fe mn cu and zn in interstitial water of near shore sediments abstract the concentrations of fe mn cu and zn were studied in the pore water of reduced muddy sediments off the gironde estuary in the area representing the deposit of the river load fe and mn concentrations were also determined in the intertidal zone in the marennes oleron bay all samples were formed of an oxidized surface layer overlying a reduced sediment the mn data from the pore water support the idea of the recycling of mn by dissolution mobilization migration and reprecipitation fe did not present a depth gradient but surface sediment showed fe depletion relative to the underlying sediment which may represent the boundary between the oxidized surface layer and the reduced underlying sediment where iron can be easily mobilized in the pore water mn and fe concentrations were higher in the pore water off the gironde estuary sediments than in those of the intertidal zone the reasons may be the degree of reducing conditions caused by bacterial activity the tide cycle with alternative oxidations and reduction of the surface sediments of the tidal flats and the association of metal fe or mn in an easily soluble phase under reducing conditions e g the effect of different complexing agents in the two regions or the equilibrium of metals with different mineral phases the cu distribution shows no trend with depth cu concentrations in the different intervals may depend upon the diagenetic reaction and quality of organic matter sulphide concentration and complex formation the zinc concentrations decreased with depth which supports the idea of sulphide precipitation and or decrease of the potential mobilized zinc with depth

synoptic wave perturbations and convective systems over equatorial africa abstract spectral analysis of the outgoing longwave radiation olr time series over equatorial africa reveals large oscillations of the convection with periods of between 3 and 6 days in march and april when the intertropical convergence zone itcz migrates northward and crosses equatorial africa this periodic behavior is most pronounced with a marked peak at 5 6 days robust horizontal and vertical patterns consistent with a convectively coupled kelvin wave can be extracted by a simple composite technique based only on the phase of the convective oscillations over equatorial africa the composite reveals differences between continental and adjacent oceanic regions over the continent the stronger oscillation of the convection is associated with larger temperature and moisture anomalies near the surface suggesting an influence of diabatic processes on the amplitude of the perturbations some convective events over equatorial africa are triggered by waves propagating eastward over the equatorial a

air sea interaction effects in the lower troposphere across the north wall of the gulf stream abstract significant atmospheric and oceanographic differences have often been observed in the regions lying on either side of the north wall of the gulf stream satellite visible range sensors have revealed changes in reflectivity across the gulf stream believed to be due to changes in sea state that are related to these atmospheric and oceanic differences in addition lines of low level cloud paralleling the edge of the gulf stream have been observed to persist in satellite imagery for several days at a time anomalous radar propagation also has been noted to occur in these same regions in an attempt to document the interrelation of these phenomena the lower atmosphere over the north wall of the gulf stream was studied on two separate occasions sea state photography and measurements of sea surface temperature air temperature dew point temperature and atmospheric pressure were made by an instrumented rp 3a aircraft flying at various altitudes from the gulf stream across the north wall and onto

the influence of sea surface temperature on surface wind in the eastern equatorial pacific weekly to monthly variability abstract temporal correlations between near equatorial surface wind and sea surface temperatures sst at 11 w in the eastern pacific ocean are investigated using data from an array of moored sensors between 5 n and 5 s the signature of tropical instability waves with periods of 20 30 days is apparent in time series of sst and both the meridional and zonal wind components results indicate the existence of a band of pronounced horizontal divergence in the surface wind field associated with the large meridional sst gradient equatorial front normally located just north of the equator perturbations of the equatorial front by the instability waves induce fluctuations in the overlying winds evidence of the air sea coupling is stronger in time series of the meridional gradients of wind and sst than between time series of the variables themselves the meridional differencing serves as a high pass filter in the space domain which removes planetary scale wind fluctuations that are unrelated to the local sst

on the relationship between sst gradients boundary layer winds and convergence over the tropical oceans a linear mixed layer model that skillfully reproduces observed surface winds and convergence over the tropicaloceans is used to examine the relative influenceof boundarylayer and free tropospheric processes on the distribution of climatological surface winds and convergence the semiempirical model assumes a subcloud layer momentum force balance between pressure gradients coriolis acceleration linearized friction and downward momentum mixing and it utilizes boundary conditions from the 40 yr ecmwf re analysis era 40 observed pressure gradients are linearly decomposed into boundary layer defined as the region below 850 hpa and free tropospheric components and the surface winds and convergence associated with these components are computed results show that surface zonal winds are predominantly associated with a combination of free tropospheric pressure gradients and downward momentum mixing whereas the distribution of convergence is primarily due to boundary layer temperature gradients which are closely related to sst gradients the authors conclude that the climatological distribution of boundary layer convergence is primarily a function of the pattern of sst gradients and is better regarded as a cause rather than a consequence of deep convection

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

modeling and analysis of ageostrophic circulation over the azores oceanic front during the semaphore experiment abstract in the conventional quasigeostrophic qg form of the equation developed by hoskins et al the unique forcing of vertical velocity is the geostrophic deformation as the qg or even the semigeostrophic sg hypotheses are not adapted to study the frontal dynamics in the atmospheric boundary layer this paper proposes a generalized expression of the hoskins et al form of the vertical velocity two thermal and three dynamical sources of the vertical velocity are identified these forcings allow for identification of each of the physical processes acting simultaneously on the ageostrophic circulation in the boundary layer this new form of the equation is used to explain wind increase in the atmospheric boundary layer over the warm waters of the sea surface temperature sst front observed during a fair anticyclonic day of the semaphore experiment 1993 and simulated with a nonhydrostatic mesoscale atmospheric model since the sst gradients are weak of the order of 1 5 c 100 km 1 the surfac

midlatitude wind stress sea surface temperature coupling in the vicinity of oceanic fronts the influences of strong gradients in sea surface temperature on near surface cross front winds are explored in a series of idealized numerical modeling experiments the atmospheric model is the naval research laboratory coupled ocean atmosphere mesoscale prediction system coamps model which is fully coupled to the regional ocean modeling system roms ocean model a series of idealized two dimensional model calculations is carried out in which the wind blows from the warm to cold side or the cold to warm side of an initially prescribed ocean front the evolution of the near surface winds boundary layer and thermal structure is described and the balances in the momentum equation are diagnosed the changes in surface winds across the front are consistent with previous models and observations showing a strong positive correlation with the sea surface temperature and boundary layer thickness the coupling arises mainly as a result of changes in the flux richardson number across the front and the strength of the coupling coefficient grows quadratically with the strength of the cross front geostrophic wind the acceleration of the winds over warm water results primarily from the rapid change in turbulent mixing and the resulting unbalanced coriolis force in the vicinity of the front much of the loss gain of momentum perpendicular to the front in the upper and lower boundary layer results from acceleration deceleration of the flow parallel to the front via the coriolis term this mechanism is different from the previously suggested processes of downward mixing of momentum and adjustment to the horizontal pressure gradient and is active for flows off the equator with sufficiently strong winds although the main focus of this work is on the midlatitude strong wind regime calculations at low latitudes and with weak winds show that the pressure gradient and turbulent mixing terms dominate the cross front momentum budget consistent with previous work

on the coupling of wind stress and sea surface temperature a simple quasi equilibrium analytical model is used to explore hypotheses related to observed spatial correlations between sea surface temperatures and wind stress on horizontal scales of 50 500 km it is argued that a plausible contributor to the observed correlations is the approximate linear relationship between the surface wind stress and stress boundary layer depth under conditions in which the stress boundary layer has come into approximate equilibrium with steady free atmospheric forcing warmer sea surface temperature is associated with deeper boundary layers and stronger wind stress while colder temperature is associated with shallower boundary layers and weaker wind stress two interpretations of a previous hypothesis involving the downward mixing of horizontal momentum are discussed and it is argued that neither is appropriate for the warm to cold transition or quasi equilibrium conditions while one may be appropriate for the cold to warm transition solutions of a turbulent large eddy simulation numerical model illustrate some of the processes represented in the analytical model a dimensionless ratio a is introduced to measure the relative influence of lateral momentum advection and local surface stress on the boundary layer wind profile it is argued that when a 1 and under conditions in which the thermodynamically induced lateral pressure gradients are small the boundary layer depth effect will dominate lateral advection and control the surface stress

arroyo channel head evolution in a flash flood dominated discontinuous ephemeral stream system we study whether arroyo channel head retreat in dryland discontinuous ephemeral streams is driven by surface runoff seepage erosion mass wasting or some combination of these hydrogeomorphic processes we monitored precipitation overland flow soil moisture and headcut migration over several seasonal cycles at two adjacent rangeland channel heads in southern arizona erosion occurred by headward retreat of vertical to overhanging faces driven dominantly by surface runoff no evidence exists for erosion caused by shallow groundwater related processes even though similar theater headed morphologies are sometimes attributed to seepage erosion by emerging groundwater at our field site vertical variation in soil shear strength influenced the persistence of the characteristic theater head form the dominant processes of erosion included removal of grains and soil aggregates during even very shallow 1 3 cm overland flow events by runoff on vertical to overhanging channel headwalls plunge pool erosion during higher discharge runoff events immediate postrunoff wet mass wasting and minor intra event dry mass wasting on soil tension fractures developing subparallel to the headwall multiple stepwise linear regression indicates that the migration rate is most strongly correlated with flow duration and total precipitation and is poorly correlated with peak flow depth or time integrated flow depth the studied channel heads migrated upslope with a self similar morphologic form under a wide range of hydrological conditions and the most powerful flash floods were not always responsible for the largest changes in landscape form in this environment

response of the atmospheric boundary layer to a mesoscale oceanic eddy in the northeast atlantic fields of air sea turbulent fluxes and bulk variables were derived from satellite sensor data from february to april 2001 over a region of the northeast atlantic where a field experiment programme oce an multidisciplinaire meso echelle pomme was conducted the satellite products are in good agreement with in situ data in terms of heat fluxes sea surface temperature and wind speed the central part of the experimental domain presented a cyclonic eddy in the ocean which corresponded to a cold sea surface temperature sst anomaly winds were weaker within the eddy than outside of it with lower latent and sensible heat loss in order to analyze the relationship between the sst and wind anomalies three numerical experiments were conducted with a regional atmospheric model three 3 month runs of the model were performed using a realistic sst field a smoothed sst field in which the cold sst was not present reference run and an sst field where the cold anomaly was increased by two degrees successively the fields simulated with the realistic sst were consistent with satellite sensor derived observations in particular the weak wind area over the cold sst anomaly was successfully rendered whereas it was not present in the forcing fields taken individually the three runs did not reveal the presence of secondary circulations however anomalous secondary circulations were clearly identified with respect to the reference run the origin of the latter circulations was investigated with the giordani and planton generalization of the sawyer eliassen equations according to our results differential heating induced by the cold sst anomaly mostly altered the vertical wind through the effect of friction and only marginally through pressure gradient forces in the upper part of the boundary layer the wind speed increased decreased over downstream the cold sst we found that stability was the main factor that induced the simulated patterns of the friction term in the diagnostic equations therefore our results show that mesoscale wind patterns were significantly affected by sst gradients through the effect of stability in a region of low oceanic eddy activity

shallow meridional circulations in the tropical atmosphere abstract a shallow meridional circulation smc in the tropical atmosphere features a low level e g 700 hpa flow that is in the opposite direction to the boundary layer monsoon or trade wind flow and is distinct from the meridional flow above representations of the smc in three global reanalyses show both similarities and astonishing discrepancies while the smc over west africa appears to be the strongest it also exists over the eastern atlantic and eastern pacific oceans and over the indian subcontinent with different strength and structure all smcs undergo marked seasonal cycles the smcs are summarized into two types one associated with the marine itcz and the other with the summer monsoon the large scale conditions for these two types of smcs are similar a strong meridional gradient in surface pressure linked to surface temperature distributions and an absence of deep moist convection the processes responsible for these conditions are different for the two types of smcs as are their str

air sea interaction in the gulf of guinea at intraseasonal time scales wind bursts and coastal precipitation in boreal spring the role of air sea interaction in the boreal spring precipitation of the west african monsoon is explored through the wind variability in the gulf of guinea linear regressions are performed in may june during the decade 2000 2009 to investigate the origin and effect of the surface wind strengthening north of the equator it appears that the equatorial sea surface temperature cooling intensifies a surface wind equatorial divergence coastal convergence circulation and generates a cross equatorial pressure gradient which both strengthen the southerlies north of the equator this increases subsidence above the ocean and convection in the northern gulf of guinea in addition an abrupt change is observed in the surface wind pattern in the eastern equatorial atlantic eea between april and july to investigate the transition mechanisms a reference date is defined as the date when the surface wind north of the equator becomes and remains stronger than south of the equator thus the maintenance of strong southerlies north of the equator is linked to a coincident installation of a deep circulation on the whole troposphere and a northward shift of the low atmospheric local circulation the resulting sharp seasonal transition coincides each year with a southeasterly wind burst suggesting that the equatorial sst cooling plays a role in the precipitation along the african coast during boreal spring copyright 2012 royal meteorological society

the global monsoon as seen through the divergent atmospheric circulation a comprehensive description is given of the global monsoon as seen through the large scale overturning in the atmosphere that changes with the seasons and it provides a basis for delimiting the monsoon regions of the world the analysis focuses on the mean annual cycle of the divergent winds and associated vertical motions as given by the monthly mean fields for 1979 93 reanalyses from the national centers for environmental prediction national center for atmospheric research ncep ncar and european centre for medium range weather forecasts ecmwf which are able to reproduce the dominant modes a complex empirical orthogonal function analysis of the divergent circulation brings out two dominant modes with essentially the same vertical structures in all months of the year the first mode which depicts the global monsoon has a simple vertical structure with a maximum in vertical motion at about 400 mb divergence in the upper troposphere that is strongest at 150 mb and decays to zero amplitude above 70 mb and convergence in the lower troposphere with a maximum at 925 mb ecmwf or 850 mb ncep however this mode has a rich three dimensional spatial structure that evolves with the seasons it accounts for 60 of the annual cycle variance of the divergent mass circulation and dominates the hadley circulation as well as three overturning transverse cells these include the pacific walker circulation an americas atlantic walker circulation both of which comprise rising motion in the west and sinking in the east and a transverse cell over asia the middle east north africa and the indian ocean that has rising motion in the east and sinking toward the west these exist year round but migrate and evolve considerably with the seasons and have about a third to half of the mass flux of the peak hadley cell the annual cycle of the two hadley cells reveals peak strength in early february and early august in both reanalyses a second monsoon mode which accounts for 20 of the variance features relatively shallow but vigorous overturning with the maximum vertical velocities near 800 mb outflow from 750 to 350 mb and inflow peaking at 925 mb it is especially strong over africa where the shallow mostly meridional overturning migrates back and forth across the equator with the seasons it influences the middle east has a signature over australia and is also an important component of the overturning in the tropical eastern pacific and atlantic and thus of the convergence zones in these regions the relationship of the global monsoon to the regional monsoons is described over six zonal sectors africa australia asia north america south america and the pacific and atlantic oceans only the two ocean areas do not undergo a seasonal reversal required for monsoons although they have direct overturning cells and they nevertheless participate in the global monsoon through the changes in large scale overturning the regional meridional cross sections highlight the importance of the shallow overturning cell in lower troposphere monsoon activity the steadiness of the overturning circulation is determined by comparing the signal of the seasonal mean vertical motions at 500 mb with the standard deviation of the transient daily variations locations where this signal exceeds 60 of the daily noise correspond closely with the regional centers of the monsoon

seasonal heat balance in the upper 100 m of the equatorial atlantic ocean the variability of sea surface temperature sst in the equatorial atlantic is characterized by strong cooling in may june and a secondary cooling in november december a numerical simulation of the tropical atlantic is used to diagnose the different contributions to the temperature tendencies in the upper ocean right at the equator the coolest temperatures are observed between 20 w and 10 w due to enhanced turbulent heat flux in the center of the basin this results from a strong vertical shear at the upper bound of the equatorial undercurrent euc cooling through vertical mixing exhibits a semiannual cycle with two peaks of comparable intensity during the first peak in may june vertical mixing drives the sst while during the second peak in november december the strong heating due to air sea fluxes leads to much weaker effective cooling than during boreal summer seasonal cooling events are closely linked to the enhancement of the vertical shear just above the core of the euc which appears to be not driven directly by the strength of the euc but by the strength and the direction of the surface current the vertical shear is maximum when the northern branch of the south equatorial current is intense the surface cooling in the eastern equatorial atlantic is not as marked as in the center of the basin mean thermocline and euc rise eastward but a strong stratification caused by the presence of warm and low saline surface waters limits the vertical mixing to the upper 20 m and disconnects the surface from subsurface dynamics

dynamics of the west african westerly jet the west african westerly jet wawj is a low level westerly jet located at 88 118n over the eastern atlanticandthewestafricancoast itis clearlydistinguishedfromthemonsoonwesterlyflowbyitsstructure and dynamics and plays an important role in transporting moisture from the tropical eastern atlantic to sahelian west africa during boreal summer the wawj develops in early june sustains maximum wind speeds of 5 6 m s 21 from late july to early september and weakens and dissipates by mid october in its mature stage the wawj is located within the atlantic itcz it extends from the surface to 700 hpa with maximum speed at 925 hpa the jet has a weak semidiurnal cycle with maxima at 0500 and 1700 local time a momentum budget analysis reveals that the wawj forms when a region of strong westerly acceleration is generatedbythesuperpositionoftheatlanticitczandthewestwardextensionofthecontinentalthermallow the wawj is supergeostrophic at its maximum with zonal pressure gradient and coriolis accelerations both pointingeastward whilemuchofthewawj sseasonalvariationcanbeexplainedbythegeostrophicwind the ageostrophic wind contributes more than 40 of the wind speed during the jet s formation and demise the westward extension of the thermal low is associated with the formation of an offshore low which is related to seasonal warming of the ocean between 68 and 188n along the coast the coastal ssts vary in response to a net surface heating pattern with warming to the north and cooling to the south which is mainly controlled by solar radiative and latent heat fluxes

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

atmospheric response to sea surface temperature in the eastern equatorial atlantic at quasi biweekly time scales the surface wind response to sea surface temperature sst and sst meridional gradient is investigated in the gulf of guinea by using daily observations and re analyses in the 2000 2009 decade with a focus on boreal spring and summer months may to august where quasi biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi biweekly equatorial sst anomalies following a large scale wind acceleration deceleration an equatorial sst cold warm anomaly is created within a few days in order to explain the local atmospheric response to this sst anomaly the two following mechanisms are invoked first a colder warmer ocean decreases increases the vertical stability in the marine atmospheric boundary layer which favours a weaker stronger surface wind and second a negative positive anomaly of sst meridional gradient induces a positive negative anomaly of the sea level pressure meridional gradient which decelerates accelerates the surface wind the first mechanism has an immediate effect in the equatorial belt between 1 s and 1 n and to a lesser extent between 3 s and 1 s whereas the second takes 1 or 2 days to adjust and damps anomalous southeasterlies up to 800 hpa in the low troposphere between 7 s and 1 n through reversed anomalies of meridional sst and pressure gradient this negative feedback leads to weaker stronger winds in the southeastern tropical atlantic which forces the opposite phase of the oscillation within about 1 week around the equator where the amplitude of the oscillation is found to be maximal both mechanisms combine to maximize the wind response to the front fluctuations between the equator and the coast a low level secondary atmospheric circulation takes control of the surface wind acceleration or deceleration around 3 n which reduces the influence of the sst front fluctuations

intraseasonal variability of tropical atlantic sea surface temperature air sea interaction over upwelling fronts tropical atlantic sea surface temperatures ssts maximum intra seasonal variability isv and their interaction with local surface winds are investigated applying statistical analysis to observations and to a recent coupled reanalysis over the 2000 2009 decade five cores of strong isv emerge with standard deviation reaching about 1 c in frontal areas of the three main upwelling systems equatorial angola benguela and senegal mauritania the southern side of the canary upwelling west of 10 w along the equator a 20 60 day peak caused by tropical instability waves is shown to generate surface wind anomalies through the adjustment of the horizontal surface pressure gradient in addition to the modification of near surface atmospheric stratification east of 10 w along the equator an intense biweekly oscillation increases the ocean and atmosphere isv in the two coastal upwelling fronts intraseasonal sst anomalies resemble each other they are shown to be influenced by coastal kelvin waves in addition to large scale wind forcing over the angola benguela upwelling coastal wind bursts controlling the sst isv are associated with anomalously strong pressure patterns related to the madden julian oscillation the st helena anticyclone and the antarctic oscillation in the senegal mauritania upwelling the wind anomalies mainly linked to the azores anticyclone in the southern front during november to may appear to be connected to the saharan heat low in the northern front from june to september in all five regions and as expected for such upwelling regimes vertical oceanic mixing represents the dominant term in the mixed layer heat budget in the equatorial band as found in previous studies horizontal advection is equally important while it appears surprisingly weak in coastal fronts finally a striking result is the general lack of surface wind signal related to the sst isv in the coastal upwellings

on the role of sea surface temperature gradients in forcing low level winds and convergence in the tropics abstract we examine the importance of pressure gradients due to surface temperature gradients to low level p 700 mb flow and convergence in the tropics over time scales 1 month the latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall our approach is to consider a simple one layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically consistent with ecmwf analyzed data the top of the layer is taken at 700 mb the influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer horizontal convergence within the layer is taken up by cumulonimbus mass flux however the development of the cumulonimbus mass flux is associated with a short relaxation time o hr roughly the development time for such convection during this short time horizontal convergence acts to redistribute mass so as to reduce horizontal pressur

on the annual cycle of equatorial upwelling in the central atlantic ocean abstract the annual cycle of the upper ocean s vertical velocity component w on the equator at 28 w is examined by integrating the continuity equation using current meter data from the seasonal response of the equatorial atlantic experiment the annual cycle consists in part of an intense but brief 1 month upwelling season beginning with the onset of strong easterly wind stress in boreal spring this upwelling is followed by weaker downwelling during the summer despite the persistence of strong easterly wind stress the record length averaged w profile shows that maximum upwelling 0 6 10 3 cm s 1 is located slightly above the core of the equatorial undercurrent and downwelling is located below the base of the thermocline the standard deviations are about tenfold the magnitude of the means independent evidence supporting these results are that 1 sea surface temperature sst is related to w during the springtime changes in easterly wind stress with the observed and computed isotherm displacem

the influence of sea surface temperature on surface wind in the eastern equatorial pacific seasonal and interannual variability abstract the climate of the eastern pacific exhibits a pronounced equatorial asymmetry boundary layer air originating in the southern hemisphere trades crosses the equator and flows into the intertropical convergence zone itcz whose southern limit is nearly always located at least 4 to the north of the equator the sea surface temperature sst distribution is characterized by a prominent cold tongue centered 1 s a strong frontal zone centered 2 n and a warm eastward current centered near 5 n the surface wind field exhibits a pronounced horizontal divergence as the air flows northward across the oceanic frontal zone these features vary in strength in response to the annual cycle and the el nino southern oscillation phenomenon the northward cross equatorial surface winds the cold tongue and the frontal zone all tend to be strongest during the cold season july through november during the cold season of the coldest years when the cold tongue is most prominent the cross equatorial flow t

the effects of sst induced surface wind speed and direction gradients on midlatitude surface vorticity and divergence abstract the effects of surface wind speed and direction gradients on midlatitude surface vorticity and divergence fields associated with mesoscale sea surface temperature sst variability having spatial scales of 100 1000 km are investigated using vector wind observations from the seawinds scatterometer on the quick scatterometer quikscat satellite and sst from the advanced microwave scanning radiometer for earth observing system amsr e aqua satellite the wind sst coupling is analyzed over the period june 2002 august 2008 corresponding to the first 6 years of the amsr e mission previous studies have shown that strong wind speed gradients develop in response to persistent mesoscale sst features associated with the kuroshio extension gulf stream south atlantic and agulhas return current regions midlatitude sst fronts also significantly modify surface wind direction the surface wind speed and direction responses to typical sst differences of about 2 4 c are on average about 1 2 m s 1 and 4

variability of the mixed layer heat budget in the eastern equatorial atlantic during 2005 2007 as inferred using argo floats we examine the variability of the sea surface temperatures in the eastern equatorial atlantic during 2005 2007 by using argo profiling floats prediction and research moored array in the atlantic buoys and satellite in situ and atmospheric data sets the eastern equatorial atlantic characterized by shallow mixed layers all yearlong is divided into nine boxes of nearly equal surface area with respect to the dynamics and thermodynamics in this region monthly mixed layer heat budgets are computed in each box from 10 day argo profiles in all the boxes the net surface heat flux is one of the main causes of the seasonal evolution of sea surface temperatures for the 3 studied years the amount of short wave radiation penetrating through the base of the mixed layer and horizontal heat advection may locally contribute to the temperature variability while entrainment has a weaker contribution to balance the heat budget a residual term exists which includes all processes that cannot be calculated with observations as well as the possible errors in the other terms this residual is more intense in the cold tongue and the northern region and exhibits a clear seasonal cycle with minimum negative values in boreal summer and maximum values in winter this residual compares well with available observations of vertical turbulent mixing collected during etude de la circulation oceanique et des echanges ocean atmosphere dans le golfe de guinee campaigns 2005 2007 in the eastern equatorial atlantic when assuming that the residual is mostly associated with vertical turbulent mixing it can be conjectured that turbulent mixing is a significant cooling source in the cold tongue and north of the equator

distribution of certain metals in lithochemical fractions of sediments from the arcachon basin southwest france authigenic mineral formation and pollution abstract metal fractionation was studied in the muddy surface sediments and in a sediment core from the arcachon basin the low mn content of the surface sediments m could be due to strong mn oxide dissolution close to the water sediment interface the intense anoxic sulfidic environment caused by the high organic matter content leads to a rapid pyrite formation and high percentages of fe mn and cu extracted in the organic sulfide fraction cu extracted in this fraction could be partly due to its association with organic matter nevertheless fe mn and ni in this fraction are mostly associated with sulfides the major part of pb and zn is extracted in the oxide carbonate fraction apparently sulfide does not control their mobile fractions the enrichment of zn pb and cu seems to be the result of an anthropogenic flux of mobile fraction of these metals

global observations of large oceanic eddies 1 ten years of sea surface height ssh fields constructed from the merged topex poseidon t p and ers 1 2 altimeter datasets are analyzed to investigate mesoscale variability in the global ocean the higher resolution of the merged dataset reveals that more than 50 of the variability over much of the world ocean is accounted for by eddies with amplitudes of 5 25 cm and diameters of 100 200 km these eddies propagate nearly due west at approximately the phase speed of nondispersive baroclinic rossby waves with preferences for slight poleward and equatorward deflection of cyclonic and anticyclonic eddies respectively the vast majority of the eddies are found to be nonlinear

numerical simulation of boundary layer structure and cross equatorial flow in the eastern pacific abstract recent observations from spaceborne microwave sensors have revealed detailed structure of the surface flow over the equatorial eastern pacific in the boreal fall season a marked acceleration of surface wind across the northern sea surface temperature sst front of the cold tongue is a prominent feature of the regional climate previous studies have attributed the acceleration to the effect of enhanced momentum mixing over the warmer waters a high resolution numerical model is used to examine the cross frontal flow adjustment in a comprehensive comparison the model agrees well with many observed features of cross equatorial flow and boundary layer structure from satellite tropical atmosphere ocean tao moorings and the recent eastern pacific investigation of climate processes epic campaign in particular the model simulates the acceleration across the sst front and the change from a stable to unstable boundary layer analysis of the model momentum budget indicates that the hydrostatic

seasonal influence of the sea surface temperature on the low atmospheric circulation and precipitation in the eastern equatorial atlantic the air sea interaction in the gulf of guinea and its role in setting precipitation at the guinean coast is investigated in the present paper this study is based on satellite observations and wrf simulations forced by different sea surface temperature sst patterns it shows that the seasonal cold tongue setup in the gulf of guinea along with its very active northern front tends to strongly constrain the low level atmospheric dynamics between the equator and the guinean coast underlying mechanisms including local sst effect on the marine boundary layer stability and hydrostatically changed meridional pressure gradient through changes in sst gradient are quantified in wrf regarding observations and cfsr reanalyses theses mechanisms strongly impact moisture flux convergence near the coast leading to the installation of the first rainy season of the west african monsoon wam system the current study details the mechanisms by which the atlantic equatorial cold tongue plays a major role in the pre onset of the boreal wam

cross equatorial structure and temporal modulation of intraseasonal variability at the surface of the tropical atlantic ocean intraseasonal variability 10 50 days in the equatorial atlantic ocean is analyzed from multiyear 1999 2005 satellite gridded products of sea level anomalies sla and sea surface temperature sst two regions with distinct intraseasonal variability have been identified the first one west of 10 degrees w is dominated by westward propagating anomalies with maximum values in sla along 5 degrees n and in sst along 2 degrees n they occur in boreal summer with periods of 25 50 days and are known to correspond to tropical instability waves tiws we show that tiws have also a signature though weaker south of the equator especially along 5 degrees s in sla northern and southern anomalies propagate together westward being mostly out of phase suggesting that equatorial wave dynamics is involved in tiws variability an sst signature of tiws is also observed near 2 degrees s in quadrature with sst anomalies detected in the northern hemisphere the interannual modulations of the tiw signature in sla and sst are compared and discussed the second dominant intraseasonal signal is only seen east of 10 degrees w in sst and corresponds to an equatorially trapped variability confined to the gulf of guinea with periods between 10 and 20 days this signal is present in boreal summer when an intense sst front is observed just north of the equator intraseasonal variability with comparable periods is also observed in the meridional wind stress throughout the year comparison of sst and meridional wind stress anomalies suggests that the 10 to 20 day variability in sst is forced by the wind stress but seasonally modulated by the presence of the sst front

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

atmospheric manifestation of tropical instability wave observed by quikscat and tropical rain measuring mission observations from two new spaceborne microwave instruments in 1999 clearly reveal the atmospheric manifestation of tropical instability waves north of the pacific equatorial cold tongue a unique zonal temporal band pass filter enables the isolation of the propagating signals and the determination of their phase differences the phase differences between the propagation of wind and sea surface temperature sst signals observed from space and the vertical wind profiles measured from a research ship are consistent with the hypothesis that the coupling between wind and sst is caused by buoyancy instability and mixing which reduces the wind shear in the atmospheric boundary layer the coupling causes higher evaporative cooling over the warm phase and infers a negative thermal feedback

generation of the african easterly jet and its role in determining west african precipitation abstract an examination of analyses and model simulations is used to show that the african easterly jet forms over west africa in summer as a result of strong meridional soil moisture gradients in a series of gcm experiments the imposition of realistic surface wetness contrasts between the sahara and equatorial africa leads to strong positive meridional temperature gradients at the surface and in the lower troposphere the associated easterly shear in the atmosphere is strong enough to establish easterly flow the african easterly jet above the monsoon westerlies at the surface positive temperature gradients associated with the summertime distributions of solar radiation ssts or clouds are not large enough to produce the easterly jet in the absence of soil moisture gradients a thermally direct ageostrophic circulation is identified that can accelerate the largely geostrophic zonal flow and maintain the jet while moisture converges throughout the lower troposphere over east africa moisture divergenc

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

observations of coupling between surface wind stress and sea surface temperature in the eastern tropical pacific satellite measurements of surface wind stress from the quikscat scatterometer and sea surface temperature sst from the tropical rainfall measuring mission microwave imager are analyzed for the three month period 21 july 20 october 1999 to investigate ocean atmosphere coupling in the eastern tropical pacific oceanic tropical instability waves tiws with periods of 20 40 days and wavelengths of 1000 2000 km perturb the sst fronts that bracket both sides of the equatorial cold tongue which is centered near 1 8s to the east of 1308w these perturbations are characterized by cusp shaped features that propagate systematically westward on both sides of the equator the space time structures of these sst perturbations are reproduced with remarkable detail in the surface wind stress field the wind stress divergence is shown to be linearly related to the downwind component of the sst gradient with a response on the south side of the cold tongue that is about twice that on the north side the wind stress curl is linearly related to the crosswind component of the sst gradient with a response that is approximately half that of the wind stress divergence response to the downwind sst gradient the perturbed sst and wind stress fields propagate synchronously westward with the tiws this close coupling between sst and wind stress supports the wallace et al hypothesis that surface winds vary in response to sst modification of atmospheric boundary layer stability