abrupt climate change in southeast tropical africa influenced by indian monsoon variability and itcz migration 1 the timing and magnitude of abrupt climate change in tropical africa during the last glacial termination remains poorly understood high resolution paleolimnological data from lake tanganyika southeast africa show that wind driven seasonal mixing in the lake was reduced during the younger dryas inter allerod cool period older dryas and heinrich event 1 suggesting a weakened southwest indian monsoon and a more southerly position of the inter tropical convergence zone over africa during these intervals these events in lake tanganyika coeval with millennial and centennial scale climate shifts in the high latitudes suggest that changes in itcz location and indian monsoon strength are important components of abrupt global climate change and that their effects are felt south of the equator in africa however we observe additional events in lake tanganyika of equal magnitude that are not correlated with high latitude changes indicating the potential for abrupt climate change to originate from within tropical systems

tides and tidal energetics of the strait of gibraltar a modelling approach abstract the main semidiurnal m2 and s2 and diurnal o1 and k1 tidal waves in the strait of gibraltar are simulated by employing a 2d high resolution non linear boundary fitted coordinate model agreement between observational evidence and model results is good for the m2 and s2 tidal waves and satisfactory for the o1 and k1 tidal waves the model reproduces all the known features of the spatial structure of these waves and predicts some new ones namely the general direction of the m2 and s2 mean tidal energy fluxes to the west with a clear increase at the camarinal sill the o1 amphidrome with anticlockwise rotation of cotidal lines in the tarifa narrows and small scale eddies in the m2 s2 and o1 mean tidal energy flux fields in the vicinity of the western and eastern boundaries of the strait

multi year wind dynamics around lake tanganyika lake tanganyika is the second largest freshwater lake in the world by volume and is of prime importance for the regional economy in east africa although the lake is recognized as a key component of the regional climate system little is known about atmospheric dynamics in its surroundings to understand this role we analyze winds around lake tanganyika as modeled by a high resolution 7 km regional climate model consortium for small scale modeling in climate mode over the period 1999 2008 modeled surface wind speed and direction are in very good agreement with high resolution 12 5 km quick scatterometer quikscat satellite wind observations during the dry season comparison of a control run with a model simulation where all lake pixels are replaced by representative land pixels indicates that mean surface wind speed over lake tanganyika almost doubles due to lake presence furthermore a region of higher surface wind speed in the central part of the lake is identified and confirmed by quikscat observations a combination of wind channeling along valley mountains and wind confluence on the upwind side of the lake is responsible for this speed up the lower wind speeds in the rest of the lake result from blocked conditions due to more pronounced orography finally the model captures a zone of higher wind speed at around 2 km height associated with the low level somali jet these results demonstrate that high resolution climate modeling allows a detailed understanding of wind dynamics in the vicinity of lake tanganyika

evidence of multimodal structure of the baroclinic tide in the strait of gibraltar 1 the multimodal structure of the baroclinic tides in the strait of gibraltar is studied using observations and numerical simulations observational data and model results revealed the presence of two types of tidal internal waves generated over camarinal sill cs one propagates toward the mediterranean and disintegrates into series of nonlinear short internal waves with amplitudes of 50 m and more the second type behind the first propagates slower and has a longer wavelength the vertical structure with both upward and downward displacements of isopycnals can be identified as a manifestation of higher baroclinic modes analysis of the empirical orthogonal functions of the adcp measurements performed over cs and model time series has shown that the second baroclinic mode predominates in the second type of internal waves its amplitude can reach one third that of the first baroclinic mode of the leading waves of depression

tides in the strait of gibraltar the strait of gibraltar is where the transition occurs between two distinct tidal regimes the north atlantic where tidal ranges are in excess of 2 m and the western mediterranean where tidal ranges are less than 1 m within the strait the tide is principally semidiurnal observations indicate that on average 96 of the bottom pressure and 74 of the current variability are contained in the semidiurnal band 2 cpd the structure of the local cotidal chart of the m2 tidal constituent is complicated but can be explained in terms of the along and across strait momentum balances along the strait the pressure gradient is mainly balanced by the acceleration of the flow although friction can be of appreciable magnitude corresponding to a decay time scale of 3 16 hours across the strait the momentum balance appears mostly geostrophic but better observations would be required to explain residual terms available observations indicate that the m2 tide has no net energy flux through the strait at the sill the tidal transport in the upper layer exceeds that in the lower layer by nearly a factor of 2 at the eastern end of the strait between algeciras and ceuta the tidal transport is larger in the lower layer this apparent shift in the location of maximum tidal transport is explained in terms of the kinematics of the interface between atlantic and mediterranean waters a mean transport of 0 21 sv due to subinertial and tidal flows carries atlantic water through the strait compensated by an approximately equal return in the lower layer

an experimental study of the upwelling response of stratified reservoirs to surface shear stress an experimental study of the dynamic response of non rotating stratified reservoirs to the application of surface shear stresses is presented the experiments were made using two layered and linear stratifications a moving belt was used to supply a shear stress to the fluid detailed measurements of the density field show that upwelling occurs at all values of the wedderburn number w rather than only occurring when w 1 differences between the two layer theory and the present observations are attributed to the fact that the experimental stratifications were continuous rather than layered shearflow dispersion is observed to be an important mechanism for distributing the effects of localized upwelling over the entire length of the mixed layer a model of mixed layer deepening based on upwelling and shearflow dispersion is presented and is compared to the observations of this and other experimental studies

three layer view of transports and hydraulics in the strait of gibraltar a three dimensional model study 1 the ability of a three dimensional sigma coordinate free surface model to reproduce the interfacial layer between the atlantic and mediterranean waters in the strait of gibraltar is verified the model uses a coastal following curvilinear orthogonal grid that includes the gulf of cadiz and the alboran sea with very high resolution in the strait less than 500 m it is forced by imposing the two major semidiurnal tidal components m2 and s2 along the atlantic and the mediterranean open boundaries the main experiment covers an entire spring neap cycle fortnight period results show that the model is able to reproduce a thick interfacial layer that carries a substantial fraction of the total transport the interfacial thickness is strongly modulated by tidal forcing that enhances the vertical exchanges between layers in order to take into account the thick interfacial layer a three layer approximation is used to study the hydraulics results show that the exchange regime is intermittently maximal due to cyclically controlled regions over camarinal sill and within tarifa narrows the analysis has been repeated in a two layer framework to evaluate the impact of the introduction of an interfacial layer on the calculation of hydraulics in the two layer approximation there is not any controlled region extending on the whole cross section so the exchange is always submaximal concluding the two layer approximation underestimates hydraulic controls in the strait respect to the three layer approach

tidal computations for morecambe bay summary a numerical finite difference technique for computing tides in areas of shoaling water has been established and used to calculate the m2 tide in morecambe bay an essential feature of the method is the representation of drying banks which appear as the water level falls on the ebb and which are re submerged on the flood as the level rises again since the non linear advective acceleration should be important in shallow water schemes for incorporating these terms are examined and the influence of advection on the tide assessed in a series of tests the open sea tides applied to the outer boundary of the numerical bay model were taken first from existing tidal charts then from calculations with a larger scale encompassing model and finally from observations of current in each case the computed tidal elevations within the bay turned out to be in satisfactory agreement with those obtained from observations however it is significant that in the first case inaccuracy of the input tidal distribution deduced from charts gives rise to erroneous currents near the boundary

a numerical model of internal wave refraction in the strait of gibraltar abstract the relative importance of depth cross strait interface till and horizontal current shear upon the wave front shapes of internal waves in the strait of gibraltar are examined by means of a numerical refraction model results from the model are also compared with observations of wave front shape made in the eastern strait using a shore based marine radar which imaged the surface effects of the waves the model is linear and assumes a time independent two layer fluid in which the depth of each layer may be any function of position the effect of current on the propagation is modeled by the addition of a term in the dispersion relation that increments the phase speed by an amount equal to the current component in the propagation direction at the depth of maximum internal wave displacement this current may also be any function of position the results suggest that in the strait of gibraltar interface tilt and horizontal shear both have a strong influence on the refraction whereas total depth is

differential cooling drives large scale convective circulation in lake tanganyika field data scaling analysis and three dimensional numerical experiments demonstrate that the dominant large scale circulation pattern during the southeast trade winds in lake tanganyika east africa is a downwind flow in the metalimnion with a returning southward upwind flow in the upper region of the epilimnion this is in the opposite direction to all prior literature available on the lake which has assumed that the southeast trade winds must drive a northward surface flow due to momentum induced by the wind the gradient in wind speed and humidity along the length of the lake and the resulting gradient in heat exchange with the atmosphere result in warm water accumulating at the northern end of the lake the north basin of lake tanganyika thus acts as a heat sink while the south basin is a net source of heat to the atmosphere the resulting variation in buoyancy flux is strong enough to drive a convective flow in the upper layer in the opposite direction to the wind a large scale convective circulation results in an annual exchange of 6 2 1012 m3 of water and an annual net transport of heat averaging 0 2 terawatt from the northern to the southern region of the lake currents in the upper 100 m layer with a mean flow of 0 2 sverdrup and flow speed of 0 08 m s 1 can be described by the superposition of a parallel convective flow with the internal seiche motion

the influence of sediment load on tidal dynamics a case study cadiz bay a two dimensional non linear finite difference hydrodynamic model was applied to cadiz bay to study the influence of sediment load on tidal dynamics the sediment load effect is represented parametrically as a dependence of the drag coefficient on the relative settling velocity the ratio of the settling velocity of suspended particles to the bottom friction velocity and the relative friction velocity the ratio of the bottom friction velocity to its critical value at which sediment particles begin to go into suspension this dependence is derived from a solution of the equations describing the vertical structure of the sediment stratified bottom logarithmic layer a comparison of the model predictions with and without allowance for the sediment load effect shows that the latter is responsible for small local changes of the amplitude and phase of tidal elevation and the maximum depth averaged tidal velocity the result counting in favour of the conventional approach whereby the influence of sediment load on tidal dynamics is considered to be negligible however it is apparent after close inspection of the model predictions that the sediment load effect tends to enhance the time space variability of the tidal characteristics in particular it results in an increase in the maximum depth averaged velocity and a decrease in the drag coefficient for the periods of flood and ebb currents thus reducing the shear bottom stress and the tidal energy dissipation by about half

the boiling water phenomena at camarinal sill the strait of gibraltar abstract a detailed description of high amplitude steady topographic internal waves recorded at camarinal sill during a survey on the r v investigador is presented these internal waves are generated during the maximum outflow westward stage of the tidal current and remain over the sill for more than 4 h until the outflow slackens then being released towards the mediterranean their amplitudes are comparable to the well known internal bore of camarinal sill occurring during maximum outflow during spring tides however they respond to a different physical origin and their spatial features are also quite different in fact the favourable hydraulic conditions for the generation of steady waves over the sill inhibit the internal bore generation and vice versa analysis of the observations suggests that steady internal waves are the result of a resonant response of the stratified fluid over the sill to the forcing of the flow on the across sill topography an important consequence of the steady internal waves with clear biological implications is the significant mixing phenomena that are induced mixing is enabled by an enhancement of the shear at the trough together with a significant induced vertical advection

a petrologic chemical and isotopic study of monument draw and comparison with other acapulcoites evidence for formation by incipient partial melting abstract we have conducted petrologic chemical and isotopic studies of acapulcoites acapulco monument draw yamato 74063 alh a77081 alh a81261 alh a81315 alh 78230 alh a81187 and alh 84190 in an attempt to constrain their genesis acapulcoites have distinctly different oxygen isotopic compositions than silicate inclusions in iab and iiicd irons winonaites and ureilites and thus formed on a different parent body oxygen isotopic compositions which are slightly heterogeneous within the group overlap with lodranites indicating a likely origin on a common parent body these groups can be distinguished on the basis of mafic silicate grain size all acapulcoites have mafic silicate compositions intermediate between e and h chondrites roughly chondritic mineralogies achondritic equigranular textures micrometer to centimeter sized veins of fe ni fes which cross cut silicate phases rapid metallographic cooling rates at 600 400 c 103 105 c myr and trapped noble gas abundances comparable to type 3 4 ordinary chondrites they exhibit variable mafic silicate zoning abundance of fe ni fes veins ree abundances and patterns and possibly cosmic ray exposure ages 5 7 ma momument draw and yamato 74063 retain rare relict chondrules phosphates are associated with fe ni fes veins or form separate veins in monument draw and acapulco heating and cooling of acapulcoites occurred very early in the history of the solar system as evidenced by the 39ar 40ar ages of 4 51 ga these ages appear distinctly younger than the likely formation time for acapulco of 4 557 ga but are older than analogous 39ar 40ar ages for most chondrites acapulcoites formed from a precursor chondrite which differs from known chondrites in mineral and oxygen isotopic compositions heating to 950 1000 c resulted in melting at the fe ni fes cotectic but silicates did not melt silicate textures resulted from extensive solid state recrystallization heating was by noncollisional sources e g 26al electromagnetic induction despite uncertainties owing to a lack of data acapulcoites may have experienced a three stage thermal history of slow cooling at high temperature rapid cooling at intermediate temperatures and slow cooling at low temperatures possibly resulting from breakup and gravitational reassembly of the parent body the complex thermal history is also reflected in disequilibrium ree abundances one or at most two impact events 7 ma and possibly 5 ma ago are consistent with the cosmic ray exposure ages of all four acapulcoites for which cosmogenic noble gas data exist

a study of internal wave propagation in the strait of gibraltar using shore based marine radar images abstract an x band marine radar on windmill hill gibraltar was used to monitor the propagation of internal waves in the strait of gibraltar during march april and june 1986 surface roughness features of the waves were observed out to a range of 15 km from photographic images of the radar screen the positions of image features were measured manually using a digitizing table and these data were then computer processed to give wave arrival times and phase speeds during most tidal cycles an internal undular bore generated at camarinal sill was observed to pass eastward a waves these occurred throughout the experiments although less frequently near neap tides the time of travel to gibraltar varied significantly so that the bore arrived between four and nine hours after high water arrival times were found to be closely related to the amplitude of the upper layer tidal current this current had a strong diurnal component which was manifested as an inequality between the arrival times of success

wind and buoyancy driven horizontal exchange in shallow embayments of a tropical reservoir lake argyle western australia many factors including depth vegetation density wind and gyres may act to influence the littoral exchange in a water body but very few studies have investigated the interaction between more than two of these factors at any time to investigate these controls on horizontal exchange in a large tropical reservoir we conducted a 9 d intensive field experiment in lake argyle western australia the experiment began with a 7 d cooling period that generated water in the shallows of the reservoir embayments that was persistently cooler than the interior this led to an underflow of dense water that moved from the lake boundary toward the center of the reservoir a three dimensional hydrodynamic model elcom was able to adequately reproduce this thermal structure and was used to demonstrate its sensitivity to wind sheltering effects and submerged macrophyte presence further analysis of the predictions indicated that when the ratio of the shear and buoyancy force averaged over 6 h b6h and 6 h averaged wind speed in the direction of the embayment u6h was 1 greater than 0 5 ms 1 and 4 5 ms 1 respectively the exchange was dominated by a topographic gyre formation 2 when 0 1 b6h 0 5 and 2 4 ms 1 u6h 4 5 ms 1 the resulting circulation was a combination of differential cooling flows and a topographic gyre circulation and 3 when u6h fell below 2 4 ms 1 purely buoyancy driven flow occurred but only if the buoyancy forces across the embayments were an order of magnitude greater than wind induced velocity shear

vertical structure of tidal currents over the camarinal sill at the strait of gibraltar an acoustic doppler current profiler adcp is used to estimate the vertical structure of currents at the camarinal sill at the strait of gibraltar the most important tidal components namely m2 s2 o1 k1 m4 m3 msf and mm are examined the semidiurnal tides are the most energetic with along strait speeds of up to 120 cm s 1 for m2 and 48 cm s 1 for s2 the diurnal tides have along strait speeds of 15 35 cm s 1 m4 increases from 4 cm s 1 in the upper layer to 15 cm s 1 below 220 m the fortnightly component msf reaches speeds in excess of 30 cm s 1 at the upper 120 m the vertical structure of the tidal signal is complicated differs between semidiurnal and diurnal tides and varies between spring and neap tides plausible assumptions about the density profile and the bottom friction yield tidal components similar to those observed the effects of friction are described by the use of an analytical constant eddy viscosity model and the effects of stratification by estimating the internal modes at each tidal frequency the major part of the tidal signal is barotropic with the internal tide modes and the frictional effects contributing 20 cm s 1 for the semidiurnal tides nevertheless individual modes reach 50 cm s 1 the internal tides for the diurnal components are 5 cm s 1 with individual modes reaching 8 cm s 1 the vertical profile of msf can be explained by the use of internal modes reaching amplitudes of 40 cm s 1 the fitting of the internal wave modes to the vertical profiles also explains the observed phase change with depth of the semimajor axis the comparison with pressure difference across the strait is consistent with previous studies this report is the first on the adcp time series on the camarinal sill

simulation of the semidiurnal tides in the strait of gibraltar the m2 and s2 surface tides in the strait of gibraltar are simulated using a two dimensional nonlinear boundary fitted coordinate model with a nominal resolution of 0 5 km good agreement is achieved with tide gauge and bottom pressure observations as well as with current measurements made during the gibraltar experiment the cotidal charts and the maps of tidal current ellipse parameters which have been constructed on the basis of the model results reproduce all of the known features of the spatial structure of the m2 and s2 tidal waves these results also show that a 90 phase difference between tidal velocity and elevation is detected in much of the strait of gibraltar thus suggesting a small mean tidal energy flux through the strait the model results give evidence of the general direction for the m2 and s2 net tidal energy fluxes to the west this finding is consistent with an observed southwestern tidal phase propagation and remains qualitatively unchanged when varying the strait s geometry as well as boundary and astronomical forcings

interbasin exchange and mixing in the hypolimnion of a large lake the role of long internal waves we conducted a combined field and numerical study of the effects of episodic internal kelvin type waves on bottom boundary turbulence and the exchange of a passive tracer between the main basin and the side basin of a large lake lake geneva high resolution measurements of the vertical current structure near the entrance of the 25 km long and 70 m deep side basin revealed that hypolimnetic current speeds frequently exceed 0 2 m s 21 leading to a turbulent bottom boundary layer several meters thick with logarithmic current profiles and to a region of strong shear across the thermocline the time series and vertical structure of the currents were reproduced by a threedimensional numerical model of the lake it was demonstrated that after episodes of strong winds from the northeast and southwest exchange flows due to internal kelvin waves were able to temporarily half or double the hypolimnetic volume of the side basin leading to an irreversible exchange of up to 40 of the hypolimnetic water in the side basin of lake geneva within only a few wave cycles with the help of the numerical model it was shown that the key mechanisms of exchange are horizontal dispersion by resolved scales and to a small extent shear dispersion in the bottom boundary layer it is suggested that bottom boundary layer turbulence and the interbasin exchange can explain the structural differences in the oxygen profiles observed in the side basin and the main basin respectively

linking watershed disturbance with nearshore sedimentation and the shell beds of lake tanganyika mahale mountains tanzania extensive deposits of shell rich sediments in the nearshore environment of lake tanganyika africa form a unique habitat for a diverse group of endemic gastropods crustaceans fish and sponges among other organisms anthropogenic alteration of the lake s hinterland from deforestation fires and agriculture threaten this crucial habitat through sediment pollution of the littoral environment this study examines the sedimentology of these nearshore shell beds along a study area north of the mahale mountains tanzania to test whether varying levels of watershed disturbance determined by analysis of satellite imagery within three moderately sized watersheds 100 km2 results in sedimentological differences offshore the results show that shell beds adjacent to disturbed watersheds lagosa and rukoma contain more fine grained sediment compared to shell beds offshore of the undisturbed watershed katumbi widespread burning clearance and land cultivation and rains of the wet season are causal factors that result in significant differences in fine grained sedimentation larger sediment plumes offshore and increased shoreline progradation in areas most affected by sedimentation sponges are largely absent and shell coverage which is a measure of fish nesting capabilities is lower the discovery of a small population of live neothauma tanganyicense offshore of the katumbi river suggests that the presence of shell bed forming gastropods in front of a relatively low disturbance watershed and their conspicuous absence offshore of the other two watersheds could be related to recent watershed disturbance and increased sedimentation the increased fine grained sedimentation and limited shell production by the reduced modern gastropod populations may signal that the shell bed ecosystem of lake tanganyika is at risk

analysis of short period internal waves using wave induced surface displacement a three dimensional model approach in algeciras bay and the strait of gibraltar 1 a three dimensional nonlinear high resolution sigma coordinate hydrodynamic model was applied to study the sea surface manifestation of short period internal waves measured in algeciras bay and the strait of gibraltar model results reproduce the tidally induced generation of the internal bore over the camarinal sill and its disintegration into wave trains as it moves eastward while propagating along the strait of gibraltar toward the mediterranean sea the wave trains partly penetrate into algeciras bay with typical oscillation periods of 20 and 40 min the modeled wave induced surface train structures are compared with satellite images and in situ observational data obtained from two pressure sensors located inside the bay results demonstrate that wave induced sea surface displacements are indicators of the presence of internal waves and may be used in the context of the internal wave analysis when surface oscillations are captured with sufficient precision

origin of intraseasonal variability in lake tanganyika 1 intraseasonal thermocline oscillations in lake tanganyika are analysed using observations near mpulungu and simple analytical numerical models in order to understand their origin the region around the lake is characterised by strong and persistent southeast winds during the four months dry season lasting from may to august the associated wind stress causes the tilting of the thermocline which oscillates for the whole year the wavelet transform spectra of temperature at 30 m depth of the lake near mpulungu indicates the presence of various scales of motion localised in frequency and time the dominant modes of thermocline oscillations are intraseasonal variability with 3 4 weeks periods similar results are obtained from a reduced gravity model with various wind forcing including the observed forcing and a simple analytical solution in addition the model results indicates that the dominant mode of oscillation exhibits one node only from the study it is inferred that the free modes of oscillations of the lake are in resonance with wind pulses

numerical modeling of an estuary a comprehensive skill assessment i numerical simulations of the hudson river estuary using a terrain following three dimensional model regional ocean modeling system roms are compared with an extensive set of time series and spatially resolved measurements over a 43 day period with large variations in tidal forcing and river discharge the model is particularly effective at reproducing the observed temporal variations in both the salinity and current structure including tidal spring neap and river discharge induced variability large observed variations in stratification between neap and spring tides are captured qualitatively and quantitatively by the model the observed structure and variations of the longitudinal salinity gradient are also well reproduced the most notable discrepancy between the model and the data is in the vertical salinity structure while the surface to bottom salinity difference is well reproduced the stratification in the model tends to extend all the way to the water surface whereas the observations indicate a distinct pycnocline and a surface mixed layer because the southern boundary condition is located near the mouth the estuary the salinity within the domain is particularly sensitive to the specification of salinity at the boundary a boundary condition for the horizontal salinity gradient based on the local value of salinity is developed to incorporate physical processes beyond the open boundary not resolved by the model model results are sensitive to the specification of the bottom roughness length and vertical stability functions insofar as they influence the intensity of vertical mixing the results only varied slightly between different turbulence closure methods of k e k and k kl

the flow of atlantic water through the strait of gibraltar abstract we describe and analyze observations of the water exchange through the strait of gibraltar the primary observations were taken in april 1986 and included data from moored recording instruments at four locations in the strait together with an intensive survey by ship these measurements included extensive ctd profiling acoustic doppler current profiling numerous profiles from expendable instruments and imaging using a high frequency echo sounder the analysis focuses on the internal hydraulics of the strait and in particular the presence of hydraulic controls and their influence on the exchange our observations during april 1986 show that the maximal exchange condition in which a subcritical flow is bounded by supercritical flow at both ends of the strait did occur although with various subtleties not explicitly incorporated in our previous theoretical developments the atlantic water moving east along the surface encounters a control at tarifa narrows in the eastern part of the strait the mediterranean water passes through controls both at camarinal sill and at spartel sill further west at our westernmost mooring at spartel the outflowing mediterranean water is continuously supercritical with little tidal variability the control at camarinal sill is periodically lost due to tidal action and reappears on a reverse flow during a falling spring tide tangier basin bounded by camarinal and spartel sills acts as an internal reservoir for the outflowing mediterranean water the interface rising and falling through each tidal cycle during spring tides the tangier basin interface rises high enough to flood the control at camarinl sill and produces a reverse flow of the lower layer when control is lost at camarinal sill a travelling bore is released this bore may modify the location of the control acting on the surface layer in tarifa narrows the atlantic water enters the alboran sea as a jet which may have a well defined northern boundary where it separates from the european coast this boundary may be identified by the presence of a slick although the internal jump west of camarinal sill is almost certainly an area of intense mixing frictional effects do not appear to be dominant in the subcritical portion of the flow