intraseasonal variability of the saharan heat low and its link with midlatitudes abstract the saharan heat low shl is thought to be a key feature of the west african monsoon and its variations during the summer season have not yet been systematically assessed to characterize the intraseasonal variations of the shl real and complex empirical orthogonal function analyses were applied to the 850 hpa potential temperature field over northern africa and the mediterranean using ncep department of energy doe atmospheric model intercomparison project amip ii reanalysis results a robust quasi propagative mode was highlighted over north africa and the mediterranean this mode consists of two phases the west phase corresponds to a maximum temperature over the coast of morocco mauritania propagating southwestward and a minimum between libya and sicily propagating southeastward the east phase corresponds to the opposite temperature structure which propagates as in the west phase a lag composite analysis revealed that this shl mode was preceded by large scale midlatitude intrase

modeling basin scale internal waves in a stratified lake basin scale internal waves provide the driving forces for vertical and horizontal fluxes in a stratified lake below the wind mixed layer thus correct modeling of lake mixing and transport requires accurate modeling of basin scale internal waves examining this capability with a hydrostatic z coordinate three dimensional 3d numerical model at coarse grid resolutions is the focus of this paper it is demonstrated that capturing the correct thermocline forcing with a 3d mixed layer model for surface dynamics results in a good representation of low frequency internal wave dynamics the 3d estuary and lake computer model elcom is applied to modeling lake kinneret israel and is compared with field data under summer stratification conditions to identify and illustrate the spatial structure of the lowest mode basin scale kelvin and poincarewaves that provide the largest two peaks in the internal wave energy spectra the model solves the unsteady reynolds averaged navier stokes equations using a semi implicit method similar to the momentum solution in the trim code with the addition of quadratic euler lagrange dis cretization scalar e g temperature transport using a conservative flux limited approach and elimination of vertical diffusion terms in the governing equations a detailed description is provided of turbulence closure for the vertical reynolds stress terms and vertical turbulent transport using a 3d mixed layer model parameterized on wind and shear energy fluxes instead of the convential eddy viscosity diffusivity assumption this approach gives a good representation of the depth of the mixed layer at coarse vertical grid resolutions that allows the internal waves to be energized correctly at the basin scale wind stresses surface heating and density currents form the driving energy fluxes of a stratified lake the basin scale energy flux from the wind is of particular interest because of its dominant role in setting the thermocline in motion which in the absence of inflows and outflows is the primary energy store for transport and mixing below the wind mixed layer thus modeling the basin scale internal wave behavior is an a priori requirement to modeling and quantifying the flux paths of nutrients in a stratified lake imberger 1994 this paper takes a first step in this direction by analyzing our ability to model basin scale internal waves that are seen in lake kinneret israel energy flux path in a stratified lake energy flux through a stratified lake has a fundamental dependence on forced and free baroclinic motions the wind imparts both momentum and turbulent kinetic energy tke to the water in the sur face layer the tke distributes momentum vertically in the

intraseasonal variability over tropical africa during northern summer abstract the intraseasonal variability over africa during northern summer was analyzed using 25 years of ncep ncar reanalysis and satellite data the dominant pattern of variability was one of enhanced deep convection over the whole african monsoon region it appeared to arise at least partly as a remote response to the intraseasonal madden julian oscillation over the warm pool sector twenty days prior to the maximum in convection over africa there was no signal over africa but convection was reduced over the equatorial warm pool an equatorial kelvin wave response to this change in warm pool convection propagated eastward and an equatorial rossby wave response propagated westward and between them they completed a circuit of the equator and met up 20 days later over africa where the negative midtropospheric temperature anomalies in the kelvin and rossby waves favored deep convection over west africa the kelvin wave component contained lower tropospheric westerly anomalies that acted to increase t

length scale dependence of horizontal dispersion in the surface water of lakes horizontal dispersion in the surface waters of a medium sized lake was investigated based on four experiments with ensembles of 14 to 17 drifters deployed in lake constance during the winter season the experiments cover length scales between 30 and 3000 m which are typical for the grid length scales employed in 3 d models horizontal dispersion coefficients kdisp were estimated to range from 0 01 to 0 03 m2 s 1 at 100 m and from 0 1 to 0 7 m2 s 1 at 1000 m length scale l of the drifter distributions in all experiments kdisp increased with l and in three of the four experiments this increase was about linear in l however although a linear increase of kdisp with l is consistent with dispersion by shear diffusion in flow fields with constant current shear numerical modelling of the paths of the drifters and the resulting spreading of the drifter ensembles indicates that horizontal shear diffusion is not sufficient to explain the increase of kdisp with l the model results suggest that not shear diffusion due to large scale shear across the entire drifter distribution but scale dependent turbulent diffusion is dominating the scale dependence of kdisp at length scales between 100 and 1000 m according to the model results effects due to horizontal shear contribute less than 40 to the overall dispersion coefficient at length scales between 100 and 1000 m however horizontal shear is required to explain shape and orientation of the drifter distributions

numerical simulation of internal kelvin waves and coastal upwelling fronts two three dimensional primitive equation numerical ocean models are applied to the problem of internal kelvin waves and coastal upwelling in the great lakes one is the princeton ocean model pom with a terrain following sigma vertical coordinate and the other is the dietrich center for air sea technology diecast model with constant z level coordinates the sigma coordinate system is particularly convenient for simulating coastal upwelling while the z level system might be better for representing abrupt topographic changes the models are first tested with a stratified idealized circular lake 100 km in diameter and 100 m deep two bottom topographies are considered a flat bottom and a parabolic depth profile three rectilinear horizontal grids are used 5 2 5 and 1 25 km the pom was used with 13 vertical levels while the diecast model was tested with both 13 and 29 vertical levels the models are driven with an impulsive wind stress imitating the passage of a weather system in the case of the flat bottom basin the dynamical response to light wind forcing is a small amplitude internal kelvin wave for both models the speed of the kelvin wave in the model is somewhat less than the inviscid analytic solution wave speed in the case of strong wind forcing the thermocline breaks the surface full upwelling and a strong surface thermal front appears after the wind ceases the edges of this thermal front propagate cyclonically around the lake quite similar to an internal kelvin wave in the case of parabolic bathymetry kelvin wave and thermal front propagation is modified by interaction with a topographic wave and a geostrophic circulation in both models higher horizontal resolution gives higher wave and frontal speeds horizontal resolution is much more critical in the full upwelling case than in the kelvin wave case vertical resolution is not as critical the models are also applied to lake michigan to determine the response to strong northerly winds causing upwelling along the eastern shore the results are more complex than for the circular basin but clearly show the characteristics of cyclonically propagating thermal fronts the resulting northward warm front propagation along the eastern shore compares favorably with observations of temperature fluctuations at municipal water intakes after a storm although the model frontal speed was less than the observed speed

observations of near inertial energy in lake superior directly measured velocity data collected in lake superior between 2008 and 2011 show that currents in the open waters of the lake are dominated by near inertial energy the near inertial signal is composed almost entirely of clockwise rotation with vertical structure dominated by the first baroclinic mode where waters above and below the thermocline are roughly 180 out of phase with each other the strength of the oscillations is strongly related to the strength of the stratification in periods of the year when the water column is well mixed typically late autumn and late spring the near inertial signal is very weak when stratification exists near inertial oscillations can occur combining the velocity amplitudes with an estimate of the thermocline displacement allows estimation of the dominant direction and horizontal wavelength of the near inertial field showing that horizontal wavelengths are on the order of 50 100 km and the direction of the waves veers counter clockwise over the course of the season with a period of 1 month observations of backscatter suggest that inertial oscillations may be responsible for re suspension of bottom sediments which could have significant ecological consequences

relationship between cyclone tracks anticyclone tracks and baroclinic waveguides abstract this study presents observational results concerning the structure and evolution of high frequency fluctuations with periods shorter than 6 days in the geopotential height field the results are based on a statistical analysis of 1000 500 and 250 mb height fields derived from twice daily nmc final analyses over the northern hemisphere for the 20 winters 1964 65 through 1983 84 the disturbances assume the form of waves elongated in the meridional direction with a mean wavelength of 4000 km a westward tilt with height and a mean eastward phase propagation of 12 1 5 m s 1 these results support an interpretation in terms of finite amplitude baroclinic waves whose structure and evolution varies with geographical location over the continents and especially along the eastern slopes of the rockies and the tibetan plateau the waves show marked departures in structure and evolution from those over the oceans in particular we find evidence of a systematic influence of the terrain in steering the

west african convection regimes and their predictability from submonthly forecasts recurrent convection regimes are identified during the extended west african monsoon wam season may nov using a k means clustering of 1980 2013 noaa daily outgoing longwave radiation olr and are well reproduced in 1996 2015 ecmwf week 1 reforecasts despite systematic biases one regime of broad drying across the sahel in the early may jun and late oct wam is of particular interest regarding the prediction of onset date this regime is associated with an anticyclonic cell along the atlantic coast of west africa leading to a weakened monsoon flow and subsiding anomalies across the sahel teleconnections of this regime with the indian monsoon sector are identified through modulations of the walker circulation alongside relationships to mjo phase 3 more than 10 days in advance when convection is enhanced over the indian ocean other regimes are associated with westward propagating anomalous convective cells along two distinct wave trains at 15 circ hbox n and 24 circ hbox n during the core jul sep and late oct nov wam respectively and translate into wet anomalies transiting across the sahel a regime of broad sahel wetting in the core wam more frequent since the 1990s is related to global sst warming agreeing with the observed recovery of sahel rainfall ecmwf skill in forecasting regime sequences decreases from week 1 to 4 leads except in the case of the above mentioned regime associated with early season dry spells translating into the potential for skillful wam onset date predictions our analysis suggests that sources of predictability include relationships to the mjo and the indian monsoon sector which need to be further examined to benefit subseasonal forecasting efforts in west africa and ultimately agricultural planning and food security across the sahel

response of the west african monsoon to the madden julian oscillation abstract observations show that rainfall over west africa is influenced by the madden julian oscillation mjo a number of mechanisms have been suggested 1 forcing by equatorial waves 2 enhanced monsoon moisture supply and 3 increased african easterly wave aew activity however previous observational studies are not able to unambiguously distinguish between cause and effect carefully designed model experiments are used to assess these mechanisms intraseasonal convective anomalies over west africa during the summer monsoon season are simulated in an atmosphere only global circulation model as a response to imposed sea surface temperature sst anomalies associated with the mjo over the equatorial warm pool region 1 negative sst anomalies stabilize the atmosphere leading to locally reduced convection the reduced convection leads to negative midtropospheric latent heating anomalies that force dry equatorial waves these waves propagate eastward kelvin wave and westward rossby wave reachin

the dynamics of the west african monsoon part v the detection and role of the dominant modes of convectively coupled equatorial rossby waves abstract this study is the last in a series of papers addressing the dynamics of the west african summer monsoon at intraseasonal time scales between 10 and 90 days the signals of convectively coupled equatorial rossby er waves within the summer african monsoon have been investigated after filtering noaa outgoing longwave radiation olr data within a box delineated by the dispersion curves of the theoretical er waves two families of waves have been detected in the 10 100 day periodicity band by performing a singular spectrum analysis on a regional index of er filtered olr for each family the first eof mode has been retained to focus on the main convective variability signal within the periodicity band of 30 100 days an er wave pattern with an approximate wavelength of 13 500 km has been depicted this er wave links the mjo mode in the indian monsoon sector with the main mode of convective variability over west and central africa this confirms the investigations carried out in previous studies wi

numerical model of the geothermal regime on the beaufort shelf arctic canada since the last interglacial 1 a finite element geothermal model is developed for the outer mackenzie delta beaufort sea shelf to predict permafrost evolution since the last interglacial 130 116 kabp cal the purpose is to reconcile sparse observations of the depth and extent of ice bonded permafrost with sediment properties and the paleoenvironment sea level curves determine as a function of time areas of the shelf that were subaerially exposed promoting permafrost aggradation and areas that were submerged promoting permafrost degradation assuming as a model starting point that a paleoclimate similar to today persisted through the last interglacial permafrost subsequently aggrades in depth and advances seaward from the present shoreline to the shelf slope bathymetric break by the last glacial maximum lgm 26 kabp cal modeled permafrost exhibits reduced growth in depth and seaward progression that correlate with early and middle wisconsin stillstands in sea level following the lgm and rise in sea level offshore permafrost degrades and permafrost base rises 100 m to its present depth of 600 m the offshore limit of modeled ice bonded permafrost lies at the 95 m isobath within 1 km of the bathymetric shelf slope break the model replicates features of offshore permafrost body observed seismically and demonstrates that warm outflow from the mackenzie river depresses the upper surface of offshore permafrost by tens of meters to the 20 m isobath although pleistocene permafrost predated the wisconsinan the model demonstrates that the paleoenvironment of the last 125 000 years is sufficient to develop the depth seaward extent and principal features of the permafrost body

a synoptic scale wave of 6 9 day period in the atlantic tropical troposphere during summer 1981 abstract using the ecmwf analyses this study documents a tropospheric synoptic scale wave in the tropical atlantic during summer 1981 it has a 6 9 day period and a westward velocity of about 8 5 longitude per day the wave structure has features similar to a mixed rossby gravity wave it is suggested that it may be a normal mode of the earth atmosphere

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

characteristics of north african 6 9 day waves during summer 1981 abstract this study applies compositing methods to ecmwf model analyses during july and august 1981 to determine the structure of synoptic scale waves with periods of 6 9 days over northern africa these ware intermittent in time are most regular in period and amplitude west of the greenwich meridian in a narrow longitude band though there is a suggestion of their existence between 15 e and 0 the average wavelength is 5000 km and the westward velocity is 6 7 longitude day 1 the horizontal wind pattern in midtroposphere 700 and 500 hpa consists of two vortices of opposite signs on either side of 12 5 n with maximum perturbation of the zonal wind component at this latitude and maximum perturbation of the meridional wind component at 5 7 5 n and at 17 5 20 n temperature is modulated by the passage of the waves throughout the troposphere this structure is very different from that of the 3 4 day african wave

marine heat flow observations on the canadian arctic continental shelf and slope abstract we present 36 new heat flow measurements grouped within five separate sites on the canadian arctic continental shelf and slope between 95 115 w and 80 82 n observations consist of temperature gradients and in situ thermal conductivity using a modified marine type multiple penetration probe with a length of 4 6 m repeat measurements from nearby locations at different times of the year indicate that for depths deeper than 200 m the bottom water remains sufficiently stable to allow shallow measurements to characterize the deeper geothermal flux in this respect the cold ice covered deep water arctic margin is unique among the world s passive continental margins using these data we construct a preliminary geothermal transect between the continental sverdrup basin to the south and the oceanic canada basin to the north values of 35 45 mw m 2 exist on the axel heiberg headland nansen channel and on the continental slope and indicate a broad scale reduction from higher values of 55 60 mw m 2 within the canadian basin to values of 40 50 mw m 2 within the sverdrup basin in contrast there is a zone of high heat flow with well constrained values of 80 mw m 2 within peary channel low values in the northeastern sector of the margin suggest that the early tertiary eurekean orogen near ellesmere island has not thermally affected the margin based on calculations for pure shear extension the 15 mw m 2 reduction across the continental margin suggests that the canada basin proper may not have opened until late cretaceous unless the pre existing continental crust contains an extremely low amount of radiogenic material lithospheric models cannot account for the high values in peary channel which may indicate water movement within the sedimentary basin

impact of the madden julian oscillation on the summer west african monsoon in amip simulations at intraseasonal timescales convection over west africa is modulated by the madden julian oscillation mjo in this work we investigate the simulation of such relationship by 11 state of the art atmospheric general circulation models runs with prescribed observed sea surface temperatures in general the atmospheric model intercomparison project simulations show good skill in capturing the main characteristics of the summer mjo as well as its influence on convection and rainfall over west africa most models simulate an eastward spatiotemporal propagation of enhanced and suppressed convection similar to the observed mjo although their signal over west africa is weaker in some models in addition the ensemble average of models composites gives a better performance in reproducing the main features and timing of the mjo and its impact over west africa the influence on rainfall is well captured in both sahel and guinea regions thereby adequately producing the transition between positive and negative rainfall anomalies through the different phases as in the observations furthermore the results show that a strong active convection phase is clearly associated with a stronger african easterly jet aej but the weak convective phase is associated with a much weaker aej our analysis of the equatorial waves suggests that the main impact over west africa is established by the propagation of low frequency waves within the mjo and rossby spectral peaks results from the simulations confirm that it may be possible to predict anomalous convection over west africa with a time lead of 15 20 day

observation of the marine atmospheric boundary layer in the gulf of guinea during the 2006 boreal spring in situ satellite and model analyses data in april july 2006 are used to investigate the links between sea surface temperature sst and the marine atmospheric boundary layer mabl in the gulf of guinea the study region between 10 w and 6 e is divided into three areas with different characteristics the north area 4 5 n to 1 n with the wettest atmosphere and the warmest sst the upwelling area 1 n to 4 s with the strongest sst decrease and the south area 4 s to 8 s with a drier atmosphere and a more slowly decreasing sst than in the upwelling area the key zone of the air sea interactions in this region seems to be the sst front between north and upwelling areas on the one hand the study of the mabl on either side of the front shows a well mixed layer between the surface and about 500 m high sensitive to surface variations which gets shallower deeper when the sst decreases increases the mabl height about 1500 m follows the same variations but is not exactly collocated with the sst variations on the other hand the observation of the mabl across the sst front shows a strengthening of southeasterlies in the south area coinciding with a strong sst decrease in the upwelling area in the latter the wind weakens above the colder sst besides in the north area the wind strengthens above the warmer sst however the wind acceleration spans from the equator to 2 n in april and as far as 4 n in june a convergence zone is observed in the vicinity of 2 n in april suggesting a convection activity there favoured by the sst front copyright 2011 royal meteorological society

the energy flux from the wind to near inertial motions in the surface mixed layer abstract time series of wind stress computed from long term meteorological buoy data off north america are used to examine the forcing of surface inertial currents by the wind a simple damped slab model of the mixed layer is used to compute h the average flux of energy from the wind to mixed layer inertial currents in a mixed layer of fixed depth h the forcing of mixed layer inertial motions is highly intermittent most of the forcing occurs during the winter months with a few dozen events accounting for typically over half the total energy flux major forcing events are usually associated with translating cold fronts or small lows with scales of about 100 km the larger synoptic scale features have little energy at the inertial frequency and thus result in only weak forcing of inertial currents a strong seasonal signal exists in the inertial forcing at ows p 50 n 145 w 50 m is largest from october to february and is a factor of 12 above the june and july values if seasonally varying

a study of the low frequency inertio gravity waves observed during the pyrenees experiment the purpose of this paper is to analyze and interpret the onset of some well characterized transient inertio gravity waves observed during the pyrex pyrenees experiment campaign the data used are the high resolution soundings launched every 6 hours on both sides of the ridge in agreement with other observations made during the field campaign certain among those soundings often show steady mountain waves it appears that those waves are often transients and are followed by well characterized inertio gravity waves propagating upward and downward from the level where the initial mountain waves are large to interpret those observations it is suggested that the observed inertio gravity waves result from the geostrophic adjustment of the large scale flow that follows the transit and eventually the breaking of the main mountain wave this hypothesis is supported by an analytical calculation of inertio gravity waves emission from a momentum deposit that lasts a finite time the spatial extent of this forcing are also finite and its maximum amplitude is a fraction of the measured mountain drag

impact of the indian part of the summer mjo on west africa using nudged climate simulations observational evidence suggests a link between the summer madden julian oscillation mjo and anomalous convection over west africa this link is further studied with the help of the lmdz atmospheric general circulation model the approach is based on nudging the model towards the reanalysis in the asian monsoon region the simulation successfully captures the convection associated with the summer mjo in the nudging region outside this region the model is free to evolve over west africa it simulates convection anomalies that are similar in magnitude structure and timing to the observed ones in accordance with the observations the simulation shows that 15 20 days after the maximum increase decrease of convection in the indian ocean there is a significant reduction increase in west african convection the simulation strongly suggests that in addition to the eastward moving mjo signal the westward propagation of a convectively coupled equatorial rossby wave is needed to explain the overall impact of the mjo on convection over west africa these results highlight the use of mjo events to potentially predict regional scale anomalous convection and rainfall spells over west africa with a time lag of approximately 15 20 days

inertial oscillations and related internal beat pulsations and surges in lakes michigan and ontario records of water temperature current and wind made during campaigns on lakes michigan 1963 and ontario 1972 are searched for inertial responses to wind action i circle tracking currents and ii isotherm depth undulations i e long internal wave manifestations of forcing by wind with stratification absent in winter only response i is seen and then only rarely with periods very close to the local inertial period tin after whole basin stratification is complete both responses i and ii occur frequently as internal kelvin waves shore trapped and not further treated here and cross basin poincare internal seiche modes mode periods range between 1 and 15 less than tin depending on which of the here modeled modes have responded and on differences between modes in their partitioning of kinetic and potential energy when in both lakes short duration wind impulses were followed by a week or more of relative calm poincare mode combinations produced beat pulsations in both responses i and ii diagnostic features of which sometimes permitted participating modes to be identified at a nearshore downwelled front in lake ontario another type of poststorm adjustment was seen periodically released from the front internal surges migrated across the basin through fields of inertially rotating response i currents

cross shelf thermal variability in southern lake michigan during the stratified periods 1 results from a field experiment in southern lake michigan are used to quantify the cross shelf nearshore variability in great lakes temperatures during the stratified season the experiment was conducted along the indiana coast of southern lake michigan with temperature and velocity moorings arranged in a cross shelf transect that extended to approximately 20 km from shore 40 m depth the field site is noteworthy because of its location at the end of a major axis of an elliptical great lake the relatively mild bathymetric slope and local shoreline orientation that is perpendicular relative to the dominant summer winds measurements demonstrate that the location of the thermocline bottom intersection is highly variable causing a wide zone of extreme thermal variability in the nearshore region with time scales of variability ranging from hours to months near inertial internal poincare waves are shown to cause large thermocline excursions but primarily only during periods of elevated activity several full upwelling events were observed but in general they were brief lasting only 1 2 days and had very limited spatial extent 2 5 km or less nonetheless the offshore extent of the upwelling front was shown to be reasonably estimated with a simple estimate of the cross shelf transport caused by alongshore wind events a persistent feature that determined the zone of elevated thermal variability the thermocline shelf intersection point was the strongly tilted thermocline which resulted in the thermocline being located very close to shore no evidence was found to support the hypothesis that internal kelvin waves affect thermal variability at the study location

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

african easterly waves and convection part i linear simulations abstract a linearized version of a nonhydrostatic model is used to study the normal mode selection and the structure of the african easterly waves in dry and moist environments associated with an idealized african easterly jet structure the dry mode reproduces the principal observed characteristics of the lower troposphere in particular the low level ascent and convergence in the trough a simple cisk type parameterization improves the upper level circulation a budget of kinetic energy shows barotropic and baroclinic contributions and adequately reproduces the analysis of norquist et al the scale selection and the structure of the most unstable mode appears sensitive to the jet structure in particular to the meridional and vertical shear several discrepancies between observations and these linear modes emphasize the importance of an accurate description of convection

a reexamination of the genesis of african waves abstract the problem of genesis of easterly african waves is reexamined as a linear eigenvalue instability problem of a supposedly realistic basic zonal shear flow with and without a cumulus heating evidence will be presented to show how the growth rate the propagation speed and the overall composite structure of the observed tropical easterly waves can he well reproduced in the model in conjunction with a cisk representation of the latent heating the degree of resemblance between the structure of the model unstable wave and the observed african waves is found to be significantly dependent upon the secondary features of the basic shear flow this finding is based on a detailed comparison of the results using different structures for the basic flow including the one used by mass much of the kinetic energy of the unstable wave is generated via the barotropic instability process associated with the horizontal shear of the sahara jet centered at the 650 mb level but the vertical shear of the zonal flow

observational evidence of african desert dust intensification of easterly waves 1 this paper presents indirect observational evidence that desert dust can modulate the amplitude of easterly waves in the atlantic ocean twenty two years of ncep ncar reanalysis and dust from a global transport model are used to characterize the evolution of enhanced easterly waves lag composites of analysis increments analysis minus first guess of geopotential height 700 hpa anomalies indicate larger amplitudes in the analysis than in the first guess fields the results indicate that the temperature structure in the lower troposphere associated with easterly waves is warmer in the analysis than in first guess fields by about 0 25 k per day we hypothesize that the differences in the amplitudes are due to radiative effects associated with african dust which have been incorporated in the reanalysis by data assimilation but absent in the model first guess

evidence of two independent modes of convection at intraseasonal timescale in the west african summer monsoon 1 two main independent modes of convection at intraseasonal timescale in the west african summer monsoon are highlighted one depicts a meridional dipole of convection over west africa linked to a modulation of the latitude of the inter tropical convergence zone itcz it is associated with a westward propagative signal of the convection over the sahel and to a modulation of cyclonic vorticity at these latitudes the other one is characterized by a stationary uniform modulation of convection in the itcz without any significant modulation of its latitudinal location it is associated with a modulation of the zonal wind component over the eastern equatorial atlantic the dominant periodicity of these two modes is around 15 days it is necessary to take into account the relative parts of these two modes in order to accurately analyze the mechanisms involved in intraseasonal variability of the african monsoon

the west african monsoon dynamics part i documentation of intraseasonal variability intraseasonal variability in the west african monsoon is documented by using daily gridded datasets of rainfall and convection and reanalyzed atmospheric fields over the period 1968 90 rainfall and convection over west africa are significantly modulated at two intraseasonal timescales 10 25 and 25 60 day leading to variations of more than 30 of the seasonal signal a composite analysis based on the dates of the maximum minimum of a regional rainfall index in wet dry sequences shows that these sequences last on average 9 days and belong to a main quasiperiodic signal of about 15 days a secondary periodicity of 38 days is present but leads to a weaker modulation during a wet dry sequence convection in the itcz is enhanced weakened and its northern boundary moves to the north south while the speed of the african easterly jet decreases increases the speed of the tropical easterly jet increases decreases and the monsoon flow becomes stronger weaker all these features being similar to the ones associated with interannual variability characterizing wet and dry years this modulation of convection at intraseasonal timescales is not limited to west africa but corresponds to a westward propagating signal from eastern africa to the western tropical atlantic an enhanced monsoon phase is associated with stronger cyclonic activity in the low levels over the sahel associated with stronger moisture advection over west africa five days before the full development of the wet phase a stronger cyclonic circulation at 20 e induces enhanced southerly winds along 25 e where convection enhances while another westwardpropagating cyclonic circulation is located at 0 this atmospheric pattern is linked to the enhancement of the subsiding branch of the northern hadley cell at 35 n northerly advection of drier air over west africa and to increased dry convection in the heat low at 20 n it propagates westward leading to a zonally extended area of enhanced monsoon winds over west africa consistent with the occurrence of the wet phase

intraseasonal variability in the equatorial atlantic west africa during march june intraseasonal 30 80 days variability in the equatorial atlantic west african sector during march june is investigated using various recently archived satellite measurements and the ncep doe amip ii reanalysis daily data the global connections of regional intraseasonal signals are first examined for the period of 1979 2006 through lag regression analyses of convection olr and other dynamic components against a regional intraseasonal convective olr index the eastward propagating features of convection can readily be seen accompanied by coherent circulation anomalies similar to those for the global tropical intraseasonal mode i e the madden julian oscillation mjo the regressed trmm rainfall 3b42 anomalies during the trmm period 1998 2006 manifest similar propagating features as for the regressed olr anomalies during 1979 2006 these coherent features hence tend to suggest that the regional intraseasonal convective signals might be mostly a regional response to or closely associated with the mjo and probably contribute to the mjo s global propagation atmospheric and surface intraseasonal variability during march june of 1998 2006 are further examined using the high quality trmm microwave imager tmi sea surface temperature sst columnar water vapor and cloud liquid water and the quikscat oceanic winds 2000 2006 enhanced suppressed convection or positive negative rainfall anomalies approximately cover the entire basin 0 10 n 30 w 10 e during the passage of intraseasonal convective signals accompanied by anomalous surface westerly easterly flow furthermore a unique propagating feature seems to exist within the tropical atlantic basin rainfall anomalies always appear first in the northwestern basin right off the coast of south america and gradually extend eastward to cover the entire basin a dipolar structure of rainfall anomalies with cross equatorial surface wind anomalies can thus be observed during this evolution similar to the anomaly patterns on the interannual time scale discovered in past studies coherent intraseasonal variations and patterns can also be found in other physical components

evidence of two regimes of easterly waves over west africa and the tropical atlantic synoptic scale easterly waves at 700 hpa have been studied over west africa and the tropical atlantic for the summers of 1979 1995 spectral analyses fast fourier and wavelet transforms of 700 hpa meridional wind component along 17 5 n enable to point out two band periods between 3 and 5 days and between 6 and 9 days an example of each wave is shown composite analysis confirms the evidence of two such regimes of easterly waves during summer over west africa and the tropical atlantic the 6 9 day wave regime differ from the 3 5 day wave regime by larger anticyclonic cells originated from the libyan and the azores areas and by more northern tracks

near inertial waves on the new england shelf the role of evolving stratification turbulent dissipation and bottom drag abstract energetic variable near inertial internal waves were observed on the springtime new england shelf as part of the coastal mixing and optics cmo project surface warming and freshwater advection tripled the average stratification during a 3 week observational period in april may 1997 the wave field was dominated by near inertial internal waves generated by passing storms wave evolution was controlled by a balance among wind stress bottom drag and turbulent dissipation as the stratification evolved the vertical structure of these near inertial waves switched from mode 1 to mode 2 with associated changes in the magnitude and location of wave shear the growth of mode 2 waves was attributable to a combination of changing wind stress forcing and a nonlinear coupling between the first and second vertical modes through quadratic bottom stress to explore both forcing mechanisms an open ocean mixed layer model is adapted to the continental shelf in this model surface wind stress and bottom stre

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

energy partitioning and horizontal dispersion in a stratified rotating lake abstract the response of a stratified rotating basin to the release of a linearly tilted interface is derived this case is compared with a uniformly forced basin in the two limits when the duration of the forcing is much greater than the period of the dominant internal waves and when it is much smaller energy partitioning is studied as a function of the burger number s relative importance of stratification versus rotation showing the dominance of a geostrophic component over the wave field for low s trajectories are integrated numerically revealing the stokes drift of the waves to be always cyclonic transport properties are classified in terms of s and the wedderburn number w relative importance of the disturbance versus stratification the geostrophic flow is the main source of advection but only the waves allow particles to break the barrier to transport between the two geostrophic gyres ultimately leading to stretching and folding for low values of w advection can become chaotic conserva

pressure driven shoreline currents in a perennially ice covered pro glacial lake in antarctica identified from a licl tracer injected into a pro glacial stream the distribution of streamwater within ice covered lakes influences sub ice currents biological activity and shoreline morphology perennially ice covered lakes in the mcmurdo dry valleys antarctica provide an excellent natural laboratory to study hydrologic limnologic interactions under ice cover for a 2 h period on 17 december 2012 we injected a lithium chloride tracer into andersen creek a pro glacial stream flowing into lake hoare over 4 h we collected 182 water samples from five stream sites and 15 ice boreholes geochemical data showed that interflow travelled west of the stream mouth along the shoreline and did not flow towards the lake interior the chemistry of water from andersen creek was similar to the chemistry of water below shoreline ice additional evidence for westward flow included the morphology of channels on the ice surface the orientation of ripple marks in lake sediments at the stream mouth and equivalent temperatures between andersen creek and water below shoreline ice streamwater deflected to the right of the mouth of the stream in the opposite direction predicted by the coriolis force deflection of interflow was probably caused by the diurnal addition of glacial runoff and stream discharge to the eastern edge of the lake which created a strong pressure gradient sloping to the west this flow directed stream momentum away from the lake interior minimizing the impact of stream momentum on sub ice currents it also transported dissolved nutrients and suspended sediments to the shoreline region instead of the lake interior potentially affecting biological productivity and bedform development copyright 2014 john wiley sons ltd

remotely forced biweekly deep oscillations on the continental slope of the gulf of guinea current meter measurements on the continental slope of the gulf of guinea at 7 20 s and 1300 m depth have revealed biweekly oscillations of the currents bottom intensified and oriented along the bathymetry we develop a three dimensional primitive equation model of the gulf of guinea to study the oscillations and their forcing mechanism the high resolution 1 12 regional model reproduces remarkably well the main characteristics of the deep currents on the continental slope experiments with different forcings demonstrate that the biweekly variability at 1300 m depth is remotely forced by equatorial winds deep yanai waves generated by the wind propagate eastward along the equator upon reaching the african coast the energy propagates poleward in both directions as coastal trapped waves the selection of the dominant biweekly period is explained by the absence of equatorial waves with westward group velocities in that frequency band contrary to a previous hypothesis involving tidal forcing our interpretation is coherent with the significant interannual variability of the biweekly energy

near inertial waves in lake erie near inertial poincare waves with a period t p similar to 17 h are the dominant wind induced internal wave motions in central lake erie and consequently have a substantial influence on lake circulation mixing and biogeochemistry however due to the complex three basin bathymetry in lake erie the vertical and horizontal modal structure of these waves remain poorly understood in this study we analyze field data to show wind events energize frequent vertical mode one poincare waves the horizontal modal structure was also investigated in a sensitivity analysis using a calibrated three dimensional hydrodynamic transport model forced with observed and idealized spatially uniform wind events strong horizontal mode one poincare wave cells form in both the central and eastern basins when wind events have a duration of 0 25 t p to 0 5 t p are impulsive and periodic at t p or have anticyclonic rotation with a duration of t p momentum transfer from longer wind events 0 5 tp will oppose the coriolis force rotated currents and damp poincare wave generation in agreement with theory the most efficient wind events are observed and computationally modeled to have a duration of 0 25 t p causing an excitation peak at similar to 0 4 t p and converting similar to 0 8 of the wind energy input to poincare waves the efficiency of wind work in generating poincare wave kinetic energy is given by 1 cos 2 pi f t t 1 where f is the inertial frequency and t is the wind duration therefore the efficiency peaks during each nt p period where n is a non negative integer and decreases significantly for longer wind events

vertical and horizontal particle transport in the coastal waters of a large lake an assessment by sediment trap and thorium 234 measurements 1 the horizontal and vertical flux of particulate material in the nearshore of southern lake michigan 0 40 m was estimated with the naturally occurring radionuclide 234th horizontal fluxes of 234th supplemented apparent vertical fluxes of 234th in the water column based on local 234th 238u disequilibria by a factor of 7 14 reinforcing the importance of lateral transport in coastal environments calculated onshore transport of particulate material across the 40 m isobath was as high as 1 1 106 kg km 1 d 1 and exceeds estimates of terrigenous riverine and bluff erosion loading estimates of onshore flux of organic carbon exceeded areal primary productivity by as much as 300 and should be considered in nearshore carbon budgets bottom tethered sediment traps placed 5 m above the bottom measured sedimentation rates that were 1 order of magnitude lower than 234th derived mass fluxes from the water column and 2 orders of magnitude lower than 234th derived mass fluxes to the lakebed we ascribe this difference to under collecting by the sediment trap either because of trap hydrodynamics or flux occurring below the trap capture plane cross shore fluxes showed a periodicity of 4 days and correlated strongly with a topographic vorticity wave that is present throughout the year in southern lake michigan the impact of this wave as a driver of bidirectional cross shore flux on biogeochemical cycling and both nearshore and offshore food webs has not yet been explicitly considered

horizontal structure and seasonality of large scale circulations associated with submonthly tropical convection the relationship between deep tropical convection and large scale atmospheric circulation in the 6 30 day period range is examined regression relationships between filtered outgoing longwave radiation at various locations in the tropics and 200 and 850 mb circulation are mapped for the standard seasons and the spatial structure and seasonal dependence of the results are interpreted in view of the basic state circulation in regions where the convection is embedded in upper level easterlies anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal along with patterns of meridional outflow into subtropical anticyclonic perturbations lagged relationships suggest that the convection is forcing the circulation in many of these cases the outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons with more symmetric signals about the equator seen in the equinoctial seasons the longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection there tends to be a first baroclinic mode vertical structure to these circulations such that equatorial westerlies are present at 850 mb within the convection with closed circulations on either side of the equator resembling equatorial rossby modes especially common over the atlantic and pacific sectors as a contrast in regions located within upper level westerlies or along the margin of influence of upper westerly disturbances convection appears to be forced by upper level wave energy propagating into the deep tropics with the heating located in the upward motion region ahead of upper level troughs this occurs over the atlantic and eastern pacific sectors during northern winter and spring and over australia the south pacific and south america during southern summer when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection the results confirm theoretical and modeling ideas that suggest that rossby wave energy is able to propagate into the deep tropics in regions where upper level westerlies exist in the tropics

the madden julian oscillation s influence on african easterly waves and downstream tropical cyclogenesis abstractthe influence of the madden julian oscillation mjo over tropical africa and atlantic is explored during the northern hemisphere summer months the mjo is assessed by using real time multivariate mjo rmm indices these indices divide the active convective signal of the mjo into 8 phases convection associated with the mjo is enhanced over tropical africa during rmm phases 8 1 and 2 convection becomes suppressed over tropical africa during the subsequent rmm phases phases 3 7 african convective signals are associated with westward propagating equatorial rossby waves the mjo modulates african easterly wave aew activity aew activity is locally enhanced during rmm phases 1 3 and suppressed during rmm phases 6 8 enhanced aew activity occurs during periods of enhanced convection over tropical africa consistent with stronger or more frequent triggering of aews as well as more growth associated with latent heat release enhanced aew activity occurs during the low level westerly wind phase of

deep water dynamics and boundary mixing in a nontidal stratified basin a modeling study of the baltic sea recent results from a tracer release experiment have shown that similar to many lakes and ocean basins deep water mixing in the baltic sea is largely determined by mixing processes occurring in the energetic near bottom region due to the complexity and small vertical extent of this region however previous modeling studies of the baltic sea have so far not been able to provide a numerically and physically sound representation of boundary mixing here we discuss first results from a nested high resolution simulation of the central baltic sea that aims at a realistic description of the turbulent bottom boundary layer with the help of new numerical techniques adaptive coordinates and state of the art turbulence modeling using a comprehensive data set from the baltic sea tracer release experiment we show that the model is able to reproduce the key dynamical processes near inertial waves topographic waves and a rim current with excellent accuracy boundary mixing triggered by these processes was found to result in simulated basin scale mixing rates in close agreement with observations including a seasonal variability that has been emphasized in previous studies these results may be relevant also for the description of mixing in large lakes and other stratified basins

shallow sediment temperature perturbations and sediment thermal conductivities canadian beaufort shelf a regional survey of sediment temperatures and thermal conductivities was conducted at 33 stations across the outer shelf of the canadian beaufort sea techniques developed for deep ocean heat flow investigations were used to study the upper 3 m of sediments temperature depth profiles exhibit curvatures that may be explained by seasonal changes in bottom water temperature some curvatures may arise from other causes it is unlikely that thermal effects of the underlying degradational permafrost can be detected from such shallow temperatures because of the magnitude of and lack of independent knowledge of these transient and local influences thermal conductivities measured on sediment cores and corrected to 1 c range from 0 9 to 2 4 w m 1 k 1 average 1 26 0 2 w m 1 k 1 these values are higher than typical conductivities of deep ocean sediments the wide range of thermal conductivities observed across the outer beaufort shelf may be explained by the presence of a varying fraction of quartz sand

the west african monsoon dynamics part iii the quasi biweekly zonal dipole abstract this paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the african monsoon during northern summer here identified as the quasi biweekly zonal dipole qbzd the qbzd is primarily characterized by a quasi stationary zonal dipole of convection whose dimension is larger than the west african monsoon domain with its two poles centered along the guinean coast and between 30 and 60 w in the equatorial atlantic the qbzd dynamical processes within the atlantic africa domain are examined in some detail the qbzd has a dipole pattern associated with a walker type circulation in the near equatorial zonal plane it is controlled both by equatorial atmospheric dynamics through a kelvin wave like disturbance propagating eastward between its two poles and by land surface processes over africa inducing combined fluctuations in surface temperatures surface pressure and low level zonal winds off the coast of west africa when convection is at a min

late quaternary history of the mackenzie beaufort region arctic canada from modelling of permafrost temperatures 1 the onshore offshore transition five offshore drill holes northeast of richards island reveal permafrost conditions which are interpreted in terms of the sea level and paleoenviromental history of the canadian beaufort shelf the top of ice bonded permafrost lies 76 88 m below the seabed within 12 km of the shore 11 m below the seabed 20 km offshore and at 60 m some 50 km offshore the base of permafrost is 500 m to over 700 m deep temperature depth gradients are negative or nearly isothermal and permafrost is degrading geothermal modelling of the temperatures and permafrost observations indicates that the sites were subaerially exposed throughout most of the wisconsinan and during the early holocene three sites 2 10 km from the present shoreline experienced inundation at 3 5 4 0 ka these times support dates of marine transgression predicted by the sea level curve but the sites may have been occupied by thermokarst lakes prior to transgression in contrast the site 20 km offshore with shallow ice bonding was inundated about 0 4 ka this area may have persisted as an island until recently shoals observed nearby may be analogues of this process being outliers that are now being eroded below sea level marine transgression occurred about 6 ka some 50 km offshore rbsumb cinq trous de forage en mer au nord est de l ile richards rtvklent la presence de conditions de pergelisol elles sont interpr6t6es en fonction de l histoire du niveau marin et du paltomilieu de la plate forme de beaufort au canada a moins de 12 km vers le large le sommet du perg6lisol cimentk par la glace se trouve entre 76 et 88 m sous le fond marin i 20 km en mer il n est qu i 11 m sous le fond marin et a quelques 50 km en mer il passe b 60 m la

dynamics of the west african monsoon part iv analysis of 25 90 day variability of convection and the role of the indian monsoon this paper is part of a series of studies addressing the dynamics of the west african summer monsoon at intraseasonal time scales between 10 and 90 days the dominant mode of 25 90 day convective variability in the african monsoon was investigated starting from previous results involving the excitation of dry equatorial kelvin and rossby waves by a negative diabatic heat source located over the warm pool this evolution is consistent with a significant contribution by a convectively coupled equatorial rossby wave and the mjo on the other hand convectively coupled kelvin waves as well as the dry kelvin wave signal have a very weak impact however there is more to the global control of the african summer monsoon than convectively coupled wave dynamics the active break cycle of the indian monsoon controlled by a northward moving dipole of diabatic heating in the indian sector can also influence the african monsoon through atmospheric teleconnections simulations performed with a dry primitive equation model show that this influence may be transferred through the northern indian heat source which excites a rossby cyclonic circulation propagating westward over north africa that is cut off by the northward arrival of the equatorial indian heat source and the associated intrusion of an anticyclonic ridge low level westerly winds and moisture advection within the itcz consequently increase over africa the mean time lag between an active phase over india and over africa is about 15 20 days

a year of internal poincare waves in southern lake michigan 1 a unique set of full year deep water observations from the middle of lake michigan s southern basin are analyzed to quantify the seasonal variability of the dominant near inertial internal poincare wave at this mid lake location the poincare wave is seen to describe more than 80 of the observed surface current variability for much of the year with characteristic near inertial frequency and clockwise rotating velocities the dominance of the near inertial seiche on the flow decreases with depth the wave persists during the stratified period roughly may through late december and is supported by as few as 1 2 degrees of thermal stratification over 150 m only after complete water column mixing does the wave go dormant for january through april the strongest poincare wave activity is seen to correspond to the period of strongest summer thermal stratification august in spite of the relatively weak winds at this time a simple inertial slab model optimized with linear friction is shown to capture the seasonal variability of the near inertial energy at this location reasonably well the vertical structure of the wave shows good agreement with that calculated with a standard normal modes formulation which is in turn used to characterize the potential shear and mixing caused by the wave late spring and summer events of elevated poincare wave activity are shown to generate sufficiently strong shear with persistent periods of sub 1 richardson numbers within the thermocline suggesting that the near inertial seiche is likely generating thermocline instabilities in the lake s interior

seasonal evolution of the basin scale internal wave field in a large stratified lake the response of the water column to varying conditions of stratification and wind forcing was investigated in lake kinneret israel using data collected from thermistor chains and acoustic doppler current profilers during 1997 and 1998 the strong daily sea breeze was found to generate a vertical mode 1 internal kelvin wave and basin scale internal poincarewaves of vertical modes 1 2 and 3 the kelvin wave the dominant component of the internal wave field was responsible for alongshore velocities in the nearshore regions in the upwind nearshore regions velocities were dominated by the forced response to the wind and were cross shore in nature in the lake interior the kelvin wave effect on the horizontal velocity field was minor compared to the higher vertical mode poincarewaves the kelvin wave is shown to exist in resonant and nonforced states with the wind whereas the vertical mode 1 poincarewave energy remained relatively constant despite large variability in the forcing condi tions the energy in the higher mode poincarewaves varied greatly both on daily and seasonal timescales the results demonstrate that the wind energy forces multiple basin scale internal wave modes and that prior motion in the water column must be considered when determining the subsequent internal wave response in periodically forced systems lake kinneret israel has long been an important regional resource providing a substantial supply of drinking water as well as supporting a commercial fishery the monomictic lake is forced daily during the summer by a powerful sea breeze with stratification and wind patterns showing little variation from year to year these factors along with a fa vorable bathymetric pattern combine to make lake kinneret an ideal place for the field investigation of large scale inter nal waves and related phenomena such as the benthic bound ary layer horizontal transport and vertical mixing processes in addition to providing substantial insight into the local pro cesses and hence influencing local management study of the lake also provides fundamental knowledge of the dynam ics of enclosed basins on a geophysical scale during summer the internal rossby radius in lake kin neret is typically one half the basin width hence large scale motion in the lake will be affected by the earth s rotation and basin scale internal waves may be of topographic ross by kelvin or poincareform topographic rossby waves are sub inertial frequency oscillations that exist only in rotating fluids with variable bathymetry mysak et al 1985 stocker and hutter 1987 and these waves result from changes in depth affecting the relative vorticity of fluid elements in the absence of vertical boundaries wavelike solutions to the lin ear inviscid equations of motion for a flat bottomed region of homogeneous fluid with a constant rate of rotation are 1

observations of seven african easterly waves in the east atlantic during 2006 abstract the african monsoon multidisciplinary analyses amma experiment and its downstream nasa extension namma provide an unprecedented detailed look at the vertical structure of consecutive african easterly waves during august and september 2006 seven easterly waves passed through the namma domain two waves developed into tropical cyclones debby and helene two waves did not develop and three waves were questionable in their role in the development of ernesto florence and gordon ncep global data assimilation system gdas analyses are used to describe the track of both the vorticity maxima and midlevel wave trough associated with each of the seven easterly waves dropsonde data from namma research flights are used to describe the observed wind structure and as a tool to evaluate the accuracy of the gdas to resolve the structure of the wave finally satellite data are used to identify the relationship between convection and the organization of the wind structure results support a necessary d

a mechanism for african monsoon breaks mediterranean cold air surges 1 surges of cold air from the mediterranean into northern africa during the boreal summer are documented and their influence on monsoon breaks is analyzed using tropical rainfall measuring mission rainfall estimates and reanalysis products between 1998 and 2006 6 10 cold air surges occurred each summer with low level temperature anomalies ranging from less than 1 k to over 6 k composite analysis indicates that cold air surges over northern africa persist for 2 10 days and travel equatorward at approximately 5 5 m s 1 which is 0 5 1 5 m s 1 faster than the observed climatological low level meridional flow northern african cold surges have characteristics similar to surges observed elsewhere in the world including a hydrostatically induced ridge of surface pressure and an amplified upper tropospheric ridge trough pattern the african cold surge is preceded by the passage of a shortwave trough and an intensification of the upper tropospheric subtropical westerly jet streak over the mediterranean sea these events are associated with increased confluence in the jet entrance region over the central mediterranean an enhanced direct secondary circulation subsidence and low level ageostrophic northerly flow over northeastern africa composite analysis shows that the passage of a cold surge is associated with an enhancement in convective activity over southern algeria western niger northern mali and mauritania 2 to 5 days before the surge reaches the eastern sahel 17 5 n when northeasterly flow channeled between the atlas and ahaggar mountains strengthens and transports relatively moist air from the western mediterranean and eastern north atlantic over the region and increases moisture convergence over western africa north of 20 n over the eastern sahel of sudan and eastern chad the composite results reveal a break in convective activity and decrease in low level convergence when the surge arrives that persists for about 6 days these results offer great promise for improving the short range prediction of rainfall over northern africa

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

spatial structure of the dominant basin scale internal waves in lake kinneret field data and numerical simulations were used to investigate the effects of basin shape continuous stratification and rotation on the three dimensional structure of the dominant natural basin scale internal wave modes in lake kinneret a stratified lake large enough so that the earth s rotation influences the wave motion the structure of the modes was inferred from power spectral density of measured and simulated isotherm vertical displacements and from rotary power spectral density of isopycnal velocities obtained from numerical simulations the shape of the lake at the level of the thermocline in conjunction with the dispersion relationship determines the horizontal configuration of the natural modes the dominant response to wind forcing was an azimuthal and vertical mode 1 kelvin wave with a natural period of 22 6 h that propagated around the entire basin the second most dominant response was a vertical mode 1 wave with a natural period close to 10 5 h and composed of two counter rotating

composite analysis of dust impacts on african easterly waves in the moderate resolution imaging spectrometer era 1 this study examines the synoptic scale impacts of african dust on easterly waves in the tropical northeast atlantic moderate resolution imaging spectrometer aerosol optical depth aod national oceanic and atmospheric administration products and national center for environmental prediction reanalysis fields in the atlantic main hurricane development region mdr form the basis for statistical analysis of a limited set of cases objectively selected for the 2000 2008 hurricane seasons when thresholds are exceeded for sea surface temperature sst easterly wind shear cyclonic vorticity and upward motion after ranking african easterly waves by aod the top dusty and bottom clean cases are studied as composite differences african dust and subsidence cause temperatures to warm 3 c in the 700 hpa layer while ssts cause temperatures to cool stabilizing the atmosphere increased aod and strong 10 m s 1 600 hpa easterly winds limit cloud efficiency through shear and oversupply of condensation nuclei vertical section composites demonstrate that warm dry subsident air coincides with the african dust plume in the latitudes 18 n 30 n hurricane reanalysis data indicate that higher aod in the mdr reduces chances for the intensification of african easterly waves

the impact of deep convection on the west african summer monsoon climate a regional climate model sensitivity study the role of the representation of deep convection on key elements of the west african summer monsoon climate is addressed using the regional climate model regcm3 two simulations in which a scheme of deep convection is activated and then turned off are performed and intercompared results show that the presence of deep convective heating along the intertropical convergence zone sustains increased lower level baroclinicity favoring intensification of the jet core and leading to a more realistic african easterly jet in addition although the isentropic potential vorticity ipv is lower when the convection scheme is switched off african easterly waves aews are still generated and propagate westwards but they dissipate around the west coast substantial differences between the two simulations occur mainly at the 6 to 9 day time scale over land when much weaker activity is simulated in the absence of convection this indicates that orographic friction and low level large scale moisture convergence generating high values of latent heat and ipv may play the dominant role in the genesis and growth of aews and that deep convection acts to strengthen the overall wave activity and to favor their west coast development copyright 2011 royal meteorological society