observations of variability on synoptic timescales in the east pacific itcz abstract data obtained in the eastern pacific intertropical convergence zone itcz during the tropical eastern pacific process study tepps show a 3 6 day variability the noaa ship ronald h brown collected surface meteorological observations c band doppler radar volumes atmospheric soundings and rainfall data while on station at 7 8 n 125 w from 8 23 august 1997 the 3 6 day variability was a prominent timescale in the meridional wind and humidity data the maxima of the surface to 700 mb meridional wind anomalies were 5 12 m s 1 maxima of the moisture anomalies at the same levels were 1 3 g kg 1 in phase with the low level southerlies while upper level moisture lagged the southerlies by less than a day the radar indicated a close connection between precipitation in the vicinity of the ship and the meridional wind with the precipitation occurring when the surface southerlies were strongest the meridional and zonal wind components had a maximum of 850 450 mb wind shear during the southerly e

effect of iron content on electrical conductivity of ringwoodite with implications for electrical structure in the transition zone abstract electrical conductivity of ringwoodite with various iron contents fe fe mg 0 09 0 2 and 0 3 was measured at pressure 20 gpa and temperature up to 1900 k conditions of the lower part of the mantle transition zone in a kawai type multi anvil apparatus the conductivity increased with increasing total iron content all electrical conductivity data were fitted to the formula of electrical conductivity 0 xfe exp h kt where 0 is the pre exponential term xfe is the mole fraction of iron content in the mg site h is the activation enthalpy k is the boltzmann constant and t is absolute temperature the activation enthalpy becomes higher at a certain temperature at high temperatures the activation enthalpy decreased from 1 44 to 0 92 ev with increasing total fe content at low temperatures less than 1000 k the activation enthalpy also decreases from 1 15 to 0 74 ev with total fe content dependence of the activation enthalpy on fe content suggests that the dominant mechanism of charge transport is fe2 fe3 hopping small polaron recent electrical conductivity depth profiles of the transition zone beneath the pacific ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660 km depths may be explained by ringwoodite with fe fe mg 0 10 on the other hand assuming a normal geotherm conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with fe fe mg 0 10 taking into consideration results from the global seismic tomographic studies relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle rather than the presence of iron poor ringwoodite or a combination of both

the annual cycle in equatorial convection and sea surface temperature abstract the coupled atmosphere ocean system in the equatorial eastern pacific and atlantic exhibits a distinct annual cycle that is reflected in contrasting conditions at the times of the two equinoxes the contrasts are so strong that they dominate the annual march of zonally averaged outgoing longwave radiation for the equatorial belt the march equinox corresponds to the warm season when the equatorial cold tongues in the eastern pacific and atlantic are absent with the onset of summer monsoon convection over colombia central america and west africa in may june northward surface winds strengthen over the eastern pacific and atlantic the equatorial cold tongues reappear and the marine convection shifts from the equatorial belt to the intertropical convergence zones itczs along 8 n as the northern summer program the itczs remain strong and shift northward to new 10 n while sea surface temperature sst continues to drop over the cold tongues and the southern tropics perhaps in response to the

upscale impact of mesoscale disturbances of tropical convection on 2 day waves abstractwestward propagating 2 day waves with embedded mesoscale disturbances contribute a large portion of synoptic variability of tropical convection over the western pacific it is of crucial im

composite patterns of easterly disturbances over west africa and the tropical atlantic a climatology from the 1979 95 ncep ncar reanalyses the horizontal and vertical structure of the 3 5 day and 6 9 day easterly waves over west africa and tropical atlantic are investigated ncep ncar reanalyses are used for the period 1979 1995 to produce a 17 year climatology of both 3 5 day and 6 9 day easterly waves composite patterns of convection wind temperature and vertical velocity are analysed with respect to the following the modulation by 3 5 day and 6 9 day wave regimes the contrasts between the itcz 5 n 10 n and the sahelo saharan band 15 n 20 n the difference between land and ocean and seasonal variations similarities and differences in the characteristics of the two wave regimes are identified

the madden julian oscillation barotropic dynamics and north pacific tropical cyclone formation part i observations low level barotropic dynamics may help to explain the modulation of eastern and western north pacific tropical cyclones by the madden julian oscillation mjo during northern hemisphere summer the mjo is characterized by alternating periods of westerly and easterly 850 mb zonal wind anomalies across the tropical pacific ocean when mjo 850 mb wind anomalies are westerly small scale slow moving eddies grow through barotropic eddy kinetic energy eke conversion from the mean flow these growing eddies together with strong surface convergence 850 mb cyclonic shear and high mean sea surface temperatures create a favorable environment for tropical cyclone formation periods of strong mjo easterlies over the pacific are characterized by lesser eke and negligible eddy growth by barotropic conversion the term 2 x is a leading contributor to low level barotropic eke conversion during mjo westerly 2 u9 u periods across the pacific indicating the importance of zonal variations in the westerly jet for producing concentrations of eddy energy this mechanism can be described as wave accumulation associated with variations of the low level zonal flow the conversion term 2 y contributes a smaller portion of the total conversion u9y9 u over the eastern pacific but is of comparable importance to 2 x during westerly mjo events in the 2 u9 u western pacific

tracking and mean structure of easterly waves over the intra americas sea abstract easterly waves ews are prominent features of the intertropical convergence zone itcz found in both the atlantic and pacific during the northern hemisphere summer and fall where they commonly serve as precursors to hurricanes over both basins a large proportion of atlantic ews are known to form over africa but the origin of ews over the caribbean and east pacific in particular has not been established in detail in this study reanalyses are used to examine the coherence of the large scale wave signatures and to obtain track statistics and energy conversion terms for ews across this region regression analysis demonstrates that some ew kinematic structures readily propagate between the atlantic and east pacific with the highest correlations observed across costa rica and panama track statistics are consistent with this analysis and suggest that some individual waves are maintained as they pass from the atlantic into the east pacific whereas others are generated locally in the caribbean a

resonant forcing of mixed layer inertial motions by atmospheric easterly waves in the northeast tropical pacific abstract westward propagating atmospheric easterly waves contribute to much of the variability of the low level wind fields within the northeast tropical pacific with the dominant period of these waves 3 5 days close to the local inertial period 2 4 days at 12 n to 5 7 days at 5 n there is the expectation that the associated winds may resonantly force mixed layer inertial motions in this region the authors test this hypothesis using a simple slab model and roughly 4 yr of wind data from four noaa tropical atmosphere ocean eastern pacific investigation of climate processes tao epic buoys along 95 w at 12 10 8 and 5 n the degree of resonance is determined by comparing model simulations using observed wind stress with simulations forced with reversed rotation wind stress results strongly indicate that pacific easterly waves pews resonantly force inertial motions in the region this resonance shows both significant seasonality and latitudinal dependence that appears to be related to the me

horizontal and vertical structure of easterly waves in the pacific itcz abstract outgoing longwave radiation olr and low level wind fields in the atlantic and pacific intertropical convergence zone itcz are dominated by variability on synoptic time scales primarily associated with easterly waves during boreal summer and fall this study uses spectral filtering of observed olr data to capture the convective variability coupled to pacific easterly waves filtered olr is then used as an independent variable to isolate easterly wave structure in wind temperature and humidity fields from open ocean buoys radiosondes and gridded reanalysis products the analysis shows that while some pacific easterly waves originate in the atlantic most of the waves appear to form and strengthen within the pacific pacific easterly waves have wavelengths of 4200 5900 km westward phase speeds of 11 3 13 6 m s 1 and maximum meridional wind anomalies at about 600 hpa a warm moist boundary layer is observed ahead of the waves with moisture lofted quickly through the troposphere by deep co

synoptic scale disturbances near the equator abstract a spectral analysis of winds analyzed and initialized at the european centre for medium range weather forecasts reveals an abundance of power in the 850 mb meridional wind along the equator with periods near four days the power is mostly in the westward propagating component using high pass filtered data it is shown that the waves have westward phase and eastward group propagation relative to the mean wind the longest wavelengths are found over the pacific ocean while the shortest are found over the convectively variable regions of indonesia south america and africa mean phase speeds at 850 mb are positively correlated with the mean wind on the equator at 500 mb and below and negatively correlated with the mean wind above that level the effective advecting zonal wind of the disturbances seems to be the density weighted average of the lower troposphere the structure of the disturbances bears resemblance to the expected structure of an equatorially trapped mixed rossby gravity wave over t

synoptic scale waves in sheared background flow over the western north pacific abstracttropical depression td type waves are the dominant mode of synoptic scale fluctuations over the western north pacific by applying spatiotemporal filters to the observed olr data and the ncep doe amip ii reanalysis data for 1979 2013 this study reveals the characteristics and energetics of convectively coupled td type waves under the effects of different circulation patterns in association with vertical wind shear results exhibit that different ambient sheared flows significantly affect the vertical structure of westward propagating td type waves with a lower tropospheric mode in an easterly sheared background and an upper tropospheric mode in a westerly sheared background energetic diagnoses demonstrate that when the disturbance is trapped in the lower upper level by easterly westerly shear the horizontal mean flow in the lower upper level favors wave growth by converting energy from the shear of the zonal mean flow from the convergence of the meridional mean flow during the penet

observation of a quasi 2 day wave during toga coare abstract detailed structure of the quasi 2 day oscillation observed in the active phase of the madden julian oscillations during the intensive observation period of tropical ocean and global atmosphere coupled ocean atmosphere response experiment toga coare iop was described a variety of observational platforms is used including high resolution gms infrared histogram rain rate estimate from toga and mit radar measurements upper air soundings and boundary layer profiler winds from the integrated sounding system and surface data from the imet buoy the quasi 2 day mode had a westward propagation speed of 12 15 day 1 a horizontal wavelength of 25 30 longitude a coupling with the westward propagating n 1 inertio gravity waves was hypothesized from the space time power spectral distribution of the cloud field the wind disturbance structure was consistent with the hypothesis the vertical wave structure had an eastward phase tilt with height below 175 hpa and vice versa above indicating the wav

an observational study of easterly waves over the eastern pacific in the northern summer using fgge data abstract fgge revel iii b data provided by the european centre for medium range weather forecasts and the outgoing longwave radiation data measured by satellites are used to investigate observationally relationships between deep cloud activity and large scale meteorological fields during the northern hemisphere summer may september of 1979 over the eastern pacific aside from the summer monsoon area over southern and eastern asia the eastern pacific is the area where strong deep convection frequently develops in the northern summer this is also one of the tropical ocean areas that have been least explored meteorologically a power spectral analysis using the maximum entropy method is made for the meridional wind component at 850 mb at various grid points in the analysis domain for each mouth from may through september the waves with a period of 4 6 days are not only stronger in july and august than those of other months but active in the regions of 100 130 w in the eastern pacific and 130 160 e i

generation of african easterly wave disturbances relationship to the african easterly jet abstract the relationship between african easterly waves and the background climatology in which they form is studied using a regional climate model the surface and lateral boundary conditions in the model are manipulated to modify the background climatology especially the african easterly jet and the itcz and the behavior of the waves in these different settings is evaluated three climate simulations are presented with monthly mean lateral and surface boundary conditions one has a strong jet and a strong itcz the second has a strong jet and a weak itcz and the third has a weak jet and a strong itcz in these simulations the presence of wave activity is more closely associated with the concentration of the itcz than the strength of the african easterly jet in particular the simulation with a strong jet accompanied by a weak itcz does not produce significant wave activity but a weak jet with a strong itcz has realistic wave disturbances both the charney stern and the fjortoft necessary conditi

a study of the energetics of african easterly waves using a regional climate model abstract the evolution and spatial distribution of the energetics of african waves are studied complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation it is found that baroclinic overturning is the dominant energy source although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet the generation of active waves usually results from the nearly in phase evolution of baroclinic and barotropic conversions which are associated with significant rainfall over africa significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet

horizontal and vertical structure of observed tropospheric equatorial rossby waves equatorial rossby er waves are identified in an 8 year data set of national meteorological center operational analyses the westward moving waves have symmetric circulations about the equator and eastward energy dispersion with maximum zonal wind perturbations along the equator and meridional wind maxima roughly 10 15 off the equator they have maximum amplitude in the lower troposphere and are associated with convective signals at roughly the mean latitude of the tropical convergence zones the circulations thus possess many of the features of the equatorially trapped n 1 rossby modes derived by matsuno 1966 they are generally around zonal planetary wavenumber 6 scale and have a deep nearly equivalent barotropic structure up to 100 mbar this is in contrast with the madden julian oscillation and mixed rossby gravity waves which generally have a first baroclinic mode vertical structure during the intensive observing period iop of the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare and in other years westward propagating er disturbances along the equator are observed by using 6 to 30 day filtered circulation data at 850 mbar zonal wind anomalies at 850 mbar are preferentially associated with like signed anomalies in a deep layer at this frequency as was observed during coare the er waves are most common during northern winter in the central pacific although they can be detected sporadically over the other ocean basins and in other seasons as well one source for the equatorial rossby waves appears to be upper level extratropical rossby wave activity propagating into the eastern pacific although the equatorial waves are sometimes present during the northern summer when the extratropical source is absent we cite evidence that propagation of er wave activity from the central into the western pacific is related to occurrences of westerly wind bursts

the interaction of easterly waves orography and the intertropical convergence zone in the genesis of eastern pacific tropical cyclones the interaction of an idealized easterly wave with an orographic feature representing the sierra madre of mexico and a large scale meridionally sheared zonal flow representing the itcz in the eastern north pacific is examined through numerical integration of the shallow water equations on an equatorial beta plane channel the model simulations show that as a wave approaches the orography from the east the relative vorticity in the lee increases due to the wave being modified by the orography this relative vorticity maximum increases the northward advection of the itcz in the lee of the mountain that occurs due to the influence of the mountain anticyclone the result of this interaction is the development of a localized region of enhanced cyclonic vorticity that moves parallel to the contours of the model orography there is a minimal increase in the amplitude of the vorticity occuring during interactions between the wave and orography itcz and orography and wave and itcz the position of the incident wave upstream of the orography at the time that the disturbance in the lee begins to develop and the subsequent motion of the leeside vorticity maximum are in agreement with an observed case of eastern pacific tropical cyclogenesis which is also presented

energetics of east pacific easterly waves during intraseasonal events abstractthe background atmospheric state of the east pacific warm pool in which easterly waves develop varies dramatically on intraseasonal time scales east pacific intraseasonal variability is well known to modulate local convective and circulation patterns westerly easterly intraseasonal phases are associated with westerly easterly low level wind and positive negative convective anomalies this study investigates the perturbation available potential energy pape and perturbation kinetic energy pke budgets of easterly waves composited during westerly easterly and neutral intraseasonal phases respectively the mechanisms magnitudes locations and vertical structures of easterly waves are shown to strongly vary as a function of intraseasonal phase easterly waves draw energy from low level barotropic conversion regardless of phase although the location and magnitude of the conversion varies strongly during neutral and westerly intraseasonal phases the generation of pape associated with

the origin and dispersion characteristics of the observed tropical summertime synoptic scale waves over the western pacific abstract the origin initiation and dispersion behavior of the observed summertime synoptic scale disturbances in the tropical western pacific are studied these westward propagating disturbances have the strongest growth rate over the region of 130 160 e off the equator the three dimensional wave activity flux associated with a wave packet in the vicinity of this region is computed in general wave activity is directed westward there is accumulation of activity flux which gives rise to the amplification of waves in the low levels such accumulation can be attributed to the convergence of both the mean flow and the intrinsic group velocity diabatic forcing also contributes to the growth of disturbances and is most important in the 500 600 hpa layer along the east west oriented storm tracks of the synoptic scale disturbances there are two different dynamical regimes west of 150 e enhanced convection is associated with increased specific humidity at the top of the planetary boundary layer an

the interaction of planetary scale tropical easterly waves with topography a mechanism for the initiation of tropical cyclones abstract the interaction of a basic state flow consisting of a planetary scale easterly wave superimposed on a uniform easterly zonal wind with an isolated topographic feature is examined by numerical integration of the shallow water equations in an equatorial beta plane channel the topographic parameters are chosen to represent the sierra madre mountains of mexico the zonal wavelength and phase speeds of the basic state easterly waves are chosen to correspond with previously published observations the topographic modification of a slow rossby mode in which the basic state vorticity maximum is located near 15 latitude is characterized by the generation of a cyclonic vorticity maximum in the lee of the mountain which extends the entire length of the ridge and a secondary maximum which propagates downstream a fast mixed rossby gravity mode in which the vorticity maximum is located at the equator interacts less strongly with the mountain with a lee vorticity maximum that is confined near the southern

observations of a convectively coupled kelvin wave in the eastern pacific itcz abstract a case study of a convectively coupled kelvin wave in the eastern pacific intertropical convergence zone itcz is presented as observed during the 1997 pan american climate studies pacs tropical eastern pacific process study tepps the large scale convective envelope associated with this disturbance with a zonal scale of approximately 1000 2000 km propagates eastward at 15 m s 1 along the mean convective axis of the itcz this envelope consists of many smaller scale westward moving convective elements with zonal scales on the order of 100 500 km as the convectively coupled kelvin wave disturbance propagates eastward it exerts a strong control on local convection radar and vertical profiler data collected aboard the noaa r v ronald h brown during the wave passage show that convection deepens rapidly as the kelvin wave approaches from the west progressing from isolated shallow cumuli to organized deep convective features within just 12 h initially rainfall in the vicinity of the s

oscillations of the intertropical convergence zone and the genesis of easterly waves part i diagnostics and theory we examine the mean and transient state of the intertropical convergence zone itcz by analyzing data and using simple theory we concentrate on the tropical eastern pacific ocean noting that there exists in this region a well developed mean itcz furthermore it is a region where there has been considerable discussion in the literature of whether easterly waves develop in situ or propagate westwards from the atlantic ocean the region is typical of tropical regions where there is a strong cross equatorial pressure gradient cepg mean convection well removed from the equator but located equatorward of the maximum sea surface temperature sst and minimum sea level pressure mslp further to the west near the dateline where the cepg is much smaller convection is weaker and collocated with sst and mslp extrema it is argued that in regions of significant cepg that the near equatorial tropical system is inertially unstable and that the rectification of the instability for a given cepg determines the location and intensity of the climatological itcz using simple theoretical arguments we develop an expression for the mean latitude of the itcz as a function of the cepg we note on a day by day basis that the itcz is highly transient state with variability occurring on 3 8 day time scales transients with amplitudes about half of the mean itcz propagate northwards from the near equatorial southern hemisphere as anomalous meridional oscillations eventually amplifying convection in the vicinity of the mean itcz it is argued that in these longitudes of strong cepg the mean itcz is continually inertially unstable with advections of anticylonic vorticity across the equator resulting in the creation of an oscillating divergence convergence doublet the low level convergence produces convection and the resultant vortex tube stretching generates cyclonic vorticity which counteracts the northward advection of anticylonic vorticity during a cycle the mid troposphere heating near 10on oscillates between 6 and 12 k day at the inertial frequency of the latitude of the mean convection as a result there exists an anomalous and shallower oscillating meridional circulation with a magnitude about 50 of the mean itcz associated with the stable state following the generation of anticylonic vorticity further it is argued that the instabilities of the itcz are directly associated with in situ development of easterly waves which occur with the inertial period of the latitude of the mean itcz the dynamical sequences and the genesis of easterly waves are absent in the regions further to the east where the cepg is much weaker or absent altogether in a companion study part ii numerical experiments are conducted to test the hypothesis raised in the present study

three dimensional structure and dynamics of african easterly waves part i observations the mean structure of african easterly waves aews over west africa and the adjacent atlantic is isolated by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered satellite derived outgoing longwave radiation these results are compared with previous studies and an idealized modeling study in a companion paper which provides evidence that the waves bear a close structural resemblance to the fastest growing linear normal mode of the summertime basic state flow over africa there is a significant evolution in the three dimensional structure of aews as they propagate along 10 n across west africa at this latitude convection occurs in northerly flow to the east of the greenwich meridian then shifts into the wave trough and finally into southerly flow as the waves propagate offshore into the atlantic itcz in contrast to the north of the african easterly jet along 15 n convection remains in southerly flow throughout the waves trajectory along 10 n over west africa the location of convection is consistent with the adiabatic dynamical forcing implied by the advection of perturbation vorticity by the mean thermal wind in the zonal direction as in the companion paper offshore and along 15 n the relationship between the convection and dynamics is more complex and not as easily explained in terms of dynamical forcing alone

westward propagating synoptic scale disturbances and the itcz abstract in an attempt to understand the dynamics of the intertropical convergence zone itcz this study explores the extent to which the itcz is causally related to zonally propagating synoptic scale disturbances the itcz measured by its mean convection is represented by mean outgoing longwave radiation olr synoptic scale disturbances measured by their deep convective signals are represented by the spectral power of the olr that is significantly above its red noise background time mean spatial distributions as well as annual and interannual variability of the itcz are compared with those of synoptic scale disturbances which are dominated by westward propagating signals in general they match each other well in their mean distributions and annual cycles but in detail discrepancies between the two fields exist some of them substantial the maximum disturbance activity tends to be located at the polar side of the itcz the seasonal cycles of the two share many similarities but the variatio

modulation of eastern north pacific hurricanes by the madden julian oscillation abstract hurricane and tropical storm statistics verify the modulation of eastern pacific tropical systems by the madden julian oscillation mjo as hypothesized by maloney and hartmann over twice as many named tropical systems hurricanes and tropical storms accompany equatorial 850 mb westerly anomalies than accompany equatorial easterly anomalies and the systems that do exist are stronger hurricanes are over four times more numerous during westerly phases of the mjo than during easterly phases the current study constructs a composite life cycle of the mjo during may november 1979 95 using an index based on the 850 mb equatorial zonal wind equatorial kelvin waves propagating eastward from convective regions of the western pacific ocean alter dynamical conditions over the eastern pacific ocean westerly easterly equatorial 850 mb wind anomalies are accompanied by enhanced suppressed convection over the eastern pacific hurricane region convection locally amplifies the wind anomalies over the ea

mixed rossby gravity waves and western pacific tropical cyclogenesis part i synoptic evolution abstract a large amplitude mixed rossby gravity wave packet is identified in the western pacific using 6 10 day bandpass filtered winds individual disturbances of 2300 3000 km wavelength propagated westward as the packet moved slowly eastward the packet first appeared and subsequently amplified within a region of active convection associated with the madden julian oscillation mjo which was isolated by low pass filtered outgoing longwave radiation the packet lasted about 5 weeks then rapidly dispersed as the active mjo moved away from it to the east west of 150 e individual disturbances within the packet turned northwestward away from the equator indicating an apparent transition from mixed rossby gravity waves to off equatorial tropical depression td type disturbances cyclones filled with cloud and anticyclones cleared during the transition nevertheless convective structure consistent with mixed rossby gravity waves remained outside the circulation centers and three tropical cyclones for

a climatology of waves in the equatorial region abstract propagating anomalies of moisture and moist deep convection in the tropics are organized into a variety of large scale modes these include but are not limited to the so called intraseasonal oscillations convectively coupled waves similar to those predicted by shallow water theory on the equatorial beta plane and tropical depression type disturbances along with the annual and diurnal cycles these modes act and interact to control much of the variance of tropical convection analyses of 10 yr of outgoing longwave radiation olr and precipitable water pw data are carried out to develop comparative climatologies of these wavelike modes the analysis relaxes the commonly used cross equatorial symmetry constraints which allows study of the portions of the wavelike processes that are asymmetric across the equator mean background states are found for olr and for pw as functions of day of the year examination of anomalies together with the background reveals much about how the waves are affec

itcz breakdown in three dimensional flows the intertropical convergence zone itcz is observed to undulate and at times break down into a series of tropical disturbances in several days some of these disturbances may develop into tropical cyclones and move to higher latitudes while others dissipate and the itcz may reform in the original region it has been proposed that the itcz may break down because of its heating induced potential vorticity pv anomalies here this process is examined in three dimensional simulations using a primitive equation model a simulation of the itcz in a background state of rest is compared to simulations in different background flows the effect of different vertical structures of the prescribed heating is also examined deep heating induces a positive pv anomaly in the lower troposphere leading to a reversal of the pv gradient on the poleward side of the heating while the induced pv anomaly at upper levels is negative leading to a reversal of the pv gradient on the equatorward side of the heating the response at upper levels leads to a weaker pv gradient change but the response is greater in areal extent than the lowertropospheric response for shallow heating the lower tropospheric pv response is greater than that for deep heating and there is no upper tropospheric pv response the itcz lasts longer before breaking in this case than in the deep heating case effects of the background flow are mainly felt in the deep heating cases when the background flow enforces the pv induced wind field itcz breakdown occurs more rapidly whereas when the background flow is opposite to the pv induced flow itcz breakdown takes longer and the itcz may dissipate before breakdown

a multiscale model for the modulation and rectification of the itcz abstractthe authors introduce the modulation of the itcz equations m itcz which describes the multiscale dynamics of the itcz on diurnal to monthly time scales in which mesoscale convectively coupled rossby waves in the itcz are modulated by a large scale gravity wave that is also generated by convection westward propagating disturbances are observed to cause itcz breakup over the course of a few days and the m itcz meso planetary scale coupled waves provide a mechanism for this interaction thereby providing a framework to study the modulation and rectification of the hadley circulation over long zonal length scales in the itcz the authors consider examples of zonally symmetric heating profiles in the m itcz system and generate a hadley circulation consistent with the observed winds zonally localized heating creates a wind response throughout the tropics that is carried by a pair of zonally propagating gravity bores driving mean easterlies at the base and mean westerlies at the top of the troposph

energetics of easterly wave disturbances over west africa and the tropical atlantic a climatology from the 1979 95 ncep ncar reanalyses our purpose is to compute the main terms of the energy budget following reed et al to characterize the maintenance of the easterly wave regimes although previous studies have been done on some well observed cases here we use ncep ncar reanalyses over the period 1979 1995 to produce a 17 year climatology of both 3 5 day and 6 9 day easterly waves conversions of energy are analyzed through the contrasts between land and ocean at 10 e 10 w and 30 w and through seasonality similarities and differences in the characteristics of these two wave regimes are identified the results relating to the 3 5 day easterly waves confirm the previous results based on smaller samples over land the waves travelling in the sahel sahara band develop most strongly at 900 hpa through barotropic and baroclinic energy conversions located below the jet level we have found some indication of a possible contribution of the zonal wind core located at 5 s along the central africa coast to the 3 5 day wave development along the southern trajectory both the dry convection north of 15 n in the heat low mainly in june july and deep convection in the itcz in august september are modulated by the 3 5 day easterly waves over the land the overall variance and covariance patterns are rather similar for the 6 9 day and the 3 5 day wave regimes however over the ocean the wind variance maxima are displaced in mid levels as for the 3 5 day waves and remain greater for the 6 9 day regime consistent with greater barotropic energy conversion and the more tilted waves at 700 hpa this 6 9 day wave regime could be connected with variance maxima in the high troposphere at the location of the subtropical westerly jet

subkilometer simulation of a torrential rain producing mesoscale convective system in east china part i model verification and convective organization abstracta nocturnal torrential rain producing mesoscale convective system mcs occurring during the mei yu season of july 2003 in east china is studied using conventional observations surface mesoanalysis satellite and radar data and a 24 h multinested model simulation with the finest grid spacing of 444 m observational analyses reveal the presence of several larger scale conditions that were favorable for the development of the mcs including mei yu frontal lifting moderate cold advection aloft and a moist monsoonal flow below and an elongated old cold dome left behind by a previously dissipated mcs results show that the model could reproduce the evolution of the dissipating mcs and its associated cold outflows the triggering of three separate convective storms over the remnant cold dome and the subsequent organization into a large mcs and the convective generation of an intense surface meso high and meso scale radar reflectivity morphologies in particular the model reproduces the passage of

eddy mean flow interaction in the gulf stream at 68 w part i eddy energetics abstract from june 1988 to august 1990 twelve tall high performance current meter moorings measured the gulf stream s velocity and temperature fields at nominal depths of 400 m 700 m 1000 m and 3500 m along three lines centered at 68 w the overall eddy variability during the 26 month experiment was dominated by six large amplitude trough formation events with each event lasting approximately one month to determine the source s of energy for this eddy variability the eddy energy budget is evaluated traditionally the baroclinic conversion of mean potential energy to eddy potential energy is defined as a downgradient heat flux however because a nondivergent heat flux can be downgradient even under conditions in which there is no barolinic conversion occurring a better more dynamically correct definition of a baroclinic conversion rate is a downgradient horizontally divergent heat flux the horizontally divergent heat flux component is estimated to be approximately half the full horizontal hea

planetary and synoptic scale influences on eastern pacific tropical cyclogenesis abstract the structure and evolution of lowpass filtered background flow and synoptic scale easterly waves were examined during the 1991 eastern pacific hurricane season active and inactive cyclogenesis periods conformed well to the sign of the near equatorial lowpass filtered 850 mb zonal wind anomaly consistent with the recent results of maloney and hartmann this behavior emphasizes the importance of westerly wind bursts associated with the madden julian oscillation mjo in creating an environment favorable for eastern pacific tropical cyclogenesis synoptic scale easterly waves reached the western caribbean and eastern pacific regularly from upstream usually from africa the amplitude of waves leaving africa had little correlation with the likelihood of a wave producing an eastern pacific storm rather easterly waves intensified and tropical depressions formed during the convectively active phase of the mjo in the western caribbean and eastern pacific wave growth measured by strengthening o

observation of 3 6 day meridional wind oscillations over the tropical pacific 1973 1992 horizontal structure and propagation abstract twenty years of rawinsonde data 1973 1992 were examined in conjunction with european centre for medium range weather forecasts ecmwf analyses and outgoing longwave radiation olr in 1980 1989 to determine the horizontal structure propagation and convective coupling of 3 6 day meridional wind oscillations over the tropical pacific wave properties from ecmwf data determined by lag correlation with respect to rawinsonde or ecmwf principal components were consistent with what could be determined from the sparse rawinsonde network alone gridded analyses allowed a clearer distinction between equatorially trapped rossby gravity waves rgw and off equatorial tropical depression td disturbances so that the contrasting properties of these waves including their seasonal and interannual variation could be studied in better detail significant correlations with olr were found increasing in magnitude from eastern to western pacific the apparent group propagation of disturbances was equator

convection and easterly wave structures observed in the eastern pacific warm pool during epic 2001 abstract during september october 2001 the east pacific investigation of climate processes in the coupled ocean atmosphere system epic 2001 intertropical convergence zone itcz field campaign focused on studies of deep convection in the warm pool region of the eastern pacific this study combines c band doppler radar sounding and surface heat flux data collected aboard the r v ronald h brown during epic to describe the kinematic and thermodynamic states of the itcz environment together with tendencies in convective structure lightning rainfall and surface heat fluxes as a function of 3 5 day easterly wave phase three easterly waves were observed at the location of the r v brown during epic 2001 wind and thermodynamic data reveal that the wave trough axes exhibited positively correlated u and winds a slight westward phase tilt with height and relatively strong weak northeasterly tropospheric shear following the trough ridge axis temperature and humidity perturbations exhibited mid to

potential vorticity easterly waves and eastern pacific tropical cyclogenesis abstract a significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310 k isentropic surface near 700 mb over the caribbean sea the charney stern necessary condition for instability of the mean flow is met in this region it is speculated that the sign reversal permits either invigoration of african waves or actual generation of easterly waves in the caribbean during the same season a correlation existed between the strength of the negative potential vorticity gradient in the caribbean and subsequent cyclogenesis in the eastern pacific the meridional pv gradient convective heating measured by outgoing longwave radiation data and eastern pacific cyclogenesis all varied on the timescale of the madden julian oscillation mjo it is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the mjo or any other convective forcing and the associated enhanced downstream tropical cyclogenesis

eastward propagating 2 to 15 day equatorial convection and its relation to the tropical intraseasonal oscillation anomalies of outgoing longwave radiation olr in 1980 1989 were examined in a narrow latitude band about the equator to elucidate the propagation and interaction of tropical intraseasonal oscillations tios and synoptic scale convective activity hovmoller diagrams of olr data reconstructed from two frequency bands corresponding to periods of 30 60 days and 2 15 days respectively revealed a clear distinction between the phase speed of eastward moving tios 3 7 m s 1 and that of eastward synoptic scale convection 10 13 m s 1 coherent propagation of tios from the indian to the western pacific oceans was often observed with temporary diminution of amplitude over the maritime continent propagation of high frequency anomalies was generally confined to an individual basin dominance of eastward propagating synoptic scale convection in the equatorial zone contrasts sharply with the westward propagation of off equatorial convergence zones interaction between the tio and high frequency activity resolved by daily olr was visible as a modest enhancement of eastward synoptic scale events and of high frequency activity in general during the convectively active phase of the tio

analysis of the dominant mode of convectively coupled kelvin waves in the west african monsoon abstract the dominant mode of convectively coupled kelvin waves has been detected over the atlantic and africa during northern summer by performing composite analyses on observational fields based on an eof reconstructed convection index over west africa propagating eastward many waves originate from the pacific sector interact with deep convection of the marine itcz over the atlantic and the continental itcz over west and central africa and then weaken over east africa and the indian ocean it has been shown that they are able to modulate the life cycle and track of individual westward propagating convective systems their mean kinematic characteristics comprise a wavelength of 8000 km and a phase speed of 15 m s 1 leading to a period centered on 6 to 7 days the african kelvin wave activity displays large seasonal variability being highest outside of northern summer when the itcz is close to the equator facilitating the interactions between convection and these equatorially trapped waves the co

origins and mechanisms of eastern pacific tropical cyclogenesis a case study the genesis of hurricane hernan 1996 in the eastern pacific was investigated using gridded analyses from the european centre for medium range weather forecasts and gridded outgoing longwave radiation hernan developed in association with a wave in the easterlies that could be tracked back to africa in longitude time plots of the filtered y component of the wind 2 6 day period at 700 mb the wave crossed central america near lake nicaragua with little change in its southwest northeast tilt but the most intense convection shifted from near the wave axis in the caribbean to west of the wave axis in the pacific the wave intensified as it moved through a barotropically unstable background state defined by a low pass filter with a 20 day cutoff in the western caribbean and eastern pacific a surge in the southwesterly monsoons and enhanced convection along 108n occurred to the west of the 700 mb wave in the pacific and traveled with the wave this had the effect of enhancing low level vorticity over a wide region ahead of the 700 mb wave available evidence suggests that additional low level vorticity was produced by enhanced flow from the north through the isthmus of tehuantepec as the 700 mb wave approached depression formation did not occur until 6 12 h after the 700mb wave reached this region of large low level vorticity in the gulf of tehuantepec eastern pacific sst and vertical wind shear magnitude are typically favorable for tropical cyclone development in northern hemisphere summer and early fall because the favorable mountain interaction and the surge in the low level monsoons appear to relate directly to the wave in the easterlies it is argued that the strength of such waves reaching central america from the east is the single most important factor in whether subsequent eastern pacific cyclogenesis occurs possible parallels with western pacific cyclogenesis are discussed

development of synoptic scale disturbances over the summertime tropical northwest pacific abstract this study addresses the origin of the synoptic scale disturbances that occur in the tropical western north pacific ocean wp region in northern hemisphere summer these have been called easterly waves and tropical depression type td disturbances this analysis uses the national center for environmental prediction national center for atmospheric research reanalysis dataset by performing a regression analysis on several terms in the vorticity equation at 850 hpa it is shown that the td disturbances propagate approximately as barotropic rossby waves at 850 hpa given this ray tracing calculations and the wave activity diagnostic introduced by plumb are used to show that wave accumulation is a promising candidate for the initial development mechanism of the td disturbances the expected local growth rate from this mechanism is simply the convergence of the group velocity which reaches values corresponding to a growth timescale of 3 days this convergence is dominated by but somewhat lar

barotropic aspects of itcz breakdown abstract in satellite images the itcz intertropical convergence zone is sometimes observed to undulate and break down into a series of tropical disturbances tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the itcz to reform it has been proposed that itcz breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow an unstable mean flow can be produced by itcz convection in just a couple of days in this sense the itcz produces favorable conditions for its own instability and breakdown in this study a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of itcz breakdown in the shallow water model the itcz is simulated by a prescribed zonally elongated mass sink near the equator the mass sink produces a cyclonic potential vorticity pv anomaly that has a reversed meridional pv gradient on its poleward side according to the ripa theorem a flow that has a