the two flavors with temperature precipitation and salinity are distinct and proxies from different regions of the pacific might be recording variability associated with only one of the flavors or some combination of their relative effects

evidence of reduced mid holocene enso variance on the great barrier reef australia this study was funded by the national environmental research programme nerp tropical ecosystems hub project 1 3 characterising the cumulative impacts of global regional and local stressors on the present and past bio diversity of the gbr to j xz jmp sgs trc y xf and others and australian research council lief grant le0989067 for the purchase and installation of the mc icp ms essential for this study to jxz jmp y xf and others as well as an australian postgraduate award apa to ndl we are grateful to eric matson from the australian institute of marine science aims for technical assistance in obtaining the fossil coral cores and to hard rock earth works rod morelli and associates in townsville for allowing access to the corals we also thank st vincent s private hospital brisbane for ct scans and queensland diagnostic imaging indooroopilly for x rays we thank an anonymous reviewer for helpful comments on an earlier manuscript the data used in this study are available through the cited references figures and tables as well as in the supporting information to this manuscript

optimal growth of central and east pacific enso events central pacific cp type and eastern pacific ep type el ni no and the southern oscillation enso events are investigated using linear inverse modeling optimal initial conditions and growth rates for cp or ep enso events are identified explicitly using a cp or ep enso norm the dominant difference in initial conditions that lead to cp and ep enso events is the role of the second empirical orthogonal function of tropical sea surface temperature which represents the pacific meridional mode pmm optimal initial conditions for cp type enso events include warm sst anomalies in the central subtropical pacific a characteristic of the pmm while optimal initial conditions for ep type enso events are focused in the eastern equatorial pacific and southern hemisphere subtropics along about 25 s thermocline anomalies differ in initial structures and in their influence on sst for cp and ep events results point to different roles of the pmm and thermocline variations in the evolution of cp and ep enso events

a novel approach to diagnosing southern hemisphere planetary wave activity and its influence on regional climate variability abstractsouthern hemisphere mid to upper tropospheric planetary wave activity is characterized by the superposition of two zonally oriented quasi stationary waveforms zonal wavenumber 1 zw1 and zonal wavenumber 3 zw3 previous studies have tended to consider these waveforms in isolation and with the exception of those studies relating to sea ice little is known about their impact on regional climate variability a novel approach is taken to quantifying the combined influence of zw1 and zw3 using the strength of the hemispheric meridional flow as a proxy for zonal wave activity the methodology adapts the wave envelope construct routinely used in the identification of synoptic scale rossby wave packets and improves on existing approaches by allowing for variations in both wave phase and amplitude while zw1 and zw3 are both prominent features of the climatological circulation the defining feature of highly meridional hemispheric states is an enhancement of the zw3 component composites of the mea

seismic imaging of upper crustal structure using travel times from the passcal ouachita experiment a first arrival travel time inversion applied to the 1986 passcal ouachita experiment has been used to image upper crustal structure twenty one shot points every 10 km were recorded along two 100 km long segments with 400 seismographs on each segment a splined velocity model parameterization allowed for the inversion of laterally inhomogeneous models but not velocity discontinuities over 800 travel times were used in the inversion with a variable data covariance weighting the multiple source nature of the experiment design allowed the use of reciprocity relationships among 21 shot points to enhance data correlation a succession of models with velocities specified at 30 90 171 and 221 nodes were inverted for whose horizontal node spacing decreased from 100 to 12 5 km in the final 221 node model vertical node spacings of 3 0 and 1 5 km were used the results of the inversions indicate changes in depth of isovelocity lines which are interpreted as triassic fault structure based on nearby well data the final rms travel time residual is 0 045 s resolution using first arrivals deteriorates rapidly below 9 km depth three distinct velocity gradients can be correlated with geologic units surface exposed cretaceous and jurassic sediments with velocities from 3 0 tomore 3 5 km s extend to 1 5 km depth triassic rift fill sediments which do not outcrop have inverted velocities ranging from 3 5 to 4 75 km s between depths of 1 5 3 0 km paleozoic sediments are imaged by a decreased velocity gradient below a depth of 3 0 km lack of resolution at depths greater than 9 km necessitates detailed modeling of wide angle reflected phases to further constrain deeper crustal structure less

is climate variability over the north pacific a linear response to enso abstract indices of the dominant spatial patterns of wintertime northern hemisphere 500 mb height and north pacific sea surface temperature are strongly correlated with one another on interannual and interdecadal timescales and both am correlated with indices of the el nin southern oscillation one possible interpretation of these relationships is that the tropical sst anomalies associated with enso force the atmospheric circulation over the north pacific and these atmospheric anomalies in turn give rise to the observed sst anomalies over the extratropical north pacific in this study linear relationships between enso and extratropical variables are examined in two different ways first the component of the observed extratropical variability that is linearly dependent upon enso is removed the dominant spatial patterns in the residual variability of 500 mb height and sst anomalies over the north pacific are shown to be similar to their counterparts in the total fields and remain strongly coupled on

andean uplift and climate change sedimentological data indicate that a semi arid arid climate prevailed across the central andes from 15 ma to 4 ma between 4 and 3 ma a switch to hyperaridity occurred along the western margin of south america palaeoaltitude data suggest that a substantial proto central andean mountain range was in place between 15 and 9 ma these data support the idea that the andean rain shadow existed by 15 ma and that it reinforced the pre existing climatic regime rather than changing it the change to hyperaridity in western south america is attributed to a combination of global climate cooling and enhanced upwelling of the humboldt current generated by closure of the central american seaway between 3 5 and 3 ma and not to the andean rain shadow

influence of el nino modoki on spring rainfall over south china 1 using observed data sets from 1979 to 2006 the relationship between el nino modoki and spring rainfall over south china sc is investigated of particular interest is the difference in the influence on spring rainfall of typical el nino events and the recently recognized el nino modoki events which are characterized by distinct warm sea surface temperature anomalies ssta in the central pacific and weaker cold anomalies in the western and eastern parts of the basin associated with the ssta anomalous ascent occurs over the central pacific and downward flow is observed over the eastern and western pacific the anomalous flow is associated with anomalous convergence in the upper troposphere over the western pacific sc is influenced by an anomalous anticyclonic circulation with prevailing northeasterly anomalies the convective activity in sc becomes weaker resulting in reduced rainfall however the situation is different in the case of el nino in terms of the influence on rainfall over sc while el nino modoki events are accompanied by a significant reduction in rainfall over sc there is enhanced rainfall associated with el nino events moreover there exists a strong asymmetry in the relationship between sc spring rainfall typical el nino southern oscillation enso and enso modoki events it appears that these relationships are only statistically significant for positive events the asymmetric influence of positive and negative in two enso phenomena may explain the difference in their respective relationships with spring rainfall over sc

el nino modoki and its possible teleconnection 1 using observed data sets mainly for the period 1979 2005 we find that anomalous warming events different from conventional el nino events occur in the central equatorial pacific this unique warming in the central equatorial pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly ssta on both sides along the equator empirical orthogonal function eof analysis of monthly tropical pacific ssta shows that these events are represented by the second mode that explains 12 of the variance since a majority of such events are not part of el nino evolution the phenomenon is named as el nino modoki pseudo el nino modoki is a classical japanese word which means a similar but different thing the el nino modoki involves ocean atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution analogous to the southern oscillation in the case of el nino hence the total entity is named as el nino southern oscillation enso modoki the enso modoki events significantly influence the temperature and precipitation over many parts of the globe depending on the season the impacts over regions such as the far east including japan new zealand western coast of united states etc are opposite to those of the conventional enso the difference maps between the two periods of 1979 2004 and 1958 1978 for various oceanic atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline this appears to be a cause of more frequent and persistent occurrence of the enso modoki event during recent decades

indices of el nino evolution abstract to characterize the nature of el nino southern oscillation enso sea surface temperature sst anomalies in different regions of the pacific have been used an optimal characterization of both the distinct character and the evolution of each el nino or la nina event is suggested that requires at least two indices i sst anomalies in the nino 3 4 region referred to as n3 4 and ii a new index termed here the trans nino index tni which is given by the difference in normalized anomalies of sst between nino 1 2 and nino 4 regions the first index can be thought of as the mean sst throughout the equatorial pacific east of the date line and the second index is the gradient in sst across the same region consequently they are approximately orthogonal tni leads n3 4 by 3 to 12 months prior to the climate shift in 1976 77 and also follows n3 4 but with opposite sign 3 to 12 months later however after 1976 77 the sign of the tni leads and lags are reversed

contrasting the termination of moderate and extreme el nino events in coupled general circulation models as in the observed record the termination of el nino in the coupled ipcc ar4 climate models involves meridional processes tied to the seasonal cycle these meridional processes both precondition the termination of el nino events in general and lead to a peculiar termination of extreme el nino events such as those of 1982 83 and 1997 98 in which the eastern equatorial pacific warm sea surface temperature anomalies ssta persist well into boreal spring early summer the mechanisms controlling the peculiar termination of extreme el nino events which involves to the development of an equatorially centred intertropical convergence zone are consistent across the four models that exhibit extreme el ninos and observational record suggesting that this peculiar termination represents a general feature of extreme el ninos further due to their unusual termination extreme el ninos exhibit an apparent eastward propagation of their ssta which can strongly influence estimates of the apparent propagation of enso over multi decadal periods interpreting these propagation changes as evidence of changes in the underlying dynamical feedbacks behind el nino could therefore be misleading given the strong influence of a single extreme event

interannual and decadal variability of the western pacific sea surface condition for the years 1787 2000 reconstruction based on stable isotope record from a guam coral 1 we present a monthly resolved 213 year stable isotope time series from a coral from guam 13 n 145 e which is located on the northern edge of the western pacific warm pool oxygen isotopic composition of the coral skeleton 18ocoral shows seasonal interannual and decadal variability which documents significant oceanographic changes related to thermal and hydrologic variations in this region the 18ocoral anomaly reflects sea surface temperature sst anomaly and sea surface salinity sss anomaly with significant r values of 0 69 and 0 49 respectively which are strongly linked to oceanographic changes that occur during el nino southern oscillation enso warm and cool phases we identified 46 enso warm el nino and 53 cool phases la nina in the coral record which are consistent with those phases reconstructed by nino 3 4 sst anomaly spectral analyses of the 18ocoral anomaly record for the years 1790 1999 identified significant peaks around 3 to 7 years these results indicate that the guam coral has recorded enso periodicity the 18ocoral anomaly shows decadal variability of 15 to 45 year periodicity with significant shifts 0 2 from warmer to cooler condition and vice versa an accumulative decrease in 18ocoral time series may imply 0 75 c warming of sst and 0 23 freshening of seawater 18o corresponding to a decrease of sss by 0 85 in the northwestern tropical pacific over the last 2 centuries

ocean chlorophyll response to two types of el nino events in an ocean biogeochemical coupled model based on a long term simulation of an ocean biogeochemical coupled model we investigate the biogeochemical response to the two types of el nino events a cold tongue ct el nino and a warm pool wp el nino in which a local maximum of anomalous sea surface temperature sst is located in the eastern and central tropical pacific our model is able to reasonably simulate the characteristics of the biological variables in a way comparable to the observations during the developing period anomalous low chlorophyll appears in the eastern pacific while it appears in the central pacific in the wp el nino the difference in the spatial temporal response of chlorophyll for the two types of el nino events is mainly due to the eastward zonal advection of upper ocean currents which plays a role in bringing nutrient poor water from the western pacific during the decaying period of the wp el nino anomalous high chlorophyll appears concurrently with anomalous low sst in the eastern pacific conversely anomalous high chlorophyll appears in the central pacific prior to the decaying period of the ct el nino in particular the anomalous low sea level from the northwestern pacific shifts to the southern equatorial region during the decaying period of the ct el nino this drives anticyclonic boundary currents which enhance the equatorial undercurrent playing a role in the supply of nutrients to the central equatorial pacific resulting in an increase in chlorophyll concentration in the same region

subtropics related interannual sea surface temperature variability in the central equatorial pacific abstract interannual sea surface temperature sst variability in the central equatorial pacific consists of a component related to eastern pacific sst variations called type 1 sst variability and a component not related to them called type 2 sst variability lead lagged regression and ocean surface layer temperature balance analyses were performed to contrast their control mechanisms type 1 variability is part of the canonical which is characterized by sst anomalies extending from the south american coast to the central pacific is coupled with the southern oscillation and is associated with basinwide subsurface ocean variations this type of variability is dominated by a major 4 5 yr periodicity and a minor biennial 2 2 5 yr periodicity in contrast type 2 variability is dominated by a biennial periodicity is associated with local air sea interactions and lacks a basinwide anomaly structure in addition type 2 sst variability exhibits a strong connection to the subtropics of both hemisphere

variability of salt flux in the indian ocean during 1960 2008 abstract this paper describes near surface horizontal salt flux in the indian ocean during 1960 2008 using the simple ocean data assimilation reanalysis version 2 2 4 the long term mean salt flux shows seasonal reversals that are more pronounced in the northern indian ocean than in the southern indian ocean mean zonal salt flux is of a higher magnitude than the mean meridional salt flux wyrtki jets influence zonal salt flux in the equatorial region meridional salt flux is northward along the northeast ne african coast and western bay of bengal bob during the southwest monsoon season the opposite holds during the ne monsoon season empirical orthogonal function analyses of salt flux show that the indian ocean dipole iod primarily controls interannual variability with flux being westward during positive dipole events and eastward during negative dipole events the co occurrence of iod with the el nino southern ocean results in enhanced salt flux especially along the equatorial region spatially the most variable regions are the ne african coast eastern arabian sea bob and equatorial regions this study will help interpret salinity data for the indian ocean from the recently launched soil moisture ocean salinity and aquarius satellite salinity missions

dipolar redistribution of summertime tropical cyclone genesis between the philippine sea and the northern south china sea and its possible mechanisms 1 recent observational records show that the dipole oscillation between the philippine sea ps and the northern south china sea nscs is a leading empirical orthogonal function eof of summertime tropical cyclone tc genesis in the western north pacific wnp this ps nscs oscillation is characterized by a distinguished decadal variability in addition to an interannual variability meanwhile the typical el nino southern oscillation enso related mode is found in the second eof mode which is predominantly interannual with regard to the ps nscs oscillation its interannual component appears to be linked with the previous wintertime enso event but the linkage is not so robust in the sense that about half of its significant events are classified as the enso related case whereas its decadal component is coupled to a sea surface temperature anomaly ssta in the central pacific with its equatorial core near the nino 4 region which is flanked by an opposite ssta on both sides along the latitudinal belt interestingly this ssta pattern resembles that related to the enso modoki and equivalently the transitional phase of enso the enso modoki is regionally manifested by the anomalous zonal sst gradient between the equatorial western and central pacific thus the decadal modulation of this anomalous zonal sst gradient can be regarded as an actual forcing that forms and modulates the decadal ps nscs oscillation in addition the decadal ps nscs oscillation is compared with the decadal basin wide tc genesis variation in terms of the pacific ssta pattern and the associated anomalous large scale environments

enso regimes reinterpreting the canonical and modoki el nino 1 we propose that the first two empirical orthogonal function eof modes of tropical pacific sea surface temperature sst anomalies do not describe different phenomena i e el nino southern oscillation enso and el nino modoki but rather the nonlinear evolution of enso we introduce two new uncorrelated indices e and c based on the leading eofs that respectively account for extreme warm events in the eastern and cold moderate warm events in the central equatorial pacific corresponding to regimes with different evolution recent trends in enso can be described as an increase in the central pacific c variability that is associated with stronger cold events as well as a reduction in the eastern pacific e variability within the cold moderate warm regime consistent with model projections however little can be said observationally with respect to the extreme warm regime