differences in atmospheric circulation between the development of weak and strong warm events in the southern oscillation abstract in this study temporal and spatial aspects of el nino warm event development are explored by comparing composite sequences of sea level pressure slp surface wind and sea surface temperature sst anomalies leading into strong and weak events el nino strength is found to be related to the magnitude and spatial extent of large scale slp anomalies that move in a low frequency mode in association with this it is also intricately linked to the amplitude and wavelength of the rossby waves in the southern midlatitudes the primary signature of the southern oscillation is a more pronounced standing wave of pressure anomalies between southeastern australia and the central south pacific leading into stronger events a strong reversal in the strength of the annual cycle between these two regions causes a stronger weaker slp gradient that drives southwesterly northwesterly wind stress forcing toward away from the western equatorial pacific in austral winter spring of year 0 1 thus pressur

statistical occurrence and mechanisms of the 2014 2016 delayed super el nino captured by a simple dynamical model the recent 2014 2016 period was marked by a failed el nino favoring a subsequent extreme el nino with dramatic worldwide impacts here this new type of major event so called the delayed super el nino is realistically captured by a simple dynamical model for the equatorial pacific with emphasis on the role of state dependent stochastic wind bursts we analyze qualitative analogues for this event compared and contrasted with the 1997 1998 super el nino in ensemble experiments based on the simple model in agreement with recent studies the timing and intensity of such an event is strongly controlled by atmospheric wind bursts both easterly and westerly in particular the early stalling by easterly wind bursts and subsequent development by westerly wind bursts as observed during 2014 2016 is consistently retrieved we show in addition that individual wind bursts may control the main characteristics of the event only during its early development while sequences of consecutive wind bursts have more important cumulative effects another important result from the present analysis is the significant statistical occurence of the delayed super el nino around 20 30 compared with the one of directly formed super events as that of 1997 1998 such a high occurence is directly linked to the random evolution of wind bursts and is retrieved here for all phases of the el nino southern oscillation used for the initiation of the ensemble experiments these results suggest that the delayed super el nino is not an unusual type of super event and could reoccur in the future

about the role of westerly wind events in the possible development of an el nino in 2014 similarities between early 1997 and 2014 has prompted climate scientists to wonder if an el nino matching the 1997 el nino of the century could develop in 2014 until april 2014 the equatorial pacific exhibited positive heat content anomalies along with an eastward warm pool displacement similar to those found during the onset of strong el nino events yet in july 2014 the warm pool had retreated back to its climatological positions and equatorial temperature anomalies were much weaker than in mid 1997 dedicated oceanic simulations reveal that these weak interannual anomalies can be attributed to differences in westerly wind event wwe sequences in contrast with 1997 the lack of wwes from april to june significantly limited the growth of eastern pacific anomalies and the eastward warm pool displacement in 2014 with the absence of additional wwe activity prospects for a mature el nino in late 2014 are fading

evidence of an extratropical atmospheric influence during the onset of the 1997 98 el nino the major 1997 98 el nino episode was initiated by a series of very energetic westerly wind bursts in late 1996 and early 1997 downwelling oceanic kelvin waves were subsequently generated and propagated rapidly eastward across the equatorial pacific ocean and induced significant warming in the eastern basin by analyzing newly available wind products derived from ssm i satellite observations we found indications that these westerly wind bursts were embedded within the active phase of the tropical intraseasonal oscillation the madden julian oscillation but their amplitudes were greatly enhanced in the western pacific sector the local enhancement involved the development of equatorial twin cyclones which themselves were induced by northerly cold surges from east asia western north pacific into the tropical pacific demonstrating an extratropical atmospheric influence on tropical processes

rectification of the madden julian oscillation into the enso cycle an ocean general circulation model forced with idealized purely oscillating wind stresses over the western equatorial pacific similar to those observed during the madden julian oscillation mjo developed rectified low frequency anomalies in sst and zonal currents compared to a run in which the forcing was climatological the rectification in sst resulted from increased evaporation under stronger than normal winds of either sign from correlated intraseasonal oscillations in both vertical temperature gradient and upwelling speed forced by the winds and from zonal advection due to nonlinearly generated equatorial currents the net rectified signature produced by the mjo like wind stresses was sst cooling about 0 48c in the west pacific and warming about 0 18c in the central pacific tending to flatten the background zonal sst gradient it is hypothesized that in a coupled system such a pattern of sst anomalies would spawn additional westerly wind anomalies as a result of sst induced changes in the low level zonal pressure gradient this was tested in an intermediate coupled

westerly wind events in the tropical pacific 1986 95 abstract based on examination of 10 yr of 10 m winds and wind anomalies from european centre for medium range weather forecasts ecwmf analysis definitions for westerly wind events wwes of eight different types are proposed the authors construct a composite for each type of event show that a simple propagating gaussian model satisfactorily describes the evolution of zonal wind anomaly for each type of event and determine the scales of each composite event by fitting the model to each composite the authors discuss the wwes that occurred during the tropical oceans global atmosphere coupled ocean atmosphere response experiment toga coare intensive observing period iop and show the extent to which these composite events are able to reproduce the major westerly wind features of the iop the frequency of occurrence of each type of wwe for each year of this record and by calendar month are described the authors find several types of events are negatively correlated with the annual mean troup souther

paleolakes and impact basins in southern arabia terra including meridiani planum implications for the formation of hematite deposits on mars 1 the hematite deposit in meridiani planum was selected for a mars exploration rover mer landing site because water could be involved in the formation of hematite and water is a key ingredient in the search for life our discovery of a chain of paleolake basins and channels along the southern margin of the hematite deposits in meridiani planum with the presence of the strongest hematite signature adjacent to a paleolake basin supports the possible role of water in the formation of the hematite and the deposition of other layered materials in the region the hematite may have formed by direct precipitation from lake water as coatings precipitated from groundwater or by oxidation of preexisting iron oxide minerals the paleolake basins were fed by an extensive channel system originating from an area larger than texas and located south of the schiaparelli impact basin on the basis of stratigraphic relationships the formation of channels in the region occurred over much of mars history from before the layered materials in meridiani planum were deposited until recently the location of the paleolake basins and channels is connected with the impact cratering history of the region the earliest structure identified in this study is an ancient circular multiringed basin 800 1600 km diameter that underlies the entire meridiani planum region the mer landing site is located on the buried northern rim of a later 150 km diameter crater this crater is partially filled with layered deposits that contained a paleolake in its southern portion

the south pacific meridional mode as a thermally driven source of enso amplitude modulation and uncertainty abstractthis study seeks to identify thermally driven sources of enso amplitude and uncertainty as they are relatively unexplored compared to wind driven sources pacific meridional modes are argued to be wind triggers for enso events this study offers an alternative role for the south pacific meridional mode spmm in enso dynamics not as an enso trigger but as a coincident source of latent heat flux lhf forcing of enso ssta that if correctly incorrectly predicted could reduce increase enso prediction errors we utilize a coupled model simulation in which enso variability is perfectly periodic and each el nino experiences identical wind stress forcing differences in el nino amplitude are primarily thermally driven via the spmm when el nino occurs coincidentally with positive phase spmm the positive spmm lhf anomaly counteracts a fraction of the lhf damping of el nino allowing for a more intense el nino if the spmm phase is instead negative the spmm lhf amplifies the lhf damping of el nin

el nino phase dependent high frequency variability in western equatorial pacific the intensities of high frequency hf variability with period less than 90 days at different phases of el nino events were investigated through observational data analysis a large asymmetry in the hf variability intensity between the developing phase and decaying phase i e pre peak stage versus post peak stage of eastern pacific ep el nino is revealed while the amplitude and spatial pattern of the sea surface temperature anomaly during these two stages are almost same the diagnosis shows that the asymmetry is significant not only on intraseasonal time scale 20 90 days but also on synoptic time scale less than 20 days the anatomy analysis further unveils that the asymmetric synoptic variability between the two episodes arises from the asymmetric intensities of the equatorial rossby and mixed rossby gravity mrg waves we suggest that the stronger vertical easterly wind shear in the pre peak stage than that in the post peak stage plays a vital role in causing the stronger synoptic equatorial rossby and mrg waves in the pre peak stage meanwhile the drier atmosphere and more descending motion in the post peak stage contribute to the weakened intraseasonal and synoptic variabilities in that stage the aforementioned weakened easterly wind shear drier atmosphere and more descending motion in the post peak stage can be traced back to the occurrence of the anomalous anticyclone circulation over the western north pacific since the decaying phase of el nino the essential role of large scale environmental conditions in modulating the hf variability during the two episodes is further confirmed by modeling experiments

the nature of the stochastic wind forcing of enso abstractel nino southern oscillation enso is commonly viewed as a low frequency tropical mode of coupled atmosphere ocean variability energized by stochastic wind forcing despite many studies h

a tropical stochastic skeleton model for the mjo el nino and dynamic walker circulation a simplified gcm abstracta simple dynamical stochastic model for the tropical ocean atmosphere is proposed that captures qualitatively major intraseasonal to interannual processes altogether including el nino south

is enso a cycle or a series of events 1 after early ideas that saw el ninos as isolated events the advent of coupled models brought the conception of enso as a cycle in which each phase led to the next in a self sustained oscillation twenty two years of observations that represent the el nino and la nina peaks east pacific sst and the memory of the system zonal mean warm water volume suggest a distinct break in the cycle in which the coupled system is able to remain in a weak la nina state for up to two years so that memory of previous influences would be lost similarly while the amplitude of anomalies persists from the onset of a warm event through its termination there is no such persistence across the la nina break these observations suggest that el ninos are in fact event like disturbances to a stable basic state requiring an initiating impulse not contained in the dynamics of the cycle itself

equatorial pacific easterly wind surges and the onset of la nina events abstractthe processes responsible for the onset of la nina events have not received the same attention as those responsible for the onset of el nino events for which westerly wind events wwes in the tropical pacific have been identified as important contributors results here show that synoptic scale surface easterly wind surges ewss play an important role in the onset of la nina events akin to the role of wwes in the onset of el nino events it is found that ewss are a substantial component of zonal wind stress variance along the equatorial pacific using reanalysis wind stress fields validated against buoy measurements 340 ews events are identified between 1986 and 2012 their distributions in space time and el nino southern oscillation enso state are described about 150 ewss occur during enso neutral conditions during the months associated with la nina initiation and growth april december composites of changes in sea surface temperature anomaly ssta following these 150 events show s

geologic versus wildfire controls on hillslope processes and debris flow initiation in the green river canyons of dinosaur national monument abstract as in many areas of high relief debris flows are an important process linkage between hillslopes and the green river in the canyons of the eastern uinta mountains yet the physical conditions that lead to debris flow initiation are unknown a recent episode of enhanced debris flow and wildfire activity provided an opportunity to examine the geomorphic impact of fire and the processes by which weathered bedrock is transported to the green river field investigations and analysis of elevation and precipitation data were undertaken in 15 catchments with recent debris flows to determine how surficial geology wildfire topography bedrock strength and meteorology influence hillslope processes the recent debris flows were triggered by intense summer rainstorms the dominant debris flow initiation mechanism the firehose effect occurred when overland flow generated on bedrock hillslopes cascaded down steep cliffs onto colluvium causing failure sixty percent of the debris flows occurred in unburned catchments however 15 of the burned catchments in the study area produced debris flows over the study period whereas only 7 of the unburned catchments did thus fire was not the primary driver of debris flows but fire related events did contribute to the increased debris flow activity the geomorphic impact of wildfire in the eastern uinta mountains is not as great as in transport limited settings with regolith mantled hillslopes the strong rocks and dry climate of the study area cause hillslopes to be very steep and weathering limited with high runoff ratios and a dearth of regolith as a result there is little vegetation and thus we hypothesize that burning does little to change hillslope processes the suite of hillslope processes in the eastern uintas are like those documented in the similarly dry grand canyon but differ from other locations in the western u s where wildfire is a primary control on debris flow processes

a coupled gcm analysis of mjo activity at the onset of el nino abstract the ocean dynamics of the madden julian oscillation mjo and its interaction with el nino southern oscillation enso are assessed using a flux corrected coupled model experiment from the australian bureau of meteorology the model demonstrates the correct oceanic kelvin wave response to the mjo related westerly winds in the western pacific although there may be a role for the mjo in influencing the strength of el nino its impact is difficult to separate from that of strong heat content preconditioning of enso hence the mjo enso relationship is assessed starting from a background state of low heat content anomalies in the western pacific that are also characteristic of recent observed el nino events the model shows a strong relationship between enso and the mjo near the peak of el nino at this time the sea surface temperature sst anomaly is largest in the central pacific and it is difficult to separate cause and effect near the onset of el nino however when pacific ocean sst anomali

the impact of convection on enso from a delayed oscillator to a series of events abstract the ncar community climate system model version 3 ccsm3 exhibits persistent errors in its simulation of the el nino southern oscillation enso mode of coupled variability the amplitude of the oscillation is too strong the dominant 2 yr period too regular and the width of the sea surface temperature response in the pacific too narrow with positive anomalies extending too far into the western pacific two changes in the parameterization of deep convection result in a significant improvement to many aspects of the enso simulation the inclusion of convective momentum transport cmt and a dilution approximation for the calculation of convective available potential energy cape are used in development integrations and a striking improvement in enso characteristics is seen an increase in the periodicity of enso is achieved by a reduction in the strength of the existing short circuited delayed oscillator mode the off equatorial response is weaker and less tropically confined largely as a

low frequency modulation of intraseasonal equatorial kelvin wave activity in the pacific from soda 1958 2001 intraseasonal equatorial kelvin wave activity iekw at a low frequency in the pacific is investigated using the simple ocean data assimilation soda oceanic reanalyses a vertical and horizontal mode decomposition of soda variability allows estimation of the kelvin wave amplitude according to the most energetic baroclinic modes a wavenumber frequency analysis is then performed on the time series to derive indices of modulation of the iekw at various frequency bands the results indicate that the iekw activity undergoes a significant modulation that projects onto baroclinic modes and is not related in a straightforward manner to the low frequency climate variability in the pacific linear model experiments corroborate that part of the modulation of the iekw is tightly linked to change in oceanic mean state rather than to the low frequency change of atmospheric equatorial variability

the association of the evolution of intraseasonal oscillations to enso phase abstract the pacific ocean intraseasonal kelvin wave is a leading oceanic mode that links intraseasonal tropical atmospheric variations with interannual variations in the coupled ocean atmosphere system this study considers the premise that these waves may evolve differently with their associated weather patterns during different phases of el nino southern oscillation enso if atmospheric and oceanic intraseasonal modes interact and evolve differently during various stages of enso this result may provide useful information with regard to the role of these intraseasonal processes in enso evolution this work utilizes signals of the oceanic kelvin wave as a statistical basis for a simple composite averaging technique that is applied during different phases of enso to objectively analyze the evolution of oceanic and the associated portions of atmospheric intraseasonal oscillations results confirm the above premise and suggest that coupling between kelvin waves and atmospheric convection evolves differen

subseasonal atmospheric variability and el nino waveguide warming observed effects of the madden julian oscillation and westerly wind events abstractwesterly wind events wwes have previously been shown to initiate equatorial pacific waveguide warming the relationship between wwes and madden julian oscillation mjo activity as well as the role of mjo events in initiating waveguide warming is reconsidered here over the 1986 2010 period wwes are identified in observations of near surface zonal winds using an objective scheme mjo events are defined using a widely used index and 64 are identified that occur when the el nino southern oscillation enso is in its neutral state of these mjo events 43 have one or more embedded wwes and 21 do not the evolution of sea surface temperature anomaly over the equatorial pacific waveguide following the westerly surface wind phase of the mjo over the western equatorial pacific is examined waveguide warming is found for the mjo with wwe events in similar magnitudes as following the wwes not embedded in an mjo there is very little statistically significant waveguide warming following mjo events that

charging el nino with off equatorial westerly wind events the buildup of the warm water in the equatorial pacific prior to an el nino event is considered a necessary precondition for event development while the event initiation is thought to be triggered by bursts of westerly wind however in contrast to the view that warm water slowly builds up years before an el nino event the volume of warm water in the equatorial pacific doubled in the first few months of 2014 reaching values that were consistent with the warm water buildup prior to the extreme 1997 1998 el nino it is notable that this dramatic warm water buildup coincided with a series of westerly wind bursts in the western tropical pacific this study uses linear wave theory to determine the effect of equatorial and off equatorial westerly wind events on the warm water volume wwv of the pacific it is found that westerly wind events have a significant impact on equatorial wwv with all events initially acting to increase wwv which highlights why wwes are so effective at exciting enso in fact our results suggest that the single westerly wind burst which peaked in the first few days of march in 2014 was largely responsible for the coincident dramatic observed increase in wwv how long the equatorial region remains charged however depends on the latitude of the westerly wind event for instance a single equatorially symmetric westerly wind event ultimately acts to discharge wwv via the reflection of upwelling rossby waves which makes it difficult to more gradually build wwv given multiple wwes in contrast when the wind events occur off the equator the subsequent discharge is significantly damped and in some cases the equatorial region can hold the heat charge for the duration of the simulations 6 months as such off equatorial wwes can not only charge equatorial region wwv in the short term but are also a mechanism to more gradually build equatorial region wwv in the longer term given that these off equatorial wwes have a relatively small projection onto the equatorial kelvin wave we argue these events can be considered as a mechanism to modulate the background state in which enso operates

strong influence of westerly wind bursts on el nino diversity el nino diversity and its genesis are debated an overview of existing work along with a fuzzy clustering analysis and simulations suggest that the asymmetry irregularity and extremes of el nino result from westerly wind bursts

influence of the state of the indian ocean dipole on the following year s el nino el nino southern oscillation enso consists of irregular episodes of warm el nino and cold la nina conditions in the tropical pacific ocean 1 with significant global socio economic and environmental impacts 1 nevertheless forecasting enso at lead times longer than a few months remains a challenge 2 3 like the pacific ocean the indian ocean also shows interannual climate fluctuations which are known as the indian ocean dipole 4 5 positive phases of the indian ocean dipole tend to co occur with el nino and negative phases with la nina 6 9 here we show using a simple forecast model that in addition to this link a negative phase of the indian ocean dipole anomaly is an efficient predictor of el nino 14 months before its peak and similarly a positive phase in the indian ocean dipole often precedes la nina observations and model analyses suggest that the indian ocean dipole modulates the strength of the walker circulation in autumn the quick demise of the indian ocean dipole anomaly in november december then induces a sudden collapse of anomalous zonal winds over the pacific ocean which leads to the development of el nino la nina our study suggests that improvements in the observing system in the indian ocean region and better simulations of its interannual climate variability will benefit enso forecasts

tropical pacific sea surface temperature anomalies el nino and equatorial westerly wind events abstract the authors examine global statistical relationships between westerly wind events wwes and sea surface temperature anomaly ssta variability using a compositing technique for the period 1986 98 the authors describe the extent to which equatorial wwes are associated with central and eastern equatorial pacific waveguide warming and with local ssta changes under the wwe their goal is to quantify the extent to which equatorial wwes are fundamental to the onset and maintenance of warm el nino southern oscillation conditions in order to understand the effect of wwes on ssta evolution they begin by examining how ssta changes in the absence of equatorial wwes they find that ssta tends toward mean climate values in the absence of equatorial wwes whether the eastern equatorial pacific has close to normal ssta or warmer than normal ssta the two equatorial wwe types whose main surface wind anomalies are west of the date line are associated with weak local surface cooling the equatorial wwe type t

westerly wind bursts and their relationship with enso in cmip3 models 1 equatorial westerly wind bursts wwbs and their relationship with el nino southern oscillation enso in the 18 climate models presented in the world climate research programme s coupled model intercomparison project phase 3 cmip3 are examined some models depict a realistic eastward shift of collective occurrences of wwbs over the pacific as the warm pool expands eastward these models that depict the frequent western pacific wwbs preceding el nino peak known to trigger or enhance el nino tend to reproduce westerly background states and enso more accurately thus the reproducibility of the westerly background states is suggested to be fundamental for wwb occurrences as well as the following el nino although wwbs generate with active convection in most of the models as observations various kinds of intraseasonal disturbances that cause the active convection are found it is suggested that organized convection is essential for the wwb generation but is prepared by each model s own dominant mode in the tropics under global warming wwbs tend to increase over the eastern pacific and decrease over the indian ocean whereas the total number of wwbs does not change consistently this might arise from an increase of short period convective disturbances over the eastern pacific due to a sea surface temperature increase although there is a weak relationship between changes in the enso amplitude and the eastern pacific wwbs in general good models reproducing the wwb enso relationship in the current climate tend to show consistent changes suggesting the possibility of the eastern pacific wwbs to intensify enso

predictability of a coupled model of enso using singular vector analysis part i optimal growth in seasonal background and enso cycles the fastest initial error growth optimal growth in the zebiak and cane zc forecast model for the el nino southern oscillation enso is analyzed by singular value decomposition of a forward tangent model along a trajectory in a reduced eof space in this paper part i of ii optimal growth about the seasonally varying background and enso cycles from a long model run are discussed among the many forms of nonlinearity in zc the discontinuity of the slope in subsurface temperature at zero thermocline depth and the nonlinear advection of sst are the most significant that positive perturbations grow much faster than negative perturbations around the seasonally varying background is first attributable to the discontinuity and second attributable to nonlinear advection about the seasonally varying background 6 month optimal growth is largest for early boreal spring starts which is related to the enhanced atmospheric heating due to equatorward movement of the itcz one dominant growing structure is found characterized by north south and east west sst dipoles convergent winds on the equator in the eastern pacific and a deepened thermocline in the whole equatorial belt this structure is insensitive to start month and optimization time optimal growth about enso cycles in a long model run is generally much smaller than that about the seasonally varying background as before one dominant growing structure insensitive to start time and optimization time is found during the warm phase of enso optimal growth is modulated by season as is that about the seasonal varying background during the onset and mature phases of enso the final pattern of the optimal structure in 6 months is confined to the eastern pacific during the decay phase of enso it spreads to the western pacific as well during the cold phase of enso optimal growth has two maxima in a year early spring and fall the optimal perturbation propagates westward associated with surface layer wind interaction the authors also compare the singular vector analysis in eof space and the standard one in physical space the importance of norm definition to optimal growth and optimal structure is discussed

processes driving intraseasonal displacements of the eastern edge of the warm pool the contribution of westerly wind events we investigate the processes responsible for the intraseasonal displacements of the eastern edge of the western pacific warm pool wpee which appear to play a role in the onset and development of el nino events we use 25 years of output from an ocean general circulation model experiment that is able to accurately capture the observed displacements of the wpee sea level anomalies and upper ocean zonal currents at intraseasonal time scales in the western and central pacific ocean our results confirm that wpee displacements driven by westerly wind events wwes are largely controlled by zonal advection this paper has also two novel findings first the zonal current anomalies responsible for the wpee advection are driven primarily by local wind stress anomalies and not by intraseasonal wind forced kelvin waves as has been shown in most previous studies second we find that intraseasonal wpee fluctuations that are not related to wwes are generally caused by intraseasonal variations in net heat flux in contrast to interannual wpee displacements that are largely driven by zonal advection this study hence raises an interesting question can surface heat flux induced zonal wpee motions contribute to el nino southern oscillation evolution as wwes have been shown to be able to do

holocene debris flows on the colorado plateau the influence of clay mineralogy and chemistry holocene debris flows do not occur uniformly on the colorado plateau province of north america debris flows occur in specific areas of the plateau resulting in general from the combination of steep topography intense convective precipitation abundant poorly sorted material not stabilized by vegetation and the exposure of certain fine grained bedrock units in cliffs or in colluvium beneath those cliffs in grand and cataract canyons fine grained bedrock that produces debris flows contains primarily single layer clays notably illite and kaolinite and has low multilayer clay content this clay mineral suite also occurs in the colluvium that produces debris flows as well as in debris flow deposits although unconsolidated deposits have less illite than the source bedrock we investigate the relation between the clay mineralogy and major cation chemistry of fine grained bedrock units and the occurrence of debris flows on the entire colorado plateau we determined that 85 mapped fine grained bedrock units potentially could produce debris flows and we analyzed clay mineralogy and major cation concentration of 52 of the most widely distributed units particularly those exposed in steep topography fine grained bedrock units that produce debris flows contained an average of 71 kaolinite and illite and 5 montmorillonite and have a higher concentration of potassium and magnesium than nonproducing units which have an average of 51 montmorillonite and a higher concentration of sodium we used multivariate statistics to discriminate fine grained bedrock units with the potential to produce debris flows and we used digital elevation models and mapped distribution of debris flow producing units to derive a map that predicts potential occurrence of holocene debris flows on the colorado plateau

modulation of equatorial pacific westerly easterly wind events by the madden julian oscillation and convectively coupled rossby waves synoptic wind events in the equatorial pacific strongly influence the el nino southern oscillation enso evolution this paper characterizes the spatio temporal distribution of easterly ewes and westerly wind events wwes and quantifies their relationship with intraseasonal and interannual large scale climate variability we unambiguously demonstrate that the madden julian oscillation mjo and convectively coupled rossby waves crw modulate both wwes and ewes occurrence probability 86 of wwes occur within convective mjo and or crw phases and 83 of ewes occur within the suppressed phase of mjo and or crw 41 of wwes and 26 of ewes are in particular associated with the combined occurrence of a crw mjo far more than what would be expected from a random distribution 3 wind events embedded within mjo phases also have a stronger impact on the ocean due to a tendency to have a larger amplitude zonal extent and longer duration these findings are robust irrespective of the wind events and mjo crw detection methods while wwes and ewes behave rather symmetrically with respect to mjo crw activity the impact of enso on wind events is asymmetrical the wwes occurrence probability indeed increases when the warm pool is displaced eastward during el nino events an increase that can partly be related to interannual modulation of the mjo crw activity in the western pacific on the other hand the ewes modulation by enso is less robust and strongly depends on the wind event detection method the consequences of these results for enso predictability are discussed

debris fan formation and rapid modification at warm springs rapid yampa river colorado in 1965 a debris flow from warm springs draw colorado deposited approximately 15 000 metric tons of sediment in the channel of the yampa river constricting the river against the opposite cliff and forming warm springs rapid radiocarbon ages associated with several older debris flow deposits bisected by the 1965 flow indicate a time interval of approximately 1000 years between large channel constricting debris flows from warm springs draw the older debris flow deposits probably constricted the yampa river in a manner analogous to that of the 1965 flow the configuration of these tributary debris fan rapids which occur along canyon rivers throughout the western u s is a function of the sediment characteristics of the debris flow and subsequent hydraulic conditions within the channel constriction erosion of the debris flow deposits and transport of the boulders through the rapid during different discharges was estimated with step backwater modeling shear stress and critical flow best indicate the

a simple approach to quantifying the noise enso interaction part ii the role of coupling between the warm pool and equatorial zonal wind anomalies stochastic forcing has been used conceptually to explain enso irregularity more recently the concept of state dependent multiplicative stochastic forcing has been explored as an explanation of a number of enso properties by calculating the state dependence factor of enso zonal wind stress noise forcing on sst we are able to separate the additive and multiplicative components of the wind stress noise forcing of enso spatially the months with large additive or multiplicative components all resemble previous studies on westerly wind bursts they differ from each other in that the wind stresses are significantly stronger during months with a large multiplicative noise component it is further shown that when the multiplicative noise component is large there have been large values of the wind stress noise in the preceding months this is not true of the months when the additive component is large the multi month growth of the wind stress from the multiplicative noise process is shown to be related to an eastward migration of the western pacific warm pool which is coupled to the wind stress through convection this process is shown to be significantly weakened in a climate model when the ocean and atmosphere are uncoupled

influence of westerly wind events stochasticity on el nino amplitude the case of 2014 vs 2015 the weak el nino of 2014 was preceded by anomalously high equatorial pacific warm water volume wwv and strong westerly wind events wwes which typically lead to record breaking el nino like in 1997 and 2015 here we use the cnrm cm5 coupled model to investigate the causes for the stalled el nino in 2014 and the necessary conditions for extreme el ninos this model is ideally suited to study this problem because it simulates all the processes thought to be critical for the onset and development of el nino it captures el nino preconditioning by wwv the wwes characteristics and their deterministic behaviour in response to warm pool displacements our main finding is that despite their deterministic control wwes display a sufficiently strong stochastic component to explain the distinct evolutions of el nino in 2014 and 2015 a 100 member ensemble simulation initialized with early spring equatorial conditions analogous to those observed in 2014 and 2015 demonstrates that early year elevated wwv and strong wwes preclude the occurrence of a la nina but lead to el ninos that span the weak with few wwes to extreme with many wwes range sensitivity experiments confirm that numerous strong wwes shift the el nino distribution toward larger amplitudes with a particular emphasis on summer fall wwes occurrence which result in a five fold increase of the odds for an extreme el nino a long simulation further demonstrates that sustained wwes throughout the year and anomalously high wwv are necessary conditions for extreme el nino to develop in contrast we find no systematic influence of easterly wind events ewes on the el nino amplitude in our model our results demonstrate that the weak amplitude of el nino in 2014 can be explained by wwes stochastic variations without invoking ewes or remote influences from outside the tropical pacific and therefore its peak amplitude was inherently unpredictable at long lead time

linking the pacific meridional mode to enso coupled model analysis abstract the occurrence of a boreal spring phenomenon referred to as the pacific meridional model mm is shown to be intimately linked to the development of el nino southern oscillation enso in a long simulation of a coupled model the mm characterized by an anomalous north south sst gradient and anomalous surface circulation in the northeasterly trade regime with maximum variance in boreal spring is shown to be inherent to thermodynamic ocean atmosphere coupling in the intertropical convergence zone itcz latitude and the mm existence is independent of enso the thermodynamic coupling enhances the persistence of the anomalous winds in the deep tropics forcing energetic equatorially trapped oceanic waves to occur in the central western pacific which in turn initiate an enso event the majority of enso events in both nature and the coupled model are preceded by mm events

the formation and impact of landslide dams state of the art abstract the blocking of river courses by mass movements is very common in mountainous areas with deep and narrow valleys landslide dams may pose serious threats to people and their livelihoods downstream in the case of abrupt dam failure since the publication of benchmark reviews of costa and schuster 1988 and korup 2002 there is a growing number of studies focusing on the formation stability and short term impacts of landslide dams this review combines the insights of all these studies builds on current concepts of landslide dams and suggests ways to unify terminologies and classifications we furthermore present a new worldwide database compiled from literature data it contains 410 landslide dams 1 million m3 in volume that were formed since 1900 since these have the most complete data entries these data show that dam longevity is among other factors correlated with the type of landslide forming the dam those formed by rock debris avalanches and rockslides have longest lifespans however the influence of landslide type or material on dam longevity decreases with time after dam formation to ensure consistency in the next database generation we suggest guidelines for data collection to provide a solid basis for evaluating dam stability and governing factors a preliminary classification matrix for landslide dam stability that combines topographic setting and the internal structure of the dam body is another outcome of our review furthermore an evaluation of the various geomorphic stability indices proposed in the literature regarding their suitability and limitations in assessing dam formation and stability shows that they predict the probability of dam formation reasonably well but that their application to longevity estimates requires further assessment the geomorphic impacts of landslide dams in the short medium and long term are summarized and illustrated with key examples finally for a better understanding of the factors controlling dam stability we recommend to 1 include dam composition and sedimentary structures in future case studies 2 maintain and update the worldwide database for sound statistical analyses 3 refine landslide dam stability indices and test them for different landslide types and 4 study hazard cascades related to multiple dams in one watershed for long term landscape evolution studies we suggest to 5 quantify terrestrial sediment flux related to landslide dams 6 detect ancient landslide dams in river profiles and 7 further exploit the sediment archives in former impoundment areas

the role of cross equatorial tropical cyclone pairs in the southern oscillation abstract the locations and frequencies of cross equatorial tropical cyclone pairs over the pacific ocean are compared between years of contrasting equatorial pacific sea surface temperature sst and atmospheric circulation anomalies i e high versus low southern oscillation so indices during the period 1971 79 weak southeast trade winds associated with low so indices allow warm 29 c surface water in the equatorial 0 5 s zone to extend eastward across the dateline encouraging formation of cyclone pairs near the dateline the cyclone pairs in turn provide the linkage between warm water and equatorial convection as indicated by canton island rainfall during southern summer in general the occurrences of cyclone pairs vary in longitude with the eastward excursions of warm equatorial water furthermore cyclone pairs appear to provide a positive feedback by advancing the eastward warm water extent in 1974 75 when no cyclone pairs developed near the dateline in spite of sufficiently high sst

on the role of anomalous ocean surface temperatures for promoting the record madden julian oscillation in march 2015 a madden julian oscillation mjo event dramatically amplified at the beginning of march 2015 as the convective phase traversed an unusually warm central pacific ocean this record amplification also resulted in record amplitude of the mjo based on index measurements since 1974 we explore the possible role of the anomalously high ocean surface temperatures in the equatorial central pacific for promoting the extraordinary amplification of this mjo event forecast sensitivity experiments with the predictive ocean atmosphere model for australia show that the enhanced growth of the mjo resulted from amplification of the convective anomaly as it encountered the unusually warm central pacific our results indicate that anomalous sea surface temperature sst at the onset of el nino 2015 promoted the intensification of the mjo we suggest a two way interaction whereby initial sst anomalies promoted enhanced mjo activity which then possibly led to enhanced el nino development

seasonal dependence of the mjo enso relationship abstract observations of the development of recent el nino events suggest a pivotal role for the madden julian oscillation mjo previous attempts to uncover a systematic relationship between mjo activity and the el nino southern oscillation enso however have yielded conflicting results in this study the mjo enso relationship is stratified by season and the focus is on mjo activity in the equatorial western pacific the results demonstrate that mjo activity in late boreal spring leads el nino in the subsequent autumn winter for the period 1979 2005 spring is the season when mjo activity is least asymmetric with respect to the equator and displays the most sensitivity to sst variations at the eastern edge of the warm pool enhanced mjo activity in the western pacific in spring is associated with an eastward expanded warm pool and low frequency westerly surface zonal wind anomalies these sustained westerly anomalies in the western pacific are hypothesized to project favorably onto a developing el n

enso diversity and the recent appearance of central pacific enso enso diversity refers to the appearance in recent decades of different el nino types in the tropical pacific in addition to the canonical el nino the newly discovered type called central pacific cp el nino has its center of warming shifted more to the central equatorial pacific whether these enso types are really distinct or are different manifestations of a continuum is currently under debate the mechanisms for the recent appearance of the additional type of enso have not been clarified while previously it was claimed that the increased cp el nino occurrence in recent decades were due to tropical trade winds weakened by global warming the observed trade winds actually intensified we systematically study the variability of the pacific sea surface temperature sst using the new method of rotated principal component analysis rpca the essential statistical characteristics of these two el nino types are found to be describable by only three fundamental modes of pan pacific sst with different proportions the enso cycle the pan pacific counterpart of the regionally defined pacific decadal oscillation pdo and a new climate mode we call the central pacific variability cpv it is shown that cp el nino can be described by the pdo and the cpv the latter has a horseshoe shaped warm sst pattern in the central pacific flanked by cold sst to the east roughly similar to the original definition of enso modoki by ashok et al j geophys res oceans 112 c11007 2007 as the second empirical orthogonal function in a narrow tropical domain decomposition the principal component pc of this mode called modoki pc mpc was thought to characterize the occurrence of this new phenomenon but it has been shown to be unable to separate it from the canonical enso in contrast we demonstrate that central pacific el nino occurs when the cpv index is larger than 0 5 the intensity of the cpv mode has increased dramatically since 1970s likely as a result of the intensification of the easterly trade winds in the tropical pacific which tilts the thermocline along the tropical pacific and raises the sst in the central equatorial pacific moderate el ninos are more prone to stalling in the central pacific becoming cp el ninos

modulation of westerly wind bursts by sea surface temperature a semistochastic feedback for enso abstract westerly wind bursts wwbs in the equatorial pacific are known to play a significant role in the development of el nino events they have typically been treated as a purely stochastic external forcing of enso recent observations however show that wwb characteristics depend upon the large scale sst field the consequences of such a wwb modulation by sst are examined using an ocean general circulation model coupled to a statistical atmosphere model i e a hybrid coupled model an explicit wwb component is added to the model with guidance from a 23 yr observational record the wwb parameterization scheme is constructed such that the likelihood of wwb occurrence increases as the western pacific warm pool extends a semistochastic formulation which has both deterministic and stochastic elements the location of the wwbs is parameterized to migrate with the edge of the warm pool it is found that modulation of wwbs by sst strongly affects the characteristics of enso in particular coupled fe

combined role of high and low frequency processes of equatorial zonal transport in terminating an enso event abstractthis study investigates the sudden reversal of anomalous zonal equatorial transport above thermocline at the peak phase of enso the oceanic processes associated with zonal transport are se

a review of eastern tropical pacific oceanography summary abstract the collection of articles in this volume reviewing eastern tropical pacific oceanography is briefly summarized and updated references are given the region is an unusual biological environment as a consequence of physical characteristics and patterns of forcing including a strong and shallow thermocline the itcz and coastal wind jets equatorial upwelling the costa rica dome eastern boundary and equatorial current systems low iron input inadequate ventilation of subthermocline waters and dominance of enso scale temporal variability remaining unanswered questions are presented

variability of el nino southern oscillation related noise in the equatorial pacific ocean 1 this study documents the variability of noise in the equatorial pacific associated with the el nino and la nina events based on the ensemble retrospective forecasts of the climate forecast system it is found that the noise measured by the ensemble spread in the western equatorial pacific zonal wind stress is enhanced around and before the peak of el nino events and weakened around 2 3 months after the peak of el nino events the change in the wind stress noise is communicated to the thermocline depth in the eastern equatorial pacific with about 1 month time lag the eastern equatorial pacific sst noise however decreases during the decay stages of el nino events while the corresponding noise in the surface zonal wind and heat flux increases this decrease in the sst noise is related to the weakening of the sst front and the suppression of the tropical instability waves along the flanks of the equatorial pacific cold tongue that is associated with the sst warming due to el nino events similar relations are seen during la nina events except that the changes are in opposite sense as such the signal to noise ratio and thus the predictability for the eastern equatorial pacific sst is relatively high during warm events compared to cold events

effects of westerly wind bursts on el nino a new perspective daily observations from 1971 to 2010 reveal that every el nino during this period was accompanied by congregated westerly wind bursts suggesting a close relationship of these bursts with both cold tongue and warm pool el nino events with the addition of burst like multiplicative noise to an intermediate ocean atmosphere coupled model it is shown that westerly wind bursts by generating eastward equatorial surface currents and downwelling kelvin waves could be responsible for the existence of the warm pool el nino and for the irregularity and extremes of the cold tongue el nino whether these bursts give rise to one type of el nino or the other depends on the timing of their occurrence relative to the phase of the recharge discharge cycle of the equatorial upper ocean heat content

the central pacific el nino intraseasonal kelvin wave in this study we document and interpret the characteristics of the intraseasonal kelvin wave iskw in the pacific over the 1989 2011 period based on observations a linear model and the outputs of an ocean general circulation model ogcm we focus on the wave activity during the central pacific cp el nino events contrasting with the extraordinary el nino of 1997 1998 we find that the iskw activity is enhanced in austral summer spring in the central pacific west of 120 w during cp el nino events the linear model experiment indicates that the austral summer peak is wind forced while the austral spring peak is not and consequently results from nonlinear processes in addition a strong dissipation of the iskws is observed east of 120 w which cannot be accounted for by a linear model using a rayleigh friction a vertical and horizontal mode decomposition of the ogcm simulation further confirms the sharp changes in characteristics of the iskws as well as the reflection of the latter into first meridional rossby wave at the longitude where the maximum zonal gradient of the thermocline is found 120 w our analysis suggests that the confinement of cp el nino warming in the central pacific may result from the reinforcement of the zonal gradient in stratification associated with the la nina like conditions since the late of the 1990s leading to scattering of the energy of the iskws in the eastern pacific

triggering of el nino by westerly wind events in a coupled general circulation model two ten members ensemble experiments using a coupled ocean atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event wwe when the tropical pacific has initial conditions favourable to the development of a warm event in the reference ensemble cref no wind perturbation is introduced whereas a strong westerly wind event anomaly is introduced in boreal winter over the western pacific in the perturbed ensemble cwwe our results demonstrate that an intense wwe is capable of establishing the conditions under which a strong el nino event can occur first it generates a strong downwelling kelvin wave that generates a positive sea surface temperature sst anomaly in the central eastern pacific amplified through a coupled ocean atmosphere interaction this anomaly can be as large as 2 5 c 60 days after the wwe secondly this wwe also initiates an eastward displacement of the warm pool that promotes the occurrence of subsequent wwes in the following months these events reinforce the initial warming through the generation of additional kelvin waves and generate intense surface jets at the eastern edge of the warm pool that act to further shift warm waters eastward the use of a ten members ensemble however reveals substantial differences in the coupled response to a wwe whereas four members of cwwe ensemble develop into intense el nino warming as described above four others display a moderate warming and two remains in neutral conditions this diversity between the members appears to be due to the internal atmospheric variability during and following the inserted wwe in the four moderate warm cases the warm pool is initially shifted eastward following the inserted wwe but the subsequent weak wwe activity when compared to the strong warming cases prevents to further shift the warm pool eastwards the seasonal strengthening of trade winds in june july can therefore act to shift warm waters back into the western pacific reducing the central eastern pacific warming this strong sensitivity of the coupled response to wwes may therefore limit the predictability of el nino events as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead

the extreme el nino of 2015 2016 the role of westerly and easterly wind bursts and preconditioning by the failed 2014 event at the beginning of 2015 as one year earlier in 2014 the scientific community anticipated that el nino conditions could develop in the tropical pacific by year end such projections were related to the occurrence of westerly wind bursts during winter spring of each year that generated strong downwelling kelvin waves indicative of an emerging el nino however the event s progression quickly stalled in 2014 but actively continued in 2015 leading to an extreme warm event comparable to 1997 or 1982 here we compare climate evolution during these two years using satellite observations and numerical simulations we show that during 2014 el nino development was interrupted mid year by an exceptionally strong easterly wind burst whereas during the second year it continued through the summer further we show that the failed 2014 event created favorable conditions for el nino development during the next year as it kept ocean heat content recharged and the western pacific warm pool extended eastward subsequently the winter spring westerly wind bursts in 2015 were followed by a series of state dependent westerly bursts as part of a strong positive bjerknes feedback analogue simulations with a coupled gcm wherein we superimpose the observed sequences of westerly and easterly wind bursts support these conclusions stressing the role of the failed 2014 event in preconditioning the ocean atmosphere system for the development of an extreme el nino in our simulations the probability of an extreme event following early year westerly wind bursts increases from 14 to nearly 60 due to this preconditioning thus the interplay between westerly and easterly wind bursts shapes el nino development and diversity

westerly wind bursts enso s tail rather than the dog westerly wind bursts wwbs in the equatorial pacific occur during the development of most el nino events and are believed to be a major factor in enso s dynamics because of their short time scale wwbs are normally considered part of a stochastic forcing of enso completely external to the interannual enso variability recent observational studies however suggest that the occurrence and characteristics of wwbs may depend to some extent on the state of enso components implying that wwbs which force enso are modulated by enso itself satellite and in situ observations are used here to show that wwbs are significantly more likely to occur when the warm pool is extended eastward based on these observations wwbs are added to an intermediate complexity coupled ocean atmosphere enso model the representation of wwbs is idealized such that their occurrence is modulated by the warm pool extent the resulting model run is compared with a run in which the wwbs are stochastically applied the modulation of wwbs by enso results in an enhancement of the slow frequency component of the wwbs this causes the amplitude of enso events forced by modulated wwbs to be twice as large as the amplitude of enso events forced by stochastic wwbs with the same amplitude and average frequency based on this result it is suggested that the modulation of wwbs by the equatorial pacific sst is a critical element of enso s dynamics and that wwbs should not be regarded as purely stochastic forcing in the paradigm proposed here wwbs are still an important aspect of enso s dynamics but they are treated as being partially stochastic and partially affected by the large scale enso dynamics rather than being completely external to enso it is further shown that wwb modulation by the large scale equatorial sst field is roughly equivalent to an increase in the ocean atmosphere coupling strength making the coupled equatorial pacific effectively self sustained

the role of the southward wind shift in both the seasonal synchronization and duration of enso events near the end of the calendar year when el nino events typically reach their peak amplitude there is a southward shift of the zonal wind anomalies which were centred around the equator prior to the event peak previous studies have shown that enso s anomalous wind stresses including this southward shift can be reconstructed with the two leading eofs of wind stresses over the tropical pacific here a hybrid coupled model is developed featuring a statistical atmosphere that utilises these first two eofs along with a linear shallow water model ocean and a stochastic westerly wind burst model this hybrid coupled model is then used to assess the role of this meridional wind movement on both the seasonal synchronization as well as the duration of the events it is found that the addition of the southward wind shift in the model leads to a christmas peak in variance similar to the observed timing although with weaker amplitude we also find that the added meridional wind movement enhances the termination of el nino events making the events shorter while this movement does not appear to play an important role on the duration of la nina events thus our results strongly suggest that the meridional movement of enso zonal wind anomalies is at least partly responsible for seasonal synchronization of enso events and the duration asymmetry between the warm el nino and cool la nina phases

essential ingredients to the dynamics of westerly wind bursts abstractwesterly wind bursts wwbs are brief anomalously westerly winds in the tropical pacific that play a role in the dynamics of enso through their forcing of ocean kelvin waves they have bee

the linear response of enso to the madden julian oscillation the possibility that the tropical pacific coupled system linearly amplifies perturbations produced by the madden julian oscillation mjo is explored this requires an estimate of the low frequency tail of the mjo using 23 yr of ncep ncar reanalyses of surface wind and reynolds sst we show that the spatial structure that dominates the intraseasonal band i e the mjo also dominates the low frequency band once the anomalies directly related to enso have been removed this low frequency contribution of the intraseasonal variability is not included in most enso coupled models used to date its effect in a coupled model of intermediate complexity has therefore been studied it is found that this mjo forcing mjo can explain a large fraction of the interannual variability in an asymptotically stable version of the model this interaction is achieved via linear dynamics that is it is the cumulative effect of individual events that maintains ensos in this model the largest coupled wind anomalies are initiated after a sequence of several downwelling kelvin waves of the same sign have been forced by mjo the cumulative effect of the forced kelvin waves is to persist the small sst anomalies in the eastern pacific just enough for the coupled ocean atmosphere dynamics to amplify the anomalies into a mature enso event even though mjo explains just a small fraction of the energy contained in the stress not associated with enso a large fraction of the modeled enso variability is excited by this forcing the characteristics that make mjo an optimal stochastic forcing for the model are discussed the large zonal extent is an important factor that differentiates the mjo from other sources of stochastic forcing

debris fan reworking during low magnitude floods in the green river canyons of the eastern uinta mountains colorado and utah the magnitude and frequency of tributary debris flows and the historical range of main stem river discharges are the main factors that create and modify rapids in the colorado river system monitoring of two recently aggraded debris fans in the green river canyons of the eastern uinta mountains shows that main stem floods with magnitudes between 40 and 75 of the predam 2 yr flood cause significant reworking of fan deposits cutbanks formed at fan margins during both small and large flows indicating that lateral bank erosion is an important reworking mechanism armoring of the debris fan surface limited the degree of reworking by successive floods even when subsequent flood magnitudes were similar to those that caused significant reworking peak discharges increased the width of the reworked zone decreased fan constrictions and lowered the water surface elevation of the ponded backwater contrary to predam geomorphic evidence monitoring indicated that eroded material from recently aggraded debris fans was deposited in bars adjacent to the downstream parts of both fans this change in the organization of the fan eddy complex has the potential to alter the location of recirculating eddies and associated areas of fine grained sediment deposition and storage

multiscale variability of deep convection in relation to large scale circulation in toga coare deep convection over the indo pacific oceanic warm pool in the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare occurred in cloud clusters which grouped together in regions favoring their occurrence these large groups of cloud clusters produced large scale regions of satellite observed cold cloud top temperature this paper investigates the manner in which the cloud clusters were organized on time and space scales ranging from the seasonal mean pattern over the whole warm pool region to the scale of individual cloud clusters and their relationship to the large scale circulation and sea surface temperature sst the dominant convective variability was associated with the intraseasonal oscillation iso a large eastward propagating ensemble of cloud clusters marked the iso s progress the meridional structure of the iso was strongly affected by the seasonal cycle with a southward shift from the northern hemisphere in october november to the southern hemisphere in january february the sst had an intraseasonal signal in lagged quadrature with the cold cloudiness and rainfall in coare the sst increased decreased during the convectively suppressed active phases of the iso enhanced low level westerly winds occurred toward the later stages of the enhanced convection periods of the iso though not always centered at the equator the strongest westerlies tended to be located between two synoptic scale cyclonic gyres which were often not symmetric about the equator in the low level wind field this asymmetry in the anomalous equatorial low level westerlies may have different implications for the oceanic response in the coupled atmosphere ocean system than those centered on the equator the cyclonic gyres contained highly concentrated deep convection and in four cases the gyres developed into tropical cyclones within the envelope marking the convectively active phase of the iso cloud clusters were frequently concentrated into westward propagating disturbances with a local periodicity of similar to 2 days these 2 day disturbances have been identified in earlier spectral studies and appear to be related to westward propagating inertio gravity waves in coare they typically contained numerous cloud clusters which underwent a distinct diurnal cycle most of the cloud clusters embedded in the 2 day disturbances moved westward though some were stationary and a few moved eastward a cloud duster tracking program identified groups of clusters lime dusters that exhibited continuity in time and space in the most convectively active period of the iso the tracking program identified almost the entire iso cloud ensemble as a long lasting trackable superconvective system this observation indicates the lack of a distinct scale separation between convection and large scale disturbances during the most intense convective periods in coare

importance of background seasonality over the eastern equatorial pacific in a coupled atmosphere ocean response to westerly wind events coupled atmosphere ocean response to westerly wind events wwes which sometimes trigger el ninos was investigated using a coupled general circulation model to clarify its dependence on the timing and location of wwes twelve sets of 20 member ensembles were made with an idealized single wwe imposed in different months from january to july and at different longitudes from 160 circ e to 160 circ w the initial ocean states are set to be near neutral to el nino la nina so that the lagged response to wwes can be isolated the results show that sea surface temperature sst in the nino3 4 region increases largely and persistently favorable for el nino growth when a wwe is imposed in may whereas a wwe in march increases sst only in the easternmost pacific in both cases an oceanic kelvin wave propagates eastward to warm the eastern pacific when forced by the wwe in may a positive nino3 4 sst anomaly appears in boreal summer according to the seasonal outcrop of the equatorial thermocline and it can strongly interact with seasonally active tropical rain belt to amplify the sst response further a favorable combination of timing and location that maximize the impact of wwes on el ninos in the subsequent winter is suggested albeit its relevance to nature should severely be tested another experiment with initial ocean states in el nino la nina years showed that the wwes in may are efficient to amplify el nino similar to the neutral case but are not efficient to suppress la nina

lack of westerly wind bursts in unmaterialized el nino years abstractthe lack of westerly wind bursts wwbs when atmospheric intraseasonal variability isv events occur from boreal spring to autumn is investigated by comparing two types of el nino years with unmaterialized el nino uen years although high ocean heat content buildup and several isv events propagating eastward are observed in all three types of years few wwbs accompany these in the uen years the eddy kinetic energy budget analysis based on isv shows that mean westerly winds in the lower troposphere facilitate the development of eddy disturbances including wwbs through convergence and meridional shear of zonal winds in the uen years these westerly winds are retracted westward and do not reach the equatorial central pacific mainly due to interannual components in addition positive sea surface temperature anomalies in the western pacific which are conducive to active convection spread widely in a meridional direction centered on 15 n both westward retracted mean westerlies and off equator

role of non linear oceanic processes in the response to westerly wind events new implications for the 1997 el nino onset in march 1997 a strong westerly wind event wwe occurred in the western equatorial pacific prior to the 1997 1998 el nino event it produced downwelling kelvin waves that interacted non linearly with the surface temperature salinity and zonal current fronts located at the eastern edge of the warm fresh pool eewp this non linear interaction locally increased zonal currents by a factor of three compared to a theoretical linear response and advected the eewp at an unexpected rate 1m s to which the ocean atmosphere coupled system may have been responding rapidly to trigger el nino conditions