a statistical study of emic rising and falling tone emissions observed by themis electromagnetic ion cyclotron emic waves with rising or falling frequency variations have been studied intensively because of their effects on energetic particles in the earth s magnetosphere we develop an automated classification method of emic events based on the characteristics of frequency variations we report some basic statistical properties of frequency variations in emic waves observed over 5 10 re by three time history of events and macroscale interactions during substorms probes from january 2012 to december 2014 we clarify whether rising tones or falling tones are observed in each chosen 20 min time segment in the present analysis we find that the occurrence rate of emic rising or falling tone events is more than 30 of the total emic wave events the dayside magnetosphere is a preferential region for the emic frequency variations the occurrence rate of rising tone events is slightly greater than that of falling tone events we examine the relation between the frequency characteristics and the magnetospheric conditions the solar wind pressure strongly controls the occurrence rates of frequency variations we also calculate ranges of frequency variations large amplitude emic waves tend to have wider frequency variations and the range of frequency variation is largest around the prenoon region in addition rapid variations in wave amplitudes called subpacket structures are found in 70 of the emic rising or falling tone events in the dayside region subpacket structures appear mainly in large amplitude emic rising or falling tones these features are consistent with nonlinear wave growth theory

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

african easterly wave variability and its relationship to atlantic tropical cyclone activity abstract automatic tracking of vorticity centers in european centre for medium range weather forecasts analyses has been used to develop a 20 yr climatology of african easterly wave activity the tracking statistics at 600 and 850 mb confirm the complicated easterly wave structures present over the african continent the rainy zone equatorward of 15 n is dominated by 600 mb activity and the much drier saharan region poleward of 15 n is more dominated by 850 mb activity over the atlantic ocean there is just one storm track with the 600 and 850 mb wave activity collocated based on growth decay and genesis statistics it appears that the 850 mb waves poleward of 15 n over land generally do not get involved with the equatorward storm track over the ocean instead there appears to be significant development of 850 mb activity at the west african coast in the rainy zone around 10 n 10 w which it is proposed is associated with latent heat release based on the tracking statistics it has been shown th

daily states of the march april east pacific itcz in three decades of high resolution satellite data abstractzonally elongated areas of cloudiness that make up the east pacific intertropical convergence zone itcz can take on several configurations in instantaneous observations a novel statistical model is used to automatically assess the daily state of the east pacific itcz using infrared satellite images from 1980 to 2012 four itcz states are defined based on itcz location relative to the equator north nitcz and south sitcz of the equator simultaneously north and south of the equator ditcz for double itcz and over the equator eitcz a fifth itcz state is used to classify days when no zonally elongated area of cloudiness is present aitcz for absent itcz the itcz states can occur throughout the year except for the eitcz which is not present during june october with the nitcz state dominating in terms of frequency of occurrence interannual variability of the state distribution is large the most striking variability in itcz states is observed in spring during march april the ditcz

double intertropical convergence zones a new look using scatterometer 1 the high resolution wind vectors observed by the space based scatterometer quikscat from 1999 to 2002 show that the double intertropical convergence zones itcz exist in the atlantic and the eastern pacific oceans for most of the annual cycle and are far more extensive than previously recognized for most of the time the southern itcz is weaker than the northern one the stronger itcz occurs when the northerly trade winds meet the southerly trade winds over warm water resulting in deep convection the weaker itcz over cooler water is caused by the deceleration of the surface winds as they approach the cold upwelling water near the equator decreases in vertical mixing and increases in vertical wind shear in the atmospheric boundary layer are suggested to be the causes of the deceleration of the trade winds as they move from warmer to colder water

evolutionary structure of the eastern pacific double itcz based on satellite moisture profile retrievals abstract for about a month near the boreal vernal equinox the eastern pacific intertropical convergence zone itcz is observed to form two troughs quasi symmetrically situated about the equator near 5 7 latitude during years when an equatorial sea surface cold tongue is present e g la nina years the three dimensional structure and temporal evolution of the eastern pacific double itcz is documented using weekly cloud liquid and ice water fields and relative humidity profiles retrieved from special sensor microwave temperature 2 measurements the depth of convection in the southern branch of the double itcz as determined by the coincident presence of cloud liquid and ice as well as by upward motion inferred from the relative humidity field is observed to be sensitive to both the underlying sst and subsidence from the northern branch the equatorial sea surface cold tongue appears to be the determining factor regulating the formation of a double itcz in the eastern pacific areas of deep convectio

a high resolution seismic stratigraphy from a scottish sea loch and its implications for loch lomond stadial deglaciation a detailed high resolution seismic stratigraphy calibrated by core data and terrestrial geomorphological mapping has been constructed for loch ainort isle of skye this study has provided a palaeoenvironmental history of the area as well as important corroborative evidence for the stepped deglaciation of the loch lomond stadial ice field on skye the ainort glacier reworked pre loch lomond glacial deposits terminating in a grounded tidewater ice front potentially 800 m beyond the previously extrapolated limit the first stage of deglaciation was characterised by the formation of de geer moraines indicative of a period of interrupted retreat the second phase by contrast produced hummocky relief with sporadic linear moraines suggesting periods of uninterrupted retreat with occasional stillstands readvances paraglacial reworking of terrestrial slopes resulted in the deposition of thick subaqueous debris flows which graded into fluvioglacial dominated sediments and ultimately modern fjordic deposits the identification of an initial period of active retreat punctuated by numerous readvances correlates directly with the terrestrial record however the offshore stratigraphy suggests that although the second phase was dominated by uninterrupted retreat occasional stillstands readvances did occur

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

shallow meridional circulation in the tropical eastern pacific abstract based on four independent datasets this study presents observational evidence of a shallow meridional circulation cell in the eastern tropical pacific in this shallow meridional circulation cell the northerly cross equatorial return flow from the itcz into the southern hemisphere is found immediately above the atmospheric boundary layer in contrast to the classic concept of the hadley type deep meridional circulation whose northerly return flow resides in the upper troposphere the strength and depth of this shallow meridional circulation undergo a distinct annual cycle possible causes and climatic implications of this shallow meridional circulation are discussed

the itcz in the central and eastern pacific on synoptic time scales the itcz in the central and eastern pacific on synoptic time scales is highly dynamic the active season extends roughly from may through october during the active season the itcz continuously breaks down and re forms and produces a series of tropical disturbances the life span of the itcz varies from several days to 3 weeks sixty five cases of itcz breakdown have been visually identified over five active seasons 1999 2003 in three independent datasets itcz breakdown can be triggered by two mechanisms 1 interaction with westward propagating disturbances wpds and 2 the vortex rollup vr mechanism results show that the frequency of occurrence of itcz breakdown from these two mechanisms is the same the vr mechanism may have been neglected because the produced disturbances are rather weak and they may dissipate quickly the itcz shows a strong tendency to re form within 1 2 days in the same location the itcz may break down via the vr mechanism without any other support and thus it may continuously generate numerous tropical disturbances throughout the season there are two main differences between the two mechanisms 1 the wpds induced itcz breakdown tends to create one or two vortices that may be of tropical depression strength the vr induced itcz breakdown generates several nearly equal sized weak disturbances 2 the wpds tend to disturb the itcz in the eastern pacific only disturbances generally move along the mexican coast after shedding off from the itcz and do not further disturb the itcz in the central pacific therefore the vr mechanism is observed more clearly and is the dominating mechanism for itcz breakdown in the central pacific

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

vertical and latitudinal variation of the intertropical convergence zone derived using gps radio occultation measurements abstract using gps radio occultation refractivity data collected over the period of 2002 2013 we present a new method for identification of the intertropical convergence zone itcz the itcz is identified by estimating the maximum in the monthly meridional refractivity and specific humidity field by applying a gaussian fit at each longitude the interannual variability and climatology of the itcz is presented from 12 years of refractivity data this new method captures all the general features of itcz extent and its variability we also examine the effects of the itcz vertically during different seasons the itcz is observed mostly at eastern pacific in may month and it is zonally distributed in the september and october months of each year the zonal variability is large between lower and higher levels particularly over the indian monsoon and western pacific the latitudinal difference in the vertical extent between 850 hpa and higher levels is larger during the northern hemisphere nh summer than nh winter

diurnal cycle of the itcz in dynamo abstractduring the 2011 special observing period of the dynamics of the madden julian oscillation dynamo field experiment two sounding arrays were established over the central indian ocean one

the low level structure of african easterly waves in 1995 abstract the existence of african easterly waves aews north of the african easterly jet aej core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the u k meteorological office global model analysis from the hurricane season of 1995 at bamako 12 5 n 8 0 w the aews were characterized mainly by maximum amplitudes at the level of the aej around 700 mb whereas at dakar 14 7 n 17 5 w the waves were characterized by maxima between 850 and 950 mb the low level waves to the north of the aej arise in association with baroclinic interactions between the negative meridional potential vorticity pv gradients in the jet core and the positive low level gradient of potential temperature enhanced by the presence of low static stability air north of the aej these waves follow the positive meridional gradients over northern africa in contrast to the jet level aews that follow the meridional pv gradients at the level of the aej cross correlation analysis

equatorial atmospheric waves and their association to convection equatorial wave systems and their relationships with convective activity are analyzed in the western and central pacific regions during the coupled ocean atmosphere response experiment coare intensive observation periods the study uses geostationary meteorological satellite infrared temperature observations and the operational european centre for medium range weather forecasts analyses supplemented with coare observations spectral and complex principal component analysis are applied to the data using the linear theory of equatorially trapped waves as a guideline the existence of three types of waves is detected in the 7 10 day period range n 5 1 rossby waves are found to the east of the date line in a region of weak convective activity over the western equatorial pacific where intense convection occurs the 7 10 day waves do not possess the general characteristics of linear rossby waves but they are strongly linked to the active phases of westerly wind bursts and of convection analysis of the meridional wind reveals intense mixed rossby gravity waves with a mean 5 day period and westward phase and eastward group velocities over the western pacific the convection is found to be strongly correlated with the antisymmetric structure of the divergent field as predicted by the linear theory in the 200 hpa divergence field n 5 1 gravity waves are visible having periods shorter than 2 5 days they rapidly propagate about 25 m s21 both westward and eastward and have strong correlations with convective clusters

mean state and wave disturbances during phases i ii and iii of gate based on era 40 abstract using ecmwf s second generation reanalysis era 40 the large scale mean state and synoptic scale features associated with african easterly wave disturbances aews are examined over west africa and the adjacent eastern atlantic ocean during the three 21 day observing periods of the global atmospheric research program garp atlantic tropical experiment gate in 1974 phase i 26 june 16 july phase ii 28 july 17 august phase iii 30 august 19 september results are partitioned into four geographical boxes in order to highlight differences among the aew vortices as they propagate westward along two tracks northern and southern over west africa land and the adjacent eastern atlantic ocean water this marks the first time that a detailed diagnosis of the northerly track aews has been conducted results are also compared to previous gate studies and a 30 yr climatology is extracted from era 40 in general the subjectively analyzed wind fields presented in earlier studies compare favorabl

influence of the basic state zonal flow on convectively coupled equatorial waves observational data are used to test the hypothesis that the basic state modulates the dispersion properties of convectively coupled equatorial waves ccews this hypothesis is based on shallow water theory which predicts that the zonal speed of propagation of such tropospheric equatorial modes is altered by the equivalent depth and the basic zonal flow localized space time spectra are calculated to investigate how ccew spectral peaks vary across the tropics and how they are affected by the variations in zonal wind observed geographically and by season doppler shifting by the basic state barotropic zonal flow is readily identified once this doppler shifting is taken into account the equivalent depths of ccews inferred from global power spectra are surprisingly uniform both geographically and temporally however there are also detectable modulations that appear consistent with changes in vertical shear of the zonal flow along with other shifts that are not as easily explained

intertropical convergence zones during the active season in daily data synoptic scale variability of vorticity structures in the lower troposphere of the tropics is analyzed in 23 yr of daily averaged high resolution reanalysis data the vorticity structures can be divided into zonally elongated vorticity strips classified as intertropical convergence zones itczs and more localized maxima termed westward propagating disturbances a composite of such variability is presented for the east to central pacific and for the east atlantic africa region both in summer the composite in the east pacific is zonally elongated and itcz like propagating westward over a number of days before dissipating the spatial structure of the vorticity strip shows the characteristic cyclonic tilt into the latitudinal direction with time that is also seen in modeling experiments the composite over the atlantic africa region shows two active regions that are correlated on synoptic time scales the disturbances in the southern region are better developed and longer lasting even though the time and space scales are smaller than over the east pacific overall variability over the atlantic is consistent with variability due to african easterly waves the double itcz in spring in the east pacific is different from the few earlier studies available it is stronger south of the equator and located at 10 s which is farther poleward than earlier studies have indicated the northern branch that is weak in comparison is located at 5 n the two branches of the double itcz tend to appear in tandem on the 2 week time scale

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