average seasonal variation of the atlantic equatorial currents from historical ship drifts abstract surface currents in the tropical atlantic were studied using historical ship drift data these are the only available data capable of resolving the long term seasonal fluctuations of currents over a broad geographical region the north equatorial countercurrent was found to extend as a continuous eastward flowing current across the atlantic in the band 5 8 n from july to december the maximum average eastward velocity was 30 cm s 1 during july september east of 20 w the countercurrent continued throughout the year west of 20 w and from january to june the countercurrent disappeared and westward currents were observed average westward velocity reached 20 cm 1 in april and may centered near 40 w the annual cycle in surface velocity 20 cm s 1 amplitude agrees closely with that of geostrophic velocity inferred from variations in thermocline depth across the counter current jets centered near 2 n and 4 s separated by a minimum near 1 s the northern jet has a large 14 cm s 1 semiannual v

the zonal currents and transports at 35 w in the tropical atlantic the total of 13 existing cross equatorial shipboard current profiling sections taken during the woce period between 1990 and 2002 along 35 w are used to determine the mean meridional structure of the zonal top to bottom circulation between the brazilian coast near 5 s and 5 n and to estimate mean transports of the individual identified shallow intermediate and deep current branches one of the results is that on the equator a mean westward equatorial intermediate current below the equatorial undercurrent exists

mean meridional currents in the central and eastern equatorial atlantic ship based acoustic doppler current profiler adcp velocity measurements collected by several major field programs in the tropical atlantic are averaged and combined with estimates of the mean near surface velocity derived from drifters and argo float surface drifts adcp d to describe the mean cross equatorial and vertical structure of the meridional currents along 23 w and 10 w data from moored adcps and fixed depth current meters a satellite derived velocity product and a global ocean reanalysis were additionally used to evaluate the mean adcp d meridional velocity the dominant circulation features in the long term mean adcp d meridional velocity in the upper 100 m are the tropical cells tcs located approximately between 5 s and 5 n with near surface poleward flow and subsurface equatorward flow that is stronger and shallower in the northern cell compared to the southern cell the thickness of the surface limb of the tcs decreases and the northern cell is found to shift further south of the equator from the central to eastern tropical atlantic analysis of two season means estimated from the ship based adcp near surface drift and moored velocity data as well as the simulated fields indicates that the maximum poleward velocity in the surface limb of the tcs intensifies during december may along 23 w largely due to seasonal compensation between the geostrophic and ageostrophic or wind driven components of the meridional velocity whereas the maximum equatorward flow in the subsurface limb of the northern cell intensifies during june november along both 23 w and 10 w due to the seasonality of the geostrophic meridional velocity

circulation in the central equatorial atlantic mean and intraseasonal to seasonal variability 1 the zonal equatorial circulation of the upper 700 m in the central tropical atlantic is studied based on 11 cross equatorial ship sections taken at 23 29 w during 1999 to 2005 and on data from a pair of moored acoustic doppler current profilers deployed on the equator at 23 w during february 2004 to may 2005 the observations on the equator reveal the existence of two mean westward cores of the equatorial intermediate current below the equatorial undercurrent in contrast to the 2002 moored observations at the same position the intraseasonal variability during the mooring period is dominated by zonal instead of meridional velocity fluctuations

thermostads and circulation in the upper layer of the atlantic ocean abstract there are three major permanent thermostads with roughly the same potential densities in the upper layer of the atlantic ocean one is the thermostad of the 13 c water in the equatorial atlantic the original type of the 13 c water is formed in the thermocline in the eastern sector of the south atlantic subtropical gyre by vertical mixing of dense low salinity water from the winter outcrop farther south and overlying less dense high salinity water there might also be a lateral contribution of relatively high salinity water from the indian ocean the original 13 c water thus formed is transported northwestward along the northern edge of the subtropical gyre and fed into the north brazilian current which flows equatorward along the coast of brazil in the region of the equator the equatorial undercurrent and the subsurface north and south equatorial countercurrents branch off from the north brazilian current and carry the 13 c water eastward to the thermostad region vertical mixing does not explain the development of the thermostad but is found to be essential in determining the ultimate characteristics of the 13 c water the other two thermostads are those of the 18 c water in the sargasso sea and the subantarctic mode water in the western south atlantic unlike the 13 c water both of these mode waters are formed as thermostads in the surface layer by winter convection but vertical mixing in the subtropical gyres may play a role in determining their characteristics all the three thermostads appear to be required to balance the system of flows in opposing directions

the equatorial undercurrent in the central atlantic and its relation to tropical atlantic variability seasonal to interannual variations of the equatorial undercurrent euc in the central atlantic at 23 w are studied using shipboard observation taken during the period 1999 2011 as well as moored velocity time series covering the period may 2005 june 2011 the seasonal variations are dominated by an annual harmonic of the euc transport and the euc core depth both at maximum during september and a semiannual harmonic of the euc core velocity maximum during april and september substantial interannual variability during the period of moored observation included anomalous cold warm equatorial atlantic cold tongue events during 2005 2008 the easterly winds in the western equatorial atlantic during boreal spring that represent the preconditioning of cold warm events were strong weak during 2005 2008 and associated with strong weak boreal summer euc transport the anomalous year 2009 was instead associated with weak preconditioning and smallest euc transport on record from january to july but during august coldest sst anomalies in the eastern equatorial atlantic were observed the interannual variations of the euc are discussed with respect to recently described variability of the tropical atlantic ocean

merging of convective clouds cloud initiation bridging and subsequent growth abstract this study examines the growth of radar echoes from the time of their initiation to several minutes after they have merged to ascertain what factors are important in determining the frequency of merging events the manner in which echo cores join together and the effect of merging on subsequent echo core growth three dimensional radar reflectivity data were examined for two convective periods 25 july 1986 and 26 august 1986 it was found that echoes that merged were initially taller and slightly larger than those that dissipated without merging suggesting they were more vigorous and thus more likely to grow and join with another echo while meteorological conditions on the two days were quite different 25 of the mergers occurred between young echoes 65 occurred between a young echo and a parent storm and 10 occurred between echo cores from two different systems previous case studies have indicated that merging is accomplished through differential storm motion or through the growth of

doppler radar wind spectra of supercell tornadoes abstract a storm intercept team from the university of oklahoma using the los alamos national laboratory portable continuous wave frequency modulated continuous wave 3 cm doppler radar collected close range data at and below cloud base in six supercell tornadoes in the southern plains during the springs of 1990 and 1991 data collection and analysis techniques are described wind spectra from five weak to strong tornadoes and from one violent tornado are presented and discussed in conjunction with simultaneous boresighted video documentation photogrammetric analysis and damage surveys maximum doppler wind speeds of 55 105 m s 1 were found in five of the tornadoes wind speeds as high as 120 125 m s were found in a large tornado during an outbreak on 26 april 1991 these may be the highest wind speeds ever measured by doppler radar and the first radar measurements of f 5 intensity wind speeds the variation in the spectrum across the 26 april 1991 tornado is presented standard and mobile soundings

isopycnal and diapycnal circulation of the upper equatorial atlantic ocean in 1983 1984 isopycnal circulation and diapycnal processes in the equatorial atlantic ocean are investigated using eight cruises from november 1982 to july 1984 an eastward decrease of the transport of the equatorial undercurrent is observed averaging 18 2 106 m3 s 1 at 35 w to 10 2 106 m3 s 1 at 4 w there is also a meridional convergence at the undercurrent level and a larger meridional divergence in the surface layer of 15 106 m3 s 1 between 35 w and 4 w in the eastern equatorial atlantic the undercurrent core density as well as the salinity above the undercurrent experience a large seasonal cycle during the season where the eastern atlantic thermocline is closest to the surface and surface waters are coldest the current peaks in denser waters and subsurface salinity maximum are weaker an analysis of the salinity on isopycnal surfaces indicates that there is significant diapycnal mixing in the upper thermocline in the eastern atlantic the upper thermocline vertical heat diffusivity coefficient scaled over 1 of latitude varies between 3 cm2 s 1 during the upwelling season and nearly 0 early in the year this seasonal mixing is an important element of undercurrent dynamics the turbulent heat flux at the base of the surface layer averages 50 w m 2 between 1 5 n and 1 5 s the surface layer heat budget implies an average oceanic gain of 60 w m 2 between 1 5 n and 1 5 s which could result from exchanges with the atmosphere the heat flux and fresh water seasonal cycles needed to close the budgets are not realistic however the eastern equatorial atlantic thermocline seasonal upwelling is associated with a large inflow into the surface layer over the two years this flow averaging 11 12 106 m3 s 1 originates from the upper thermocline and diapycnal transports near the core of the undercurrent or below are found to be small below the core of the undercurrent we also find that mixing is not intense for instance at 26 5 in the upper part of the thermostad vertical heat diffusivity is only 0 6 cm2 s 1 assuming that mixing takes place over 1 of latitude the uncertainties on these budgets are however large and assumptions on the dynamics that were used could not be checked

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

upper equatorial atlantic variability during 2002 and 2005 associated with equatorial kelvin waves on the basis of moored 35 degrees w 0 degrees and 23 degrees w 0 degrees and satellite observations we contrast the upper equatorial atlantic variability during 2002 with those during 2005 inspection of the interannual boreal summer cold tongue variability revealed a warm cold event in 2002 2005 with relaxed intensified winds in the west and the equatorial undercurrent euc embedded in a shallower deeper thermocline at 23 degrees w during both years 2002 and 2005 equatorial kelvin waves are present in moored observations as 20 degrees c isotherm depth anomalies and dynamic height anomalies basin wide sea surface height anomalies are used to derive an equatorial kelvin wave mode the time evolution of this mode represents the basis for a regression analysis to investigate related oceanic variability compared to the exceptionally strong wave activity in 2002 equatorial kelvin waves were generally weaker during 2005 the main effect of equatorial kelvin waves on zonal velocity anomalies at 23 degrees w 0 degrees is evident well below the euc core their direct influence on cold tongue sea surface temperature is small but they are found to affect the equatorial thermocline slope prior to the cold tongue onset in 2002 2005 the presence of equatorial kelvin waves is associated with a flattened steeper thermocline slope that is crucial for the shallowing deepening of the euc core at 23 degrees w and that might precondition the development of the warm cold event

the role of forcing in cell morphology and evolution within midlatitude squall lines abstract this study assesses the role of mesoscale forcing on cell morphology and early evolution of midlatitude squall lines the forcing chosen was a cold front simulated to frontal collapse to produce a specific set of thermodynamic profiles at the leading edge of the front use of a realistic balanced and persistent forced state allowed a unique evaluation of its importance in thunderstorm evolution compared with a traditional homogeneous environment without forcing three dimensional squall lines were modeled with and without the front present in low and high bulk richardson number environments the forced convection evolved in significantly different ways than their isolated unforced counterparts in low shear conditions the line of isolated convective cells split with the adjacent split cells interfering destructively with neighboring cells in the line with forcing present differences in anticyclonic cell intensity and propagation prevented this interaction from occurring leading to longe

observations of the sea breeze front during cape part i single doppler satellite and cloud photogrammetry analysis abstract an analysis of two sea breeze events on 6 august an onshore flow event and 12 august an offshore flow event 1991 is presented using single doppler observations satellite images and cloud pictures collected during the convection and precipitation electrification cape experiment documentation of the alongfrontal variability at the leading edge of the sea breeze circulation is presented for the first time the horizontal structure of the front was strongly modulated by the near perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 august these intersection points also appeared to be preferred locations for cloud development along the front horizontal convective rolls were also documented on 6 august however their orientation was nearly parallel to the sea breeze front as a result extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection po

predicting head cut erosion and migration in concentrated flows typical of upland areas 1 soil erosion due to head cut development and migration can devastate agricultural lands yet current prediction technology does not address this important erosion process here an analytical model of this erosional phenomenon is presented realistic physically based approximations to the laws governing mass momentum and energy transfer in the neighborhood of the scour hole result in closed form predictive algorithms for the magnitude of the plunge pool erosion and the rate of head cut migration the model introduces a special treatment of nonventilated overfall conditions is limited to homogeneous unbounded soil layers and is validated by available experimental measurements

seasonal modes of surface cooling in the gulf of guinea a numerical simulation of the tropical atlantic ocean indicates that surface cooling in upwelling zones of the gulf of guinea is mostly due to vertical mixing at the seasonal scale the spatial structure and the time variability of the northern and southern branches of the south equatorial current sec and of the guinea current are correlated with the timing and distribution of turbulent heat fluxes in the gulf of guinea through modulation of the velocity shear at the subsurface these surface currents control the vertical turbulent exchanges bringing cold and nutrient rich waters to the surface this mechanism explains the seasonality and spatial distribution of surface chlorophyll concentrations better than the generally accepted hypothesis that thermocline movements control the nutrient flux the position of the southern sec explains why the cold tongue and high chlorophyll concentrations extend from the equator to 4 s in the southeastern part of the basin

the structure and classification of numerically simulated convective storms in directionally varying wind shears abstract using a three dimensional numerical cloud model we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes as with a previous series of experiments using unidirectional wind shear profiles weisman and klemp the current results evince a spectrum of storm types ranging from short lived single cells at low shears multicells at intermediate shears to supercells at high shears with a clockwise curved hodograph the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank an analysis of the dynamic structure of the various cells reveals that the quasi steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system we use this feature along with other related storm characteristics such as updraft rotation to propose a dynamically based storm classification scheme following

variability in horizontal current velocities in the central and eastern equatorial atlantic in 2002 1 near surface current velocity data from the pilot research moored array in the tropical atlantic at 23 w in the year 2002 were re examined and compared to simultaneous unpublished velocity data at 10 w near the surface strong 7 day period fluctuations were observed in the zonal velocity component and in temperature records at 23 w the large temperature fluctuations near the thermocline depth suggest a vertical velocity component at the same 7 day period the 7 day period signals occurred in june july simultaneously with the seasonal upwelling the meridional velocity component presented similar spectral contents at 23 w and at 10 w two distinguishable quasiperiodic signals were observed in the meridional component at both locations the first signal occurred in the spring had a periodicity of about 14 days was subsurface intensified and had the characteristics of a mixed rossby gravity wave the second signal occurred three times in the year once in winter 2001 2002 another in summer and another during fall 2002 had a periodicity of about 20 days was strongest close to the surface and was the result of the passage of tropical instability waves tiws satellite sea surface temperature sst data show the same periodicities as the meridional velocity records and present a spatial structure in accordance with the tiw sst signature the summer of 2002 showed different tiw sst patterns depending upon location suggesting the existence of different types of tiws overall variability in the current meter data for the year 2002 was mainly due to current instabilities except around boreal spring where intraseasonal variability in surface winds was strongest

diagnosing the annual cycle of the equatorial undercurrent in the atlantic ocean from a general circulation model abstract ten year long output series from a general circulation model forced by daily realistic winds are used to analyze the annual cycle of the equatorial undercurrent euc in the atlantic ocean two well defined transport maxima are found one present during boreal summer and autumn in the central part of the basin is generally recognized and regarded as a near equilibrium response to the equatorial easterly trades that culminate in this period another one most pronounced near the western boundary occurs in april may when the trades relax this second maximum is less patent in the observations but concomitant signals in previously published analyses of the north brazil current and surface velocity seasonal variations might be indirect manifestations of its reality because this intensification appears at periods when the boundary between the tropical and equatorial gyres nears the equator the authors relate its existence to wind stress curl variations at subequatorial latitudes a link between

the mean flow field of the tropical atlantic ocean the mean horizontal sow eld of the tropical atlantic ocean is described between 20in and 20is from observations and literature results for three layers of the upper ocean tropical surface water central water and antarctic intermediate water compared to the subtropical gyres the tropical circulation shows several zonal current and countercurrent bands of smaller meridional and vertical extent the wind driven ekman layer in the upper tens of meters of the ocean masks at some places the sow structure of the tropical surface water layer as is the case for the angola gyre in the eastern tropical south atlantic although there are regions with a strong seasonal cycle of the tropical surface water circulation such as the north equatorial countercurrent large regions of the tropics do not show a signi cant seasonal cycle in the central water layer below the eastward north and south equatorial undercurrents appear imbedded in the westward sowing south equatorial current the antarcic intermediate water layer contains several zonal current bands south of 3in but only weak sow exists north of 3in the sparse available data suggest that the equatorial intermediate current as well as the southern and northern intermediate countercurrents extend zonally across the entire equatorial basin due to the convergence of northern and southern water masses the western tropical atlantic north of the equator is an important site for the mixture of water masses but more work is needed to better understand the role of the various zonal under and countercurrents in cross equatorial water mass transfer 1999 elsevier science ltd all rights reserved

response of the surface tropical atlantic ocean to wind forcing abstract we use 10 years of satellite data sea level pressure surface winds and absolute dynamic topography adt together with argo inferred monthly mean values of near surface velocity and water transport to examine how the tropical system of near surface zonal currents responds to wind forcing the data is analyzed using complex hilbert empirical orthogonal functions confirming that most of the variance has annual periodicity with maximum amplitudes in the region spanned by the seasonal displacement of the inter tropical convergence zone itcz the adt mirrors the shape of the upper isopycnals hence becoming a good indicator of the amount of water stored in the upper ocean within about 3 from the equator where the coriolis force is small there is year long meridional ekman transport divergence that would lead to the eastward transport of the equatorial undercurrent and its northern and southern branches beyond 3 of latitude and at least as far as 20 the convergence of the ekman transport generally causes a poleward positive adt gradient which sustains the westward south equatorial current sec the sole exception occurs in summer between 8 n and 12 n when an ekman transport divergence develops and depletes de amount of surface water resulting in an adt ridge valley system which reverses the adt gradient and drives the eastward north equatorial countercurrent necc at latitudes 4 9 n in late fall divergence ceases and the necc drains the adt ridge so the adt gradient again becomes positive and the sec reappears the seasonal evolution of a tilted itcz controls the surface water fluxes the wind induced transports set the surface divergence convergence which then drive the adt and through the adt gradients create the geostrophic jets that close the water balance

zonal structure and seasonal variability of the atlantic equatorial undercurrent simultaneous mooring arrays were maintained along the path of the equatorial undercurrent euc at three longitudes 23 w 10 w and 0 e from october 2007 to june 2011 as part of the clivar tropical atlantic climate experiment the measurements allow for the first time a description of the seasonal cycle and interannual variability of the euc across the atlantic basin the mean transport of the euc at 23 w is 14 3 0 6 sv decreasing to 12 1 0 9 and 9 4 0 6 sv at 10 w and 0 e respectively the euc shows a changing seasonal cycle across the basin at 23 w the strongest euc transport occurs in boreal fall in association with maximum easterly wind stress at 10 w the euc transport shows a semiannual cycle with a maximum in boreal spring and fall while at 0 e the euc has a single spring maximum at all locations the euc core exhibits a similar seasonal vertical migration with shallowest core depths occurring in boreal spring and deepest core depths in boreal fall the maximum core intensity occurs in boreal spring all across the basin when the euc is shallow during the annual wind relaxation the weakest euc core intensity occurs during the boreal summer cold tongue phase especially in the eastern part of the basin at both 23 w and 10 w a deep extension of the euc occurs in boreal summer which increases the transport in the lower thermocline and partially offsets the weaker upper euc transport during boreal summer no clear linkage could be established between the interannual variability of the euc in the eastern part of the basin and the intensity of the summer cold tongue despite evidence for such a linkage in the western part of the basin

seasonal and regional variability of upper ocean diapycnal heat flux in the atlantic cold tongue sst variability within the atlantic cold tongue act region is of climatic relevance for the surrounding continents a multi cruise data set of microstructure observations is used to infer regional as well as seasonal variability of upper ocean mixing and diapycnal heat flux within the act region the variability in mixing intensity is related to the variability in large scale background conditions which were additionally observed during the cruises the observations indicate fundamental differences in background conditions in terms of shear and stratification below the mixed layer ml for the western and eastern equatorial act region causing critical froude numbers fr to be more frequently observed in the western equatorial act the distribution of critical fr occurrence below the ml reflects the regional and seasonal variability of mixing intensity turbulent dissipation rates e at the equator 2 n 2 s are strongly increased in the upper thermocline compared to off equatorial locations in addition e is elevated in the western equatorial act compared to the east from may to november whereas boreal summer appears as the season of highest mixing intensities throughout the equatorial act region coinciding with act development diapycnal heat fluxes at the base of the ml in the western equatorial act region inferred from e and stratification range from a maximum of 90 wm 2 in boreal summer to 55 wm 2 in september and 40 wm 2 in november in the eastern equatorial act region maximum values of about 25 wm 2 were estimated during boreal summer reducing to about 5 wm 2 towards the end of the year outside the equatorial region inferred diapycnal heat fluxes are comparably low rarely exceeding 10 wm 2 integrating the obtained heat flux estimates in the ml heat budget at 10 w on the equator accentuates the diapycnal heat flux as the largest ml cooling term during boreal summer and early autumn in the western equatorial act elevated meridional velocity shear in the upper thermocline contributes to the enhanced diapycnal heat flux within this region during boreal summer and autumn the elevated meridional velocity shear appears to be associated with intra seasonal wave activity

rill erosion in natural and disturbed forests 2 modeling approaches 1 as forest management scenarios become more complex the ability to more accurately predict erosion from those scenarios becomes more important in this second part of a two part study we report model parameters based on 66 simulated runoff experiments in two disturbed forests in the northwestern u s the 5 disturbance classes were natural 10 month old and 2 week old low soil burn severity high soil burn severity and logging skid trails in these environments the erosion rates were clearly detachment limited and the rill erodibility parameters calculated from four hydraulic variables increased by orders of magnitude the soil shear stress based erodibility parameter kr was 1 5 10 6 s m 1in the natural plots 2 0 10 4 s m 1 in the high soil burn severity plots and 1 7 10 3 s m 1 in the skid trail plots kr values for the low soil burn severity plots had negative sign the erodibility value for the skid trail plots fell within ranges reported for tilled agricultural fields and also for forest roads the kr values decreased as erosion occurred in the plots and therefore should not be a constant parameter the stream power produced the largest r2 value 0 41 when hydraulic predictors and the sediment flux were log transformed but none of the four hydraulic variables soil shear stress stream power unit stream power and unit length shear force explained much of the variability in sediment flux rates across the five levels of disturbance when evaluated in the linear form of the erosion models under consideration

on the global estimates of geostrophic and ekman surface currents surface currents in oceanic environments are of crucial importance because they transport momentum heat salt and tracers over large distances that regulate both the local and large scale climate conditions and because they contribute to the lagrangian displacement of floating material ranging from living resources to marine pollution in recent decades the understanding of surface currents has benefited from the opportunity of observing sea level and wind stress via satellite derived measurements combining these parameters into geostrophic and wind driven components provides an estimate of surface currents with a quarter degree horizontal resolution at a global scale and at a daily time scale in the present study improvements are made on the consideration of the time dependence of the main parameters implied in the determination of the ekman wind driven component and on the treatment of the equatorial singularity the resulting geostrophic and ekman current observatory gekco estimates were validated with independent observations from both lagrangian and eulerian perspectives the statistics of comparison were significant over the globe for the 2000 2008 period the only exception was the estimation of meridional current along the equator which requires further developments of the dynamic model and probably more accurate measurements applications using our gekco surface current estimates in cross disciplinary approaches from physical oceanography to marine ecology are presented and discussed

do deterministic sediment detachment and transport equations adequately represent the process interactions in eroding rills an experimental field study abstract this paper tackles two main questions by linking observations determination of hydraulic parameters and measurement of sediment transport with the formulae used in soil erosion models first do constant shear stress values in different rills with constant soil parameters result in the same soil detachment values secondly deterministic soil erosion models make the assumption that there is a relationship often further assumed to be linear between shear stress and soil detachment is this suitable for representing real erosion process combinations in natural rills following most process based deterministic soil erosion models derived hydraulic and erosion parameters should be similar however the results from the different experiments showed clear differences in sediment concentration transport rates and other measured as well as calculated values in contrast to our experimental results a model simulation would produce erosion parameters with low variations represented by the relative measurement error and the empirical variation coefficient this reveals the general problems of using process based deterministic models for erosion in shallow rills while soil erosion models simulate the processes resulting from the shear forces of flowing water on the soil surface other processes like side wall failure headcut retreat and plunge pool dynamics are not taken into account our results suggested that these other processes may contribute substantially to rill erosion processes the results of this study strongly suggested that the model concept of most physical based soil erosion models is inadequate for modeling rill erosion processes measured total rill erosion rates were the sum of erosion rates caused by a combination of different soil erosion processes with different spatial and temporal distribution this combination cannot be described by a single equation

spatial patterns of convection in south florida abstract although they are a fairly consistent feature the sea breeze and lake breeze convergence lines and the associated convection over south florida during the summer may vary considerably from one day to the next daily radar maps indicate a few basic recurring patterns analyses of radiosonde data show significant differences corresponding to the different patterns in the local thermodynamic parameters most notably the mixing ratio changes in the synoptic scale wind field correspond closely to changes in the observed radar patterns and the local thermodynamic conditions explanation of the formation and development of the different patterns of convection is given in terms of the complex interaction between the regional synoptic peninsular and local scale circulations

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

why were sea surface temperatures so different in the eastern equatorial atlantic in june 2005 and 2006 abstract a comparison of june 2005 and june 2006 sea surface temperatures in the eastern equatorial atlantic exhibits large variability in the properties of the equatorial cold tongue with far colder temperatures in 2005 than in 2006 this difference is found to result mainly from a time shift in the development of the cold tongue between the two years easterlies were observed to be stronger in the western tropical atlantic in april may 2005 than in april may 2006 and these winds favorably preconditioned oceanic subsurface conditions in the eastern atlantic however it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid may 2005 over a broad region extending from 20 w to the african coast and from 6 s to the equator this particular event underscores the ability of local intraseasonal wind stress variability in the gulf of guinea to initiate the cold tongue seaso

new insights on the upper layer circulation north of the gulf of guinea the oceanic circulation in the upper layers of the north of the gulf of guinea is analyzed as inferred from in situ observations and numerical simulations this particular region in spite of the presence of a coastal upwelling and its impact on resources and regional climate is still poorly documented cruises carried out in the framework of different international programs e g egee amma pirata allowed to show the existence of an eastward flowing undercurrent found under the guinea current named the guinea undercurrent guc numerical results from high resolution simulation allowed the description of the seasonal variability of this current it appears that the guc is stronger in spring and reverses westward in august september we also depict the fate and the sources of the guc based on selected trajectories from numerical particle tracking the simulated trajectories reveal i a preferred route of the guc along 4 n from cape palmas to cape three points and following the coast east of cape three points ii strong recirculations in the most eastern part of the gulf of guinea and off cape palmas including warm and salty waters of the south equatorial current iv a weak inflow from northern latitudes through a subsurface current flowing southward along the west african coast in addition lagrangian experiments show that the guc is not an extension of the north equatorial undercurrent and confirm that this current does not penetrate into the gulf of guinea this article is protected by copyright all rights reserved

cell merger potential in multicell thunderstorms of weakly sheared environments cell separation distance versus planetary boundary layer depth using high resolution three dimensional numerical experiments this paper shows that the cell separation distance scales as 0 75 times the planetary boundary layer pbl depth for successful cell mergers between constructively interacting cells within multicell thunderstorms this boundary layer scaling is determined from several simulations of convective cell pairs with a fixed pbl depth and is shown to be valid for other sensitivity simulations with larger pbl depths this research establishes a robust and quantitative relation between prestorm ambient conditions and cell merger potential useful for research efforts on the multifaceted cell merger process of multicell thunderstorms the weakly sheared ambient prestorm conditions of the 9 august 1991 convection and precipitation electrification experiment cape multicell thunderstorm are used to initialize the cell pair simulations since ambient wind and wind shear are assumed to be zero only simple cell mergers defined in this study as those between cell updraft cores joined but not overlapping in the convective stage are shown to be possible the coarse resolution simulations of stalker suggest that ambient wind shear may be necessary for forced cell mergers defined in this study as those in which the initial updraft cores are found apart the scenarios of overlapping initial updraft cores for cell merger are considered physically invalid in this study

soil erosion by surface water flow on a stony semiarid hillslope soil erosion on hillslopes occurs by processes of soil splash from raindrop impacts and sediment entrainment by surface water flows this study investigates the process of soil erosion by surface water flow on a stony soil in a semiarid environment a field experimental method was developed whereby erosion by concentrated flow could be measured in predefined flow areas without disturbing the soil surface the method allowed for measurements in this study of flow erosion at a much wider range of slopes 2 6 to 30 1 per cent and unit discharge rates 0 0007 to 0 007 m2 s 1 than have been previously feasible flow velocities were correlated to discharge and hydraulic radius but not to slope the lack of correlation between velocity and slope might have been due to the greater rock cover on the steeper slopes which caused the surface to be hydraulically rougher and thus counteract the expected effect of slope on flow velocity the detachment data illustrated limitations in applying a linear hydraulic shear stress model over the entire range of the data collected flow detachment rates were better correlated to a power function of either shear stress r2 0 51 or stream power r2 0 59 published in 1999 by john wiley sons ltd

response of the equatorial atlantic ocean to the 1983 1984 wind from the programme francais ocean et climat dans l atlantique equatorial cruise data set variations of the zonal pressure gradient zpg and the equatorial undercurrent euc are described in the equatorial atlantic ocean between 34 30 w and 6 30 e as observed during eight pairs of programme francais ocean et climate dans l atlantique equatorial cruises between october 1982 and august 1984 in 1982 and 1983 the annual cycle of weak trades early in the year followed by comparatively strong and sustained trades were relatively normal in the boreal winter of 1984 the extent of the collapse of the trade winds was exceptional and consequences were observed in the upper layers of the ocean along the equator the 20 c isotherm depth representative of the thermocline depth was unusually deep along the equator in the gulf of guinea a near zero value of the zpg was observed from the greenwich meridian westward an eastward surface current separated from the euc by a layer of reduced velocity was observed over much the same fetch west of 10 w the maximum velocity of the euc showed very little seasonal variation usually between 70 and 80 cm s 1 except in the boreal fall of 1983 when it exceeded 100 cm s 1 during this period in both 1983 and 1984 the euc transport at 23 w reached 25 106 m3 s 1 or twice its annual average which derived not only from the increase in speed but also for a deepening of the current to below 100 m it was also the season when the zpg was strongest and has a similar amplitude at both the surface and 50 dbar relative to 500 dbar the seasonal variations of the euc in the gulf of guinea were even more pronounced during the boreal summer and fall the current weakened while the local zpg reversed direction compared to the west the stronger penetration of the euc into the gulf occurred in the alternate seasons when there was no local reversal in the eastward acting zpg force the large changes in the oceanic and atmospheric circulation during the 1983 1984 period are hypothesized to be related to the suppression of the coastal period are hypothesized to be related to the suppression of the coastal benguela upwelling along the namibia coast this atlantic equivalent to el nino occurred 1 year after the exceptionally strong event in the pacific

a linear analysis of equatorial atlantic ocean thermocline variability abstract observed variations in the atlantic ocean s equatorial thermocline are compared at four locations with simulations using an analytical reduced gravity model the comparison shows the essential features of the seasonal wind forced thermocline response to be accounted for by a linear superposition of equatorial long waves evolving basinwide tending to bring the zonal pressure gradient into balance with the wind stress a frequency response function is derived whose properties provide a basis for discussing the large scale features of the equatorial atlantic ocean s seasonal cycle for example its evolution along the equator the maximum upwelling region observed in the gulf of guinea and the secondary upwelling season also observed there clarification is also given to the issue of remote versus local forcing for these features

small scale cyclicity in alpine proglacial fluvial sedimentation abstract this paper compares contemporary sedimentation in an alpine proglacial braided river with deposits from inactive areas of the braidplain this approach allows confidence in determining the key characteristics of alpine proglacial braided river deposits the diagnostic features are 1 repeated coarse fine couplets in fine grained sediment

development of gullies on the landscape a model of headcut retreat resulting from plunge pool erosion 1 head advance due to plunge pool erosion is a common process in gullies incising resistant soils a model of headcut retreat resulting from plunge pool erosion is developed and implemented in the channel hillslope integrated landscape development child model an existing three dimensional landscape evolution modeling framework the model estimates horizontal headcut retreat as a function of discharge height of the headcut upstream slope and relevant land surface and soil properties for soil erosion we analyze the sensitivity of headcut retreat to flow discharge upstream slope and surface roughness and headcut height child simulations indicate that headcut retreat is most significant in zones with either gentle slopes or large headcut heights model parameters have contrasting effects on the retreat rates depending on the size and depth of the pool beneath the headcut and upstream flow hydraulics making the process difficult to predict as a function of topographic thresholds and simple geomorphic transport laws

observations of the sea breeze front during cape part ii dual doppler and aircraft analysis abstract the three dimensional kinematic structures of offshore and onshore flow sea breeze fronts observed during the cape experiment are shown using high resolution dual doppler and aircraft data the fronts interact with horizontal convective rolls hcrs that develop within the convective boundary layer nearly perpendicular intersections between the hcrs and sea breeze front were observed during the offshore flow case close to the front the hcr axes were tilted upward and lifted by the frontal updrafts consequently a deeper updraft was created at the intersection points providing additional impetus for cloud development furthermore clouds forming at periodic intervals along the ncrs intensified as they propagated over the front during the onshore flow case the hcr orientation was nearly parallel to the front extended sections of the front merged with the hcrs this process strengthened the front and is explained as the merger of like sign vortices associated with both the front and hcrs

a further study of cumulus interactions and mergers three dimensional simulations with trajectory analyses abstract a total of nine three dimensional experiments are made to study cloud interaction and merging under the influence of different imposed conditions large scale lifting forcing environmental wind shear and cloud microphysical processes are the three parameters to be varied the basic design of the study is to generate several convective clouds randomly inside the model domain and then to observe and analyze the interactions and merging between the simulated clouds the locations as well as the intensities of simulated clouds while they interact with each other are not predetermined a two dimensional version of the model has been used to investigate the effects upon merging produced by varying large scale conditions with a gate dataset in this study we continue studying the cloud interactions and merging problems through using a fully three dimensional model and the same dataset ten merged systems involved precipitating clouds are identified in this numerical study eight mergers involve two

measurements in a braided river chute and lobe 1 flow pattern sediment transport and channel change this paper and its companion ashworth et al this issue discuss measurements of channel change and associated flow and sediment transport processes in a representative chute and bar reach within a proglacial gravel bed river the sunwapta river in the canadian rockies during a week in which water discharge through the reach increased then decreased a sediment wave passed into and partly along the reach at first the chute aggraded then sediment was eroded from the chute and deposited in a prograding lobe to one side of the original bar head between two distributaries measurements of velocity shear stress and gravel transport rate revealed day to day changes in the divergent pattern of flow and sediment transport the off center location of the lobe reflected an initial asymmetry in the pattern of flow and sediment transport but as the lobe grew the flow and transport gradually switched away to the other distributary width averaged bed load transport rates do not agree well with average rates inferred from bar head deposition or from volumes and spatial patterns of scour and fill the indirect estimates are considered more reliable than those based on direct sampling for necessarily brief durations by the end of the study the new lobe had almost accreted onto the original bar head supporting the idea that most braid bars are of compound origin temporal and spatial patterns in grain size distributions of the bed bed load and deposited sediment are discussed in the companion paper

observational evidence for flow between the subtropical and tropical atlantic the atlantic subtropical cells abstract this study determines the mean pathways and volume transports in the pycnocline and surface layer for water flowing between the subtropical and tropical atlantic ocean using potential vorticity salinity geostrophic flow maps on isopycnal surfaces and surface drifter velocities in both hemispheres subducted salinity maximum waters flow into the tropics in the pycnocline along both interior and western boundary pathways the north atlantic ventilating trajectories are confined to densities between about 23 2 and 26 0 and only about 2 sv sv 106 m3 s 1 of water reaches the tropics through the interior pathway whereas the western boundary contributes about 3 sv to the equatorward thermocline flow flow on shallower surfaces of this density range originates from the central atlantic near 40 w between 12 and 16 n whereas flow on the deeper surfaces originates from near 20 w just off the coast of africa at higher latitudes the pathways skirt around the potential vorticity barrier located

seasonal variability of the equatorial undercurrent at 10 w as inferred from recent in situ observations 1 eighteen cross equatorial shipboard current profiling sections along 10 w with conductivity temperature depth measurements taken between 1997 and 2007 are used to analyze the mean meridional structure and the seasonal variability of the equatorial undercurrent euc at 10 w our analysis suggests a seasonal cycle for the euc transport at 10 w with a well defined annual harmonic and some indication of a semiannual component with a first maximum in january and a second stronger maximum from june to september the mean euc transport at 10 w is estimated to be 12 1 sv and compared to earlier estimates farther in the west at 35 w 20 9 sv and 26 w 13 8 sv the eastward flow transport exhibits a strong variability at 10 w with a total range of transports from 7 1 sv to more than 31 7 sv the seasonal amplitude of the eastward flow variability is 8 9 sv from a minimum of 8 2 sv in november to 25 9 sv in august the eastward flows within the thermocline are divided in two parts a permanent part within the 24 5 26 5 isopycnal layer with a semiannual cycle known as the euc and a nonpermanent part in the deep thermocline beneath 26 5 isopycnal associated with a strong eastward transport up to 15 sv during the boreal summer that is not observed during the rest of the year the current at the equator in the deep thermocline is even westward during boreal fall the disappearance of the salinity core of the euc during the boreal summer associated with the upwelling of the hydrological structure at 10 w reveals that the saline subtropical waters carried by the euc within the thermocline no longer flow into the gulf of guinea during the boreal summer our data also show the presence of the south equatorial undercurrent seuc at 10 w in the gulf of guinea with a strong latitudinal and depth variability throughout the year the mean seuc at 10 w is centered near 5 s and is farther south than observed at 35 w and 26 w suggesting its poleward shift from west to east

rill hydraulics on a semiarid hillslope southern arizona seventy field experiments were conducted in seven rills located on a semiarid rangeland hillslope underlain by gravelly soils at walnut gulch arizona the rills which are characterized by wide shallow cross sections and gravel covered beds have mean at a station hydraulic geometry exponents of b 0 33 f 0 34 and m 0 33 although the differences between these values and typical values of b 0 30 f 0 40 and m 0 30 for cropland rills are not statistically significant they are thought to be real as cropland rills often have more rectangular cross sections and steeper sides than the rangeland rills under study for rills formed in silty loamy soils govers developed an empirical relation between mean flow velocity and discharge emphasizing the generality of this relation he suggested that it may be used as a simple means of routing runoff through rills he also noted that this relation appeared to be unaffected by either slope or soil materials the present data represent rills underlain by coarser and somewhat more varied gravel rich soils these data do not conform to govers relation and a multiple regression analysis reveals that slope and soil materials either directly or indirectly through bed roughness exert almost as much influence on flow velocity as does discharge three alternative methods are developed for predicting flow velocity in the rills under study all three methods give good results with the largest root mean square deviation being 3 115 cm s 1

some overlooked features of tropical atlantic climate leading to a new nino like phenomenon abstract the atlantic nino an equatorial zonal mode akin to the pacific el nino southern oscillation enso is phase locked to boreal summer when the equatorial easterly winds intensify and the thermocline shoals in the gulf of guinea a suite of satellite and in situ observations reveals a new mode of tropical atlantic variability that displays many characteristics of the zonal mode but instead peaks in november december nd this new mode is found to be statistically independent from both the atlantic nino in the preceding summer and the pacific enso the origin of this nd zonal mode lies in an overlooked aspect of the seasonal cycle in the equatorial atlantic in november the equatorial easterly winds intensify for the second time increasing upwelling and lifting the thermocline in the gulf of guinea an analysis of high resolution climatological data shows that these dynamical changes induce a noticeable sst cooling in the central equatorial atlantic the shoaling thermocline and increased upwelli

using multiple passive tracers to identify the importance of the north brazil undercurrent for atlantic cold tongue variability the use of multiple passive tracers in a dynamical ocean model is presented as a method to study water mass origins the multiple passive tracer method is successfully applied to determine the origin regions of the atlantic cold tongue act using 31 passive tracers at the time of the act minimum in sea surface temperature 38 3 of the surface water in the central equatorial atlantic is found to originate in the western side of the basin with only 11 2 originating in the east conversely for the act surface cooling off the west coast of africa only 2 1 is from the west of the atlantic basin with 71 7 from the east the region of the north brazil undercurrent is found to be the most important origin region for the central atlantic act region for a 6 8 month lead time using the met office en4 observational dataset of ocean temperatures correlations are found between january temperatures in the north brazil undercurrent region and both the act mean temperature and act spatial extent in july august significant at the 0 95 level this suggests that the 6 month lead time predictability of the act may be increased by improved knowledge of the north brazil undercurrent temperature results from previous studies suggest that such increased predictability of the act could potentially help improve predictions of the west african monsoon

stochastic sediment transport in soil erosion a stochastic model governing the downslope transport of a sediment particle during an erosion event is described particles alternate between resting on the soil bed and being transported by the overland flow probability densities and moments are constructed for the distribution of a particle s position at a given time and also for time of passage to a given location we show that a suitable averaging of the stochastic particle motions in our model gives rise to the deterministic erosion differential of hairsine and rose hairsine p rose c 1991 soil sci soc am j 55 2 320 324 the model generalized h einstein s stochastic model der geschiebebetrieb als wahrscheinlichkeitsproblem verlag rascher zurich 1937 for bedload transport in streams

variability of the atlantic off equatorial eastward currents during 1993 2010 using a synthetic method 1 we have developed validated and applied a synthetic method to monitor the off equatorial eastward currents in the central tropical atlantic this method combines high density expendable bathythermograph xbt temperature data along the ax08 transect with altimetric sea level anomalies slas to estimate dynamic height fields from which the mean properties of the north equatorial countercurrent necc the north equatorial undercurrent neuc and the south equatorial undercurrent seuc and their variability can be estimated on seasonal to interannual timescales on seasonal to interannual timescales the synthetic method is well suited for reconstructions of the necc variability reproduces the variability of the neuc with considerable skill and less efficiently describes variations of the seuc which is located in a region of low sla variability a positive correlation is found between interannual variations of the necc transport and two indices based on an interhemispheric sea surface temperature sst gradient and southeasterly wind stress in the central tropical atlantic the neuc is correlated on interannual timescales with ssts and meridional wind stress in the gulf of guinea and zonal equatorial wind stress this study shows that both altimetry and xbt data can be effectively combined for near real time inference of the dynamic and thermodynamic properties of the tropical atlantic current system

coupling between the atlantic cold tongue and the west african monsoon in boreal spring and summer 1 the formation of the atlantic cold tongue act is the dominant seasonal sea surface temperature signal in the eastern equatorial atlantic eea a comprehensive analysis of variability in its spatial extent temperature and onset is presented then the physical mechanisms which initiate act onset as well as the feedbacks from the act to the maritime boundary layer and how the act influences the onset of the west african monsoon wam are discussed we argue that in the eea the air sea coupling between the act and wam occurs in two phases from march to mid june the act results from the intensification of the southeastern trades associated with the st helena anticyclone steering of surface winds by the basin shape of the eea imparts optimal wind stress for generating the maximum upwelling south of the equator during the second phase mid june august wind speeds north of the equator increase as a result of the northward progression of the intensifying trades and as a result of significant surface heat flux gradients produced by the differential cooling between the act and the tropical waters circulating in the gulf of guinea gg it is anticipated that the atmospheric divergence induced at low levels north of the equator reduces convection over the gg and that increased northward winds shift convection over land correlations between the act and the wam onset dates over the last 26 years 1982 2007 measure as much as 0 8 this suggests that the act plays a key role in the wam onset

cell development and merger in an illinois thunderstorm observed by doppler radar abstract a reflectivity and triple doppler radar study of the development of several cells and their successive union within a nonsevere thunderstorm is presented two characteristic separations were found between the newly formed cells and the parent thunderstorm with the closer cells forming in response to the collapse of an active cell and the more distant cells forming in a previously existing storm modified area characterized by mesoscale convergence and rain cooled air the manner in which these cells evolved appeared to be partially related to differences in the environment in which they formed as suggested by peterson the cells that formed closer to the main storm resembled the weakly evolving cells of foote and frank the updraft of the weakly evolving cell analyzed here merged with the updraft in a cell in the main storm as one cell was decreasing in intensity and the other was increasing later in the life cycle of the storm two cells which initially formed further away from the main st

on the evolution of the thermocline and subthermocline eastward currents in the equatorial atlantic 1 eastward currents in the thermocline and subthermocline layers of the tropical atlantic ocean are described using observations obtained during two boreal summer cruises covering the whole equatorial basin the analysis focuses on the eastward evolution of zonal currents in the thermocline and sub thermocline the equatorial undercurrent is not found close to the african coast in boreal summer 2000 maybe because of interannual variability as observed in the pacific the south and north equatorial undercurrents are shown to shift poleward from west to east and the two cruises indicate that the north equatorial undercurrent does not enter the gulf of guinea the south and north intermediate countercurrents are variable with longitude and contrarily to the observations in the eastern equatorial pacific are not found in the east of the guinea basin

intraseasonal mixed layer heat budget in the equatorial atlantic during the cold tongue development in 2006 1 estimating the mixed layer heat budget is a key issue for understanding the cold tongue development in the eastern equatorial atlantic a high resolution ocean regional model is used to diagnose the mixed layer heat budget online during the egee 3 experiment from may to august 2006 the heat budget shows the major role of the horizontal advection and turbulent mixing in the mixed layer temperature balance in the cold tongue the surface net heat flux and entrainment processes play a minor role the equatorial cooling is mainly induced by low frequency advection which is balanced by high frequency zonal and meridional advections the high frequency advections are organized in patterns along the northern edge of the cold tongue where they are associated with strong sea surface temperature gradients and well developed tropical instability waves in the western atlantic special attention is paid to the wind energy flux which controls horizontal advection and turbulent mixing we suggest that the wind energy flux drives the vertical velocity which in turn adjusts the mixed layer depth its stratification and the vertical shear of the horizontal current although vertical advection is not essential in providing cold water in the atlantic cold tongue it is shown that the vertical velocity plays a central role in preconditioning the mixed layer and maximizes the turbulent mixing