oceanic equatorial waves and the 1991 93 el nino abstract equatorial kelvin and rossby waves associated with the 1991 93 el nino warm event were detected in temperature observations made by the tropical atmosphere ocean tao buoy array intraseasonal kelvin waves were a prominent part of equatorial thermocline depth variability and were well represented by a simple model consisting only of first and second mode baroclinic kelvin waves the second mode was essential to properly represent the observed amplitude thermocline depth variability at 5 n and 5 s was dominated by annual and interannual rossby waves which were found to have been largely wind forced in midbasin with little if any signal associated with eastern boundary reflection an evaluation of the wyrtki buildup hypothesis and the delayed oscillator hypothesis in connection with the 1991 93 events showed that a long lag about two years occurred between the arrival of the downwelling signal in the west and the subsequent initiation of el nino this was considerably longer than suggested

channelization on plateaus composed of weakly cohesive fine sediment 1 in this paper we present a mathematical model of the formation of gullies at the downstream end of plateaus composed of weakly cohesive fine material such as freshly deposited volcanic ash as an extension of the theory of channel inception on purely erosional slopes proposed by izumi and parker 2000 we perform a linear stability analysis using the momentum equations of flow the transport equation of suspended sediment and the exner equation for beds subject to both erosion and deposition of bed material we find from the analysis that deposition leads to uniform growth of perturbations at small wave numbers or large wavelengths and suppresses the dominant wavelength associated with the maximum growth rate when the effect of deposition increases further channel initiation itself tends to be inhibited as long as the effect of deposition is not large slopes become unstable to transverse perturbations and the dominant wavelength appears in the range of 6 15 times the froude critical depth of the sheet flow on slopes divided by the bottom friction coefficient assuming that the bottom friction coefficient is of the order of 0 01 we find that the dominant gully spacing is of the order of 1000 times the froude critical depth which is similar to the result obtained in the case without deposition we find that deposition affects channel initiation itself rather than channel spacing

the south pacific meridional mode and its role in tropical pacific climate variability abstractthe pacific meridional mode a thermodynamically coupled mode of variability links extratropical pacific oceanic and atmospheric anomalies to the tropical pacific and may give rise to el n

observations of the vertical structure of the eddy field in the kuroshio extension normalized kinetic energy frequency distributions at a site in the kuroshio extension along 152 e are independent of depth as a good first approximation the lowest frequencies are strongly vertically coherent from 350 to 6000 m depths nominal none of the other vertical sets of time series previously examined possess these characteristics to such a degree coherence being typically much lower between the thermocline and abyssal depths with spectral shape changing significantly as well the vertical distributions of eddy kinetic energy at this kuroshio extension site and for the gulf stream along 55 w are very similar if a depth independent contribution of approximately 100 cm2 s 2 be added to the former the latter observation suggests the existence of a more energetic weakly depth dependent component for the eddy field in the north atlantic than for the north pacific at special locations in the vicinity of their mid latitude jets possibly a signature of distinct instability mechanisms

erosion of tephra from the 1980 eruption of mount st helens erosion of tephra on hillslopes north of mount st helens was measured at intervals of several months between 1980 and 1983 using large arrays of stakes rill erosion was initially greater than sheetwash but its importance decreased comparatively quickly both rill erosion and sheetwash decreased by one to two orders of magnitude over the three year period because a stable rill network developed and more permeable and less erodible substrates were created or exposed the decrease occurred prior to recovery of plants the causes for the rapid decline in erosion at mount st helens explain patterns of erosion following several other recent tephra eruptions basin wide annual rates of erosion were computed by mapping the surface cover the hillslope gradient and the texture and thickness of tephra and by using statistical relations developed between them and erosion rates annual erosion decreased from 26 mm between may 1980 and may 1981 to 1 8 mm between may 1982 and may 1983 projecting this rate of decline into the future indicated that only about one sixth of the tephra on hillslopes will be removed by water erosion before soil creep and other forms of mass wasting again dominate hillslope evolution

influence of recent stratification changes on enso stability in a conceptual model of the equatorial pacific abstractchanges in the mean circulation of the equatorial pacific ocean partly control the strong decadal modulation of el nino southern oscillation enso this relationship is considered from the linear stability of a conceptual recharge discharge model with parameters tuned from the observed mean state whereas decadal changes in the mean thermocline depth alone are usually considered in conceptual enso models here focus is given to decadal changes in the mean stratification of the entire upper ocean e g the mean thermocline depth intensity and thickness those stratification changes modify the projection of wind stress forcing momentum onto the gravest ocean baroclinic modes their influence on the simulated frequency and growth rate is comparable in intensity to the one of usual thermodynamic and atmospheric feedbacks while they have here a secondary effect on the spatial structure and propagation of sst anomalies this sensitivity is evidenced in particular for the climate shift of the 1970s

why do we have el nino quantifying a diabatic and nonlinear perspective using observations el nino due to its global impact on weather patterns ecosystems agriculture and public health has become as commonly known to the public as the recent global warming but why we have el nino is not yet as well answered as it may have been assumed linear theories have been successful in explaining the transition from the warm phase to the cold phase of the eastern tropical pacific that results from the rise and fall of el nino but failed to explain the asymmetry between the two phases a nonlinear theory for el nino has suggested that there exist two equilibrium states for the tropical pacific one is zonally symmetric or nearly so with the warm pool extending all the way to the eastern pacific and the other is strongly zonally asymmetric with the warm pool confined to the western half of the tropical pacific under this hypothesis enso results from the fact that under the current radiative heating both states are unstable resulting in the apparent wandering behavior in between these two states as seen in the observations to test this hypothesis the authors have obtained the best approximations for the two equilibrium states empirically using updated ocean assimilation data and quantified the stability of these two empirically obtained equilibrium states using two stability analysis methods the results suggest that the two states are unstable offering support for the nonlinear view of why we have el nino

interdecadal change of the structure of the enso mode and its impact on the enso frequency in the late 1970s the enso cycle exhibited frequency change the oscillation period increased from 2 4 yr high frequency during 1962 75 to 4 6 yr low frequency during 1980 93 observations suggest that this frequency change was accompanied by a significant change in the structure of the coupled enso mode in comparison with the high frequency regime the structure of the coupled mode in the low frequency regime shows three distinctive features during the warm phase of enso the eastward shift of the westerly anomalies the meridional expansion of the westerly anomalies and the weaker intensity of the easterly anomalies in the eastern pacific to test the robustness of the relationship between the oscillation period and the structure of the coupled mode the authors designed empirical atmospheric models based on observations and coupled them with the ocean model of zebiak and cane numerical experiments demonstrate that the enso period is sensitive to changes in the wind anomaly pattern in a way much like the observed enso frequency structure relation the increase of the enso period after 1980 is mainly due to the eastward shift of the zonal wind stress with respect to the sst anomalies physical explanations of the dependence of enso frequency on the structure of the coupled mode are provided by diagnosing the relative contributions of the thermocline feedback and zonal advection feedback on enso evolution

sedimentation under deep sea storms fast currents high concentrations of suspended sediment and grooved mud beds are associated with erosion in frequent abyssal storms where a fast deep mean flow is augmented or reversed by intense intermittent currents this occurs about 5 km below the gulf stream or its rings the waning phase of a storm results in development of bedforms and rapid deposition of a mud blanket several other regions of the world ocean display evidence of abyssal storm activity

barotropic rossby wave radiation from a model gulf stream 1 the barotropic rossby wave field in the north atlantic ocean is studied in an eddy resolving ocean model simulation the meandering model gulf stream radiates barotropic rossby waves southward through preferred corridors defined by topographic features the smoother region between the bermuda rise and the mid atlantic ridge is a particularly striking corridor of barotropic wave radiation in the 20 50 day period band barotropic rossby waves are also preferentially excited at higher frequencies over the bermuda rise suggesting resonant excitation of topographic rossby normal modes the prevalence of these radiated waves suggests that they may be an important energy sink for the equilibrium state of the gulf stream

interannual variability in a tropical atmosphere ocean model influence of the basic state ocean geometry and nonlinearity abstract the behavior of a tropical coupled atmosphere ocean model is analyzed for a range of different background states and ocean geometries the model is essentially that of cane and zebiak for the tropical pacific except only temporally constant background states are considered here for realistic background states and ocean geometry the model solutions feature oscillations of period of 3 5 yr by comparing the full model solution with a linearized version of the model it is shown that the basic mechanism of the oscillation is contained within linear theory a simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere ocean system the analog model highlights the properties that produce coupled atmosphere ocean instability in the eastern ocean basin and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed negative feedback into this instability growth the growth rate of the local instabil

a review of numerical ocean modeling 1983 1986 midlatitude mesoscale and gyre scale for the years 1983 to the present numerical modeling of midlatitude ocean circulation on the mesoscale and gyre scale is reviewed numerical results have shown great similarity to recent thermocline theories but also some important differences in particular ventilation of the pool zone caused by convective processes is substantial also solutions employing explicitly resolved eddies differ significantly from their equilibrium counterparts inverse methods have provided improved estimates of meridional heat transport and tropical upwelling of tracers the crucial influence of the western boundary on the circulation of the gyre interior is beginning to be understood research has focused on the conditions for radiation by jets the relationship between a jet s structure and its stability and the generation of deep mean flows by eddy activity near the surface constraints on the structure of isolated vortices imposed by instability mechanisms have been explored with new balanced models which provide an inexpensive but accurate alternative to primitive equation models for many oceanic processes transport of salinity by gulf stream rings has been found to be comparable to the effects of evaporation and precipitation in the slope water it has also been shown that radial circulation in warm core rings cannot be disregarded as a possible cause of ring spin down in anticipation of a dramatic increase in the oceanic observational data base from altimetry scatterometry tomography and in situ measurement new modeling techniques for data assimilation and prediction are beginning to be developed

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

the role of mean state on changes in el nino s flavor recently many studies have argued for the existence of two types of el nino phenomena based on different spatial distributions the conventional el nino or eastern pacific ep el nino and the central pacific cp el nino here we investigate the decadal modulation of cp el nino occurrences using a long term coupled general circulation model simulation focusing in particular on the role of climate state in the regime change between more and fewer cp el nino events the higher occurrence regime of the cp el nino coincides with the lower occurrence regime of ep el nino and vice versa the climate states associated with these two opposite regimes resemble the leading principal component analysis pca modes of tropical pacific decadal variability indicating that decadal change in climate state may lead to regime change in terms of two different types of el nino in particular the higher occurrence regime of cp el nino is associated with a strong zonal gradient of mean surface temperature in the equatorial pacific along with a strong equatorial trade wind over the area east of the dateline in addition the oceanic variables the mixed layer depth and the thermocline depth show values indicating increased depth over the western to central pacific the aforementioned climate states obviously intensify zonal advective feedback which promotes increased generation of the cp el nino frequent cp el nino occurrences are not fully described by oceanic subsurface dynamics and dynamical or thermodynamical processes in the ocean mixed layer and air sea interaction are important contributors to the generation of the cp el nino furthermore the atmospheric response with respect to the ssta tends to move toward the west which leads to a weak air sea coupling over the eastern pacific these features could be regarded as evidence that the climate state can provide a selection mechanism of the el nino type

on the westward shift of tropical pacific climate variability since 2000 a profound westward shift in atmosphere ocean variability in the tropical pacific is observed when 2000 2017 is compared to 1979 1999 this westward displacement is associated with changes in oceanic kelvin wave activity along the equator especially a remarkable reduction in the eastern pacific 175 145 w this coincides with a weakening of low level westerly winds in this region and a westward shift of low level easterly winds in the central and eastern tropical pacific as well as decreased variability of vertical entrainment vertical diffusion in the ocean mixed layer in the eastern pacific the observed enhancement of the zonal contrast of the mean state across the tropical pacific seems to be associated with the westward shift of climate variability along the equator this systematically westward shift of the atmosphere ocean variability in the tropical pacific alters the impact of the tropical pacific on extra tropical climate

the relationship between thermocline depth and sst anomalies in the eastern equatorial pacific seasonality and decadal variations even though the vital role of thermocline fluctuation in el nino southern oscillation enso cycle has been established previously the direct relationship between the thermocline depth and sea surface temperature sst anomalies in the equatorial pacific is yet to be fully understood especially its seasonality thermocline depth anomalies have been found to lead sst anomalies in time with a longitude dependent delay but our study also suggests that the relationship shows considerable seasonal dependency and is strongest weakest during the boreal spring summer over the eastern equatorial pacific where there is least delay compared to that in the western and central pacific the connection between thermocline and sst is the weakest during the boreal spring this feature may be one of causes for enso spring persistence barrier furthermore the thermocline sst connections exhibit significant decadal variations which are remarkably consistent with the decadal changes in the persistence barrier of sst anomalies over the eastern pacific it is also found that the decadal shift in the timing of the thermocline sst connection barrier is caused by the changes in the seasonal cycle of tropical trade winds and thermocline depths

boundary forced nonlinear planetary radiation abstract in recent years there has been renewed interest in the gulf stream system and its interaction with the mesoscale oceanic eddy field an important question not yet adequately addressed concern the possible generation mechanisms of the mesoscale eddy field and in general the problem of radiation of mesoscale energy from a meandering current this problem has been investigated in a variety of studies the basic result of which is that in the quiescent ocean the far field can transmit energy radiated by a meandering northern current only if the latter has a westward phase speed all the proposed models are however linear nonlinear effects may be expected to modify the above results as indicated by numerical experiments carried out with fully nonlinear models in the present study the question is addressed in the context of a fully nonlinear but simple model the quasi geostrophic equivalent barotropic potential velocity equation in a zonal channel over variable relief the meandering curre

interaction of the gulf stream and deep western boundary current where they cross the long timescale variability of the deep western boundary current dwbc where it crosses under the gulf stream is analyzed using a 3 year array of bottom current meters and inverted echo sounders the relationship of this variability to fluctuations in the upper layer gulf stream is specifically addressed at periods shorter than a year the dwbc variability shows no relationship to the local gulf stream 100 day fluctuations show a pulsing of the dwbc transport whereas longer period fluctuations are indicative of a meandering type variability by contrast on timescales greater than a year variations in the orientation and transport of the dwbc are directly related to such fluctuations in the local gulf stream the coupling reveals that the dwbc is forced by changes in the gulf stream rather than causing such variations which is in contrast to earlier modeling results

the relationship of low frequency deep variability near the hebble site to gulf stream fluctuations abstract current meter measurements from two recent moored array experiments on the continental rise and in the gulf stream south of nova scotia are examined to determine horizontal and vertical structures of energetic time variable motions the intent being to put the so called abyssal storms observed at the hebble site into the context of the larger scale energetics of the region empirical orthogonal functions are used to examine characteristics of the motions as a function of frequency one array spanned the continental rise and the stream in many low frequency bands motions exhibit properties of nearly barotropic topographic rossby waves and are coherent over the array phase propagation of the waves is to the southwest indicating onshore energy propagation data from a second current meter array within the stream are examined to determine source region structure and relationship to rise variability in most frequency bands stream motions are too complex to be clearly separable into empirical modes but a significant mode does exist in the band centered at 30 days westward propagation is observed in the meridional component of the velocity at 4000 m depth and the associated zonal wavenumber matches that observed on the rise the hypothesis that westward propagating meanders can directly excite far field motions is explored and found to be consistent with observations in the 30 day band

abyssal current character determined from sediment bedforms on the nova scotian continental rise abstract bottom photographs across the nova scotian continental rise show effects of an intense bottom current at 4800 5000 m that decreases in strength upslope to tranquil near bottom conditions at 3200 4000 m apparently weak currents are present above 3200 m long term geologically significant flow parallels the bathymetric contours toward the southwest but significant short term variability in direction is indicated by orientations of small scale bedforms the western boundary undercurrent documented elsewhere along the continental margin at depths less than 4000 m has relatively little effect in modifying the sea floor on the nova scotian rise but the deep currents 4000 m have a significant effect repeated photography of small seafloor areas suggests that many bedforms especially those of smaller scale are constructed and destroyed by strong variably directed current events benthic storms on time scales of 3 6 months or less thus a significant part of the geologic response of the seabed to currents may be event controlled on time scales not very different from those in coastal environments

stochastic wave radiation by the gulf stream abstract a linear small amplitude model of rossby waves forced by idealized meanders is presented in order to ascertain whether or not the observed low frequency motions near the gulf stream are capable of being so generated two crucial ingredients are shown to be necessary the meandering activity must vary in the downstream direction and the meanders must have a transient behavior cast as a stochastic average over a large number of meanders the predicted amplitudes of the kinetic energy and reynolds stresses are similar to those observed implications for the forcing of a mean flow are discussed

profiling the gulf stream with a current meter mooring a current meter mooring was deployed for one year at the mean position of the gulf stream at 68 w with instruments from 400 m to 4000 m it provides the first unambiguous evidence that the gulf stream extends from top to bottom of the water column using the strong cross stream temperature structure to derive a horizontal coordinate related to temperature we estimate the width and transport of the gulf stream for four individual events and we determine an average profile of the vertical and cross stream structure of the gulf stream a conservative estimate of transport for this average profile indicates that the gulf stream has a transport of 103 106 m s 1 at 68 w

a description of the bottom boundary layer at the hebble site low frequency forcing bottom stress and temperature structure abstract a preliminary description of the bottom flow at the hebble site 4800 m is presented based on bottom tripod measurements in the lower 5 m of the bottom mixed layer extending to within 30 cm of the seabed and thermistor measurements extending through the mixed layer up to 110 m two different deployment periods are described covering approximately 4 days in april of 1983 and 80 days from june to september 1983 the near bottom flow during the 80 day period is composed of low frequency motions with fluctuations of 20 days or greater with less energetic fluctuations of tidal and inertial periods riding on the flow the speed at 1 m during the long deployment varies from 0 to 17 cm s 1 suggesting that the boundary layer depth should also be modulated at low frequency direct reynolds stress estimates during times of moderate flow indicate the local boundary layer height is reasonably estimated using a simple drag law and classical scaling u f the mixed layer depths observed at the site are 30 70 m corresponding to boundary layer thicknesses estimated for the strongest flows observed and are typically more than five times the estimated boundary layer thickness for moderate flows two flow events observed during the 80 day record correspond to weak and moderate benthic storms during which significant sediment resuspension occurs one event has eastward flow and the other westward the potential importance of the role of superposition of fluid motions of several frequencies in determining the magnitude of such storms is demonstrated by comparing these two events

drivers of reduced enso variability in mid holocene in a coupled model the drivers of el nino southern oscillation enso intensity change during the mid holocene mh are investigated through employing a coupled model that exhibits excellent performance in simulating the present day enso behaviors the model shows a 28 decrease in enso intensity in the mh simulation compared to the pre industrial pi simulation showing an agreement with the ranges indicated by the paleo proxies based on quantitative analyses we reveal that the changes in the oceanic dynamic processes including the thermocline zonal advection and ekman feedback terms in the order from being most to least important were the major drivers of the reduced enso intensity in the mh further diagnosis analyses show that the weakening in all three oceanic dynamic terms could be traced back to the weakened thermocline response to zonal wind stress anomaly in the mh compared to that in the pi such weakened thermocline response was due to the relatively flattened meridional structure of enso related interannual anomaly field e g zonal wind stress anomaly field in the mh which arose from the strengthening of the mean meridional overturning circulation namely the mean pacific subtropical cell stc the process oriented analyses throughout this study suggest a critical linkage between enso intensity change and mean stc change in the mh through documenting how the mean stc change altered the oceanic dynamic processes and thus drove the enso intensity change

hydrodynamic description for advective sediment transport processes and rill initiation a physically based model for advective sediment transport during rill initiation is developed based on 1 a short wavelength rescaling of the momentum balance equations for the surface water flux 2 a deforming coordinate system which characterizes local morphological changes of the surface during erosion and 3 a generalized transport function for the surface sediment flux which distinguishes the contributions of diffusive and advective transport a linear stability analysis of the coupled system indicates that an intrinsic minimum transverse length scale for rill initiation is forced by the cross slope pressure gradient in the surface water the wavelength of this initial rill spacing is dependent upon the relative significance of the advective component of sediment transport and the hydraulic efficiency of the surface water flux and is independent of the total length of the hillslope the hydrodynamic model presented can also be used to evaluate the nonlinear development of rills and rill networks subsequent to initiation

investigating the role of the tibetan plateau in enso variability abstractthe role of the tibetan plateau tp in el nino southern oscillation enso variability is investigated using coupled model experiments with different topography setups removing the tp res

channelization on plateaus with arbitrary shapes 1 the initiation of channels and the factors that control their spacing is one of the most fundamental problems in geomorphology here we present an analysis of channelization that is downstream driven so called because channelization is initiated at some point away from the divide where the froude critical flow is achieved by using two novel techniques the frozen time approach and the momentary stability concept we are able for the first time to investigate downstream driven channelization that occurs on a plateau with an evolving base profile the frozen time approach assumes that the time evolution of the base state can be ignored in the linearized equations whereas the momentary stability concept assumes that an unsteady base state becomes momentarily unstable to a disturbance when the growth of the disturbance is faster than the evolution of the base state the analysis shows that an arbitrary slope profile is momentarily unstable and incised by channels before it evolves into a steady base state if the topographic curvature at the location where the froude critical flow is achieved is sufficiently small as the curvature decreases further dominant channel spacing increases slightly in a purely erosional case while clear dominant channel spacing tends not to appear in an erosional depositional case it is also found from the analysis that in both cases the channel spacing is on the order of 1000 times the froude critical depth the results are compared with field observations and experiments and they show satisfactory agreement

sensitivity of an intermediate ocean atmosphere coupled model of the tropical pacific to its oceanic vertical structure an intermediate ocean atmosphere coupled model icm of the tropical pacific is used to investigate the sensitivity of the coupled system to the oceanic vertical structure the model consists of a three baroclinicmode tropical pacific ocean and a gill tropical atmosphere the mixed layer is similar to the zebiak and cane zc coupled atmosphere ocean model but uses a different parameterization for subsurface temperature derived from xbt data a 100 yr simulation is made and the long term variability of the icm is briefly discussed and compared with the the observations and the zc model the icm reproduces regular enso events with a spectrum peak centered at the period 3 5 yr the presence of higher order vertical modes in the oceanic component of the model allows for reduced off equatorial variability of zonal current and thermocline anomalies and for finer meridional scale for sst anomaly variability in comparison with a simulation with one baroclinic mode only the nature of the processes sustaining the oscillations in the icm is then investigated based on the coupled instabilities and delayed oscillator theories it

mass transfer between gulf stream rings experiments are performed to examine the close interaction of two anticyclonic gulf stream rings a tracer is put in one ring initially is advected and diffuses into the other as the pair evolve in time the appearance of the tracer streak lines is examined in the light of conservation of potential vorticity kinematic viscosity and tracer diffusivity are each varied over two orders of magnitude and tracer behavior changes greatly this work is analogized to the interaction of a warm core ring with the gulf stream with the streamers observed in this work corresponding to those observed in ring stream interactions

two types of el nino events cold tongue el nino and warm pool el nino abstract in this study two types of el nino events are classified based on spatial patterns of the sea surface temperature sst anomaly one is the cold tongue ct el nino which can be regarded as the conventional el nino and the other the warm pool wp el nino the ct el nino is characterized by relatively large sst anomalies in the nino 3 region 5 s 5 n 150 90 w while the wp el nino is associated with sst anomalies mostly confined to the nino 4 region 5 s 5 n 160 e 150 w in addition spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of el nino events furthermore the difference in the transition mechanism between the two types of el nino is clearly identified that is the discharge process of the equatorial heat content associated with the wp el nino is not efficient owing to the spatial structure of sst anomaly as a result it cannot trigger a cold event it is also demonstrated that zonal advective feedback i e zonal adve

assessing the twenty first century shift in enso variability in terms of the bjerknes stability index abstracta decadal change in the character of enso was observed around year 2000 toward weaker amplitude higher frequency events with an increased occurrence of central pacific el ninos here these changes are assessed in terms of the bjerknes stability index bj index which is a measure of the growth rate of enso related sst anomalies the individual terms of the index are calculated from ocean reanalysis products separately for the time periods 1980 99 and 2000 10 the spread between the products is large but they show a robust weakening of the thermocline feedback due to a reduced thermocline slope response to anomalous zonal wind stress as well as a weakened wind stress response to eastern equatorial pacific sst anomalies these changes are consistent with changes in the background state of the tropical pacific cooler mean sst in the eastern and central equatorial pacific results in reduced convection there together with a westward shift in the ascending branch of the walker circulation this shif

a comparative stability analysis of atlantic and pacific nino modes abstractel nino southern oscillation enso in the pacific and the analogous atlantic nino mode are generated by processes involving coupled ocean atmosphere interactions known as the bjerknes feedback it has been argued that the atlantic nino mode is more strongly damped than enso which is presumed to be closer to neutrally stable in this study the stability of enso and the atlantic nino mode is compared via an analysis of the bjerknes stability index this index is based on recharge oscillator theory and can be interpreted as the growth rate for coupled modes of ocean atmosphere variability using observational data an ocean reanalysis product and output from an ocean general circulation model the individual terms of the bjerknes index are calculated for the first time for the atlantic and then compared to results for the pacific positive thermocline feedbacks in response to wind stress forcing favor anomaly growth in both basins but they are twice as large in the pacific compared to the atlanti

interannual and interdecadal variability of ocean temperature along the equatorial pacific in conjunction with enso in this paper the leading modes of ocean temperature anomalies ota along the equatorial pacific ocean are analyzed and their connection with el nino southern oscillation enso and interdecadal variation is investigated the first two leading modes of ota are connected with the different phases of the canonical enso and display asymmetric features of enso evolution the third leading mode depicts a tripole pattern with opposite variation of ota above the thermocline in the central pacific to that along the thermocline in the eastern and western pacific this mode is found to be associated with so called enso modoki insignificant correlations of this mode with the first two leading modes suggest that enso modoki may be a mode that is independent to the canonical enso and also has longer time scales compared with the canonical enso the fourth mode reflects a warming cooling tendency above below the thermocline since 2000 both the first and second modes have a large contribution to the interdecadal change in thermocline during 1979 2012 also the analysis also documents that both enso and ota shifted into higher frequency since 2000 compared with that during 1979 1999 interestingly the enso modoki related ota mode does not have any trend or significant interdecadal shift during 1979 2012 in addition it is shown that first four eof modes seem robust before and after 1999 2000 suggesting that the interdecadal shift of the climate system in the tropical pacific is mainly a frequency shift and the changes in spatial pattern are relatively small although the mean states over two periods experienced some significant changes

wind stress curl and enso discharge recharge in the equatorial pacific discharge and recharge of the warm water volume wwv above the 20 c isotherm in an equatorial pacific ocean box extending across the pacific from 156 e to the eastern ocean boundary between latitudes 5 s and 5 n are key variables in enso dynamics a formula linking wwv anomalies zonally integrated wind stress curl anomalies along the northern and southern edges of the box and flow into the western end of the box is derived and tested using monthly data since 1993 consistent with previous work a wwv balance can only be achieved if the 20 c isotherm surface is not a material surface that is warm water can pass through it for example during el nino part of the wwv anomaly entering the box is cooled so that it is less than 20 c and therefore passes out of the bottom of the box the 20 c isotherm surface the observations suggest that the anomalous volume passing through the 20 c isotherm is approximately the same as t the anomalous wwv entering the western end of the box therefore the observed wwv anomaly can be regarded as being driven by the anomalous wind stress curl along the northern and southern edges of the box the curl anomaly changes the wwv both by divergent meridional flow at the edges of the box and vortex stretching that is the sverdrup balance does not hold in the upper ocean a typical amplitude for the rate of change of wwv for the 5 s 5 n box is 9 6 sv sv 10 6 m 3 s 1 the wind stress curl anomaly and the transport anomaly into the western end of the box are highly correlated with the el nino index nino 3 4 the average sea surface temperature anomaly ssta over the region 5 s 5 n 170 120 w and nino 3 4 leads minus the wwv anomaly by one quarter of a cycle based on the preceding results a simple discharge recharge coupled enso model is derived only water warmer than about 27 5 28 c can give rise to deep atmospheric convection so unlike past discharge recharge oscillator models the west central rather than eastern equatorial sstas are emphasized the model consists of two variables t the ssta averaged over the region of strong enso air sea interaction in the west central pacific equatorial strip 5 s 5 n 156 e 140 w and d the 20 c isotherm depth anomaly averaged over the

storms and flow reversals at the hebble site abstract we examine a 20 month long abyssal current meter record obtained from the hebble site about 40 27 n 62 22 w as expected the westward flowing abyssal western boundary current described by richardson et al 1981 and weatherly and kelley 1982 generally passes over this site we call this current the cold filament current cfc and conclude that it is appreciably stronger than suggested by weatherly and kelley 1984 15 cm s 1 as compared to 10 cm s 1 after comparing the abyssal current record with charts showing the surface location of the gulf stream and rings we conclude that the majority of abyssal storms at the hebble site are due to the presence of the cfc passing by and the absence of a meander or ring overhead the presence of a ring or meander overhead apparently results in the cfc being displaced out of the hebble site as suggested by kelley 1984 at such times the near bottom flow appears to respond to the barotropic component of the overlying ring or meander

weakened interannual variability in the tropical pacific ocean since 2000 abstractan interdecadal shift in the variability and mean state of the tropical pacific ocean is investigated within the context of changes in el nino southern oscillation enso compared with 1979 99 the interannual variability in the tropical pacific was significantly weaker in 2000 11 and this shift can be seen by coherent changes in both the tropical atmosphere and ocean for example the equatorial thermocline tilt became steeper during 2000 11 which was consistent with positive negative sea surface temperature anomalies increased decreased precipitation and enhanced suppressed convection in the western central and eastern tropical pacific which reflected an intensification of the walker circulation the combination of a steeper thermocline slope with stronger surface trade winds is proposed to have hampered the eastward migration of the warm water along the equatorial pacific as a consequence the variability of the warm water volume was reduced and thus enso amplitude also decreased

jet instability over smooth corrugated and realistic bathymetry abstractthe stability of a horizontally and vertically sheared surface jet is examined with a focus on the vertical structure of the resultant eddies over a flat bottom the instability is mixed

reconstructing tropical pacific sea level variability for the period 1961 2002 using a linear multimode model monthly mean sea level anomalies in the tropical pacific for the period 1961 2002 are reconstructed using a linear multi mode model driven by monthly mean wind stress anomalies from the ncep ncar and era 40 reanalysis products overall the sea level anomalies reconstructed by both wind stress products agree well with the available tide gauge data although with poor performance at kanton island in the western central equatorial pacific and reduced amplitude at christmas island the reduced performance is related to model error in locating the pivot point in sea level variability associated with the so called tilt mode we present evidence that the pivot point was further west during the period 1993 2014 than during the period 1961 2002 and attribute this to a persistent upward trend in the zonal wind stress variance along the equator west of 160 w throughout the period 1961 2014 experiments driven by the zonal component of the wind stress alone reproduce much of the trend in sea level found in the experiments driven by both components of the wind stress the experiments show an upward trend in sea level in the eastern tropical pacific over the period 1961 2002 but with a much stronger upward trend when using the ncep ncar product we argue that the latter is related to an overly strong eastward trend in zonal wind stress in the eastern central pacific that is believed to be a spurious feature of the ncep ncar product

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

a three region conceptual model for central pacific el nino including zonal advective feedback abstractthe simple zonal two region framework of the recharge paradigm can accurately manifest the traditional eastern pacific ep type of el nino southern oscillation enso as its major warming

inception of channelization and drainage basin formation upstream driven theory the ubiquitous presence of river drainage basins in the terrestrial environment suggests that distributed overland flow generated by rainfall tends to spontaneously organize itself into dendritic systems of discrete channels several recent numerical models describe the evolution of complete drainage basins from the initial condition of rainfall on a flat tilted plateau the surface of which has been provided with random elevation perturbations these analyses model overland flow via the assumption of a perfect balance between gravitational and frictional terms i e in terms of normal flow linear stability analysis applied to the normal flow model has been shown however to fail to select a wavelength corresponding to a finite distance of separation between incipient basins this suggests that the normal flow model may not be a sufficient basis for studying drainage basin development especially at the finest scales of morphologic significance here the concept of a threshold condition for bed erosion is combined with an analysis of the full equations of shallow overland flow in order to study wavelength selection classical linear stability analysis is shown to be inadequate to analyse the problem at the level of inception an alternative linear analysis of bed perturbations based on the threshold condition is developed and shown to lead to the selection of finite wavelength of the correct order of magnitude the analysis here is driven from the upstream direction in that bed erosion is first caused only when sufficient flow has gathered from upstream due to rainfall a downstream driven theory of incipient channelization that is not necessarily dependent upon rainfall is presented in izumi 1993 and is presently in preparation for publication

el nino variability in simple ocean data assimilation soda 1871 2008 1 a new ocean reanalysis that covers the period from 1871 to 2008 is used to explore the time evolving characteristics of el nino the new reanalysis assimilates all available hydrographic and sea surface temperature data into a model of the global ocean forced with surface boundary conditions from an atmospheric reanalysis that also covers the period from 1871 through 2008 using traditional measures of el nino our reanalysis shows that the timing of el nino events is in agreement with sea surface temperature reconstructions but el nino in the reanalysis is stronger particularly from 1871 to 1920 a new index based on the first moment of the temperature anomaly is introduced the new index is used to characterize the strength and location of el nino events and has the advantage that it is independent of the location of el nino using the new index el nino in the reanalysis shows prominent decadal variability of strength but relatively little long term trend el nino events were strong in the last part of the 19th century and first part of the 20th century and again in the latter part of the 20th century with weak el nino events in the middle of the 20th century the location of el nino also varies considerably ranging from the western to the eastern pacific near the coast of south america however the null hypothesis that the location of el nino can be represented as a random distribution about a central longitude of about 140 w cannot be rejected

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

cyclogenesis in the deep ocean beneath western boundary currents a process oriented numerical study 1 a two layer shallow water equation model is applied to a flat bottom ocean on the f plane to explore instability mechanisms in western boundary current wbc that lead to the formation of strong cyclones in the deep ocean underneath findings reveal a tight coupling of surface meandering and deep cyclogenesis in agreement with observational evidence barotropic cyclones develop in timescales of 5 10 days and attain swirl speeds of 50 cm s depends on initial strength of wbc on a diameter of 100 km cyclogenesis is driven by advection of relative vorticity in the surface ocean and failure of the thermocline to respond rapidly enough to the associated sea level variations findings suggest that cyclogenesis and the associated strong abyssal flows benthic storms are ubiquitous features of wbcs and other frontal flows

variability of the meridional overturning in the north atlantic from the 50 year gecco state estimation abstract the german partner of the consortium for estimating the circulation and climate of the ocean gecco provided a dynamically consistent estimate of the time varying ocean circulation over the 50 yr period 1952 2001 the gecco synthesis combines most of the data available during the entire estimation period with the ecco massachusetts institute of technology mit ocean circulation model using its adjoint this gecco estimate is analyzed here for the period 1962 2001 with respect to decadal and longer term changes of the meridional overturning circulation moc of the north atlantic a special focus is on the maximum moc values at 25 n over this period the dynamically self consistent synthesis stays within the error bars of h l bryden et al but reveals a general increase of the moc strength the variability on decadal and longer time scales is decomposed into contributions from different processes changes in the model s moc strength are strongly influenced by the southward communication of d

changes in tropical pacific thermocline depth and their relationship to enso after 1999 abstractthe characteristics of el nino southern oscillation enso variability have experienced notable changes since the late 1990s including a breakdown of the zonal mean upper ocean heat content as a precursor for enso these changes also initiated a debate on the role of thermocline variations on the development of enso events since the beginning of the twenty first century in this study the connection between thermocline variations and el nino and la nina events is examined separately for the 1980 98 and 1999 2012 periods the analysis highlights the important role of thermocline variations in modulating enso evolutions in both periods it is found that thermocline variation averaged in the central tropical pacific including both equatorial and off equatorial regions is a good precursor for enso evolutions before and after 1999 while the traditional basinwide mean of equatorial thermocline variation is a good precursor only before 1999 the new precursor including both high frequency variabili