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

effects of zonal flows on equatorially trapped waves abstract linear shallow water equations are employed to study the effects of basic zonal flows on equatorially trapped waves analytical solutions are obtained for constant basic zonal flows it is shown that changes in the wave structures due to the non doppler effect of the basic zonal flow are considerable for the rossby wave moderate for the westward propagating mixed rossby gravity wave but negligible for the other equatorial waves the rossby wave and the westward propagating mixed rossby gravity wave possess greater eigenfrequencies and are less trapped in westerlies than in easterlies the dependence of the rossby wave structure upon the basic zonal flow is interpreted in terms of potential vorticity conservation in basic zonal flows with meridional shears the eigenfrequencies are found to be larger in equatorial easterlies than in equatorial westerlies for the westward propagating waves but smaller for the eastward propagating waves while the meridional structures of the eastward propagating

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

convection over tropical africa and the atlantic ocean during northern summer part ii modulation by easterly waves abstract using meteosat data and european centre for medium range weather forecast ecmwf analyses we examine easterly waves and their relation with the cloudiness over west africa and the tropical atlantic ocean for three summers june july august and september 1983 85 spectral analysis of the low level meridional wind in the 2 8 5 1 day band reveals maximum wave amplitude near the west african coast at 20 n during august and september the wave amplitude is larger than during june and july and a secondary maximum appears around 7 5 n composites of the mean structure of the wave and the associated cloud modulation reveal consistent relationship between observed cloudiness and ecmwf analyses the phase of the wave modulation of the cloudiness is strongly dependent on the geographical location in response to changes in the mean climatological conditions this phase varies however grossly in four large land and ocean regions centered at 7 5 and 17 5 n respectively for which we derive vertical cro

a global view of equatorial waves and tropical cyclogenesis abstractthis study investigates the number of tropical cyclone formations that can be attributed to the enhanced convection from equatorial waves within each basin tropical depression td type disturbances i e easterly waves were the primary tropical cyclone precursors over the northern hemisphere basins particularly the eastern north pacific and the atlantic in the southern hemisphere however the number of storms attributed to td type disturbances and equatorial rossby waves were roughly equivalent equatorward of 20 n tropical cyclones formed without any equatorial wave precursor most often over the eastern north pacific and least often over the western north pacific the madden julian oscillation mjo was an important tropical cyclone precursor over the north indian south indian and western north pacific basins the mjo also affected tropical cyclogenesis by modulating the amplitudes of higher frequency waves each wave type reached the attribution threshold 1 5 times more often and tropi

easterly wave regimes and associated convection over west africa and tropical atlantic results from the ncep ncar and ecmwf reanalyses ncep ncar and ecmwf daily reanalyses are used to investigate the synoptic variability of easterly waves over west africa and tropical atlantic at 700 hpa in northern summer between 1979 1995 1979 1993 for ecmwf spectral analysis of the meridional wind component at 700 hpa highlighted two main periodicity bands between 3 and 5 days and 6 and 9 days the 3 5 day easterly wave regime has already been widely investigated but only on shorter datasets these waves grow both north and south of the african easterly jet aej the two main tracks noted over west africa at 5 n and 15 n converge over the atlantic on latitude 17 5 n these waves are more active in august september than in june july their average wavelength phase speed varies from about 3000 km 8 m s 1 north of the jet to 5000 km 12 m s 1 south of the jet rainfall convection and monsoon flux are significantly modulated by these waves convection in the inter tropical convergence zone itcz being enhanced in the trough and ahead of it with a wide meridional extension compared to the 3 5 day waves the 6 9 day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics the only main track is located north of the aej along 17 5 n both over west africa and the atlantic the mean wavelength is higher about 5000 km long and the average phase speed is about 7 m s 1 then the wind field perturbation is mostly evident at the aej latitude and north of it the perturbation structure is similar to that of 3 5 days in the north except that the more developed circulation centers moving more to the north lead to a large modulation of the jet zonal wind component south of the aej the wind field perturbation is weaker and quite different the zonal wind core of the jet appears to be an almost symmetric axis in the 6 9 day wind field pattern a clockwise circulation north of the aej being associated with a counter clockwise circulation south of the jet and vice versa these 6 9 day easterly waves also affect significantly rainfall convection and monsoon flux but in a different way inducing large zonal convective bands in the itcz mostly in the trough and behind it as opposed to the 3 5 day wave regime these rainfall anomalies are associated with anomalies of opposite sign over the guinea coast and the sahelian regions over the continent these waves are more active in june july and in august september over the ocean gate phase i gave an example of such an active 6 9 day wave pattern considered as a sequence of weak easterly wave activity this phase was also a sequence of high 6 9 day easterly wave activity we suggest that the 6 9 day regime results from an interaction between the 3 5 day easterly wave regime maintained by the barotropic baroclinic instability of the aej and the development of strong anticyclonic circulations north of the jet over west africa and both north and south of the jet over the atlantic significantly affecting the jet zonal wind component the permanent subtropical anticyclones azores libya st helena could help initiation and maintenance of such regime over west africa and tropical atlantic based on an a priori period band criterion our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over west africa and tropical atlantic ncep ncar and ecmwf reanalyses are in good agreement the main difference being a more developed easterly wave activity in the ncep ncar reanalyses especially for the 3 5 day regime over the atlantic

linking african easterly wave activity with equatorial waves and the influence of rossby waves from the southern hemisphere abstracta connection is found between african easterly waves aews equatorial westward moving mixed rossby gravity wmrg waves and equivalent barotropic rossby waves rws from the southern hemisphere sh the amplitude and phase of equatorial waves is calculated by projection of broadband filtered era interim data onto a horizontal structure basis obtained from equatorial wave theory mechanisms enabling interaction between the wave types are identified aews are dominated by a vorticity wave that tilts eastward below the african easterly jet and westward above the tilt necessary for baroclinic wave growth however a strong relationship is identified between amplifying vorticity centers within aews and equatorial wmrg waves although the waves do not phase lock positive vorticity centers amplify whenever the cross equatorial motion of the wmrg wave lies at the same longitude in the upper troposphere southward flow and east of this in the lower troposphere northward flow two mechanisms could

case study of an intense african easterly wave abstract the life cycle of an intense african easterly wave aew over the african continent is examined using european centre for medium range weather forecasts ecmwf operational analyses meteosat satellite images and synoptic observations this system the strongest aew of 2000 can be tracked from central north africa into the eastern atlantic ocean where it is associated with the genesis of hurricane alberto synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity pv allowing exploration at the role of multiple scales in the evolution of this aew the authors analysis promotes the division of the aew life cycle into three distinctive phases i initiation the aew development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in sudan this convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical north africa ii baroclini

enso impact on kelvin waves and associated tropical convection abstractthe impact of el nino southern oscillation enso on atmospheric kelvin waves and associated tropical convection is investigated using the ecmwf re analysis noaa outgoing longwave radiation olr and the analysis technique introduced in a previous study it is found that the phase of enso has a substantial impact on kelvin waves and associated convection over the equatorial central eastern pacific el nino la nina events enhance suppress variability of the upper tropospheric kelvin wave and the associated convection there in both extended boreal winter and summer the mechanism of the impact is through changes in the enso related thermal conditions and the ambient flow in el nino years because of sst increase in the equatorial central eastern pacific variability of eastward moving convection which is mainly associated with kelvin waves intensifies in the region in addition owing to the weakening of the equatorial eastern pacific westerly duct in the upper troposphere in el nino years

convectively coupled equatorial waves in high resolution hadley centre climate models abstract a methodology for diagnosing convectively coupled equatorial waves is applied to output from two high resolution versions of atmospheric models the hadley centre atmospheric model version 3 hadam3 and the new hadley centre global atmospheric model version 1 hadgam1 which have fundamental differences in dynamical formulation variability horizontal and vertical structures and propagation characteristics of tropical convection and equatorial waves along with their coupled behavior in the models are examined and evaluated against a previous comprehensive study of observed convectively coupled equatorial waves using the 15 yr ecmwf re analysis era 15 and satellite observed data the extent to which the models are able to represent the coupled waves found in real atmospheric observations is investigated it is shown that in general the models perform well for equatorial waves coupled with off equatorial convection however they perform poorly for waves coupled with equatorial convect

monsoons itczs and the concept of the global monsoon earth s tropical and subtropical rainbands such as intertropical convergence zones itczs and monsoons are complex systems governed by both large scale constraints on the atmospheric general circulation and regional interactions with continents and orography and coupled to the ocean monsoons have historically been considered as regional large scale sea breeze circulations driven by land sea contrast more recently a perspective has emerged of a global monsoon a global scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation this results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone regional subsystems are embedded in this global monsoon localized by surface boundary conditions parallel with this much theoretical progress has been made on the fundamental dynamics of the seasonal hadley cells and convergence zones via the use of hierarchical modeling approaches including aquaplanets here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings after summarizing the dynamical and energetic balances that distinguish an itcz from a monsoon we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets limitations of current theories are discussed including the need for a better understanding of the influence of zonal asymmetries and transients on the large scale tropical circulation

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

mesoscale convective systems in relation to african and tropical easterly jets abstractthis paper documents the interaction processes between mesoscale convective systems mcs the tropical easterly jet tej and the african easterly jet aej over west africa during the monsoon peak of 2006 observed during the african monsoon multidisciplinary analyses amma project the results highlight the importance of the cloud system localization relative to the jets in order to explain their duration and life cycle a systematical study reveals that intense and long lived mcss correspond to a particular pattern where clouds associated with deep convection are located in entrance regions of tej and in exit regions of aej a case study on a particularly well documented convective event characterizes this link and infers the importance of jet streaks in promoting areas of divergence favoring the persistence of mcss

maintenance of the african easterly jet the maintenance of the african easterly jet aej has been examined using a zonally symmetric general circulation model with simple parametrizations it is shown that the aej is maintained in association with two diabatically forced meridional circulations one associated with surface fluxes and dry convection in the saharan heat low region and one associated with deep moist convection in the intertropical convergence zone equatorward of this the heat low heating which reaches the height of the aej around 700 mb is particularly important in maintaining the aej and its associated meridional gradients in potential vorticity it is concluded that the mean observed aej results from a combination of the diabatically forced meridional circulations which maintain it and easterly waves which weaken it

a systematic comparison of tropical waves over northern africa part ii dynamics and thermodynamics abstractthis study presents the first systematic comparison of the dynamics and thermodynamics associated with all major tropical wave types causing rainfall modulation over northern tropical afric

analysis of convection and its association with african easterly waves abstract the association between convection and african easterly wave aew activity over tropical africa and the tropical atlantic during the boreal summer is examined using satellite brightness temperature tb and ecmwf reanalysis datasets spectral analysis using 18 yr of tb data shows significant variance in the 2 6 day range across most of the region within the regions of deep convection this time scale accounts for about 25 35 of the total variance the 2 6 day convective variance has similar amplitudes over western and eastern africa while dynamic measures of aew activity show stronger amplitudes in the west this study suggests that weak aew activity in the east is consistent with initial wave development there and indicates that convection triggered on the western side of the mountains over central and eastern africa near darfur western sudan and ethiopia has a role in initiating aews westward the subsequent development and growth of aews in west africa is associated with stronger cohe

energy dispersion in african easterly waves the existence of an upstream eastward group velocity for african easterly waves aews is shown based on single point lag regressions using gridded reanalysis data from 1990 to 2010 the eastward energy dispersion is consistent with the direction of ageostrophic geopotential flux vectors a local eddy kinetic energy eke budget reveals that early in the life cycle of aews growth rate due to geopotential flux convergence exceeds baroclinic and barotropic growth rates later in the life cycle eke decay due to geopotential flux divergence cancels or exceeds baroclinic and barotropic growth a potential vorticity pv budget is used to diagnose tendencies related to group propagation although both upstream and downstream group speeds are possible because of the reversal in the mean meridional pv gradient upstream propagation associated with the positive poleward pv gradient dominates wave packet evolution analogous to the concept of downstream development of midlatitude baroclinic waves new aews develop preferentially upstream of the older ones thus providing a mechanism for seeding new waves it is suggested that these results are also relevant to aew intermittency and storm track structure

atlantic tropical systems of 1993 abstract a total of 70 tropical waves also known as african or easterly waves were counted in the atlantic basin during the 1993 hurricane season these waves led to the formation of 9 of the 10 total number of tropical cyclones in the atlantic hurricane basin it appears that tropical waves led to the formation of practically all of the eastern pacific tropical cyclones in 1993

sahel rainfall tropical easterly jet relationship on synoptic to intraseasonal time scales abstractthe tropical easterly jet tej is a characteristic upper level feature of the west african monsoon wam circulation moreover the tej over west africa is significantly correlated with su

enso related variation of equatorial mrg and rossby waves and forcing from higher latitudes the contrasting behaviour of westward moving mixed rossby gravity wmrg and the first rossby r1 waves in el nino en and la nina ln seasons is documented with a focus on the northern hemisphere winter the eastward moving variance in the upper troposphere is dominated by wmrg and r1 structures that appear to be doppler shifted by the flow and are referred to as wmrg e and r1 e in the east pacific and atlantic the years with stronger equatorial westerly winds have the stronger wmrg and wmrg e in the east pacific r1 is also a maximum in ln however r1 e exhibits an eastward shift between ln and en the changes with enso phase provide a test bed for the understanding of these waves in the east pacific and atlantic the stronger wmrg e and wmrg with stronger westerlies are in accord with the dispersion relation with simple doppler shifting by the zonal flow the possible existence of free waves can also explain stronger r1 in en in the eastern hemisphere 1 d free wave propagation theory based on wave activity conservation is also important for r1 however this theory is unable to explain the amplitude maxima for other waves observed in the strong equatorial westerly regions in the western hemisphere and certainly not their enso related variation the forcing of equatorial waves by higher latitude wave activity and its variation with enso phase is therefore examined propagation of extratropical eastward moving rossby wave activity through the westerly ducts into the equatorial region where it triggers wmrg e is favoured in the stronger westerlies in ln in the east pacific and en in the atlantic it is also found that wmrg is forced by southern hemisphere westward moving wavetrains arching into the equatorial region where they are reflected the most significant mechanism for both r1 and r1 e appear to be lateral forcing by subtropical wavetrains

two contrasting african easterly wave behaviors abstractthe dominant structural variability of african easterly waves aews is explored using an empirical orthogonal function eof approach the structure of aews is obtained by projecting the w

salinity changes in the central baltic sea nw europe over the last 10000 years we attempt a reconstruction of salinity levels of the central baltic sea based on diatom assemblages the isotopic composition of organic matter and sedimentological expression of anoxia over the last 10 000 years we use the data to investigate the dependence of salinity levels on climate evolution and isostasy changes in salinity of surface and deep waters were most pronounced from 8400 to approximately 5000 cal bp density stratification between salty deep and fresher surface waters caused the frequent development of anoxic conditions and deposition of laminated sediments on large parts of the sea floor in the central baltic sea and dramatic changes in organic carbon accumulation rates from 5000 to 3100 cal bp the salinity of the basin decreased oxygenation of deep sea floors was improved and fertility of the sea surface was significantly reduced this is reflected by low accumulation rates of organic carbon in bioturbated sediments since 2800 cal bp salinity rose again and anoxic periods were more common even though the major steps in environmental evolution in the baltic sea coincide with known patterns of climatic change of the north atlantic realm over the last 10 000 years we find no conclusive evidence for synchronous changes or linear responses on submillennial timescales however we note that major variations in our salinity records agree with temporal patterns of reconstructed summer warmth and winter precipitation in southern scandinavia both types of record suggest that climate in the mid holocene was far from stable our data also confirm that climate evolution over the late holocene had significant impact on environmental conditions in the baltic sea

revisiting the 1979 st elias alaska aftershock sequence and its regional significance i have relocated seismicity occurring in the st elias alaska region between 1973 and 2002 to examine its relation to the 1979 m s 7 1 st elias mainshock and compare the relocations to recent studies of global positioning system gps geodesy structural geology geothermometry and marine geophysical studies the relocated seismicity clearly shows that much of the seismic deformation is occurring on or above a regional decollement that may be located at the top of the basement of the yakutat microplate a persistent y shaped concentration of seismicity located east and north of icy bay contains the deepest 15 30 km earthquakes within the region and reflects the present day deformational front between the yakutat microplate and southern alaska coulomb failure stress modeling suggests that static stress changes had little influence on the location of most aftershocks of the 1979 sequence

the impact of moist processes on the african easterly jet african easterly wave system the african easterly jet easterly wave aej aew system was explored in an idealised model prescribed zonally symmetric surface temperature and moisture profiles determine the aej which becomes established through meridional contrasts in dry and moist convection as in previous studies a realistic aej developed with only dry convection including moist processes increased its development rate but reduced its speed and meridional extent aews grew through barotropic baroclinic conversions negative meridional potential vorticity pv gradients arose in the zonally symmetric state through the intrusion of the low pv saharan boundary layer since moist processes strengthened this significantly through diabatically generated pv in the intertropical convergence zone moist aews were three times stronger larger barotropic conversions and faster aej development increased the moist wave growth rate jet level and northerly low level amplitudes grew but in the moist case the low level amplitudes weakened as the aew interacted with convection consistent with their absence from observations during the peak monsoon striking dependencies between the aej aew and rainfall existed two time scales governed their evolution depending on the transfer coefficients 1 the aej s replenishment rate influenced by heat fluxes and 2 the wave growth rate by damping and the slower jet development rate moist aews were characterized by intermittent growth decay with growth preceded by increased mean rainfall and later weakening aejs these dependencies established an internal 8 10 day variability consistent with intra seasonal observations of 9 day rainy sequences this internal variability offers an alternative explanation to the previously proposed external forcing and a new view of the moist aew life cycle copyright 2009 royal meteorological society